Numerical Investigation of Scouring at the Base

of a Circular Pile in a Steady Tidal Current By

Mark Donnelly-Orr

A thesis submitted to the University of Dublin, Trinity College, in partial fulfilment of the requirements for the degree of

MAI in Mechanical & Manufacturing Engineering

April 2015

Supervisor

Dr. Craig Meskell

Dept. of Mechanical and Manufacturing Engineering

Parsons Building

Trinity College Dublin

Dublin 2, Ireland

Mark Donnelly-Orr

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Declaration

I declare that I am the sole author of this dissertation and that the work present in it, unless

otherwise referenced, is entirely my own. I also declare that the work has not been submitted, in

whole or in part, to any other university as an exercise for a degree or any other qualification.

I agree that the library of Trinity College Dublin may lend or copy this dissertation upon request.

Mark Donnelly-Orr

Date: 7th April 2015

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Mark Donnelly-Orr

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Abstract

The flow around a cylindrical pile exposed to a steady current is numerically investigated and the

extent of scouring that occurs around the base is assessed.

The primary objectives of the report is to determine the extent of scouring around the base of a

wind turbine pylon installed as part of the Dublin Array on the Kish and Bray Banks in Dublin Bay,

Ireland. The effect different sea current speeds will have on the extent of the scour will also be

determined.

The secondary objective of the report is to investigate the scour mitigating effects of various

scour prevention devices designed in this report, but based of different ideas found in the

academic literature. Various collars and helical wires were considered and the effects of the

devices examined.

The methodology of the report involved investigating the types of sediment, marine conditions

and the physical nature of the turbine pylons expected in the Dublin Array; these parameters

where then implemented into ANSYS Fluent, a computational fluid dynamics (CFD) numerical

model, and a solution numerically calculated.

It was determined that localised clear-water scouring initially occurred at the base of the pile

once a sea current of 0.225-0.275m/s arose. As the sea current increased, the extent of the

scouring region increased from where it initially occurred. Once a sea current of 0.4-0.6m/s arose,

live-bed scouring was deemed to occur and the entire seabed was in motion. The sea current at

which these transitions occurred depended on the sediment size, which varied from 0.2-0.8mm

diameters on the Kish and Bray Sand Banks.

The scour prevention devices designed were shown to have a substantial effect on the flow

regime around the pile, disrupting the magnitude and momentum of the horseshoe vortices that

normally form around circular piles and cause scouring on the seabed. Irregularities in the scour

prevention device simulations results reduced the confidence of the conclusions made about the

devices designed.

The main conclusions drawn from the report is that scouring will occur around the wind turbine

pylon bases that are installed on the Kish and Bray Banks as part of the Dublin Array. But given

the self-nourishing aspect of the sand banks, the extent of scouring is deemed not to be a

permanent feature of the seabed around the wind turbine pylon bases, but will gradually increase

and decrease depending on the tidal conditions. A secondary conclusion is that the scour

prevention devices are effective at disrupting the horseshoe vortices that would otherwise occur

around a circular pile, and hence will reduce the effects of scouring.

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Keywords

Scour, Offshore Wind Turbine, Monopile, SST Transition Model, ANSYS Fluent, Sediment

Movement, Dublin Bay, Steady Current.

Acknowledgements

I would like to thank my Supervisor Dr. Craig Meskell for his support, teachings, enthusiasm, and

guidance throughout this project.

I would like to acknowledge the entire academic staff of the Department of Mechanical and

Manufacturing Engineering at Trinity College for their support and assistance during my time at

Trinity College Dublin.

In addition I would like to thank my friends Jaakko, Rupert, and Aaron for their support, advice

and welcomed distractions.

I would also like to thank my girlfriend Amy, who was always there for me.

Lastly, I would like to thank my family, especially my parents, Peter and Wendy, for their constant

encouragement and support throughout the duration of this thesis and my time at Trinity College

Dublin.

Mark Donnelly-Orr

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Table of Contents Declaration .............................................................................................................................................................. i

Abstract ................................................................................................................................................................. iii

Keywords ............................................................................................................................................................... iv

Acknowledgements ............................................................................................................................................... iv

Table of Figures ...................................................................................................................................................... x

Table of Tables.................................................................................................................................................... xviii

Nomenclature ....................................................................................................................................................... xix

Abbreviations ................................................................................................................................................... xix

Units ................................................................................................................................................................. xix

1 Introduction ................................................................................................................................................... 1

1.1 Background ........................................................................................................................................... 1

1.2 Problem Definition ................................................................................................................................ 1

1.3 Objectives ............................................................................................................................................. 2

1.4 Methodology ......................................................................................................................................... 2

1.5 Outline .................................................................................................................................................. 2

2 Context .......................................................................................................................................................... 4

2.1 Global Warming .................................................................................................................................... 4

2.2 EU Climate Goals ................................................................................................................................... 4

2.3 Ireland’s Climate Goals ......................................................................................................................... 4

2.4 Wind Energy .......................................................................................................................................... 6

2.5 Offshore Wind Energy ........................................................................................................................... 7

2.6 Successful Installations around the World .......................................................................................... 10

3 Literature Review ......................................................................................................................................... 12

3.1 Site Data .............................................................................................................................................. 12

3.1.1 Site Layout .................................................................................................................................. 12

3.1.2 Sediment .................................................................................................................................... 13

3.1.3 Tidal Flows .................................................................................................................................. 21

3.1.4 Sea Water Properties ................................................................................................................. 22

3.1.5 Boundary Layer Formations ....................................................................................................... 23

3.1.6 Foundation Type ........................................................................................................................ 28

3.2 Scouring .............................................................................................................................................. 30

3.2.1 Fundamental Fluid Mechanics ................................................................................................... 30

3.2.2 Different Features ...................................................................................................................... 32

3.2.3 Current Based ............................................................................................................................. 38

3.2.4 Maximum Scour Depth .............................................................................................................. 38

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3.2.5 Equilibrium Scour Depth............................................................................................................. 39

3.2.6 Clear Water Scour vs. Live Bed Scour Criterion .......................................................................... 39

3.2.7 Scour Protection & Prevention Measures .................................................................................. 41

3.2.8 Shear Stresses Acting on the Seabed ......................................................................................... 49

3.2.9 Sediment Movement .................................................................................................................. 50

3.3 Computational Fluid Dynamics ........................................................................................................... 58

3.3.1 Software Used ............................................................................................................................ 58

3.3.2 Turbulence Model Chosen ......................................................................................................... 58

3.3.3 Benefits of Chosen Model .......................................................................................................... 59

3.3.4 Limits of Chosen Model .............................................................................................................. 59

3.3.5 Validation of Choice ................................................................................................................... 59

3.4 Project Validation ................................................................................................................................ 60

4 Methodology................................................................................................................................................ 61

4.1 Creating the 3D Model ........................................................................................................................ 61

4.2 Meshing Development ........................................................................................................................ 62

4.2.1 Basic Mesh .................................................................................................................................. 62

4.2.2 Initial Bias ................................................................................................................................... 62

4.2.3 Symmetry ................................................................................................................................... 64

4.2.4 Hex-Dominant Meshing .............................................................................................................. 64

4.2.5 Volume Meshing......................................................................................................................... 66

4.2.6 Refined Volume Meshing ........................................................................................................... 67

4.2.7 Inflation Layer Details & Issues .................................................................................................. 70

4.2.8 Element Count Limits ................................................................................................................. 71

4.2.9 Final Meshing ............................................................................................................................. 71

4.3 ANSYS 3D Model Parameters and Boundary Conditions .................................................................... 76

4.3.1 UDF ............................................................................................................................................. 76

4.3.2 Fluid Properties .......................................................................................................................... 76

4.3.3 Inlet Velocity ............................................................................................................................... 77

4.3.4 Setup Options ............................................................................................................................. 77

4.3.5 ANSYS Model Used ..................................................................................................................... 78

4.3.6 Zero Shear on Walls .................................................................................................................... 78

4.4 Meshing Independence ....................................................................................................................... 80

4.4.1 Meshing Independence Tests Development .............................................................................. 80

4.4.2 Meshing Independence Tests Results ........................................................................................ 86

4.4.3 Refined Final Meshing ................................................................................................................ 92

4.5 Mesh Validation .................................................................................................................................. 95

4.5.1 Mesh Validation Techniques ...................................................................................................... 95

4.5.2 Mesh Validation Results ........................................................................................................... 103

4.6 Creating the Scour Prevention 3D Models ........................................................................................ 110

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4.6.1 Rectangular Collar .................................................................................................................... 110

4.6.2 Triangular Collar ....................................................................................................................... 111

4.6.3 Rounded Collar ......................................................................................................................... 111

4.6.4 Helical Wires ............................................................................................................................. 112

4.7 Meshing Development for Scour Prevention Models ....................................................................... 115

4.7.1 Volume Meshing ...................................................................................................................... 115

4.7.2 Meshing Details ........................................................................................................................ 115

4.7.3 Inflation Layer Details & Issues ................................................................................................ 116

4.7.4 Element Count, Scour Prevention Models ............................................................................... 116

4.7.5 Final Meshing ........................................................................................................................... 116

4.8 ANSYS Scour Prevention Model Parameters and Boundary Conditions ........................................... 120

4.8.1 UDF ........................................................................................................................................... 120

4.8.2 Setup Options ........................................................................................................................... 120

4.8.3 Fluid Properties ........................................................................................................................ 120

4.8.4 Inlet Velocity ............................................................................................................................ 120

4.8.5 ANSYS Model Used ................................................................................................................... 120

4.8.6 Zero Shear on Walls ................................................................................................................. 120

4.9 Determining if Scour will occur ......................................................................................................... 121

4.9.1 Stresses on Seabed ................................................................................................................... 121

4.9.2 Streamlines ............................................................................................................................... 123

4.9.3 y-Velocity Component .............................................................................................................. 125

5 Results ....................................................................................................................................................... 126

5.1 3D Model .......................................................................................................................................... 126

5.1.1 Scour Regions of 3D Model ...................................................................................................... 126

5.1.2 Streamlines of 3D Models ........................................................................................................ 139

5.1.3 y-Velocity Component of 3D Model ......................................................................................... 141

5.2 Scour Prevention Models .................................................................................................................. 143

5.2.1 Scour Regions of Scour Prevention Devices ............................................................................. 143

5.2.2 Streamlines of Scour Prevention Devices ................................................................................. 146

5.2.3 y-Velocity Components of Scour Prevention Devices .............................................................. 150

6 Discussion .................................................................................................................................................. 153

6.1 3D Model .......................................................................................................................................... 153

6.1.1 Scour Region Shape .................................................................................................................. 153

6.1.2 Streamlines ............................................................................................................................... 158

6.1.3 y-Velocity .................................................................................................................................. 159

6.2 Clear-Water/Live-Bed Criterion ........................................................................................................ 160

6.3 Downstream Vortices ....................................................................................................................... 162

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6.4 Scour Prevention Devices .................................................................................................................. 166

6.4.1 Scour Region ............................................................................................................................. 167

6.4.2 Streamlines ............................................................................................................................... 168

6.4.3 y-Velocity .................................................................................................................................. 170

6.4.4 Scour Prevention Device Logistical Factors .............................................................................. 171

6.4.5 Best Choice ............................................................................................................................... 173

6.5 Meshing ............................................................................................................................................. 174

6.6 Mesh Validation ................................................................................................................................ 176

6.6.1 Coefficient of Pressure Distribution around Pile Wall .............................................................. 177

6.6.2 Coefficient of Pressure Distribution along Upstream Pile Wall ................................................ 178

6.6.3 Coefficient of Pressure Distribution along Upstream Symmetry Line ...................................... 178

6.6.4 Wall Shear Distribution along Upstream Symmetry Line ......................................................... 179

6.6.5 Boundary Layer Formation ....................................................................................................... 179

6.6.6 Viscous Sublayer ....................................................................................................................... 180

6.6.7 Mesh Validation Summary ....................................................................................................... 181

6.7 Meshing Independence ..................................................................................................................... 182

7 Recommendations ..................................................................................................................................... 183

7.1 3D Model Improvement .................................................................................................................... 183

7.2 Scour Prevention Models Improvement ........................................................................................... 183

7.3 General Improvements ..................................................................................................................... 184

8 Conclusion .................................................................................................................................................. 185

9 References ................................................................................................................................................. 186

10 Appendices ............................................................................................................................................ 190

10.1 A - Folk’s Classification System ......................................................................................................... 190

10.2 B - Definition of Phi ........................................................................................................................... 193

10.3 C - y+ Definition ................................................................................................................................. 194

10.4 D - UDF Code ..................................................................................................................................... 195

10.5 E - Seabed Shear Stress Calculations ................................................................................................. 196

10.6 F - Seabed Shear Stress Calculations, Various Sediment Sizes .......................................................... 200

10.6.1 Wet Packed Sand; Sediment Density: 2082kg/m3 .................................................................... 200

10.6.2 Sand, Water Filled; Sediment Density: 1922kg/m3 .................................................................. 200

10.6.3 Sand with Gravel, wet; Sediment Density: 2020kg/m3 ............................................................ 200

10.7 G - Seabed Shear Stresses ................................................................................................................. 201

10.7.1 Varying Current Speed ............................................................................................................. 201

10.7.2 Varying Sediment Size .............................................................................................................. 246

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10.8 H - 3D Model Streamlines ................................................................................................................. 296

10.8.1 0.2m/s Streamlines .................................................................................................................. 296

10.8.2 0.7m/s Current Speed .............................................................................................................. 300

10.8.3 1.42m/s Current Speed ............................................................................................................ 303

10.9 I - Meshing Independence Results Tables ......................................................................................... 307

10.10 J - Scour Prevention Models Streamlines ..................................................................................... 309

10.10.1 Basic Model Streamlines ...................................................................................................... 309

10.10.2 Rectangular Collar Model Streamlines ................................................................................ 313

10.10.3 Triangular Collar Model Streamlines ................................................................................... 316

10.10.4 Rounded Collar Model Streamlines ..................................................................................... 320

10.10.5 Helical Wire (Half Wire) Model Streamlines ........................................................................ 323

10.10.6 Helical Wire (Full Wire) Model Streamlines ......................................................................... 327

10.11 K - Finite Length Pile Model Results ............................................................................................. 331

10.11.1 Model Geometry and Meshing ............................................................................................ 331

10.11.2 Finite Length Pile Model Streamlines .................................................................................. 333

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Table of Figures

Figure 2-1 - Renewable Electricity Growth to 2010 [8] ................................................................................................... 5 Figure 2-2 - Electricity Generated by Wind (GWh) 1990-2012 [9] .................................................................................. 5 Figure 2-3 - Installed Wind Generating Capacity 2000 – 2012 [9] ................................................................................... 6 Figure 2-4 Viewshed of Dublin Array on the Surrounding Area [23] ............................................................................... 9 Figure 2-5 Location of offshore wind turbine arrays. [24] ............................................................................................ 11 Figure 3-1 Site Layout on the Kish and Bray Banks, wind turbines indicated by the black dots ................................... 12 Figure 3-2 Location of Site Investigation Boreholes [25] ............................................................................................... 14 Figure 3-3 Sediment Distribution Results for Offshore Borehole 1 into Kish Sand Bank [25] ....................................... 14 Figure 3-4 Sediment Distribution Results for Offshore Borehole 2 into Kish Sand Bank [25] ....................................... 15 Figure 3-5 Sediment Distribution Results for Offshore Borehole 3 into Kish Sand Bank [25] ....................................... 15 Figure 3-6 Sediment Type Classification based on percentage sand, mud and gravel (after Folk [28]) [27] ................ 17 Figure 3-7 Spatial Distribution of Derived Sediment Types [27] ................................................................................... 18 Figure 3-8 Variation in Mean Particle Size [27] ............................................................................................................. 19 Figure 3-9 Location of Biological Trawls Traversing Proposed Development and Samples Presented in ([27]) [29] .... 20 Figure 3-10 Location of Recording Stations in Kish Banks [32]...................................................................................... 21 Figure 3-11 Development of a boundary layer as it progresses along a flat plate and the distortion of a fluid particle as it flows within the boundary layer. [38] ....................................................................................................... 23 Figure 3-12 a) Velocity profile for turbulent water flow plotted using a linear scale for both the horizontal and vertical axis. b) The same velocity data as in a), plotted using a log10 vertical scale and linear horizontal scale. [39] . 24 Figure 3-13 Velocity Profile for water flow using a Power Law. Both axis are linear scale. .......................................... 24 Figure 3-14 Velocity Profile for water flow using a Power Law. The vertical axis using a log10 scale, and the horizontal axis using a linear scale. ............................................................................................................................... 25 Figure 3-15 Boundary Layer Velocity Profile [38] .......................................................................................................... 26 Figure 3-16 Viscous Sublayer Velocity Profile [40] ........................................................................................................ 27 Figure 3-17 Share of Substructure Types for Online Wind Farms End 2011 [43] .......................................................... 28 Figure 3-18 Monopile Foundation [44] ......................................................................................................................... 29 Figure 3-19 Flow around a cylindrical pile, Isometric View [42].................................................................................... 30 Figure 3-20 Flow around a pile, Side View [55] ............................................................................................................. 31 Figure 3-21 Formation of Horseshoe Vortices [56] ....................................................................................................... 31 Figure 3-22 Flow around a cylindrical object, Top View [57] ........................................................................................ 31 Figure 3-23 Separation Distance Xs/D as function of δ/D. [42] ..................................................................................... 32 Figure 3-24 Ultimate Scour Depth (Suc) as a function of diameter of obstruction [60] ................................................. 33 Figure 3-25 Scour Depth vs. Time Curves for Pier Shape Effects Test [61].................................................................... 34 Figure 3-26 Separation Distance Xs/D as function of δ/D. [42] ..................................................................................... 35 Figure 3-27 Influence of the pile Reynolds number (a) Separation distance Xs/D. (b) Maximum bed shear stress amplification under the horseshoe vortex on the upstream symmetry line. [42] ........................................................ 36 Figure 3-28 Suc/D as a function of flow Froude number for different model sizes. [60] ............................................... 37 Figure 3-29 Equilibrium Scour Depth as a Function of Mean Approach Flow Velocity [71] .......................................... 40 Figure 3-30 Flexible Scour Protection around a Circular Pile [76] ................................................................................. 41 Figure 3-31 Flow around a Monopile with Bed Protection. [77] ................................................................................... 42 Figure 3-32 Bed Degradation Erosion around Pile with Riprap, white arrow indicates current flow direction [76] .... 42 Figure 3-33 Scour Prevention Mats, before and after installation [78] ......................................................................... 44 Figure 3-34 Description of how the Scour Prevention Mats work. [78] ........................................................................ 45 Figure 3-35 Three Dimensional Bathymetric Surveys of the seabed around a monopile foundation before and after Scour Prevention Mat installation. [79] ................................................................................................................ 46 Figure 3-36 Threaded Pile (Helical Wires or Cables wrapped spirally on the pile to form thread [80] ......................... 46 Figure 3-37 Vortex flow fields at the upstream plane of symmetry of an unprotected pile[80] .................................. 47 Figure 3-38 Vortex flow fields at the upstream plane of symmetry of a triple threaded pile [80] ............................... 47 Figure 3-39 Scour around an Unprotected Pile (current only) [83] ............................................................................... 48 Figure 3-40 Edge Scour at the pile protected by a small collar (current only) [83] ....................................................... 49 Figure 3-41 Scour at the pile protected by a large collar (current only) [83] ................................................................ 49 Figure 3-42 Modes of Sediment Transport [39] ............................................................................................................ 50 Figure 3-43 Diagram showing the range of current speeds at which sediment particles of different sizes are eroded and their form of transportation. [39] .............................................................................................................. 51 Figure 3-44 The Hjulström curve [87] ............................................................................................................................ 52 Figure 3-45 Forces acting on a sediment particle resting on a bed of similar particles. [88] ........................................ 52 Figure 3-46 Forces acting on a stationary sediment grain resting on a bed of similar grains in a flow. [39] ................ 53

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Figure 3-47 Lift and Drag on a bed sediment particle. [88, 89] ..................................................................................... 53 Figure 3-48 The Shields Diagram ................................................................................................................................... 55 Figure 3-49 A modified and updated version of Shields Diagram. [88, 92] ................................................................... 56 Figure 3-50 Updated Shields Diagram, recast in terms of shear velocity, U*, and particle diameter, D. ....................... 57 Figure 4-1 3D Model, Isometric View ............................................................................................................................ 61 Figure 4-2 Bias Meshing, viewed from below. ............................................................................................................... 63 Figure 4-3 Bias meshing and edge sizing at base of pylon, view from below. ............................................................... 63 Figure 4-4 3D Model with Symmetry Applied, Isometric View ...................................................................................... 64 Figure 4-5 Tetrahedral Meshing Structure, Isometric view ........................................................................................... 65 Figure 4-6 Hexahedral Meshing Structure, Isometric view ............................................................................................ 65 Figure 4-7 Initial Volume Meshing, isometric view ........................................................................................................ 66 Figure 4-8 Initial Volume Meshing, Reserve Isometric View ......................................................................................... 67 Figure 4-9 Revised Volume Meshing.............................................................................................................................. 68 Figure 4-10 Initial Volume Meshing around pile ............................................................................................................ 69 Figure 4-11 Revised Volume Meshing around pile ........................................................................................................ 69 Figure 4-12 y+ value on pile wall ................................................................................................................................... 71 Figure 4-13 Overview of Final Meshing ......................................................................................................................... 72 Figure 4-14 Close-Up Overview of Final Meshing .......................................................................................................... 72 Figure 4-15 Reverse View Close-Up of Final Meshing .................................................................................................... 73 Figure 4-16 Close-Up of Pile Meshing ............................................................................................................................ 73 Figure 4-17 Close-Up of Inflation Layer ......................................................................................................................... 74 Figure 4-18 Named Blocks ............................................................................................................................................. 74 Figure 4-19 ANSYS Fluent Setup Launcher Options ....................................................................................................... 77 Figure 4-20 Surfaces with zero shear stress................................................................................................................... 79 Figure 4-21 Monitoring Points, Top View ...................................................................................................................... 81 Figure 4-22 Monitoring Points, Top View, Close Up ...................................................................................................... 81 Figure 4-23 Monitoring Points, Side View, Close Up...................................................................................................... 82 Figure 4-24 Meshing, Independence Meshing Test Model 1 ......................................................................................... 83 Figure 4-25 Meshing, Independence Meshing Test Model 2 ......................................................................................... 84 Figure 4-26 Meshing, Independence Meshing Test Model 3 ......................................................................................... 84 Figure 4-27 Monitoring Points 1-8 ................................................................................................................................. 85 Figure 4-28 Monitoring Points 9-16 ............................................................................................................................... 85 Figure 4-29 Plotted Monitor Points, Pressure Values, Points 1-8 .................................................................................. 88 Figure 4-30 Plotted Monitor Points, Pressure Values, Points 9-16 ................................................................................ 88 Figure 4-31 Plotted Monitor Points, Wall Shear Values, Points 1-8 ............................................................................... 89 Figure 4-32 Plotted Monitor Points, Velocity Values, Points 9-16 ................................................................................. 89 Figure 4-33 Seabed Shear Stress, Basic Model .............................................................................................................. 90 Figure 4-34 Seabed Shear Stress, Meshing Independence Test 1 ................................................................................. 90 Figure 4-35 Seabed Shear Stress, Meshing Independence Test 2 ................................................................................. 91 Figure 4-36 Seabed Shear Stress, Meshing Independence Test 3 ................................................................................. 91 Figure 4-37 Overview of Refined Final Meshing, Isometric View .................................................................................. 93 Figure 4-38 Overview of Refined Final Meshing, Isometric View, Close Up .................................................................. 93 Figure 4-39 Overview of Refined Final Meshing, Reverse Isometric View .................................................................... 94 Figure 4-40 Overview of Refined Final Meshing, Reverse Isometric View, Close Up ..................................................... 94 Figure 4-41 Mean Pressure Distribution on the Pile [37]............................................................................................... 95 Figure 4-42 Pressure Distribution around Pile Wall Data Source .................................................................................. 96 Figure 4-43 Pressure Coefficient Distribution along the length of the upstream edge of the pile, [42]........................ 98 Figure 4-44 Pressure Distribution along Upstream Edge of Pile Data Source ................................................................ 99 Figure 4-45 Coefficient of Pressure Distribution on the Seabed along the upstream symmetry line. Note: the pressure coefficient is normalized by the pressure coefficient at the toe of the pile, [42] ......................................... 100 Figure 4-46 Pressure Distribution along Upstream Symmetry Line Data Source ........................................................ 101 Figure 4-47 Seabed Shear Stress amplification along upstream symmetry line [42]................................................... 102 Figure 4-48 Boundary Layer Data Line, Isometric view ................................................................................................ 103 Figure 4-49 Boundary Layer Data Line, Z-axis view...................................................................................................... 103 Figure 4-50 Comparison of Pressure Distribution around Pile Wall Data .................................................................... 104 Figure 4-51 Comparison of Pressure Distribution Data along Upstream Pile Wall ...................................................... 105 Figure 4-52 Comparison of Pressure Distribution Data along Upstream Symmetry Line ............................................ 106 Figure 4-53 Comparison of Wall Shear Data along Upstream Symmetry Line ............................................................. 107 Figure 4-54 Boundary Layer Formation Check, 0.2m/s ................................................................................................ 108

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Figure 4-55 Boundary Layer Formation Check, 1.42m/s ............................................................................................. 108 Figure 4-56 Velocity Contour Plane, 0.2m/s ................................................................................................................ 109 Figure 4-57 Velocity Contour Plane, 1.42m/s .............................................................................................................. 109 Figure 4-58 Rectangular Collar, Side and Isometric View ............................................................................................ 110 Figure 4-59 Triangular Collar, Side and Isometric View ............................................................................................... 111 Figure 4-60 Rounded Collar, Side and Isometric View ................................................................................................ 111 Figure 4-61 Helical Wire (Full Wire), Side and Isometric View .................................................................................... 112 Figure 4-62 Helical Wire (Full Wire), Filleted, Side and Isometric View ...................................................................... 113 Figure 4-63 Helical Wire (Half Wire), Side and Isometric View ................................................................................... 113 Figure 4-64 Helical Wire (Half Wire), Filleted, Side and Isometric View ..................................................................... 114 Figure 4-65 Volume Meshing for Scour Prevention Devices ....................................................................................... 115 Figure 4-66 Named Blocks ........................................................................................................................................... 116 Figure 4-67 Rectangular Collar, Final Meshing ............................................................................................................ 117 Figure 4-68 Triangular Collar, Final Meshing ............................................................................................................... 117 Figure 4-69 Rounded Collar, Final Meshing................................................................................................................. 118 Figure 4-70 Helical Wire (Full Wire), Final Meshing .................................................................................................... 118 Figure 4-71 Helical Wire (Half Wire), Final Meshing ................................................................................................... 119 Figure 4-72 Shields Diagram with Various Sediment Sizes .......................................................................................... 122 Figure 4-73, Streamlines Source Plane, Close Up Isometric View ............................................................................... 124 Figure 4-74 Streamlines Source Plane, Source Points ................................................................................................. 124 Figure 4-75 y-Velocity Component Source Plane, Isometric View .............................................................................. 125 Figure 5-1 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0005m ................................................. 127 Figure 5-2 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0005m ................................................. 127 Figure 5-3 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0005m ............................................... 128 Figure 5-4 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0005m ................................................. 128 Figure 5-5 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0005m ............................................... 129 Figure 5-6 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0005m ................................................. 129 Figure 5-7 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0005m ............................................... 130 Figure 5-8 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0005m ................................................. 130 Figure 5-9 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0005m ............................................... 131 Figure 5-10 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0005m ............................................... 131 Figure 5-11 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0005m ............................................... 132 Figure 5-12 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0005m ............................................... 132 Figure 5-13 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0005m ............................................... 133 Figure 5-14 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0005m ............................................... 133 Figure 5-15 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0005m ............................................. 134 Figure 5-16 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0002m ............................................... 135 Figure 5-17 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0003m ............................................... 136 Figure 5-18 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0004m ............................................... 136 Figure 5-19 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0005m ............................................... 137 Figure 5-20 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0006m ............................................... 137 Figure 5-21 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0007m ............................................... 138 Figure 5-22 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0008m ............................................... 138 Figure 5-23 Streamlines for 0.2m/s current, Isometric View ...................................................................................... 139 Figure 5-24 Streamlines for 0.7m/s current, Isometric View ...................................................................................... 140 Figure 5-25 Streamlines for 1.42m/s current, Isometric View .................................................................................... 140 Figure 5-26 y-Velocity Component, 0.2m/s Current, Symmetry Wall Plane ............................................................... 141 Figure 5-27 y-Velocity Component, 0.7m/s Current, Symmetry Wall Plane ............................................................... 141 Figure 5-28 y-Velocity Component, 1.42m/s Current, Symmetry Wall Plane ............................................................. 142 Figure 5-29 Seabed Shear Stress, Basic Model, Current Speed: 0.5m/s, Sediment Size: 0.0008m ............................. 143 Figure 5-30 Seabed Shear Stress, Rectangular Collar, Current Speed: 0.5m/s, Sediment Size: 0.0008m ................... 144 Figure 5-31 Seabed Shear Stress, Triangular Collar, Current Speed: 0.5m/s, Sediment Size: 0.0008m ...................... 144 Figure 5-32 Seabed Shear Stress, Rounded Collar, Current Speed: 0.5m/s, Sediment Size: 0.0008m ........................ 145 Figure 5-33 Seabed Shear Stress, Helical Wire (Half Wire), Current Speed: 0.5m/s, Sediment Size: 0.0008m ........... 145 Figure 5-34 Seabed Shear Stress, Helical Wire (Full Wire), Current Speed: 0.5m/s, Sediment Size: 0.0008m ........... 146 Figure 5-35 Streamlines for Basic Model, 0.5m/s current, Isometric View ................................................................. 147 Figure 5-36 Streamlines for Rectangular Collar Model, 0.5m/s current, Isometric View ............................................ 147 Figure 5-37 Streamlines for Triangular Collar Model, 0.5m/s current, Isometric View .............................................. 148 Figure 5-38 Streamlines for Rounded Collar Model, 0.5m/s current, Isometric View ................................................ 148

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Figure 5-39 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Isometric View .................................... 149 Figure 5-40 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Isometric View .................................... 149 Figure 5-41 y-Velocity Component, Basic Model, 0.5m/s Current, Symmetry Wall Plane .......................................... 150 Figure 5-42 y-Velocity Component, Rectangular Collar Model, 0.5m/s Current, Symmetry Wall Plane ..................... 151 Figure 5-43 y-Velocity Component, Triangular Collar Model, 0.5m/s Current, Symmetry Wall Plane ........................ 151 Figure 5-44 y-Velocity Component, Rounded Collar Model, 0.5m/s Current, Symmetry Wall Plane .......................... 152 Figure 5-45 y-Velocity Component, Helical Wire (Half Wire) Collar Model, 0.5m/s Current, Symmetry Wall Plane .. 152 Figure 5-46 y-Velocity Component, Helical Wire (Full Wire) Model, 0.5m/s Current, Symmetry Wall Plane ............. 153 Figure 6-1 Seabed Shear Stress Amplification. (a) Numerical Model Results, Published Study [42]. (b) Experimental Results, Published Study [86]. ...................................................................................................................................... 154 Figure 6-2 Seabed Shear Stress Amplification, CFD Results, Current Study................................................................. 154 Figure 6-3 Overview of flow around a Wall Mounted Cylindrical Pile, [110] ............................................................... 163 Figure 6-4 Mean Arched Vortices visualised by streamlines [108] .............................................................................. 163 Figure 6-5 Mean Arched Vortices visualised by streamlines, Finite length Pile Model ............................................... 165 Figure 6-6 Downward Trailing Vortices visualised by Streamlines, Finite length Pile Model, Side View ..................... 165 Figure 6-7 Meshing Irregularities ................................................................................................................................. 174 Figure 6-8 Comparison of the CFD Velocity Profile results with theoretical Linear Velocity profile and the Velocity profile developed in the UDF code .............................................................................................................................. 181 Figure 6-9 Meshing Independence Test Irregularity .................................................................................................... 182 Figure 10-1 The 15 major textural groups [28] ............................................................................................................ 190 Figure 10-2 Textural Names of Classifications seen in Figure 82 [28] .......................................................................... 191 Figure 10-3 Expansion of the bottom tier of Figure 82 [28] ........................................................................................ 192 Figure 10-4 Particle Size shown in Phi and mm, and related to the Wentworth and Folk's Classification Schemes [27] ............................................................................................................................................................................... 193 Figure 10-5 Shields Diagram showing the data line for 0.0002m sediment vs. the Shields Curve .............................. 198 Figure 10-6 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0002m ............................................... 201 Figure 10-7 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0002m ............................................... 202 Figure 10-8 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0002m ............................................. 202 Figure 10-9 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0002m ............................................... 203 Figure 10-10 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0002m ........................................... 203 Figure 10-11 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0002m ............................................. 204 Figure 10-12 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0002m ........................................... 204 Figure 10-13 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0002m ............................................. 205 Figure 10-14 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0002m ........................................... 205 Figure 10-15 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0002m ............................................. 206 Figure 10-16 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0002m ............................................. 206 Figure 10-17 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0002m ............................................. 207 Figure 10-18 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0002m ............................................. 207 Figure 10-19 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0002m ............................................. 208 Figure 10-20 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0002m ........................................... 208 Figure 10-21 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0003m ............................................. 209 Figure 10-22 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0003m ............................................. 209 Figure 10-23 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0003m ........................................... 210 Figure 10-24 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0003m ............................................. 210 Figure 10-25 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0003m ........................................... 211 Figure 10-26 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0003m ............................................. 211 Figure 10-27 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0003m ........................................... 212 Figure 10-28 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0003m ............................................. 212 Figure 10-29 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0003m ........................................... 213 Figure 10-30 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0003m ............................................. 213 Figure 10-31 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0003m ............................................. 214 Figure 10-32 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0003m ............................................. 214 Figure 10-33 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0003m ............................................. 215 Figure 10-34 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0003m ............................................. 215 Figure 10-35 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0003m ........................................... 216 Figure 10-36 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0004m ............................................. 216 Figure 10-37 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0004m ............................................. 217 Figure 10-38 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0004m ........................................... 217 Figure 10-39 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0004m ............................................. 218

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Figure 10-40 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0004m ........................................... 218 Figure 10-41 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0004m ............................................. 219 Figure 10-42 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0004m ........................................... 219 Figure 10-43 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0004m ............................................. 220 Figure 10-44 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0004m ........................................... 220 Figure 10-45 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0004m ............................................. 221 Figure 10-46 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0004m ............................................. 221 Figure 10-47 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0004m ............................................. 222 Figure 10-48 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0004m ............................................. 222 Figure 10-49 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0004m ............................................. 223 Figure 10-50 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0004m ........................................... 223 Figure 10-51 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0006m ............................................. 224 Figure 10-52 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0006m ............................................. 224 Figure 10-53 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0006m ........................................... 225 Figure 10-54 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0006m ............................................. 225 Figure 10-55 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0006m ........................................... 226 Figure 10-56 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0006m ............................................. 226 Figure 10-57 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0006m ........................................... 227 Figure 10-58 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0006m ............................................. 227 Figure 10-59 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0006m ........................................... 228 Figure 10-60 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0006m ............................................. 228 Figure 10-61 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0006m ............................................. 229 Figure 10-62 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0006m ............................................. 229 Figure 10-63 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0006m ............................................. 230 Figure 10-64 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0006m ............................................. 230 Figure 10-65 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0006m ........................................... 231 Figure 10-66 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0007m ............................................. 231 Figure 10-67 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0007m ............................................. 232 Figure 10-68 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0007m ........................................... 232 Figure 10-69 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0007m ............................................. 233 Figure 10-70 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0007m ........................................... 233 Figure 10-71 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0007m ............................................. 234 Figure 10-72 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0007m ........................................... 234 Figure 10-73 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0007m ............................................. 235 Figure 10-74 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0007m ........................................... 235 Figure 10-75 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0007m ............................................. 236 Figure 10-76 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0007m ............................................. 236 Figure 10-77 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0007m ............................................. 237 Figure 10-78 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0007m ............................................. 237 Figure 10-79 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0007m ............................................. 238 Figure 10-80 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0007m ........................................... 238 Figure 10-81 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0008m ............................................. 239 Figure 10-82 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0008m ............................................. 239 Figure 10-83 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0008m ........................................... 240 Figure 10-84 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0008m ............................................. 240 Figure 10-85 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0008m ........................................... 241 Figure 10-86 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0008m ............................................. 241 Figure 10-87 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0008m ........................................... 242 Figure 10-88 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0008m ............................................. 242 Figure 10-89 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0008m ........................................... 243 Figure 10-90 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0008m ............................................. 243 Figure 10-91 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0008m ............................................. 244 Figure 10-92 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0008m ............................................. 244 Figure 10-93 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0008m ............................................. 245 Figure 10-94 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0008m ............................................. 245 Figure 10-95 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0008m ........................................... 246 Figure 10-96 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0002m ............................................. 247 Figure 10-97 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0003m ............................................. 247 Figure 10-98 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0004m ............................................ 248

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Figure 10-99 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0005m ............................................. 248 Figure 10-100 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0006m ........................................... 249 Figure 10-101 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0007m ........................................... 249 Figure 10-102 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0008m ........................................... 250 Figure 10-103 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0002m ........................................... 250 Figure 10-104 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0003m ........................................... 251 Figure 10-105 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0004m ........................................... 251 Figure 10-106 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0005m ........................................... 252 Figure 10-107 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0006m ........................................... 252 Figure 10-108 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0007m ........................................... 253 Figure 10-109 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0008m ........................................... 253 Figure 10-110 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0002m ......................................... 254 Figure 10-111 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0003m ......................................... 254 Figure 10-112 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0004m......................................... 255 Figure 10-113 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0005m......................................... 255 Figure 10-114 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0006m ......................................... 256 Figure 10-115 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0007m ......................................... 256 Figure 10-116 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0008m ......................................... 257 Figure 10-117 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0002m ........................................... 257 Figure 10-118 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0003m ........................................... 258 Figure 10-119 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0004m ........................................... 258 Figure 10-120 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0005m ........................................... 259 Figure 10-121 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0006m ........................................... 259 Figure 10-122 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0007m ........................................... 260 Figure 10-123 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0008m ........................................... 260 Figure 10-124 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0002m ......................................... 261 Figure 10-125 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0003m ......................................... 261 Figure 10-126 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0004m......................................... 262 Figure 10-127 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0005m......................................... 262 Figure 10-128 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0006m ......................................... 263 Figure 10-129 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0007m ......................................... 263 Figure 10-130 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0008m ......................................... 264 Figure 10-131 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0002m ........................................... 264 Figure 10-132 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0003m ........................................... 265 Figure 10-133 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0004m ........................................... 265 Figure 10-134 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0005m ........................................... 266 Figure 10-135 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0006m ........................................... 266 Figure 10-136 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0007m ........................................... 267 Figure 10-137 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0008m ........................................... 267 Figure 10-138 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0002m ......................................... 268 Figure 10-139 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0003m ......................................... 268 Figure 10-140 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0004m......................................... 269 Figure 10-141 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0005m......................................... 269 Figure 10-142 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0006m ......................................... 269 Figure 10-143 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0007m ......................................... 270 Figure 10-144 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0008m ......................................... 270 Figure 10-145 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0002m ......................................... 271 Figure 10-146 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0003m ......................................... 271 Figure 10-147 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0004m......................................... 272 Figure 10-148 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0005m......................................... 272 Figure 10-149 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0006m ......................................... 273 Figure 10-150 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0007m ......................................... 273 Figure 10-151 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0008m ......................................... 274 Figure 10-152 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0002m ........................................... 274 Figure 10-153 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0003m ........................................... 275 Figure 10-154 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0004m ........................................... 275 Figure 10-155 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0005m ........................................... 276 Figure 10-156 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0006m ........................................... 276 Figure 10-157 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0007m ........................................... 277

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Figure 10-158 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0008m ........................................... 277 Figure 10-159 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0002m ........................................... 278 Figure 10-160 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0003m ........................................... 278 Figure 10-161 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0004m ........................................... 279 Figure 10-162 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0005m ........................................... 279 Figure 10-163 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0006m ........................................... 280 Figure 10-164 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0007m ........................................... 280 Figure 10-165 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0008m ........................................... 281 Figure 10-166 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0002m ........................................... 281 Figure 10-167 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0003m ........................................... 282 Figure 10-168 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0004m ........................................... 282 Figure 10-169 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0005m ........................................... 283 Figure 10-170 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0006m ........................................... 283 Figure 10-171 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0007m ........................................... 284 Figure 10-172 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0008m ........................................... 284 Figure 10-173 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0002m ........................................... 285 Figure 10-174 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0003m ........................................... 285 Figure 10-175 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0004m ........................................... 286 Figure 10-176 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0005m ........................................... 286 Figure 10-177 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0006m ........................................... 287 Figure 10-178 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0007m ........................................... 287 Figure 10-179 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0008m ........................................... 288 Figure 10-180 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0002m ........................................... 288 Figure 10-181 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0003m ........................................... 289 Figure 10-182 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0004m ........................................... 289 Figure 10-183 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0005m ........................................... 290 Figure 10-184 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0006m ........................................... 290 Figure 10-185 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0007m ........................................... 291 Figure 10-186 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0008m ........................................... 291 Figure 10-187 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0002m ......................................... 292 Figure 10-188 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0003m ......................................... 292 Figure 10-189 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0004m ......................................... 293 Figure 10-190 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0005m ......................................... 293 Figure 10-191 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0006m ......................................... 294 Figure 10-192 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0007m ......................................... 294 Figure 10-193 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0008m ......................................... 295 Figure 10-194 Streamlines for 0.2m/s current, Isometric View .................................................................................. 296 Figure 10-195 Streamlines for 0.2m/s current, X+ View ............................................................................................. 297 Figure 10-196 Streamlines for 0.2m/s current, X- View .............................................................................................. 297 Figure 10-197 Streamlines for 0.2m/s current, Y+ View ............................................................................................. 298 Figure 10-198 Streamlines for 0.2m/s current, Y- View .............................................................................................. 298 Figure 10-199 Streamlines for 0.2m/s current, Z+ View ............................................................................................. 299 Figure 10-200 Streamlines for 0.2m/s current, Z- View .............................................................................................. 299 Figure 10-201 Streamlines for 0.7m/s current, Isometric View .................................................................................. 300 Figure 10-202 Streamlines for 0.7m/s current, X+ View ............................................................................................. 300 Figure 10-203 Streamlines for 0.7m/s current, X- View .............................................................................................. 301 Figure 10-204 Streamlines for 0.7m/s current, Y+ View ............................................................................................. 301 Figure 10-205 Streamlines for 0.7m/s current, Y- View .............................................................................................. 302 Figure 10-206 Streamlines for 0.7m/s current, Z+ View ............................................................................................. 302 Figure 10-207 Streamlines for 0.7m/s current, Z- View .............................................................................................. 303 Figure 10-208 Streamlines for 1.42m/s current, Isometric View ................................................................................ 303 Figure 10-209 Streamlines for 1.42m/s current, X+ View ........................................................................................... 304 Figure 10-210 Streamlines for 1.42m/s current, X- View ............................................................................................ 304 Figure 10-211 Streamlines for 1.42m/s current, Y+ View ........................................................................................... 305 Figure 10-212 Streamlines for 1.42m/s current, Y- View ............................................................................................ 305 Figure 10-213 Streamlines for 1.42m/s current, Z+ View ........................................................................................... 306 Figure 10-214 Streamlines for 1.42m/s current, Z- View ............................................................................................ 306 Figure 10-215 Streamlines for Basic Model, 0.5m/s current, Isometric View ............................................................. 309 Figure 10-216 Streamlines for Basic Model, 0.5m/s current, X+ View ........................................................................ 310

Mark Donnelly-Orr

xvii

Figure 10-217 Streamlines for Basic Model, 0.5m/s current, X- View ......................................................................... 310 Figure 10-218 Streamlines for Basic Model, 0.5m/s current, Y+ View......................................................................... 311 Figure 10-219 Streamlines for Basic Model, 0.5m/s current, Y- View ......................................................................... 311 Figure 10-220 Streamlines for Basic Model, 0.5m/s current, Z+ View ......................................................................... 312 Figure 10-221 Streamlines for Basic Model, 0.5m/s current, Z- View ......................................................................... 312 Figure 10-222 Streamlines for Rectangular Collar Model, 0.5m/s current, Isometric View ........................................ 313 Figure 10-223 Streamlines for Rectangular Collar Model, 0.5m/s current, X+ View ................................................... 313 Figure 10-224 Streamlines for Rectangular Collar Model, 0.5m/s current, X- View .................................................... 314 Figure 10-225 Streamlines for Rectangular Collar Model, 0.5m/s current, Y+ View ................................................... 314 Figure 10-226 Streamlines for Rectangular Collar Model, 0.5m/s current, Y- View .................................................... 315 Figure 10-227 Streamlines for Rectangular Collar Model, 0.5m/s current, Z+ View ................................................... 315 Figure 10-228 Streamlines for Rectangular Collar Model, 0.5m/s current, Z- View .................................................... 316 Figure 10-229 Streamlines for Triangular Collar Model, 0.5m/s current, Isometric View ........................................... 316 Figure 10-230 Streamlines for Triangular Collar Model, 0.5m/s current, X+ View ...................................................... 317 Figure 10-231 Streamlines for Triangular Collar Model, 0.5m/s current, X- View ....................................................... 317 Figure 10-232 Streamlines for Triangular Collar Model, 0.5m/s current, Y+ View ...................................................... 318 Figure 10-233 Streamlines for Triangular Collar Model, 0.5m/s current, Y- View ....................................................... 318 Figure 10-234 Streamlines for Triangular Collar Model, 0.5m/s current, Z+ View ...................................................... 319 Figure 10-235 Streamlines for Triangular Collar Model, 0.5m/s current, Z- View ....................................................... 319 Figure 10-236 Streamlines for Rounded Collar Model, 0.5m/s current, Isometric View ............................................. 320 Figure 10-237 Streamlines for Rounded Collar Model, 0.5m/s current, X+ View ........................................................ 320 Figure 10-238 Streamlines for Rounded Collar Model, 0.5m/s current, X- View ......................................................... 321 Figure 10-239 Streamlines for Rounded Collar Model, 0.5m/s current, Y+ View ........................................................ 321 Figure 10-240 Streamlines for Rounded Collar Model, 0.5m/s current, Y- View ......................................................... 322 Figure 10-241 Streamlines for Rounded Collar Model, 0.5m/s current, Z+ View ........................................................ 322 Figure 10-242 Streamlines for Rounded Collar Model, 0.5m/s current, Z- View ......................................................... 323 Figure 10-243 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Isometric View ................................ 323 Figure 10-244 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, X+ View ........................................... 324 Figure 10-245 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, X- View ........................................... 324 Figure 10-246 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Y+ View ........................................... 325 Figure 10-247 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Y- View ............................................ 325 Figure 10-248 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Z+ View ........................................... 326 Figure 10-249 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Z- View ............................................ 326 Figure 10-250 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Isometric View ................................ 327 Figure 10-251 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, X+ View ........................................... 327 Figure 10-252 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, X- View ............................................ 328 Figure 10-253 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Y+ View ............................................ 328 Figure 10-254 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Y- View ............................................ 329 Figure 10-255 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Z+ View ............................................ 329 Figure 10-256 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Z- View ............................................ 330 Figure 10-257 Finite Length Pile Model, Geometry, Reverse Isometric ...................................................................... 331 Figure 10-258 Finite Length Pile Model, Geometry, Side View.................................................................................... 331 Figure 10-259 Finite Length Pile Model, Meshing, Reverse Isometric ......................................................................... 332 Figure 10-260 Finite Length Pile Model, Meshing, Side View ...................................................................................... 332 Figure 10-261 Streamlines for Finite Length Pile Model, 0.5m/s current, Isometric View .......................................... 333 Figure 10-262 Streamlines for Finite Length Pile Model, 0.5m/s current, X+ View ..................................................... 334 Figure 10-263 Streamlines for Finite Length Pile Model, 0.5m/s current, X- View ...................................................... 334 Figure 10-264 Streamlines for Finite Length Pile Model, 0.5m/s current, Y+ View ..................................................... 335 Figure 10-265 Streamlines for Finite Length Pile Model, 0.5m/s current, Y- View ...................................................... 335 Figure 10-266 Streamlines for Finite Length Pile Model, 0.5m/s current, Z+ View ..................................................... 336 Figure 10-267 Streamlines for Finite Length Pile Model, 0.5m/s current, Z- View ...................................................... 336

Mark Donnelly-Orr

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Table of Tables

Table 2-1 Characteristics of Offshore Wind Farm Sites [24] ......................................................................................... 11 Table 3-1 Average Sediment Distribution based on the three test boreholes, refer to Figure 5 [25] ........................... 16 Table 3-2 Dependence of Local Scour Depth at bridge piers affected by the Relative Depth of Flow [63] .................. 38 Table 3-3 Equations for Maximum Scour Depth ........................................................................................................... 38 Table 3-4 Granular materials used in the studies of threshold of motion, as seen in Figure 3-49above. [92] ............. 56 Table 4-1 Inflation Layer Options .................................................................................................................................. 70 Table 4-2 Element Sizes in Volume Meshing Blocks ...................................................................................................... 75 Table 4-3 Face Sizing on Volume Meshing Blocks ......................................................................................................... 75 Table 4-4 Fluid Properties ............................................................................................................................................. 77 Table 4-5 Solution Methods for SST Transition Model .................................................................................................. 78 Table 4-6 Solutions Methods for k-ε Model .................................................................................................................. 78 Table 4-7 Element Size in Volume Meshing Blocks, Meshing Independence Test ........................................................ 83 Table 4-8 Face Size in Volume Meshing Blocks, Meshing Independence Test .............................................................. 83 Table 4-9 Element and Node Count, Meshing Independence Test ............................................................................... 83 Table 4-10 Monitoring Points X, Y, Z Coordinates ......................................................................................................... 86 Table 4-11 Monitoring Points Values, Meshing Independence Test 1 .......................................................................... 87 Table 4-12 Element Sizes in Volume Meshing Blocks .................................................................................................... 92 Table 4-13 Face Sizing on Volume Meshing Blocks ....................................................................................................... 92 Table 4-14 Coefficient of Pressure Parameter Values ................................................................................................... 97 Table 4-15 Element Sizing of Blocks in the Volume Meshing ...................................................................................... 115 Table 4-16 Inflation Options, Helical Wire Models...................................................................................................... 116 Table 4-17 Parameters for Shields Equations.............................................................................................................. 121 Table 4-18 Interception Points and Rearranged Equations ......................................................................................... 123 Table 4-19 Critical Seabed Shear Stress required for Sediment Movement of various Sediment Sizes ..................... 123 Table 6-1 Threshold Velocity for live-bed initialisation for different sediment sizes .................................................. 160 Table 10-1 Textural Names of Classification seen in Figure 84 [28] ............................................................................ 192 Table 10-2 Shear Stress Calculation Parameters ......................................................................................................... 196 Table 10-3 Boundary Reynolds Number and Critical Shields Stress Calculations, 0.0002m........................................ 197 Table 10-4 Interception Point for 0.0002m ................................................................................................................. 199 Table 10-5 Sediment Movement Threshold Values, 0.0002m .................................................................................... 199 Table 10-6 Critical Seabed Shear Stress required for Sediment Movement of various Sediment Sizes, with a Sediment Density of 2082kg/m3.................................................................................................................................. 200 Table 10-7 Critical Seabed Shear Stress required for Sediment Movement of various Sediment Sizes, with a Sediment Density of 1922kg/m3.................................................................................................................................. 200 Table 10-8 Critical Seabed Shear Stress required for Sediment Movement of various Sediment Sizes, with a Sediment Density of 2020kg/m3.................................................................................................................................. 200 Table 10-9 Monitoring Points Values, Basic Model ..................................................................................................... 307 Table 10-10 Monitoring Points Values, Meshing Independence Test 2 ...................................................................... 307 Table 10-11 Monitoring Points Values, Meshing Independence Test 3 ...................................................................... 308

Mark Donnelly-Orr

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Nomenclature

Abbreviations

CFD – Computational Fluid Dynamics

CO2 – Carbon Dioxide

DES – Detached Eddy Simulation

DNS – Direct Numerical Simulation

EC – European Commission

EU – European Union

GWh – Gigawatts Hour

LES – Large Eddy Simulation

MW – Megawatts

MWe – Megawatt Electrical

RANS – Reynolds-Averaged Navier Stokes

SST – Shear Stress Transport

UDF – User Defined Function

Units

b – Pier Width (m)

D – Diameter (m)

D – Sediment Particle Diameter (m)

°C – Degrees Celsius

Fr – Froude Number

g – Gravity (𝑚

𝑠2)

h – Water Depth (m)

hr – Hour

L – Characteristic length of the object (m)

PSU – Practical Salinity Unit (𝑔

𝑘𝑔)

Re – Reynolds Number

Re* - Shear Reynolds Number, Boundary Reynolds Number

S – Separation Line (Figure 21)

S – Scour Depth (m)

Suc – Ultimate Scour Depth (m)

U – Flow Velocity (𝑚

𝑠)

ū – Velocity Gradient (𝑚

𝑠)

Mark Donnelly-Orr

xx

u – Velocity at height y (𝑚

𝑠)

U – Reference Velocity at Height δ (𝑚

𝑠)

U* - Shear Velocity = √𝜏𝑜

𝜌𝑤 (

𝑚

𝑠)

Uc – Depth Averaged Critical Velocity (𝑚

𝑠)

V – Mean Inlet Velocity (𝑚

𝑠)

Ws – Average Settling Velocity (𝑚

𝑠)

xS – Distance in front of pile (m) (Figure 21)

y – Depth of flow (m)

y – Height of interest in velocity profile (m)

y+ - Y Plus Value

δ – Boundary Layer Thickness (m)

Θc – Critical Shields Stress (Pa)

μ – Dynamic Viscosity (𝑃𝑎

𝑠)

ν – Kinematic Viscosity (𝑚2

𝑠)

ξ – Normalised Distance = 𝑦+

𝑅𝑒∗

ρ – Density (𝑘𝑔

𝑚3)

ρs – Density of Seabed Sediment (𝑘𝑔

𝑚3)

ρw – Density of Fluids (𝑘𝑔

𝑚3)

τs – Shear Stress (Pa)

τo – Dimensional Shear Stress (Pa)

𝜏𝑚𝑎𝑥

𝜏∞ – Maximum Shear Stress Amplification

Φ, Phi – Sediment Size Measurement

Introduction, Background Mark Donnelly-Orr

1

1 Introduction

1.1 Background

Given the increasing population in the world, estimated at 6.916 billion people as of 2010 [1], the

energy demand and consumption will increase along with it. With this increase in energy demand

comes the demand for either new sources of energy or an improvement of currently existing

sources, in terms of operational cost, efficiency, effectiveness, and environmental impact. Tied

with this demand for new and better sources is the demand for environmentally friendly energy

sources with little or no carbon emissions. With government backing, a recent package was

outlined by the European Commission called the 20-20-20 Package whereby there would be a

20% reduction in gas emissions, 20% of energy sources being renewable, and a 20% increase in

energy efficiencies would be achieved by the EU by 2020. With the impetus of this package,

Ireland has to reconsider its current energy sources and how it could reach these targets.

Given Ireland’s location in a temperate climate, there are consistently strong winds affecting it.

This would give a preference towards wind energy as a suitable source of renewable energy as

opposed to solar, biofuels, or hydro. While solar energy could be a viable source of energy, as it is

in Germany who shares a similar climate to Ireland [2], the current incentives put in place by the

Government of Ireland are very poor. There are currently no grants for a homeowner to install a

photovoltaic solar system and only a €800 grant for a homeowner to install a solar thermal

system [3], a small proportion of the overall cost of the solar thermal system. But given the

difficulty of installing onshore wind farms without causing cultural, ecological, human and noise

issues, land based wind turbine farms are also difficult to implement on a large and substantial

level due to land limitation. One option is to base the wind farms offshore, in shallow seas, where

they would be away from the population and land where all the issues arise for land based wind

farms. The efficiency of the wind turbines increases as well when installed offshore given the lack

of obstructions and more consistent, powerful winds. A project that is currently being developed

and planned is the Dublin Array, which will compose of 145 wind turbines with a potential output

of 520MW. It is being developed by Saorgus Energy Ltd and is planned to be fully functioning by

2020.

1.2 Problem Definition

An issue that arises when these wind farms are implemented is the harshness of the marine

environment; such as being exposed to strong winds and tidal currents. What this project will

investigate is the effect that the tidal current has on the erosion of sediment around the base of

Introduction, Objectives Mark Donnelly-Orr

2

the wind turbine pylons, known as scouring. Scouring can cause a reduction in the seabed

sediment that would otherwise support the wind turbine base, potentially damaging the overall

structural integrity of the wind turbine.

The primary purpose of this report is to determine if scouring will occur at the base of the wind

turbine pylons that will be installed on the Kish and Bray Banks with the implementation of the

Dublin Array Project, and to what extent at different sea current speeds. The secondary purpose

of this report is to determine the scouring mitigation effects of various scour prevention devices

that could be installed on the shaft or around base of wind turbine pylons.

1.3 Objectives

By investigating the types of sediment, marine conditions and the physical nature of the turbine

pylons, these parameters can be implemented into a computational fluid dynamics (CFD)

numerical model and conclusions can be made to determine the extent of scouring expected in

the Dublin Array. Scouring prevention devices will then be designed, developed, and

implemented in the same numerical model. The effects and scouring mitigation levels of the

devices will then be discussed.

1.4 Methodology

The CFD numerical model used will be a three-dimensional model based on the Reynolds-

Averaged Navier-Stokes (RANS) equation and uses a Shear Stress Transport (SST) Transition

model which incorporates a k-ε model in the far-field and a k-ω model close to the surface wall.

From the results of this simulation, the seabed shear stresses will be estimated. Furthermore,

these simulation results are combined with the theoretical calculations of the shear stresses

required for sediment movement. Therefore, it can be deduced as to whether scour will occur

and to what extent. The same methodology will be used for the analysis of the scour prevention

devices.

1.5 Outline

After this introduction, Section 2 looks at the overall context such as why there is a need for the

Dublin Array and the renewable energy it will provide. Section 3 will evaluate the state of the art

technologies and the current literature that is available on the relevant aspects of this report.

These aspects include data that is available about the intended Dublin Array site, such as tidal

conditions, seabed sediment, fluid properties of the Irish Sea and the types of foundations

intended to be used; these are discussed in Section 3.1. The fundamental fluid mechanics will also

be discussed in Section 3.2 in order to form an understanding on the dynamics of the scouring

Introduction, Outline Mark Donnelly-Orr

3

phenomenon. Section 3.3 discusses the merits of the chosen computation model for this report.

Section 3.4 explains the reason for the current project and the validation behind it.

Section 4 explains the methodology used for the report. Section 4.1 and 4.2 explains the process

of first developing the model then implementing a thorough meshing scheme. This is followed by

Section 4.3 which contains the explanation of the input parameters for the 3D Model and the

reasoning behind the parameter choices. The meshing independence setup, results, and

conclusion are also shown in Section 4.4. Section 4.5 discusses the methods used for the

validation of the meshing scheme, including the results of these methods. Section 4.6, 4.7, and

4.8 explains the methodology about the model and meshing development of the Scour

Prevention Models, as well as the input parameters for the Scour Prevention Models. Finally in

Section 4.9, it will be determined at what ranges scouring will occur. This will be done by

performing theoretical calculations that have been confirmed as being realistic.

Section 5 contains the results for the project, with Section 5.1 showing the results from the 3D

Model and Section 5.2 showing the results from the Scour Prevention Devices Models. The

qualities of the results are discussed in Section 6, as well as discussion of the results seen in the

meshing independence and mesh validation tests, and other important factors of the report.

Section 7 is a recommendation of further work that is required in order to obtain a more

comprehensive result, before the entire project is concluded in Section 8. Section 9 contains all

the references used in the project report and Section 10 contains the Appendices for the project.

Context, Global Warming Mark Donnelly-Orr

4

2 Context

2.1 Global Warming

Global Warming is having a major effect on the world. Some of the effects include melting the ice

on the Earth’s Poles, causing sea levels to rise, increasing average precipitation levels across the

globe, and causing hurricanes and storms to become stronger [4]. The main cause of Global

Warming is the increasing concentration of CO2 in the Earth’s atmosphere. This causes the Earth’s

atmosphere to act like a greenhouse, allowing solar energy in, but containing that solar energy

rather than letting it escape, causing an increase in temperature.

2.2 EU Climate Goals

In order to combat Global Warming, the European Commission has set a list of goals in the 2020

Climate and Energy Package [5]. The climate and energy package is a set of binding legislation

which aims to ensure the European Union meets its ambitious climate and energy targets for

2020. These targets, known as the "20-20-20" targets, set three key objectives for 2020:

A 20% reduction in EU greenhouse gas emissions from 1990 levels

Raising the share of EU energy consumption produced from renewable resources to 20%

A 20% improvement in the EU's energy efficiency

Under this Package, the EU Directive 2009/28/EC is the most important legislation influencing the

growth of renewables in Ireland and the rest of Europe. This Directive establishes a common

framework for the use of energy from renewable sources in order to limit greenhouse gas

emissions and to promote cleaner transport. To this end, national action plans are defined, as are

procedures for the use of biofuels [6]. It also requires each Member State to have a target

calculated according to the share of energy from renewable sources in its gross final consumption

for 2020. This target is in line with the overall '20-20-20' goal for the Community. Each Member

State has to prepare national action plans in line with the directive.

2.3 Ireland’s Climate Goals

Ireland’s National Renewable Energy Action Plan [7] states that Ireland’s overall target is to

achieve 16% of energy from renewable sources by 2020. Specifically, the Government has set a

target of 40% electricity consumption from renewable sources by 2020. However, as of 2010, the

electricity consumption from renewable sources stood at 14.8%. The increases in the electricity

consumption from renewable sources is mainly attributed to onshore wind energy, see Figure

Context, Ireland’s Climate Goals Mark Donnelly-Orr

5

2-1, Figure 2-2, and Figure 2-3 below. This shows the importance of wind energy and the

investment being put into it.

Figure 2-1 - Renewable Electricity Growth to 2010 [8]

Figure 2-2 - Electricity Generated by Wind (GWh) 1990-2012 [9]

Context, Wind Energy Mark Donnelly-Orr

6

Figure 2-3 - Installed Wind Generating Capacity 2000 – 2012 [9]

This investment and energy return shows that Ireland is a suitable country for wind energy,

although challenges do arise in the form of intermittent power, which occurs as a result of the

chaotic nature of wind. These challenges are being address by the Irish Transmission System

Operator, EirGrid. They are involved in detailed examination of the issues and are pioneering

several renewables facilitation studies with a view to ensuring the appropriate management of

the grid and stability of the electricity system during this transition [7].

2.4 Wind Energy

Given Ireland’s investment into Wind Energy and the trend of increasing energy usage from wind

when compared with other renewable energy sources, the Irish Government need to consider

how they are going to accommodate the rapidly expanding wind energy installations.

Ireland currently has 188 onshore wind farms with a total installed capacity of 2263MW [10].

Although this is a substantial total capacity, the capacity factor has to be taken into account

which, for Ireland, has historically been 31.7% [11]. This means that a wind turbine onshore will

generally generate about 31.7% of the theoretical maximum output over a year. The efficiency of

a turbine has a theoretical limit of 59%, known as the Betz Limit. The capacity factor is a limiting

Context, Offshore Wind Energy Mark Donnelly-Orr

7

factor of onshore wind turbines. When wind turbines are located offshore, the capacity factor

increases to 41% [12].

Land limitations also affect the ability of wind farms commissioning’s as the population

distribution in Ireland is spread out, resulting in human interactions and complaints due to the

proximity of the wind turbines [13]. Generally for an onshore wind turbine to be as effective and

efficient as possible, it’s necessary that the turbine be located in a visible and often exposed

locations, such as hill tops, which further adds to human complaints due to the visual offense.

Noise levels also must be considered, with a minimum setback of 500m considered sufficient

enough for safety [14], further adding to the land limitations issues.

Public attitudes are also important factors that must be considered when evaluating the impact of

onshore wind turbines. [15] was a survey conducted in order to find out attitudes towards wind

energy in Ireland. Its main findings summarised by [16] were:

There is a high degree of support for more wind farms in Ireland.

The public prefer fewer, larger turbines to more, smaller ones.

The public prefer more, smaller farms to fewer, bigger ones.

Large wind farms (25 turbines or more) were not favoured.

The increased capacity factor, with the addition of land limitation, cultural impact, public

attitudes, and physical limits on the turbines results in a strong argument as to why offshore wind

farms could be of great importance to Ireland’s ability to reach its goals of 40% electricity

generation from renewable sources.

2.5 Offshore Wind Energy

[17] investigates the ecological and economic impacts of offshore wind farms relative to onshore

wind farms and other means of electricity production. It lists numerous reasons as to why

offshore wind farms are both beneficial and detrimental for many areas of interest such as

financially, environmentally, culturally, and economically. Examples of the benefits include:

Location: Offshore farms can generally be located near major population centres (major

human settlements have historically developed from successful ports, such as Belfast,

Cork, and Dublin), thereby removing the need for expensive high voltage transmission

cables.

Power: Offshore winds are generally stronger and more consistent than onshore winds.

The increased wind speeds lead to a 150% increase in electricity production.

Context, Offshore Wind Energy Mark Donnelly-Orr

8

Transport and Construction: Marine cranes developed for offshore oil and natural gas

industry are capable of handling larger equipment, allowing for larger turbine to be

effectively erected at sea.

Design Considerations: Since offshore wind farms do not have to be concerned about

turbine noise unlike the onshore industry. This allows bigger turbines to be made, which

results in more power and better economies of scale.

[17] concludes that the costs, both financially and ecologically, involved for the installation of

wind farms is very site dependent. Some sites could cheaply produce electricity with negligible

environmental impacts.

While offshore wind farms may be a suitable solution to Ireland’s renewable electricity

production, there are numerous issues with offshore wind energy that must be considered. Cost

estimates of offshore wind energy compared to onshore wind energy shows why there is a

hesitancy to invest in offshore wind energy. [18] shows that the cost of electricity in America,

from new onshore wind installations, was on average $45/MWh for 2007. While [19] estimates

that an offshore wind energy would cost $54/MWh, for a hypothetical 500MW wind farm

composed of 100, 5MW turbines. The farm was in shallow water, 15 miles from the coast. This

hypothetical scenario is very similar to the installation, of the Dublin Array, considered for this

project.

The main reason for this increased cost of electricity is the capital cost involved in installing

offshore wind farms. The capital cost of offshore wind power is around twice that of onshore

wind energy projects. The higher cost is due to increased investments in laying cables offshore,

constructing expensive foundations at sea, transporting materials and turbines to the wind farm,

and installing foundations, equipment and the turbines themselves. The turbines, although based

on onshore designs, are also more expensive. They need to be designed with additional

protection against corrosion and the harsh marine environment to help reduce maintenance

costs, which are also higher offshore [20].

Environmental Impacts must also be considered as a result of an offshore wind farm. [21] goes

into much detail on the effects that the proposed Dublin Array, located in Dublin Bay, will have on

commercial fisheries, marine ecology, birds, marine animals, and turtles. [22] concludes that,

subject to the appropriate mitigation measures being put in place, there will be no significant

adverse impacts on the physical, human and biological environments resulting from Dublin Array.

The only substantial human impact comes from the visual impact of the Dublin Array. The impact

Context, Offshore Wind Energy Mark Donnelly-Orr

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of this viewshed can be seen in Figure 2-4 below, where the areas of orange shading are regions

where the Dublin Array could be seen from.

Figure 2-4 Viewshed of Dublin Array on the Surrounding Area [23]

The installation of the Dublin Array also comes with a potential export opportunity. While Ireland

could enjoy a potential surplus of wind energy, the United Kingdom faces a number of challenges.

Due to changes in policy in relation to coal and nuclear energy, the UK faces a substantial threat

to their energy supplies from 2016 onwards [22]. The Office of Gas and Electricity Markets, the UK

regulator, have made it clear, that they will require imported energy to help meet the UK’s energy

needs. The UK also faces its own 2020 climate change targets and with the closure of nuclear

plants and the decision by a number of nuclear project promoters not to pursue other projects,

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the UK faces a multiple problems. This has been made more pressing due to a slowdown in

onshore wind consenting in the UK. While the UK has ambitious long term plans for offshore

projects, many of these projects are planned for deeper waters and are, as a result, more

expensive to develop than those in Ireland. The UK consequently will have a need for energy and

a preference for green energy, provided they may not be able to meet their own objectives [22].

This need for energy could create an opportunity for employment growth and export earnings

Ireland has an excellent offshore wind resource that compares favourably with the best in the

world both in terms of consistency and strength. This makes Ireland an ideal location for the

development of both offshore wind farms, and research and development facilities. Currently,

there are five companies actively involved in the development of offshore wind energy projects in

Ireland; these are SSE Renewables, Oriel Wind farm, Codling Wind Park, Dublin Array and

Fuinneamh Sceirde Teoranta. However, this report will focus on the Dublin Array, located on the

Kish Sand Banks off Bray.

The Dublin Array will generate renewable energy from a zero carbon sustainable source. The

project offers the opportunity to harness Ireland’s exceptional wind resource to stimulate

investment and create jobs. The project will assist Ireland’s economic recovery by the creation of

a new export resource. Should the Irish Government choose, the project can also make a major

contribution to Ireland’s climate change targets as indicated in a number of policy documents;

this would include the National Renewable Energy Action Plan. It could also help increase the long

term security of energy supply in Ireland. [22]

2.6 Successful Installations around the World

Within Europe there are numerous successful offshore wind turbine arrays. These include several

sites within the Irish Sea which provide examples that can be compared to the proposed site in

Dublin Bay in terms of the type of seabed sediment and tidal conditions. Other sites around

Europe allow other details of the installation and protection details to be compared and to

understand where previous problems occurred. Figure 2-5 below shows the sites of offshore wind

arrays in Europe.

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Figure 2-5 Location of offshore wind turbine arrays. [24]

Figure 2-5 comes from [24], who performed a study on each of these sites. Table 2-1, shows how

each site is unique in its characteristics. So while observations can be made on these sites, they

cannot be directly compared with the proposed site in Dublin Bay.

Table 2-1 Characteristics of Offshore Wind Farm Sites [24]

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3 Literature Review

3.1 Site Data

The offshore wind farm being investigated for this project is the Dublin Array, an offshore wind

farm that is being developed on the Kish and Bray Banks in the Irish Sea, 10 km off the coast of

Dublin and Wicklow. It consists of 145 wind turbines with a potential installed capacity of 520

MW, and is being installed by Saorgus Energy Ltd.

3.1.1 Site Layout

Figure 3-1 below shows the site layout [23], where the black dots represent the wind turbines.

Figure 3-1 Site Layout on the Kish and Bray Banks, wind turbines indicated by the black dots

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A series of coast-parallel north-south trending offshore banks exist in the western Irish Sea at a

distance of approximately 10 km offshore. These banks stand in 20-30 m of water and rise to

within a few metres of the water surface. Dublin Array is a proposal to construct an offshore wind

farm on two of these banks, the Kish Bank and the Bray Bank. These two banks lie approximately

10 km off the coast of Dublin and Wicklow, with the Bray Bank being a southerly continuation of

the Kish Bank. The Kish Lighthouse marks the northern end of the Kish Bank and the Codling Bank

lies to the southern end of the Bray Bank. [23]

The narrow extent of the Kish and Bray Banks dictates that the turbines can only be arranged in a

north-south direction. The wind turbines would be arranged in rows, four to five deep and placed

500 m apart, which would run parallel to the coast along the natural contour lines of the sand

banks. Up to 145 offshore wind turbines can be accommodated in suitable water depths over the

extent of the banks and on this basis, the study area extends for approximately 3 km in the east-

west direction and approximately 18 km in the north-south direction giving an overall study area

in the order of 54 km2. The study area also includes the route of the sub-sea transmission cable

which will connect the wind farm to the Irish shore. [23]

3.1.2 Sediment

The sediment type around the turbine pylon will be a critical factor in the methodology of this

project. Glover Site Investigation Ltd conducted a Preliminary Site Investigation in [25], in order to

establish the ground conditions with regard to the construction of an offshore wind farm. The

report was based on the results from three boreholes created by percussion boring methods. A

borehole is a deep, narrow hole made in the ground. The location of the boreholes can be seen

below in Figure 3-2.

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Figure 3-2 Location of Site Investigation Boreholes [25]

The soil encountered in each of the boreholes, to their maximum depth of 20m, were marine

sand deposits, typically slightly silty – silty, predominantly fine to medium sand. It was classed as

the following, with their relevant depths:

Loose to medium dense soil in the upper 2.5 -6m.

Becoming medium dense at greater depth.

Very dense below approximately 12m depth.

Figure 3-3, Figure 3-4, and Figure 3-5, below, from [25], shows the Particle Size Distribution for

the first 4m at each borehole, with an average for the 4m.

Figure 3-3 Sediment Distribution Results for Offshore Borehole 1 into Kish Sand Bank [25]

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Figure 3-4 Sediment Distribution Results for Offshore Borehole 2 into Kish Sand Bank [25]

Figure 3-5 Sediment Distribution Results for Offshore Borehole 3 into Kish Sand Bank [25]

Table 3-1 below, shows the average sediment distribution based off Figure 3-3, Figure 3-4, and

Figure 3-5 above. From the table it can be taken that the average particle size ranges from 0.2mm

to 0.4mm.

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% Passing Diameter (mm) % Passing Diameter (mm)

100 3.35 90 0.415

99.9 2 80 0.306

99.1 1.18 50 0.197

96 0.6 20 0.143

90.8 0.425 10 0.089

79.3 0.3

55.4 0.212

26.5 0.15

4.1 0.063

Table 3-1 Average Sediment Distribution based on the three test boreholes, refer to Figure 5

[25]

EcoServe performed an ecological study [26] and ran tests to determine the sediment type on the

Kish and Bray Banks. Their experimental method included a series of trawls being run over the

site using a biological dredge fitted with a 1cm mesh bag, any sediment samples recovered were

briefly described based on hand sample appearance. The report based on this work goes on to

state:

“The survey showed that the shallower parts of the Kish and Bray banks consisted of fine sand

with some shell. Along the western edge of the Kish Bank the seabed was predominantly coarse

shell with sand, which graded into shell with pebbles, gravel and stones along the west of the Bray

Bank and larger cobbles and stones at the southern end of the Bray Bank. The eastern side of the

Kish Bank consisted of fine sand and coarse shell.”

A geological mapping was also conducted by [27]. Part of the mapping process included collected

sediment samples from the seabed and subjecting them to particle size analysis using laser

granulometry for grains less than 2mm, and wet sieving grains greater than 2mm. Figure 3-7

below shows the spatial distribution of derived sediment types based on Folk Classification

Scheme[28], which can be seen in Figure 3-6. The Folk Classification Scheme is explained in

Appendix 10.1.

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Figure 3-6 Sediment Type Classification based on percentage sand, mud and gravel (after Folk

[28]) [27]

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Figure 3-7 Spatial Distribution of Derived Sediment Types [27]

[27] shows a figure, seen below in Figure 3-8, which shows that most of the sediments could be

classified as ‘sand’ with a mean particle-size of 2 phi (0.25mm) to 1 phi (0.5mm). Phi is defined in

Appendix 10.2.

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Figure 3-8 Variation in Mean Particle Size [27]

These two reports, [26, 27], have relatively conflicting data in that they both state different

sediment sizes as the mean sediment size; but [26] has a substantial source of error given their

method of sediment analysis. [29] argues that caution must be used when integrating this data,

from [26], with other sources. The samples obtained as part of this work contain an unknown

"skew" in sediment size distribution due to the means used to acquire them. The 1cm mesh bag

will allow quite large sediment particles to pass through, and the samples gathered are that

portion of the seabed effectively trapped and retained by organic matter. In addition, the trawls

were run over significant distances, as such, the sample may represent inputs from several

bottom-types. A further potential source of error is the method used to describe the sample

(visual estimation of grain size vs. granulometry as used by [27]). With these caveats, locations for

several of these shorter trawls are presented in Figure 3-9.

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Figure 3-9 Location of Biological Trawls Traversing Proposed Development and Samples

Presented in ([27]) [29]

From the reports discussed above, it can be seen that average particle size in the Kish and Bray

Banks will be in the range of 0.2-0.8mm. This value will not affect the methodology of this project,

but will be critical in the analysis as to whether scour will occur or not. Once a CFD model has

been created and verified, then the seabed shear stress calculated from the model will be

correlated with the range of sediment sizes and it can be determined whether scour is predicted

to occur or not based on relevant sediment movement literature, seen in Section 3.2.9.

The density of the sand that will be used in this project will be influential on the theoretical

results obtained. The density of the sand influences the Shields Equations (see section 3.2.7.3)

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which gives the theoretical shear stress at which sediment movement occurs. From the above

reports, it can be seen that generally, the sediment type found in Dublin Bay was sand, with some

exceptions of gravel and mud being found. For the current report, the sediment was taken as wet,

packed sand with a density of 2082kg/m3.

3.1.3 Tidal Flows

The tidal current flow would be another critical factor in the thoroughness of this project, as it

would have an effect on the local flow and hence the shear stresses, that would alter the validity

of this project if it wasn’t fully understood and left unaccounted for. The main report from Dublin

Array, [23], assumes that a maximum tidal velocity of 1.13 m/s. This is obtained from an

Admiralty Chart [30], that was last updated in 1999, at a single point 2 kilometres North-East of

the top row of wind turbines on the Kish Bank.

A hydrographic survey [31], shows that there would be a substantial variance in the tidal flow on

the wind turbine pylons. The survey recorded data from both the North and South of the Kish

Sand Banks, seen in Figure 3-10.

Figure 3-10 Location of Recording Stations in Kish Banks [32]

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Both North and South sensors recorded the current speeds at three depths: sub-surface, mid-

water, and off-bottom. The North sensor recorded maximum current speeds of 1.34 m/s, 0.92

m/s, and 0.81 m/s for the sub-surface, mid-water, and off-bottom respectively. The South sensor

recorded maximum current speeds of 1.4 m/s, 1.42 m/s, and 1.05 m/s for the sub-surface, mid-

water, and off-bottom respectively. It can be clearly seen that there is a substantial difference

between the current speeds between the North and South of the Kish Sand Banks.

A conservative approach to the project would be to input the current speed into the CFD model

as 1.42 m/s. But the development of the boundary layer must be considered for the CFD and is

discussed in Section 3.1.5 further on in the literature review.

For the purpose of this project, only tidal current flow is being used in the CFD model, with the

effect of waves being ignored completely. This is due to the water height, ranging from 8 m to 13

m [31], to 30 m at some turbines [23]. It was assumed that with these water heights, waves

would have a secondary effect on the effects and extent of scouring, and hence can be ignored.

The direction of the tidal is also negligible as the dynamic fluid effects will be the same regardless

of the direction, given the circular nature of the wind turbine pylon.

3.1.4 Sea Water Properties

3.1.4.1 Temperature

The temperature of the sea water would have a slight impact on the fluid properties that follow.

The temperature was taken as the average for the entire year and was obtained from [33]. The

average for the year at Bray was 11.73°C.

3.1.4.2 Salinity

The Salinity level would also affect the fluid properties. The salinity was taken from [34] and was

found to be as low as 31 g/L, or 3.1%, approaching the English Coast.

3.1.4.3 Density

It was found that the density of the sea water would be 1023.07 kg/m3, a temperature of 10°C

and a salinity of 30 g/L were assumed so as to obtain an accurate reading from [35].

3.1.4.4 Viscosity

The dynamic viscosity was found to be 1.48x10-3 Pa s and the kinematic viscosity was found to be

1.44x10-6 m2/s. These figures were taken at a temperature of 10°C and a salinity of 35g/L from

[36].

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All these properties would be inputted into relevant boundary conditions in the CFD model when

it was designed and created.

3.1.4.5 Reynolds Number

The Reynolds Number depends on the density of the fluid (ρ), characteristic length of the object

(L), mean inlet velocity (V) and the dynamic viscosity (μ), and is related in the equation:

𝑅𝑒 = 𝜌𝑉𝐿

𝜇

With an inlet velocity of 1.42m/s and pile diameter of 5m, the Reynolds Number is 4,907,970. This

is an important number when comparing results with other papers as the Reynolds number need

to be similar. The Reynolds number calculated is very high and finding literature that comparisons

can be made with is very difficult. [37] investigates a circle in a cross flow with a Reynolds

Number of 1,000,000, which is one of the highest in literature. In order to compare papers, the

simulations run for this report must be altered to have a comparable Reynolds Number. The only

parameter that could be changed was the inlet velocity; every other parameter must be kept

constant for comparability.

3.1.5 Boundary Layer Formations

The boundary layer is the layer of fluid in the immediate vicinity of a bounding surface where the

effects of viscosity are significant. There can be a wide variety in the size of a boundary and the

structure of the flow within it. This is mainly due to the nature and shape of the surface that the

boundary layer is forming off. The simplest form of boundary layer development is the

development over a flat plate of a viscous, incompressible fluid, seen below in Figure 3-11.

Figure 3-11 Development of a boundary layer as it progresses along a flat plate and the

distortion of a fluid particle as it flows within the boundary layer. [38]

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Currents in the sea constantly change and vary in direction and speed. Tidal currents, in

particular, change direction with time and accelerate from what may be effectively zero speed at

slack water towards a maximum speed and then decelerate again. The result is that the

logarithmic velocity profile is not straight, as seen below in Figure 3-12b), for either flow

direction, but curved [39] as seen in Figure 3-14. Figure 3-13 uses a power law velocity profile,

which is discussed further on in this section.

Figure 3-12 a) Velocity profile for turbulent water flow plotted using a linear scale for both the

horizontal and vertical axis. b) The same velocity data as in a), plotted using a log10 vertical scale

and linear horizontal scale. [39]

Figure 3-13 Velocity Profile for water flow using a Power Law. Both axis are linear scale.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

He

igh

t ab

ove

se

abe

d (

m)

Current Velocity (m/s)

Velocity Profile - Power Law

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Figure 3-14 Velocity Profile for water flow using a Power Law. The vertical axis using a log10

scale, and the horizontal axis using a linear scale.

The boundary layer velocity profile used for the CFD analysis was assumed to be a power-law

profile [38] (as already seen in Figure 3-13 above):

𝑢

𝑈= (

𝑦

𝛿)

1

7= 𝑌

1

7 for 𝑌 = 𝑦

𝛿 ≤ 1 and 𝑢 = 𝑈 for 𝑌 > 1

As shown:

0.001

0.01

0.1

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

He

igh

t ab

ove

se

abe

d (

m)

Current Velocity (m/s)

Velocity Profile - Power Law

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Figure 3-15 Boundary Layer Velocity Profile [38]

Where:

u is the velocity at a height y

U is the reference velocity at a height δ

This profile, seen in Figure 3-15, is used as the flow in the model will be turbulent, due to a

Reynolds number higher than 5000. This is a reasonable approximation of experimentally

observed profiles, except very near the seabed where this formula gives ∂u/∂y = ∞ at y = 0. Note

the differences between the assumed turbulent profile and the laminar profile. [38]

It should also be noted that the flow is not invariably turbulent throughout the full thickness of

the boundary layer. There is a very thin layer adjoining the seabed, in which the flow is essentially

laminar. This layer is known as the viscous sublayer, and it is no more than a few millimetres

thick. This sublayer develops when the current velocities are not excessive and the bed is

sufficiently even [39]. The thickness of the viscous sublayer is inversely proportional to the

current speed. For the purpose of this project, the seabed is presumed to be completely flat.

Although ripples will inherently form on the seabed, they are of such a small scale in comparison

to the pile that they are deemed negligible in the CFD model.

Within the viscous sublayer, the velocity profile is effectively linear, so the velocity gradient is

constant. This can be seen in Figure 3-16 below. The curved blue line represents a linear velocity

profile and is curved due to the logarithmic axis. The straight brown line represents a velocity

profile that follows a log law, or a power law in the case of this report. The red line represents the

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true velocity profile when the viscous sublayer is taken into account; this effect is known as the

law of the wall. This phenomenon is explained in more detail in Section 4.4.3.

Figure 3-16 Viscous Sublayer Velocity Profile [40]

Another velocity profile that could be used in the project is the Van Driest Velocity Profile. In [41],

van Driest performs an analysis that forms the basis for the theoretical calculation of the velocity

profiles, and finds that the theory checks well with experimental data. This velocity profile is used

in [42], because they found that the velocity profile improved the stability of the model and

allowed a coarser mesh resolution at the bed. The velocity profile equation is much more

complex than the profile that will be used, as the equation for the profile requires the roughness

Reynolds Number, and changing y+ (the y+ variable is explained in the Appendix 10.3) values as

the profile is developed. For the purpose of this report, the power law velocity profile was

deemed appropriate.

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3.1.6 Foundation Type

The foundation type that the Dublin Array envisages will be adopted is the Monopile Foundation.

Monopile foundations are the preferred foundation for offshore with 75% of all units installed

since 1991 being monopile foundations [43], as Figure 3-17 shows.

Figure 3-17 Share of Substructure Types for Online Wind Farms End 2011 [43]

The monopile foundation is a simple structure, as seen in Figure 3-18, which can be constructed

at almost any near shore location and can be towed by tug or carried by barges to the wind farm

construction site. The foundation consists of a steel pile with a diameter in the range 3.5 to 5.5 m

and wall thickness of 60 to 80 mm. The mass of the pile tends to be between 350 and 500 tonnes.

Where soil conditions allow, piles are driven or vibrated into the seabed to a depth of between 20

and 40 metres by a large special purpose ship or rig [23].

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Figure 3-18 Monopile Foundation [44]

The monopile foundations being used in the Dublin Array could vary in diameter from 4m to

6.5m, depending on the type of wind turbine selected. For the purpose of this project, an average

diameter of 5m was chosen as the pylon diameter for the CFD model.

With the installation of a marine structure foundation, an erosion process known as scouring

tends to occur on the seabed. It is caused by the erosive action of currents that increase in speed

as the current is forced to move around such obstacles. The level of scouring around a structure

placed on the seabed depends on a number of factors, including the composition of the seabed

material and the speed of localised currents. [23]

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3.2 Scouring

3.2.1 Fundamental Fluid Mechanics

Investigation of the fluid mechanics of scours shows numerous descriptions by many authors, in

particular, the mechanics of scour around a circular pile in a steady current. All cover the same

basis as to the development and features of scouring. [42] gives a list on the numerous papers

that review scour around piles in steady currents:

[45]

[46]

[47]

[48, 49]

[50]

[51]

[52]

[53]

[54]

Scour is caused by the interaction of a flow field and the geometry of a structure resting on the

seabed. For the purpose of this project, consider a vertical cylindrical on a seabed, as seen in

Figure 3-19 below.

Figure 3-19 Flow around a cylindrical pile, Isometric View [42]

As the seabed boundary layer that has formed approaches the vertical pile and passes along the

seabed past the pile, a stagnation pressure gradient develops. Since this pressure gradients

decreases towards the seabed, towards the decreasing velocity (as a result of the boundary

layer), a secondary flow is generated towards the seabed, as seen in Figure 3-20 and Figure 3-21

below.

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Figure 3-20 Flow around a pile, Side View [55]

Figure 3-21 Formation of Horseshoe Vortices [56]

A recirculating eddy, known as a horseshoe vortex, then forms from the downward flow at the

midplane of the intersection of the pile and seabed, resulting in a shear stress on the seabed. The

resulting vortex wraps around the pile and trails off downstream. As seen below in Figure 3-22.

Figure 3-22 Flow around a cylindrical object, Top View [57]

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If the seabed is erodible, then the likelihood of sediment transport is increased, resulting in scour.

The scour occurs when the seabed shear stress exceeds the critical shear stress at which the bed

material starts to move.

3.2.2 Different Features

3.2.2.1 Boundary Layer Interaction/Separation & Horseshoe Vortices

Upstream of the circular pile, if the pressure field induced by the pile is sufficiently strong, the

natural turbulent boundary layer undergoes a three-dimensional separation [58], seen along the

dotted S line in Figure 3-19. The result is that the lower regions of the separated boundary layer

roll up to form a vortex system upstream of the building [58]. The ends of this vortex system are

swept downstream and assume a characteristic shape when viewed from above, leading to its

name – the horseshoe vortices.

[59] suggests that for piles whose height exceeds the thickness of the bottom boundary layer, the

region affected by the horseshoe vortices extends approximately one pile diameter upstream of

the obstruction. [55] indicates that the acceleration caused by the downward flow in front of a

pile increases the seabed shear stresses by a factor of up to four over the seabed shear stress in

the undisturbed approach flow, and that the amplification decreases approximately inversely

with the square of the distance from the pile.

These notions are validated in Figure 3-23 below from [42]. As the separation point moves further

away from the pile, the increasing boundary layer thickness has no effect on the separation point

location, with it remaining at roughly 0.95 diameters away (i.e. 1 diameter away).

Figure 3-23 Separation Distance Xs/D as function of δ/D. [42]

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3.2.2.2 Lee-Wake Vortices

Flow patterns in the wake of the pile also influence the effects of scour. Wake vortex systems are

formed by the rolling up of the unstable shear layers generated at the surface of the pier and

these are detached from both the sides of the pier at the separation line [60]. They are shed

alternately from the pier and are trailed downstream, as seen in Figure 3-22. The dimensions of

the vortex system primarily depend on the diameter of obstruction, and their strength is a

function of the pier shape and flow velocity. The roughness of the surface can also affect the lee-

wake vortices, but for this project, the pile wall roughness will not be considered.

3.2.2.3 Factors Affecting Scouring

[60] concludes that the main parameters governing the scouring process include the diameter of

the obstruction, water height and the flow velocity, which influences the Reynolds number,

Froude Number, velocity profile and the resistance of the bed material to erosion.

3.2.2.3.1 Pile Diameter & Velocity

Figure 3-24 below, from [60], shows the relationship between the diameter and scour depth. Also

included in the figure is the relationship between diameter and scour depth at various current

velocities.

Figure 3-24 Ultimate Scour Depth (Suc) as a function of diameter of obstruction [60]

Figure 3-24 shows that as the diameter of the pile is increased, there is an increase in scour

depth. Similarly, as the velocity is increased, the scour depth increases, with greater increases in

scour depth per velocity increase at higher diameters.

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3.2.2.3.2 Shape of Pile

The shape of the geometry will affect the scouring process due to its alteration of the flow field.

This project will only consider a circular pile, but the relationship between the shape and the

scour depth and pattern is worth understanding. [61] investigated the effects of pier shapes on

scour depth in a flume test. The shapes tested were a circular pier and various rectangular shapes

piers with various lengths. Figure 3-25 shows what was found.

Figure 3-25 Scour Depth vs. Time Curves for Pier Shape Effects Test [61]

It can be seen that the circular pier shape had the lowest scour depth for the given time when

compared with the square and rectangular shape. It must also be considered that the shapes

investigated in [61] were aligned with the river flow, which will always be its flow direction.

Whereas in a marine instalment, the direction of the current will vary hour by hour, and

completely reverse twice a day.

3.2.2.3.3 Boundary Layer & Velocity Profile

The boundary layer and velocity profile are interchangeable. The chosen velocity profile chosen

for this project is discussed earlier in the report in Section 3.1.5.

The velocity profile will have an important impact on the effects of scouring as the amplitude of

the pressure gradient that arises from the velocity profile hitting the pile front will determine the

amplitude of the horseshoe vortices that form as a result. This will increase the shear force that

the vortices will induce on the seabed. If the inlet current was uniform, i.e. no boundary layer

formed, then the velocity profile would be a vertical line approaching the pile, resulting in no

adverse pressure gradient and no horseshoe vortices.

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[42] states that the separation of the bed boundary layer (to form the horseshoe vortex) will be

delayed if the boundary-layer-thickness-to-pile-diameter ratio, δ/D, is small (i.e. a more uniform

velocity distribution in the incoming boundary layer), presumably leading to a smaller-size

horseshoe vortex. For very small values of δ/D, the boundary layer may not even separate, and

hence no horseshoe vortex will be formed. This can be seen in Figure 3-26 below, with the trend

for the boundary layer thickness decreasing towards zero as the separation point moves closer to

the pile.

Figure 3-26 Separation Distance Xs/D as function of δ/D. [42]

3.2.2.3.4 Reynolds Number

Figure 3-27 from [42] shows the effect the Reynolds number has on the boundary layer

separation point and the seabed shear stress amplitude upstream of the pile. Figure 3-27 a)

shows that as the Reynolds number is increased, the separation point moves further away until a

Reynolds number of 500 is reached. At this point the separation point will move closer to the pile

as Reynolds number is increased, before levelling out at approximately one diameter from the

pile face at a Reynolds number of 105.

Figure 3-27 a) explains that for a Reynolds number of less than 500, the size of the horseshoe

vortices decreases with decreasing Reynolds number. While for Reynolds greater then 500, the

size of the horseshoe vortices decreases with increasing Reynolds number. This is due to the

increased momentum exchange between the layers of fluid in the separating turbulent boundary

layer (and therefore due to the delay in the boundary-layer separation) with increasing Reynolds

number [42].

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Figure 3-27 Influence of the pile Reynolds number (a) Separation distance Xs/D. (b) Maximum

bed shear stress amplification under the horseshoe vortex on the upstream symmetry line. [42]

3.2.2.3.5 Froude Number

The Froude Number is the ratio of inertial force to gravitational forces and can be applied to a

flow with a free surface. The dimensionless group equation for Froude Number is:

𝐹𝑟 = 𝑉

√𝑔𝐿

Where:

V is the flow velocity

g is gravity

L is the characteristic length, i.e. flow depth

The acceleration of gravity becomes an important variable in a fluid dynamics problem in which

the fluid weight is an important force. It will generally be important in problems involving flows

with free surfaces since gravity principally affects this type of flow [38].

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[60] states that the two most important parameters influencing scour are the approach flow

velocity and the depth of flow. These two parameters can be represented by the model Froude

Number. Figure 3-28 below shows the variation of relative scour depth with Froude Number. The

Suc/D is found to increase with increases in Fr (Froude Number). Significant variation of Suc/D is

found at higher flow Froude numbers than at lower flow Froude numbers, i.e. the influence of

diameter is felt more at higher Froude numbers.

Figure 3-28 Suc/D as a function of flow Froude number for different model sizes. [60]

Given that for the project the CFD model is constrained along the top by a flat surface with a zero

shear boundary condition, i.e. the air above the sea, and waves are not allowed to form, the

Froude number will have no effect on the project.

3.2.2.3.6 Sediment Resistance

As explained in Section 3.2.9 later on, sediment resistance is central as to when sediment

movement will actually occur. There are forces acting on each grain of sand that must be

overcome if movement is to occur.

3.2.2.3.7 Flow Depth

Scouring processes are influenced by the relationship of pile diameter to depth of flow. The scour

depth increases with the depth of flow until a threshold is reached, and no further incision occurs

[62]. The width of the pile also comes into effect, if the pile is narrow, scour depth is proportional

to pile width and independent of flow depth, if the pile is wide, the scour depth is related to flow

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depth without influence of pile width [62]. For intermediate sized piles, both factors affect scour

depth. Table 3-2 below shows these relationships.

Class of Pier Width Pier Width (b) / Depth of Flow (y) Local Scour Dependence

Narrow b/y < 0.7 ys ∝ b

Intermediate 0.7 < b/y <5 ys ∝ (by)0.5

Wide b/y > 5 ys ∝ y

Table 3-2 Dependence of Local Scour Depth at bridge piers affected by the Relative Depth of

Flow [63]

The ratio of the pile width to the depth of flow in this project is 5/10 = 0.5 (given a pile diameter

of 5m and a water height of 10m). Therefore, regarding this project, the flow depth will not have

an effect on scour depth.

3.2.3 Current Based

It should be noted that while scouring is affected by both currents and waves, this project will

look solely at the scouring caused by current, with no attention being paid to the effects of wave

interaction. There are many forms of scour depth prediction formulae that cover the flow

conditions such as current only, waves only, current and waves, or current or waves.

3.2.4 Maximum Scour Depth

There are numerous formulae that have been developed in order to establish the maximum scour

depth. The dimensionless scour depth (S/D, S = Scour depth and D = Pile diameter) is a parameter

that allows non-dimensional studies on the scour process around a pile to be carried out.

[64] summarises numerous formulae that can be used to calculate maximum scour depth, seen in

Table 3-3 below.

Equation No. Authors Year Maximum Scour Depth Prediction

1 [45] 1977 𝑆

𝐷max = 1.5 × 𝐷 × tanh (

ℎ

𝐷)

2 [65] 1982 𝑆

𝐷max = 2.5 (1 − 0.5 ×

𝑢𝑐

𝑢)

3 [66] 1988 𝑆

𝐷max = 2.4

4 [67] 1992 𝑆

𝐷max = 1.3

Table 3-3 Equations for Maximum Scour Depth

Where:

h is the water depth

D is the pile diameter

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U is the current velocity

Uc is the depth averaged critical velocity

Again, the purpose of this project is not to calculate the maximum scour depth, but whether

scour will occur. These equations can be used to give an understanding as to the theoretical scour

depth that will occur at the turbine base.

3.2.5 Equilibrium Scour Depth

Equilibrium scour depth is the depth of scour after which the scour depth does not noticeably

change with time. [68] states the equilibrium scour depth to be the depth at which the rate of

increase of scour does not exceed 5% of the pier diameter in the succeeding 24 h period. [69]

defines it as when the rate of scour reduces to 5% of the smaller of the foundation length (pier

diameter or abutment length) or the flow depth in the succeeding 24-h period.

For the purpose of this project, the equilibrium scour depth will be ignored as neither time

dependent scour is investigated nor the equilibrium depth. But it should be noted that the rates

of scour do fade as time passes.

3.2.6 Clear Water Scour vs. Live Bed Scour Criterion

There are two types of scour that can occur in this project. [70] identified the two types of scour

according to the type of sediment transport in the approach flow:

Clear-Water Scour: Where material is removed from the scour hole, but not replenished by the

approach flow.

Live-Bed Scour: Where the scour hole is continually supplied with sediment by the approach flow.

Initially the scour criterion predicted for this project was a clear-water scour as opposed to a live-

bed criterion given the relatively low current speeds expected in Dublin Bay. Upon reviewing the

results obtained through the CFD models, it appeared that after a certain current velocity is

reached that the sediment on the entire seabed essentially becomes mobilised and the type of

scour switches from a clear-water scour criterion to a live-bed scour criterion.

When this switch occurs, the scour that is occurring at the foot of the pile will change in its

development. Instead of sediment just being transported out of the scour hole, sediment will now

also be transported into it as a result of the live-bed development.

The effects that this switch has on the extent of the scouring can be seen in Figure 3-29 below,

where equilibrium scour depth is shown for a given pier and sediment size as a function of

approach velocity.

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Figure 3-29 Equilibrium Scour Depth as a Function of Mean Approach Flow Velocity [71]

[71] summarises Figure 3-29 by suggesting that the maximum scour depth occurs at the condition

of threshold of particle motion for the approach flow. With clear-water scour, the equilibrium

scour depth is approached asymptotically when the flow is no longer capable of removing bed

sediment from the hole. With live-bed scour, an equilibrium depth is attained when, over a

period of time, the average amount of sediment transported into the scour hole by the approach

flow is equal to the average amount of sediment removed from the scour hole due to local scour

action. Under this condition, the local scour depth fluctuates periodically about a mean value, the

fluctuations corresponding to the passage of bed forms through the scour hole. [71]

The threshold value will be estimated from the results seen in Section 5.1. Since the scour depth

is not being estimated in the simulations and is being ignored for the purpose for this project, the

seabed shear stress will be evaluated for the various inlet speeds. Looking solely at the seabed

shear stress in the far field, i.e. away from the fluid flow that is affected by the pile, and applying

the limits at which sediment transport is deemed to occur for various sediment sizes, it can be

judged and estimated at what current velocities live-bed scour conditions will arise in Dublin Bay.

Regardless of whether the criterion is deemed to be clear-water or live-bed, scour still occurs but

to different magnitudes. Since this project is not investigating the scour depth but rather if and

where scour will occur, the simulations run for this project show to what extent the scour will

occur for the various velocities.

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3.2.7 Scour Protection & Prevention Measures

Since scour is deemed to occur for a range of current velocities expected in Dublin Bay, scour

prevention measures should be investigated in order to protect the wind turbine pylons from

scouring. While the purpose of this project isn’t to design and implement a working scour

prevention device, rudimentary and basic designs can be numerically investigated to form an

understanding of how effective simple geometry additions could be at mitigating the effects of

scour.

3.2.7.1 Different Types of Scour Protection

From investigating the literature, it can be seen that numerous reports are available on the

implementation of different scour prevention devices and how effective they are at preventing

scour. There are two main types of scour prevention, armour or flow alteration techniques.

3.2.7.1.1 Riprap or Bed Protection

Riprap is one of the most common types of scour protection and is investigated by the

following papers: [72-76]. The process of applying riprap at the base of a turbine pylon is

relatively simple and is achieved by placing large course rocks around the base, as seen in

Figure 3-30 below.

Note: Figure 3-30 below is a scale down version of the riprap that is normally used for offshore

wind turbines, but the ratio of the riprap size to the pylon is similar to that of a full scale model.

Figure 3-30 Flexible Scour Protection around a Circular Pile [76]

Riprap works by causing the development of smaller horseshoe vortices in front of the bed

protection as shown in Figure 3-31.

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Figure 3-31 Flow around a Monopile with Bed Protection. [77]

While riprap is deemed to be an effective scour prevention technique it is limited in its ability to

prevent scour though due to several mechanism such as winnowing, shear failure, edge scour,

and mainly bed degradation. Figure 36 shows the effect, over time, that the current has on the

riprap.

Figure 3-32 Bed Degradation Erosion around Pile with Riprap, white arrow indicates current

flow direction [76]

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Due to these mechanisms, the ripraps ability to prevent scour degrades with time until its

effectiveness is deemed unfitting. Riprap will not be investigated as a possible scour prevention

measure for this project. The ability to model riprap in the ANSYS simulations used in this project

would be very difficult to implement and also to obtain worthy results from which conclusions

could be made. A more permanent solution would also be ideal as the economic benefits of not

having to frequently rearrange the riprap are obvious.

3.2.7.1.2 Tire Mats

An intriguing scour prevention device is the Scour Prevention Mat, seen in [78] and shown in

Figure 3-33 below, developed by Scour Prevention Systems Ltd. The system forms a mattress

constructed from recycled car tyres joined together and laid onto the seabed. The company

claims that in effect they are a completely inert protective skin over the seabed and completely

eliminate scour. The mats take advantage of the fact that the density of the tyres and that of the

surrounding seabed are very similar. This results in the mats not floating away because they are

less dense, but not sinking and pushing down too much on the seabed, causing secondary scour

(seen in the riprap), because they are more dense then the seabed.

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Figure 3-33 Scour Prevention Mats, before and after installation [78]

The way the mats work is that when there is a high energy flow, sediment is transported in the

fluid. When this flow passes across the mat surface, these flows are disrupted and their speeds

reduced. As a result, sediment transport across the mats is disrupted and the sediment is instead

entrapped within the mat’s void spaces, where it is retained due to the tyre’s shape, seen in

Figure 3-34 below.

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Figure 3-34 Description of how the Scour Prevention Mats work. [78]

The effectiveness of this scour prevention measure was shown when a demonstration trial was

undertaken at E.ON’s Scroby Sands Offshore Wind Farm in 2012 and 2013. The first part of the

trial focused on the installation and removal methods proving that the mats were a viable option

in terms of installation. The second part of the trial remediated existing scour at the wind turbine

bases. The scour pit around the turbine was infilled with tyre-filled nets and Scour Prevention

Mats were laid on top. Results from multibeam echo sounder surveys undertaken before and

soon after mat installation are shown in Figure 3-35 below, illustrating the increased seabed level

around the pile. Pre-existing secondary scour caused by rock dumping used as previous scour

protection is still seen: the trial’s aim was to remediate scour immediately around the pile. The

trial successfully demonstrated that the mats stabilise the seabed onto which they are placed and

encourage seaborne sediment back into previously existing scour pits. [79]

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Figure 3-35 Three Dimensional Bathymetric Surveys of the seabed around a monopile

foundation before and after Scour Prevention Mat installation. [79]

3.2.7.1.3 Helical Wires

Helical Wires are investigated thoroughly by [80]. It found that a in a steady current, a threaded

pile, seen below in Figure 3-36, proved to be effective at controlling scour depth to a great extent.

It found that cables wrapped spirally forming threads on the pile help to weaken the down flow

and horseshoe vortices that form. It experimentally found that the maximum reduction of scour

depth observed, in a steady current, was 46.3% by using a triple threaded pile having a thread

angle of 15° and a cable–pile diameter ratio of 0.1, the effect of the wires can be seen in Figure

3-38 and Figure 3-38 below. This design is economical as it is simply a wire that must be added

onto the turbine pylon after it has been constructed, as opposed to designs that require the

addition of the scour protection device during construction.

Figure 3-36 Threaded Pile (Helical Wires or Cables wrapped spirally on the pile to form thread

[80]

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Figure 3-37 Vortex flow fields at the upstream plane of symmetry of an unprotected pile[80]

Figure 3-38 Vortex flow fields at the upstream plane of symmetry of a triple threaded pile [80]

3.2.7.1.4 Sacrificial Piles

[81] explains that sacrificial piles are piles placed upstream of a bridge pier for the purpose of

protecting it from local scour. The piles, which themselves may be subject to substantial scour,

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protect the pier from scour by deflecting the high-velocity flow and creating a wake region behind

them. The effectiveness of this method as a pier scour countermeasure is dependent on the

number of piles, the size of the piles relative to the pier, the protrusion of the piles (partly or fully

submerged), and the geometric arrangement of the piles in relation to one another and the

bridge pier. The piles can be arranged in a variety of configurations. A triangular array, with the

apex of the triangle pointing upstream, has been shown to be one of the better configurations

among those tested. [70, 82] performed laboratory testing of sacrificial piles and found up to a

50% scour reduction due to the presence of sacrificial piles, but under clear water conditions and

a limited duration.

Given that the project is investigating a pile in a sea current, and hence varying current direction,

the use of sacrificial piers is limited unless a universal design can be designed which is effective

from every flow direction.

3.2.7.1.5 Collars

Another flow alteration technique is the installation of a pre-fabricated collar installed around the

base of a turbine pylon. The purpose of the collar is to armour the seabed by preventing the

down flow and horseshoe vortices development from reaching the seabed. [83, 84] found that

collars are an effective way of reducing scour depth when a current in considered, as shown in

Figure 3-40 and Figure 3-41 below. Figure 3-39 shows the effects of scour without any protection

while Figure 3-40 and Figure 3-41 show the effects collars have on reducing the effects of scour.

[85] numerically investigates the effects of fins using ANSYS software making it a valuable source

for comparisons. While not similar to the collar investigations mentioned above, the fins are

similar in their nature and the design process is relatively to the procedure used for this project.

Figure 3-39 Scour around an Unprotected Pile (current only) [83]

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Figure 3-40 Edge Scour at the pile protected by a small collar (current only) [83]

Figure 3-41 Scour at the pile protected by a large collar (current only) [83]

3.2.8 Shear Stresses Acting on the Seabed

The shear stress acting on the seabed will be imperative in calculating whether scour will occur

around the pile being investigated for this project. If shear stresses on the seabed are greater

than the critical shear stress required for sediment movement, then scour will occur. As scour

occurs, the seabed will change shape, which will in turn affect how the horseshoe vortices will

impact the seabed and the shear stresses on it. For the purpose of this project and the limitations

of the software used, the CFD model will have a flat seabed surface that will not have its shaped

altered by the shear stresses acting on it.

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[55] summarises that data from [59, 86] indicates that the fluid shear stresses impinging on the

bottom beneath the horseshoe vortices are amplified eleven- and twelve-fold, respectively,

above the values associated with the undisturbed free stream. [55] goes on to explain that an

effect of the secondary flow, caused by the pressure gradient in front of the pile, is the downward

deflection of the horseshoe vortices passing around the pile. This has little effect on the

horseshoe vortex, except for a slight enhancement of the vortex due to the rotation caused by

the downward deflection.

3.2.9 Sediment Movement

Sediment movement is the most important aspect of scouring as scouring can’t exist without

sediment movement. The movement of sediment on the sea-bed begins when the shear stress at

the seabed becomes sufficiently strong enough to overcome the frictional and gravitational forces

holding the grains on the bed. This is known as the critical shear stress. To understand the critical

shear stress, the underlying principles sediment movement must first be investigated and

understood.

3.2.9.1 Types of Sediment Movement

There are four types of sediment movement, seen in Figure 3-42 [39],: sliding, rolling, saltation

and suspension.

Figure 3-42 Modes of Sediment Transport [39]

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Sliding particles remain in continuous contact with the bed, merely tilting to and fro as they

move. Rolling grains also remain in continuous contact with the bed. Saltating grains ‘jump’ along

the bed in a series of low trajectories. Sediment particles in these three categories collectively

form the bedload. The suspended load consists of particles in suspension, rarely coming in

contact with the seabed until they are deposited when the flow slackens.

Figure 3-43 below summaries data as to whether erosion (directly related to scour) will occur at

certain speed and grain sizes and in what form of transport it will occur in. The maximum

recorded tidal flow that is being used for this project was 1.42 m/s (not taking into account the

accelerated flow around the pile) and the average grain size was 0.2-0.8mm. These ranges are

noted on the figure with a red box in Figure 3-43. It is predicted that for flows greater than

approximately 0.1 m/s, sediment transport will occur, with erosion occur at a flow rate of

approximately 0.3 m/s.

Figure 3-43 Diagram showing the range of current speeds at which sediment particles of

different sizes are eroded and their form of transportation. [39]

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Figure 3-43 from [39] is very similar to that found in [87], seen below in Figure 3-44. Showing that

there is a general consensus as to what current flow will cause which size of sediment to

transport, and in what form of transport.

Figure 3-44 The Hjulström curve [87]

3.2.9.2 Forces on Bed Particle

Looking at the typical sediment particle’s static form, when the fluid forces aren’t strong enough

to move the particle, the forces acting on it must be in balance. [88] separates the forces into

three components; particle weight, contact forces, and fluid forces, as seen in Figure 3-45 below.

[39] simply breaks the forces acting on the particle into the gravity component (i.e. the weight)

and the fluid force component, composing of lift and drag components, not considering the

contact forces, seen in Figure 3-46 below.

Figure 3-45 Forces acting on a sediment particle resting on a bed of similar particles. [88]

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Figure 3-46 Forces acting on a stationary sediment grain resting on a bed of similar grains in a

flow. [39]

The particle weight is the submerged weight which is equivalent to its volume times the density

of the sediment minus the density of the surrounding water, and acts though the centre of gravity

of the particle. The contact forces exert upward, generally from three underlying particles, and

become adjusted in light of the contact geometry, the particle weight, and the fluid force so that

the particle remains motionless [39].

The fluid forces acting on the particle can be broken up into two components, the lift force and

the drag force, as seen in Figure 3-47 below. The drag force arises as a result of the difference in

the fluid pressure on its upstream and downstream sides. Similarly, the lift forces arise as a result

of the pressure being high around the base of the particle and relatively low over the top surface

of the particle. The lift force has been found to be almost equal to the drag force at high

boundary Reynolds numbers [88].

Figure 3-47 Lift and Drag on a bed sediment particle. [88, 89]

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3.2.9.3 Initiation of Movement & Critical Shear Stress

[39, 88, 89] all agree that particles on the seabed will start to move when the combined lift and

drag forces become strong enough to counteract the gravitational and contact forces of the

particle. [39] states it as when the shear stress at the bed becomes sufficiently strong to

overcome the particle forces, but defines shear stress as:

𝜏𝑜 ∝ 𝜌 × �̅�2

Where:

𝜏𝑜 = Shear stress at the seabed (Pa)

ρ = density of the water (kg/m3)

�̅� = time-averaged current velocity (m/s)

But as explained above, lift and drag forces result from fluid flow force, which is dictated by both

the current speed and density of the fluid.

Another way to describe the initiation of sediment movement is to use Shield’s parameter. In [90,

91], Shield’s plotted the initial-movement data from the flume experiments on a graph of

boundary shear stress, nondimensionalised by dividing by the submerged specific weight and the

mean size of the sediment, against the boundary Reynolds number. Essentially, he found a critical

value of shear stress for the initial movement of grains in a uniform current and expressed it as a

function of the boundary Reynolds number. This relation is plotted in Figure 3-48 below.

Critical Shields Stress:

𝜃𝑐 = 𝜏𝑜

(𝜌𝑠 − 𝜌𝑤)𝑔𝐷

Boundary Reynolds Number:

𝑅𝑒∗ = 𝑈∗𝐷

𝑣

Where:

τo = Bed Shear Stress (Pa)

ρs = Density of seabed sediment (kg/m3)

ρw = Density of fluid (kg/m3)

g = Gravity (m/s2)

D = sediment particle diameter (m)

U* = √𝜏𝑜

𝜌𝑤 (m/s)

v = Kinematic Viscosity (m2/s)

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By knowing what shear stress is acting on the seabed, whether by practical measures or CFD, the

Shields graph allows you to make a prediction as to whether erosion will occur. Given the amount

of variables that are taken into account, it gives a more thorough indication as to whether erosion

will occur compared to Figure 3-43. The graph shows that results below the curved line no

sediment movement will occur, while results above the curve indicate that movement of grains

will occur.

Figure 3-48 The Shields Diagram

While Shields diagram has been used right up to present with little modification. [92] updated the

Shields diagram by drawing upon various more recent data to re-plot the diagram and redraw the

curve, as seen below in Figure 3-49. The identification of the symbols used in the graph can be

seen in Table 3-4.

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Figure 3-49 A modified and updated version of Shields Diagram. [88, 92]

Table 3-4 Granular materials used in the studies of threshold of motion, as seen in Figure

3-49above. [92]

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While the Shields diagram is a useful tool to capture the physical nature of the sediment erosion,

i.e. whether it will occur or not, it is difficult to use to find the threshold stress that corresponds

to a given sediment diameter, or to find the largest sediment diameter that can be moved at a

given shear stress. This is due to both τo and D appearing in both axis variables. To solve this

problem, [92] recast the Shields diagram, with the Shields parameter and the boundary Reynolds

number recast into two equivalent dimensionless variables, one with D but not τo and the other

with τo but not D [88]. This can be seen in Figure 3-50 below.

Figure 3-50 Updated Shields Diagram, recast in terms of shear velocity, U*, and particle

diameter, D.

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3.3 Computational Fluid Dynamics

3.3.1 Software Used

The software used for this project was ANSYS Fluent R14.5 Academic. ANSYS Fluent software

contains the broad physical modelling capabilities needed to model flow and turbulence.

Advanced solver technology provides fast, accurate CFD results, flexible moving and deforming

meshes, and superior parallel scalability.

3.3.2 Turbulence Model Chosen

The turbulence model chosen for this project was the Shear Stress Transport (SST) Transition

model. The original SST k-ω model was developed by [93]. The SST Transitions model is a

modified k-ω model, developed to improve on [94]’s original model. The SST Transition model has

an even higher sensitivity to adverse-pressure gradient flows. The k-ω model was chosen because

of its better performance with a flow which contains a boundary layer and a strong adverse-

pressure gradient [93, 94]. As described above, the 3D model developed for this project contains

both a boundary layer, in the form of a power-law expansion, and an adverse-pressure gradient,

seen in front of the pile giving rise to the horseshoe vortices. These two dynamics lead to the

choice of an SST Transition model being used for the CFD model of this project.

[93] makes an extensive comparison between various models for well-documented flows, the

models compared are:

k-ε Model

k-ω Model

k-ω, Baseline Model

k-ω, SST Model

Within the flows tested, two flows had adverse-pressure gradients, a backward-facing-step flow

and an aerofoil at an angle of attack near maximum lift condition. It was found that the SST k-ω

model yielded the most accurate results, hence why it was chosen for the CFD model used for this

project. [42] came to the same conclusion, and is the model used for their comprehensive testing

for flow around a circular pile.

The choice of a SST Transition model was used because of the coupling between the SST k-ω

model transport equations with two other transport equations, one for the intermittency and one

for the transition onset criteria, in terms of momentum-thickness Reynolds number.

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Another reason the SST Transition model was chosen is because the flow being investigated is a

transitional flow, i.e. the flow is initially laminar before becoming turbulent when flowing around

the pile. A SST k-ω model assumes that the flow is fully turbulent from the inlet. An STT Transition

model calculates additional equations which capture the transition of the boundary layer flow

from laminar to turbulent.

3.3.3 Benefits of Chosen Model

As explained, the main reason for using the SST Transition model is its accuracy of determining

the turbulent shear stress and flow separation under adverse-pressure gradients. This is the result

of using a combination of a k-ω model in the inner boundary layer and a k-ε model in the region

outside the boundary layer. Another advantage of using the SST Transition Model is because of its

design. ANSYS developed it for simulation that feature upstream laminar boundary layers that

transition into a turbulent flow further downstream, which is what will occur in the simulations

run for this project.

Due to computational limits while writing this project, a model had to be chosen that had

reasonable computational cost. Models such as Large Eddy Simulation (LES) and Unsteady

Reynolds-Averaged Navier-Stokes (URANS) yield more accurate results when compared to

experimental results, as seen in [37]. But an LES or URANS model require much more

computational power to run in a similar timeframe to that seen when running a RANS model.

3.3.4 Limits of Chosen Model

SST has a number of disadvantages that limits its modelling ability. It requires a fine mesh

resolution near wall with a Y+ value on the pile wall of approximately 1. Another limit is the

models capability of not taking into account the effect of rotation on turbulence and therefore

gives poor results when there is swirl or streamline curvature.

3.3.5 Validation of Choice

As stated, the SST Transition model has been used in a limited amount of literature, but RANS

models have been used in numerous papers or have been compared against experimental data or

other modelling techniques. Such literature includes [42, 95-97], who all use a RANS model with a

k-ω turbulence closure.

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3.4 Project Validation

The validation of this project comes from the inadequacy of the Hydrodynamic Modelling

Assessment Report published by Saorgus Energy Ltd, [32]. The Report was carried out by Hydro

Environmental Ltd and its purpose was to investigate the potential hydrodynamic impact from

the proposed Dublin Array and to quantify the hydrological and sedimentological impacts of the

turbine installation on the Kish and Bray Sand Banks through a hydrodynamic model of the

subject water.

The report analyses the flow around the monopile supports using a 2 dimensional finite element

formulation model, TELEMEC 2D, to assess the likely effect of the project on sand erosion and

deposition, i.e. whether scour will occur.

The report concludes that the model calculated that the tidal flow velocities and their

corresponding bed shear stresses were high throughout the Sand Banks, enough to cause the

mobilisation of coarse to very coarse sand. This conclusion doesn’t take into account the

accelerated flow that will occur as the flow approaches the monopiles and passes around, leading

to an even higher seabed shear stress.

The report also concludes that the main effect of the structures is to reduce velocities at the

upstream stagnation point and in the downstream wake of the monopile. That these local

impacts on velocities will have no discernible impact on the sediment regime on the Sand Banks,

but that local erosion is likely to occur in the immediate vicinity of the monopile, but limited to 5

to 10m from the pile.

These two conclusions are quite conflicting, firstly with the method this report was conducted in

and secondly with the literature available on scouring. As explained by [98], the 2D model used is

problematic because the effects that this report is investigating are inherently three-dimensional,

and so by definition cannot be captured by a two-dimensional simulation of the types used in the

report. This is due to the main mechanism of scour, namely horseshoe-vortices, being a three-

dimensional fluid phenomenon [55].

The literature available on scouring and sediment movement, explains how scouring occurs and

at what current speeds sediment movement is likely to occur, and even by conservative

estimations in Dublin Bay, scouring will occur, as opposed to the report stating that it is “likely to

occur”, given the average and max current speeds, and the pile diameters involved.

Methodology, Creating the 3D Model Mark Donnelly-Orr

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This gives rise to the purpose of this project. Will scour occur at the base of an offshore wind

turbine monopile given the local conditions at the Kish and Bray Sand Banks, and to what extent

will it occur?

4 Methodology

4.1 Creating the 3D Model

A 3D model had to be created that accurately reflected the dimensions of the turbine pylon sunk

into the seabed floor in order to capture the detail needed for the numerical simulations. This

was achieved by using the Design Modeller in ANSYS to create a model as seen below in Figure

4-1.

Figure 4-1 3D Model, Isometric View

The figure represents a turbine pylon in the sea. The pylon itself is manifested as the circular pile

seen in the middle of the rectangular block, which represents the sea from the seabed up to the

sea surface. For the simulations, the inlet was taken as coming in from the upper left hand corner

of the figure above, travelling along the positive X-axis.

The dimensions for the Figure above are:

Turbine Pylon Diameter: 5m

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Water depth: 10m

Upstream Distance: 50m

Downstream distance: 200m

Distance either side of pylon: 50m

These dimensions were chosen in order to accommodate certain aspects of the free stream

velocity. The 50m upwind was required so that the boundary layer had adequate space to form

and stabilise before it hit the pylon. 200m downwind was deemed adequate to allow for the

turbulent flow, as a result of the interaction with the pylon, to develop and steady out as it

flowed downstream. 50m either side of the pylon was needed so that there was no impact on the

turbulence development around the turbine.

4.2 Meshing Development

The meshing used in this simulations for this project were critical as the quality of the meshing

has a direct and hugely influential impact on the accuracy of the results obtained from the

simulations. The development and evolution of the mesh is explained in the following sections

and how each step furthered the quality of the mesh.

4.2.1 Basic Mesh

Initially, a very basic mesh was set up in order to simplify the process of understanding how the

model should be set up and to make sure that it could be simulated. The basic mesh was simply

an element size of 1m being applied to the whole body. With this in place, simple simulations

were run in order to understand the process of the numerical simulations.

4.2.2 Initial Bias

Once the process of running the simulations was understood, the mesh was developed so as to

be more appropriate for achieving the objectives of the current study. The questions involved

knowing, in detail, the turbulence caused by the pylon turbine and the effects it had on the

seabed in the vicinity of the turbine pylon. In order to achieve this, a bias factor was applied from

the walls of the 3D model towards the turbine pylon. These bias lines were applied on the seabed

wall of the 3D model as effects of the turbulence were on the seabed. Figure 4-2 below shows

these bias lines on the seabed. The lines were bias with smaller element sizing as the line

approaches the turbine pylon.

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Figure 4-2 Bias Meshing, viewed from below.

An edge sizing was also applied to the bottom of the turbine pylon, as seen below in Figure 4-3, in

order for greater accuracy around the base of the turbine pylon in the simulations.

Figure 4-3 Bias meshing and edge sizing at base of pylon, view from below.

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4.2.3 Symmetry

Symmetry was also applied to the model in order to reduce the computational requirements and

to allow for more elements to be used on the remaining half, resulting in greater detail. This was

done as the academic license for ANSYS only allowed for a maximum of 512,000 elements in a

simulation, and given the large 3D nature of this model and the detailed results needed, the limit

would be reached very quickly with the development of a more thorough meshing. The

application of symmetry didn’t affect the free stream or the turbulence development. The

symmetry was applied along the X-axis directly down the middle of the model as seen below in

Figure 4-4.

Figure 4-4 3D Model with Symmetry Applied, Isometric View

4.2.4 Hex-Dominant Meshing

The next step in the development of the mesh was the type of meshing to use. All the previous

models had used a tetrahedral meshing structure. The problem with using a tetrahedral meshing

is that it can skew the flow lines due to its triangular nature. With the implementation of a

hexahedral meshing structure, the skewing factor will be avoided and leads to more accurate

solutions in CFD. Figure 4-5 and Figure 4-6 below show the difference between a tetrahedral

meshing structure and a hexahedral meshing structure. Many sources in literature use a hex-

dominant meshing scheme in the development of their numerical models, such as [42, 97, 99-

101].

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Figure 4-5 Tetrahedral Meshing Structure, Isometric view

Figure 4-6 Hexahedral Meshing Structure, Isometric view

The hexahedral structure was achieved in ANSYS using a CutCell Assembly Meshing. Initially a

Sweep Method was used in ANSYS, but due to issues with implementing the inflation layer

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(explained in Section 4.2.7) the CutCell method was used. It can be seen from the Figure 4-6, that

the method scales the size of the element depending on the shape of the structure. Ideally, in the

case of this model, it scales it to a finer element size as it approaches the turbine pylon, resulting

in a more detailed solution closer to the pylon wall.

4.2.5 Volume Meshing

The next step of the meshing development involved dividing the solid body up into more

manageable bodies. This was done because when the model was just one solid body, the meshing

options quickly became numerous and conflicting and resulted in compromises having to be

made. With the solid model divided up into smaller bodies, the meshing became considerably

more manageable. Figure 4-7 below shows the initial volume meshing.

Figure 4-7 Initial Volume Meshing, isometric view

Figure 4-8 below shows the initial volume meshing, but from the opposite side of the model.

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Figure 4-8 Initial Volume Meshing, Reserve Isometric View

With the volume meshing in place, smaller element sizes could be implemented near the pylon

without causing the element size of the whole body to be affected. Another benefit of this

approach was the ability to scale down the element sizing towards the pylon, but this had to be

carefully considered because if the element size change was too significant between two adjacent

bodies, then it could have an effect on the results. This could be understood by drawing a straight

line through the various blocks and plotting the velocity data from this line, if there were any

sudden velocity changes along this line when it wasn’t obvious there was an obstruction from

interaction with the pile, and then the volume meshing transition could be deemed passable.

4.2.6 Refined Volume Meshing

The volume meshing seen in Figure 4-8 was the initial setup when trying to implement the

volume meshing, but as can be seen, it is overly complex. With that in mind, the volume meshing

was refined and simplified to Figure 4-9 below. It was decided to reduce the number of blocks in

the meshing as the numerous blocks weren’t required upstream as the flow was steady and

straight. By reducing the number of blocks, the meshing procedure was simplified with less

chance of conflicting meshing parameters occurring. It was decides a single large block in the

downstream of the pile was adequate enough to capture the turbulence detail required.

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Figure 4-9 Revised Volume Meshing

The volume meshing around the pile also changed significantly as seen in the Figure 4-10 and

Figure 4-11 below. The volume meshing was changed from having horizontal blocks to vertical

blocks because the area of interest in the project was at the seabed so a finer detailed was

required around the vicinity of the pile base in order for more accurate results. With the vertical

blocks in place, a finer element size could be gradually implemented towards the base of the pile,

without having too severe changes in the element size.

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Figure 4-10 Initial Volume Meshing around pile

Figure 4-11 Revised Volume Meshing around pile

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4.2.7 Inflation Layer Details & Issues

In order to capture the detail of the viscous sub layer that arises due to boundary layer formation

and to achieve a suitable y+ value of the pile wall, a thorough and quality inflation layer was

required. There were initial problems when trying to implement the inflation layer in the model

as mentioned before. In order to implement a hexahedral meshing, a meshing method had to be

chosen. A Sweep Method was initially chosen as it is a popular option with numerous

demonstrations and tutorials on how to implement it. The results were good and what was

required, but an issue arose when trying to introduce the inflation layer.

The inflation layer that was implemented was on the entire seabed and on the pile wall. The issue

with the sweep method being used was that only allowed for an inflation layer to be used on

either the seabed or pile wall, not both. By converting to a CutCell meshing method, the inflation

layer could be implemented on both surfaces.

With the issues sorted, a detailed inflation layer could be implemented on all the required

surfaces. The details of the inflation layer are seen in Table 4-1.

Inflation Option Total Thickness

Number of Layers 20

Growth Rate 1.35

Maximum Thickness 0.1m

Table 4-1 Inflation Layer Options

This was implemented on the entire seabed and the pile wall. There couldn’t be any difference

between the inflation layers of the seabed or the pile wall due to merging issues, if they weren’t

the same, an error occurred in ANSYS and neither inflation layer was implemented. Only a Total

Thickness inflation option worked in the model, but by altering the growth rate, and increasing it,

it was possible to obtain an appropriate first layer height. The height of the first layer is

approximately 0.0006m or 0.6mm. Given the size of the geometry in question, a 5m diameter

pylon, this is a detailed first layer height, with the results proving this, as will be seen in the

Section 5 later on in the report. The y+ on the pile wall is on average 1.258 with a minimum of

0.112 and a maximum of 2.435, seen in Figure 4-12 below. This was deemed adequate for the

simulations, as explained in the literature review, a y+ value of roughly 1 was required.

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Figure 4-12 y+ value on pile wall

4.2.8 Element Count Limits

An issue that consistently arose during the meshing procedure was the limit of elements allowed

in the model. Given the version of ANSYS being used, ANSYS R14.5 Academic, there was an

element limit of 512,000. Given the size of the 3D model and the detail of accuracy required in

the results, this limit was reached very quickly in the mesh developing process. It required a lot of

reorganising in terms of element sizes in different blocks in the 3D model. In order to decrease

the element size in one block, another block had to increase its element size, resulting in

numerous compromises. The final model contained 498,188 elements and it was found that any

decrease in the element size in any block caused the element count limit to be reached.

4.2.9 Final Meshing

Figure 4-13, Figure 4-14, Figure 4-15, Figure 4-16, and Figure 4-17 below show the final meshing

chosen when there was a limit on the element count.

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Figure 4-13 Overview of Final Meshing

Figure 4-14 Close-Up Overview of Final Meshing

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Figure 4-15 Reverse View Close-Up of Final Meshing

Figure 4-16 Close-Up of Pile Meshing

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Figure 4-17 Close-Up of Inflation Layer

Figure 4-18 shows the blocks created for the volume meshing along with their allocated names,

followed by Table 4-2 detailing the size of the elements within those blocks.

Figure 4-18 Named Blocks

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Block Element Size (metres)

A 3.5

B 2

C 0.6

D 0.6

E 0.5

F 0.4

G 0.3

H 0.25

I 0.2

Table 4-2 Element Sizes in Volume Meshing Blocks

The pile blocks also had a face sizing on the inside of the pile wall in order to have a more detailed

mesh there. Again a gradual change in the face sizing was used as the meshing progressed

towards the seabed. Table 4-3 below shows the details of the face sizing. The table also includes

the face sizing details used on the base of Block D. Additional detail was needed on the seabed in

the region of Block D as this was the area where scour was likely to occur, so a more accurate

solution was needed.

Block Face Sizing (metres)

F 0.2

G 0.15

H 0.125

I 0.1

Bottom face of Block D 0.25

Table 4-3 Face Sizing on Volume Meshing Blocks

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4.3 ANSYS 3D Model Parameters and Boundary Conditions

4.3.1 UDF

The User Defined Function (UDF) that is implemented was critical to the legitimacy of the results

that ANSYS computed. By implementing a UDF, the velocity profile can be implemented, thereby

accurately replicating actual marine conditions. When no UDF is defined, ANSYS inputs a uniform

flow into the 3D model and no boundary layer will form, except for close to the 3D model walls,

but this doesn’t accurately reflect the condition that the turbine pylon will be subjected to in the

actual sea environment. As explained in the Section 3.1.5, the boundary layer formation chosen

for this project was a power-law profile using the equation:

𝑢

𝑈= (

𝑦

𝛿)

17

Where:

u is the velocity at a height y

U is the reference velocity at a height δ

In order to implement the UDF, code had to be written in C programming language. The code

used for this project can be found in the Appendix 10.4.

Even after the UDF was imported into the ANSYS simulation, the user has to manually choose the

inlet_x_velocity under the Velocity Magnitude Boundary Condition. The key line in the code is the

F_PROFILE line as this defines the velocity profile as the power-law profile that was chosen for

this project.

The UDF was successfully implemented as will be explained in the relevant section later on in the

report.

4.3.2 Fluid Properties

The fluid parameters chosen for this project have been explained in the literature review as being

in line with the conditions found in Dublin Bay. These were all implemented into the ANSYS

simulations so that the results were as accurate as possible with what the results would be in

Dublin Bay.

In summary, as seen in Section 3.1.4:

Temperature 11.73°C

Salinity 31 g/L or 3.1%

Density 1023.07 kg/m3

Dynamic Viscosity 1.48x10-3 m2/s

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Table 4-4 Fluid Properties

4.3.3 Inlet Velocity

Various inlet velocities were run in the 3D model so that a range of conditions could be

investigated and the onset of scouring determined. The range of current speeds implemented

were: 0.1m/s, 0.2m/s, 0.25m/s, 0.3m/s, 0.35m/s, 0.4m/s, 0.45m/s, 0.5m/s, 0.55m/s, 0.6m/s,

0.7m/s, 0.9m/s, 1.1m/s, 1.3m/s, and 1.42m/s. The increments decreased to 0.5m/s from 0.2m/s

to 0.6m/s as these were the critical speeds where the seabed transitioned from clear-water scour

to live-bed scour. The increments increase from 0.1m/s to 0.2m/s after 0.7m/s because at after

0.7m/s the scour condition was almost all live bed and running extra simulations was deem

unnecessary. The maximum inlet velocity was taken as 1.42m/s. As explained in Section 3.1.3, this

was the maximum recorded tidal current in the hydrographic survey. The ranges of inlet current

velocities were inputted into the UDF code in the profile equation and attached to the

accompanying UDF.

4.3.4 Setup Options

When Setup is opened in ANSYS Fluent, a pop-up window appears called Fluent Launcher, as seen

below in Figure 4-19.

Figure 4-19 ANSYS Fluent Setup Launcher Options

In this window, there are various options that can be chosen for the Setup component of ANSYS

Fluent. The Double Precision option was chosen because this uses a 64 bit floating point number,

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whereas a Single Precision option would use a 32 bit floating number, and gives a more accurate

solution. This was also possible because the computer being used to run the simulations had a 64-

bit Operating System. Under Processing Options, the Parallel (Local Machine) option was chosen

and the Number of Processes set to 4. This meant that 4 processors in the computer were run in

parallel for the simulation, resulting in quicker calculations and hence quicker simulation running

times. The computer being used had 8 processor units with a total capacity of 16 GB of RAM. For

an unknown reason, ANSYS Fluent Setup wouldn’t run if more than 4 processors were chosen

under the Processing Options. This meant that two simulations could be run simultaneously on

the computer.

4.3.5 ANSYS Model Used

As explained in the Section 3.3.2 the SST Transition model was chosen for the ANSYS simulation.

In order to help the model converge, the simulations were initially run with a k-ε model, so that

when it had converged, the model could be switched to SST Transition model. The convergence

criteria in both models were set to 0.00001 for the continuity, x-velocity, y-velocity, and z-

velocity. The Solution Methods were also changed to the following settings for the SST Transition

model, seen in Table 4-5. The k-ε model’s Solution Methods can be seen in Table 4-6.

Gradient Least Squares Cell Base

Pressure Standard

Momentum First Order Upwind

Turbulent Kinetic Energy First Order Upwind

Specific Dissipation Rate Second Order Upwind

Intermittency Second Order Upwind

Momentum Thickness Re Second Order Upwind

Table 4-5 Solution Methods for SST Transition Model

Gradient Least Squares Cell Base

Pressure Standard

Momentum First Order Upwind

Turbulent Kinetic Energy First Order Upwind

Turbulent Dissipation Rate First Order Upwind

Table 4-6 Solutions Methods for k-ε Model

4.3.6 Zero Shear on Walls

In order to reduce the impact of outside factors and unnecessary turbulence that could skew

results and increase the computational time, zero stress was applied to several surfaces within

the model. The top of the 3D model, i.e. the sea surface, and the far boundary wall, shown in

Figure 4-20 below, had a zero shear implemented on them so as to not impact on the flow field.

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With these in place, a boundary layer wouldn’t form along these surfaces, which would otherwise

potentially skew the results.

Figure 4-20 Surfaces with zero shear stress

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4.4 Meshing Independence

4.4.1 Meshing Independence Tests Development

A meshing independence test was performed in order to determine if the solution obtained was

independent of the mesh resolution. Although the model has converged for all inlet speeds, not

checking the mesh independence can lead to erroneous results. To perform the meshing

independence test, sixteen monitoring points were chosen in the basic model already used. Three

models were set up, each with a further globally refined meshing throughout the domain. The

models were fully run until convergence and the velocity, pressure, and wall shear stress values

from these points were taken. Once these values were obtained from all three models, they could

be plotted and it can be determined if the meshing used was refined enough to capture all the

detail required. If the current meshing was not refined enough, then the required meshing can be

determined from the meshing independence tests.

Eight points were chosen at various points around pile, on the seabed as seen in Figure 4-21 and

Figure 4-22 below, these were chosen at points where there was significant flow separation and

hence significant fluid property changes. Another eight monitoring pints were set up 2.5m up

from the seabed, at the same X and Z coordinates, as seen in Figure 4-23. It should be noted that

since the eight points on the seabed are on the seabed, there will be a zero value for the velocity

value at these points, and similarly there will be no wall shear stress values at the eight

monitoring points that are 2.5m up from the seabed.

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Figure 4-21 Monitoring Points, Top View

Figure 4-22 Monitoring Points, Top View, Close Up

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Figure 4-23 Monitoring Points, Side View, Close Up

Note: For the following meshing independence tests, a full ANSYS R14.5 license was obtained in

order to bypass the element limit count imposed by the academic license.

Three meshing independence test were run. All the meshing independence tests models were run

using an inlet speed of 1.13m/s. The meshing details for the three models can be seen in Table

4-7 and Table 4-8 below.

Note: the blocks in the following table refer to the blocks found in Figure 4-18

The Base Model was the model used with the initial meshing. The inflation layers used in the

independence test models was the same as that used in the basic models.

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Block Element Size (metres)

Base Model Model 1 Model 2 Model 3

A 3.5 1.5 1 0.75

B 2 1 0.75 0.5

C 0.6 0.3 0.25 0.15

D 0.6 0.3 0.25 0.15

E 0.5 0.25 0.2 0.125

F 0.4 0.2 0.095 0.1

G 0.3 0.15 0.095 0.075

H 0.25 0.125 0.085 0.0625

I 0.2 0.1 0.075 0.05

Table 4-7 Element Size in Volume Meshing Blocks, Meshing Independence Test

Block Face Size (metres)

Base Model Model 1 Model 2 Model 3

F 0.2 0.2 0.1 0.1

G 0.15 0.15 0.075 0.075

H 0.125 0.125 0.065 0.065

I 0.1 0.1 0.05 0.05

Bottom of Block D 0.25 0.15 0.1 0.075

Table 4-8 Face Size in Volume Meshing Blocks, Meshing Independence Test

Base Model Model 1 Model 2 Model 3

Element Count 498,188 1,252,677 3,786,888 5,912,981

Node Count 526,119 1,332,956 3,975,955 6,197,267

Table 4-9 Element and Node Count, Meshing Independence Test

Figure 4-24 Meshing, Independence Meshing Test Model 1

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Figure 4-25 Meshing, Independence Meshing Test Model 2

Figure 4-26 Meshing, Independence Meshing Test Model 3

Figure 4-27 and Figure 4-28 show the points from which the following figures obtain their data.

Monitoring points 1-8 are on the seabed and monitoring points 9-16 are 2.5m up from the

seabed. The coordinates of the monitoring points can be found in Table 4-10 below, with the

origin being in the centre of the pile on the seabed level.

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Figure 4-27 and Figure 4-28 show the points from which the following figures obtain their data.

Monitoring points 1-8 are on the seabed and monitoring points 9-16 are 2.5m up from the

seabed. The coordinates of the monitoring points can be found in Table 4-10 below, with the

origin being in the centre of the pile on the seabed level.

Figure 4-27 Monitoring Points 1-8

Figure 4-28 Monitoring Points 9-16

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Monitoring Point X-Coordinate Y-Coordinate Z-Coordinate

1 5 0 1

2 -1 0 2.5

3 -2.75 0 0.5

4 -1.5 0 5

5 2 0 5

6 1.5 0 2.25

7 2.5 0 1

8 0 0 25

9 5 2.5 1

10 -1 2.5 2.5

11 -2.75 2.5 0.5

12 -1.5 2.5 5

13 2 2.5 5

14 1.5 2.5 2.25

15 2.5 2.5 1

16 0 2.5 25

Table 4-10 Monitoring Points X, Y, Z Coordinates

4.4.2 Meshing Independence Tests Results

As a result of performing the meshing independence test, the meshing in the 3D Model is likely to

refined and improved. Therefore it is deemed that the results from the meshing independence

tests should be shown in this section in order to explain how the final meshing was chosen for the

3D Model, upon which the accuracy of the current study’s result hinges. Discussion of the results

of the meshing independence tests can be found in Section 6.7.

4.4.2.1 Monitoring Points Values

Table 4-11 below shows the values obtained from the monitoring points in Meshing

Independence Test 1. These values, along with the data points from Table 10-9, Table 10-10, and

Table 10-11, which can be found in Appendix 10.8, were used to create Figure 4-29, Figure 4-31,

and Figure 4-32 in Section 4.4.2.2.

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Meshing Independence Test 1: 1,252,677 Elements

Monitoring Point Pressure (Pa) Wall Shear (Pa) Velocity (m/s)

1 -126.6 0.5438 -

2 -563.5 3.049 -

3 319 6.918 -

4 -129.6 1.405 -

5 -154.4 1.297 -

6 -135.4 0.4848 -

7 -115.9 1.021 -

8 -2.078 0.3093 -

9 -131.6 - 0.2051

10 -711.9 - 1.497

11 415.6 - 0.408

12 -140.4 - 1.092

13 -168.4 - 1.105

14 -251.7 - 0.8896

15 -154.8 - 0.2111

16 -2.091 - 0.944

Table 4-11 Monitoring Points Values, Meshing Independence Test 1

4.4.2.2 Plotted Monitoring Points Values

The following Figure 4-29, Figure 4-31, and Figure 4-31 show the plotted data obtained from the

monitoring value points in the meshing independence tests. Figure 4-29 shows the pressure

values on the seabed and Figure 4-30 shows the pressure values on the plane above the seabed.

Figure 4-31 shows the wall shear values of the seabed monitoring points, and Figure 4-32 show

the velocity values of the monitoring points on the plane above the seabed. The Figures have the

number of elements as there X-axis. This was done so that the reader could see how the

monitoring point values levelled out as the number of elements was increased in the model, as

opposed to just the next model being tested.

It can be seen that as the number of elements is increased in the model, the values being

obtained from the monitoring points start to level out and the amount of change in the

monitoring point values decreases. This is a result of the finer element size being able to capture

more data. It can be seen that there is minimal change in the monitoring point values between

Test 2, containing 3,786,888 elements, and Test 3, containing 5,912,981, and that meshing

independence is achieved at roughly 4,000,000 elements.

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Figure 4-29 Plotted Monitor Points, Pressure Values, Points 1-8

Figure 4-30 Plotted Monitor Points, Pressure Values, Points 9-16

-800

-600

-400

-200

0

200

400

0 1000000 2000000 3000000 4000000 5000000 6000000 7000000

Pre

ssu

re (

Pa)

Number of Elements

Pressure Values, Monitoring Points 1-8

Point 1 Point 2 Point 3 Point 4

Point 5 Point 6 Point 7 Point 8

-800

-600

-400

-200

0

200

400

600

0 1000000 2000000 3000000 4000000 5000000 6000000 7000000

Pre

ssu

re (

Pa)

Number of Elements

Pressure Values, Monitoring Points 9-16

Point 9 Point 10 Point 11 Point 12

Point 13 Point 14 Point 15 Point 16

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Figure 4-31 Plotted Monitor Points, Wall Shear Values, Points 1-8

Figure 4-32 Plotted Monitor Points, Velocity Values, Points 9-16

0

1

2

3

4

5

6

7

8

0 1000000 2000000 3000000 4000000 5000000 6000000 7000000

Seab

ed

Sh

ear

Str

ess

(P

a)

Number of Elements

Wall Shear Stress Values, Monitoring Points 1-8

Point 1 Point 2 Point 3 Point 4

Point 5 Point 6 Point 7 Point 8

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 1000000 2000000 3000000 4000000 5000000 6000000

Ve

loci

ty (

m/s

)

Number of Elements

Velocity Values, Monitoring Points 9-16

Point 9 Point 10 Point 11 Point 12

Point 13 Point 14 Point 15 Point 16

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4.4.2.3 Seabed Shear Stresses Contours

This section shows the seabed shear stress for all the meshing independence tests. Limits have

not been applied to the wall shear results as the scouring extent is not trying to be shown, but

rather the effects of a denser mesh.

Figure 4-33 Seabed Shear Stress, Basic Model

Figure 4-34 Seabed Shear Stress, Meshing Independence Test 1

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Figure 4-35 Seabed Shear Stress, Meshing Independence Test 2

Figure 4-36 Seabed Shear Stress, Meshing Independence Test 3

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4.4.3 Refined Final Meshing

The results of the meshing independence tests can be seen in the previous section, and as a result

of these tests, the final meshing used for the 3D Model simulations was updated. It was deemed

that having roughly 2,500,000 elements in the 3D Model would result in more accurate solutions

that would in turn produce more reliable scour predictions. The details of this refined final

meshing are in Table 4-12 and Table 4-13. The blocks referred to in Table 4-12 and Table 4-13 can

be found in Figure 4-18.

Block Element Size (metres)

A 3.5

B 2

C 0.2

D 0.2

E 0.2

F 0.2

G 0.2

H 0.2

I 0.2

Table 4-12 Element Sizes in Volume Meshing Blocks

Block Face Sizing (metres)

F 0.2

G 0.15

H 0.125

I 0.1

Bottom face of Block D 0.15

Table 4-13 Face Sizing on Volume Meshing Blocks

It can be seen that the refined meshing was not scaled down proportionally. Instead, the blocks

that contained large element sizes were scaled down to match the small elements found on the

face sizing of the blocks, giving the area of interest, around and downstream of the pile, a smaller

element sizing and as a result a better ability to capture detail.

The above meshing sizing resulted in the 3D Model having 2,267,295 elements and this was the

meshing used in the for the 3D Model simulations. The inflation layer options are the same as

used in the original final meshing of the 3D Meshing seen in Section 4.2.7. Images of the refined

final meshing can be seen below.

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Figure 4-37 Overview of Refined Final Meshing, Isometric View

Figure 4-38 Overview of Refined Final Meshing, Isometric View, Close Up

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Figure 4-39 Overview of Refined Final Meshing, Reverse Isometric View

Figure 4-40 Overview of Refined Final Meshing, Reverse Isometric View, Close Up

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4.5 Mesh Validation

In order to validate the mesh that was developed, numerous methods were used so that the

meshing could be either compared with experimental data and other recognised numerical

journals that used CFD models. The following are methods used to legitimise the meshing used in

this project and that it is producing reliable results. The techniques are described first, followed

by the results from the 3D Model. Discussion of the results can be found in Section 6.6.

4.5.1 Mesh Validation Techniques

4.5.1.1 Pressure Distribution around the Pile Wall

The pressure distribution around the pile wall in the cross flow is a simple and effective way of

proving that mesh was of good quality. When a pile is exposed to a cross flow, there is a pressure

distribution that forms around the pile wall. Figure 4-41 below shows the Coefficient of Pressure

distribution around a cylinder, which is the same as a pile, using various CFD turbulence models

from a published paper, [37]. By checking the Coefficient of Pressure Distribution around the pile

in the 3D Model developed for this current project, we can check if the meshing quality is good

enough to capture the detail required for dependable results.

Figure 4-41 Mean Pressure Distribution on the Pile [37]

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In order to make the data comparable, the 3D Model used for this technique had to be altered.

The coefficient of pressure distribution around a pile that is taken from literature is when a cross

flow is uniform, with no boundary layers of turbulence. The 3D model was updated so as to make

the data comparable with literature. The UDF wasn’t implemented in the model, with a constant

inlet velocity being used. The inlet velocity was also reduced in order to achieve a lower Reynolds

number so that the results could be compared with data obtained from literature. Given the

dimension of the turbine pylon and the relative fluid properties, shown in Section 3.1.4.5, an inlet

velocity of 0.275m/s was required to achieve a Reynolds number of 1,004,433. This allowed the

coefficient of pressure distribution to be directly compared to the data from [37]. A zero shear

condition was also applied to the seabed as it already was on the top surface and the side wall.

The mesh was left as it was so that the quality of it could be validated. The pressure was checked

along the line shown in Figure 4-42 below.

Figure 4-42 Pressure Distribution around Pile Wall Data Source

With these implementations in place, the simulation was run and the pressure values along the

line shown above obtained. The data was manipulated in Windows Excel in order to allow it to be

compared to the data from Figure 4-41 which was digitally extracted and added to the Excel

Graph to allow for direct comparison.

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4.5.1.1.1 Coefficient of Pressure Equation

In order to be able to compare the data obtained from the simulation and the data from the

literature, the data from the simulation had to be converted from Pressure values to Coefficient

of Pressure values. To do this, the equation below is used for incompressible flow:

𝐶𝑝 =𝑝 − 𝑝∞

12

𝜌∞𝑉∞2

Where:

p = Pressure at the point at the pressure coefficient is being evaluated

p∞ = Pressure in the freestream (remote from any disturbance)

ρ = Freestream fluid density

V∞ = Freestream velocity of the fluid

Table 4-14 shows what values were used for the various parameters.

Parameter Value

p Varies for every data point

p∞ 1.5 Pa

ρ 1023.07kg/m3

V∞ 0.275m/s

Table 4-14 Coefficient of Pressure Parameter Values

4.5.1.2 Pressure Distribution along Upstream Pile Wall

Another method of mesh validation is to check the pressure distribution on the seabed along

symmetry line in front of the pile. By obtaining the pressure data on this line and calculating the

coefficient of pressure, the data can be compared to other literature sources. [42]investigates the

flow around a circular pile by both experimental and numerical means. In the paper is a figure,

seen below in Figure 4-43, that shows the coefficient of pressure distribution along the upstream

edge of the pile wall. The figure contains plots of both numerical and experimental investigations,

with the dots representing the experimental data, obtained from [102], and the line representing

numerical data from [42].

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Figure 4-43 Pressure Coefficient Distribution along the length of the upstream edge of the pile,

[42]

By checking the coefficient of pressure along the upstream edge of the pile in the 3D Model and

comparing it to the data found in [42], a conclusion can be made about the quality of the mesh

and its ability to capture the detail of the fluid flow.

In order to have comparable data, the 3D Model had to be altered. The Reynolds number used in

the model that is shown in Figure 4-43 is 52,000, whereas the Reynolds number for the 3D Model

used in this study has a maximum of 4,907,970. In order to have a comparable model, the inlet

current flow of the 3D Model was reduces to 0.015m/s in order to achieve a Reynolds number of

51,844. The same ANSYS parameters, UDF, and boundary conditions were implemented as the

original 3D Model. The meshing was also unmodified as this was the feature that was being

validated.

The data source line that was used is shown in below in Figure 4-44. The pressure data was taken

from this line and then manipulated in Windows Excel in order to allow the data to be compared.

The pressure was converted to the coefficient of pressure; this procedure can be found in Section

4.5.1.1.1. The plots were also digitally taken from Figure 4-43 and imposed onto the Excel graph

so the data sets could be directly compared.

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Figure 4-44 Pressure Distribution along Upstream Edge of Pile Data Source

4.5.1.3 Pressure Distribution along Upstream Symmetry Line

Another method of mesh validation is to check the pressure distribution along the upstream

symmetry line. By checking the pressure data and comparing it to the data from literature

sources, a conclusion about the quality of the mesh can be made. Using [42] again, it investigates

the coefficient of pressure distribution in front of the pile on the seabed, specifically along the

upstream symmetry plane, shown in Figure 4-45 below. The figure contains plots of both

numerical and experimental investigations, with the dots representing the experimental data,

obtained from [102], and the line representing numerical data from [42].

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Figure 4-45 Coefficient of Pressure Distribution on the Seabed along the upstream symmetry

line. Note: the pressure coefficient is normalized by the pressure coefficient at the toe of the

pile, [42]

As done in Section 4.5.1.1, the 3D Model was altered in order to obtain comparable data. The

Reynolds number used in the model that is shown in Figure 4-45 is 52,000, whereas the Reynolds

number for the 3D Model used in this study has a maximum of 4,907,970. In order to have a

comparable model, the inlet current flow of the 3D Model was reduces to 0.015m/s in order to

achieve a Reynolds number of 51,844. The same ANSYS parameters, UDF, and boundary

conditions were implemented as the original 3D Model. The meshing was also unmodified as this

was the feature that was being validated.

The data source line that was used is shown in below in Figure 4-46. The pressure data was taken

from this line and then manipulated in Windows Excel in order to allow the data to be compared.

The pressure was converted to the coefficient of pressure; this procedure can be found in Section

4.5.1.1.1. The plots were also digitally taken from Figure 4-45 and imposed onto the Excel graph

so the data sets could be directly compared.

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Figure 4-46 Pressure Distribution along Upstream Symmetry Line Data Source

4.5.1.4 Wall Shear Distribution along Upstream Symmetry Line

An additional method of mesh validation is to check the wall shear distribution along the

upstream symmetry line. By checking the wall shear values and comparing it to data from

literature, a further conclusion can be made about the quality of the mesh. Using [42] again it

investigates the wall shear distribution in front of the pile on the seabed, specifically along the

upstream symmetry plane, shown in Figure 4-47 below. The figure contains plots of both

numerical and experimental investigations, with the dots representing the experimental data,

obtained from [102], and the line representing numerical data from [42]. It should be noted that a

negative shear value is shown in Figure 4-47; this is due to the wall shear in the X-axis orientation

being plotted as opposed to the absolute value. This meant that when the wall shear data was

being taken from the CFD results in the current study, the Wall Shear in the X-axis orientation was

taken.

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Figure 4-47 Seabed Shear Stress amplification along upstream symmetry line [42]

As done in Section 4.5.1.1, the 3D Model was altered in order to obtain comparable data. The

Reynolds number used in the model that is shown in Figure 4-47 is 52,000, whereas the Reynolds

number for the 3D Model used in this study has a maximum of 4,907,970. In order to have a

comparable model, the inlet current flow of the 3D Model was reduces to 0.015m/s in order to

achieve a Reynolds number of 51,844. The same ANSYS parameters, UDF, and boundary

conditions were implemented as the original 3D Model. The meshing was also unmodified as this

was the feature that was being validated.

The data source line that was used is shown in the previous Section 4.5.1.3 in Figure 4-46. The

pressure data was taken from this line and then manipulated in Windows Excel in order to allow

the data to be compared. The wall shear was divided by the freestream wall shear in order for the

data to be comparable. The plots were also digitally taken from Figure 4-47 and imposed onto the

Excel graph so the data sets could be directly compared.

4.5.1.5 Boundary Layer Formation

The boundary layer formation was another area where the quality of the mesh could be checked.

By showing that the boundary layer formation has been fully captured, it can be proven that the

meshing is of good quality. To capture the formation of the boundary layer, a data line was

inputted into the 3D model so that the velocity gradient could be captured. Figure 4-48 and

Figure 4-49 below shows the location of this data line.

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Figure 4-48 Boundary Layer Data Line, Isometric view

Figure 4-49 Boundary Layer Data Line, Z-axis view

By plotting the velocity data along this line, the velocity profile can be seen.

Another check of the boundary layer formation is to simply check the contours of the velocity

along the model so that the boundary layer can be seen to be forming as the flow progresses

along the 3D model.

4.5.2 Mesh Validation Results

A discussion of the following results can be found in Section 6.6.

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4.5.2.1 Pressure Distribution around the Pile Wall

The coefficient of pressure distribution from around the pile wall of the 3D Model is displayed on

the following Figure 4-50 in red and is compared to the data obtained from [37], represented by

the blue line, who used a RANS model in their numerical simulation. The green dots represent

experimental data from [103] at a Reynolds Number of 1,200,000, which is slightly higher than

the two CFD data sets which were taken from numerical models with a Reynolds Number of

1,000,000.

Figure 4-50 Comparison of Pressure Distribution around Pile Wall Data

Figure 4-50 shows the coefficient of pressure distribution around the pile wall. The pressure

coefficient starts at a value of 1 for the upstream pile face before dropping down to a maximum

of -2 at 80°, at this position on the pile wall the pressure is at its greatest before rising up to a

value of -0.5 at 130° at which point separation occurs after which the pressure is steady at -0.5.

4.5.2.2 Pressure Distribution along Upstream Pile Wall

The coefficient of pressure distribution along the upstream pile wall from the various sources is

shown in Figure 4-51. The blue line represents the data taken from the 3D Model used in the

current study, the red line represents the numerical simulation data taken from [42], and the

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

0 20 40 60 80 100 120 140 160 180

Cp

Degrees (Θ)

Pressure Distribution around the Pile Wall

CFD Data, Current Study CFD Data, Published Study

Experimental Data, Published Study

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green dots represent the experimental data values taken from [102]. The y-axis is represented by

z/δ, where z is the height above the sea floor, and δ is the height of the boundary layer.

Figure 4-51 Comparison of Pressure Distribution Data along Upstream Pile Wall

The coefficient of pressure distribution along the pile wall is shown in the Figure 4-51. The

pressure coefficient should, ideally, start at 1 at the top of the pile, before gradually decreasing as

the stagnation pressure gradient increases due to the oncoming flow. The dip at the bottom of

the pile is a result of the horseshoe vortices forming.

4.5.2.3 Pressure Distribution along Upstream Symmetry Line

The coefficient of pressure distribution along the upstream symmetry line from the various

sources is shown in Figure 4-52. The blue line represents the data taken from the 3D Model used

in the current study, the red line represents the numerical simulation data taken from [42], and

the green dots represent the experimental data values taken from [102]. The x-axis is represented

by x/D, where x is equal to the distance from the centre of the pile, and D is the diameter of the

pile. 0.5 on the x-axis represents the front of the pile.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1

z/δ

Cp

Pressure Distribution along Upstream Pile Wall

CFD Data, Current Study CFD Data, Published Study

Experimental Data, Published Study

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Figure 4-52 Comparison of Pressure Distribution Data along Upstream Symmetry Line

Figure 4-52 shows the pressure coefficient distribution along the upstream symmetry line. It can

be seen that as the line approaches the pile wall, the pressure coefficient increases until

approximately 1.5m in front of the pile wall when a dip in the pressure coefficient occurs, this is a

result of the horseshoe vortices forming. The pressure coefficient then increases to 1 at the base

of the pile wall. The pressure coefficient data is normalised by the pressure coefficient data found

at the base of the pile.

4.5.2.4 Wall Shear Distribution along Upstream Symmetry Line

The wall shear distribution along the upstream symmetry line from the various sources is shown

in Figure 4-53. The blue line represents the data taken from the 3D Model used in the current

study, the red line represents the numerical simulation data taken from [42], and the green dots

represent the experimental data values taken from [42]. The x-axis is represented by x/D, where x

is equal to the distance from the centre of the pile, and D is the diameter of the pile. 0.5 on the x-

axis represents the front of the pile.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-2 -1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6

Cp

x/D

Pressure Distribution along Upstream Symmetry Line

CFD Data, Current Study CFD Data, Published Study

Experimental Data, Published Study

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Figure 4-53 Comparison of Wall Shear Data along Upstream Symmetry Line

Figure 4-53 shows the wall shear, in the x-orientation, distribution along the upstream symmetry

line. The shear stress distribution is decreasing as the line approached the pile wall. It can be seen

that at approximately 2m in front of the pile the wall shear is 0. This is due to the wall shear data

which is in the x-orientation, so as the pile wall is approached, the shear that was being caused by

the inlet flow is now being caused by the horseshoe vortices that are forming in front of the pile

and pushing back on the seabed, causing a negative shear stress value to arise. This shear stress

reaches a max at approximately 0.5m in front of the pile before returning to a 0 shear value at the

base of the pile. The point the wall shear crosses the 0 x-axis is the maximum distance away from

the pile that the horseshoe vortices have an effect on the seabed.

4.5.2.5 Boundary Layer Formation

The velocity values from the data line in the 3D Models is plotted in Figure 4-54 and Figure 4-55

below as a blue line and is plotted against the UDF velocity profile, shown as a dashed red line,

that was implemented in the UDF code for that particular inlet speed, i.e. 𝑢

𝑈= (

𝑦

𝛿)

1

7. Two

simulation boundary layers are shown to show that that regardless of whether the inlet speed is

high or low, the boundary layer formation is still occurring.

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

-2 -1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6

τ o/τ

∞

x/D

Wall Shear Distribution along Upstream Symmetry Line

CFD Data, Current Study CFD Data, Published Study

Experimental Data, Published Study

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Figure 4-54 Boundary Layer Formation Check, 0.2m/s

Figure 4-55 Boundary Layer Formation Check, 1.42m/s

Figure 4-54 and Figure 4-55 show the boundary layer formation, the UDF power simply follows

the power law equation, explained in Section 3.1.5. It can be seen that as the height above the

0

0.05

0.1

0.15

0.2

0.25

0 1 2 3 4 5 6 7 8 9 10

Inle

t Sp

ee

d (

m/s

)

Height above Seabed (m)

Boundary Layer Formation Check, 0.2m/s

CFD Velocity Profile UDF Power Law Velocity Profile

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 5 6 7 8 9 10

Inle

t Sp

ee

d (

m/s

)

Height above Seabed (m)

Boundary Layer Formation Check, 1.42m/s

CFD Velocity Profile UDF Power Law Velocity Profile

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seabed is increased that the inlet speed of both the CFD and UDF code increases according to the

power law equation.

Figure 4-56 and Figure 4-57 below shows the velocity contours along the model, showing the

boundary layer forming. Again, this is shown for an inlet speed of 0.2m/s and 1.42m/s.

Figure 4-56 Velocity Contour Plane, 0.2m/s

Figure 4-57 Velocity Contour Plane, 1.42m/s

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4.6 Creating the Scour Prevention 3D Models

Given the lack of experimental facilities used throughout this project, only numerical simulations

could be performed to investigate the effects of various scour prevention device. The designs

chosen to be investigated were three types of collar designs: a rectangular, triangular, and

rounded design; two additional models simulating a helical full wire and helical half wire.

It should be noted that the purpose of this current project is not to design a scour prevention

device that will mitigate the effects of scouring. The purpose of this design process is effectively a

proof of concept or feasibility, not of the design themselves. The meshing involved in the scour

prevention designs was difficult given the complex designs, as opposed to the original pile. This

also resulted in difficulty achieving convergence when running simulations, as the fine scale

geometry nearly always introduces small radii of curvature, and hence high local stresses which

may not be accurate. The mesh developed allowed for reliable simulation results, in some cases

the absolute convergence criteria of 1x10-5 wasn’t always reached, but a low convergence value

was generally reached.

Each of the geometries shown in the following sections was still implemented in the flow field

originally developed. Instead of the pile used before, the following geometries were placed in its

place.

4.6.1 Rectangular Collar

The rectangular collar designed can be seen in Figure 4-58 below. The collar has a height of 1m

and a width of 2m, giving the collar a total diameter of 7m.

Figure 4-58 Rectangular Collar, Side and Isometric View

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4.6.2 Triangular Collar

The triangular collar can be seen in Figure 4-59 below. The triangle has a height of 2m and a

length of 2m, giving the collar a total diameter of 7m.

Figure 4-59 Triangular Collar, Side and Isometric View

4.6.3 Rounded Collar

The rounded collar can be seen in Figure 4-60 below. The rounded collar has a height of 2m and

width of 2m, giving the collar a total diameter of 2m. The rounded edge has a radius of 2m.

Figure 4-60 Rounded Collar, Side and Isometric View

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4.6.4 Helical Wires

Two helical wire designs were developed to be tested. One model was developed using full wires

wrapped around the pile (seen in Figure 4-61), while the second had half wires wrapped around

the pile (seen in Figure 4-63). This was done as the geometry of the full wire proved difficult to

mesh and to achieve convergence on when running simulations. The wire pattern is the same for

all models, a double threaded pile with three turns of the wire around the pile.

Due to meshing issues, caused by the sharp corners where the wire met the pile wall, the edges

of these contact points had to be filleted with a radius of 0.15m. This can be seen in Figure 4-62

and Figure 4-64. The reduction in sharp corners allowed the geometry to be meshed with the

same settings as the collar models, allowing for more comparable results.

[80] found that the most effective wire size was when the wire-pile diameter ratio was 0.1. With a

pile diameter of 5m, this results in a wire with a diameter of 0.5m. This was implemented in the

full wire geometry design, while the half wire geometry still had a wire diameter of 0.5m, but only

half the wire was used

4.6.4.1 Helical Wire, Full Wire

Figure 4-61 Helical Wire (Full Wire), Side and Isometric View

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Figure 4-62 Helical Wire (Full Wire), Filleted, Side and Isometric View

4.6.4.2 Helical Wire, Half Wire

Figure 4-63 Helical Wire (Half Wire), Side and Isometric View

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Figure 4-64 Helical Wire (Half Wire), Filleted, Side and Isometric View

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4.7 Meshing Development for Scour Prevention Models

4.7.1 Volume Meshing

Similar to the original 3D Model, volume meshing was used in order to have a more detailed

mesh in regions of interest or where high turbulence was expected. A different volume meshing,

seen below in Figure 4-65, was used compared to the volume meshing seen in the original 3D

Model, seen in Figure 4-9. The main difference is the block around the pile is bigger so that it

could accommodate the scour prevention devices being put in place of where the pile is. There

also is not any block division along the pile block, as seen in Figure 4-11.

Figure 4-65 Volume Meshing for Scour Prevention Devices

4.7.2 Meshing Details

Table 4-15 below shows the element sizing in each of the block in the volume meshing. Figure

4-66 below shows the Block Names used in Table 4-15

Block Element Sizing (m)

A 2

B 1.5

C 0.5

D 0.25

E 0.2

F 0.25

Table 4-15 Element Sizing of Blocks in the Volume Meshing

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Figure 4-66 Named Blocks

Face Sizing were also implemented so that further detail could be obtained in areas of interest. A

Face sizing of 0.1m was applied on the bottom of Blocks C, D, E, and F on the seabed. A face sizing

of 0.1m was also applied to the inside of the pile wall and the accompanying scour prevention

device.

4.7.3 Inflation Layer Details & Issues

The inflation layer used was the same as in the original 3D Model as shown in Table 4-1 in Section

4.2.7. The only change was in the Helical Wire Models when the inflation layer options had to be

changed to accommodate the complex geometries, the details of this inflation layer can be seen

below in Table 4-16 and where applied to the same faces as the original 3D Model.

Inflation Option Total Thickness

Number of Layers 12

Growth Rate 1.2

Maximum Thickness 0.1m

Table 4-16 Inflation Options, Helical Wire Models

4.7.4 Element Count, Scour Prevention Models

Model Element Count

Basic 2,191,805

Rectangular Collar 2,210,493

Triangular Collar 2,187,325

Rounded Collar 2,211,612

Helical Wire (Half Wire) 2,307,406

Helical Wire (Full Wire) 2,389,579

Table 4-17 Element Count, Scour Prevention Models

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4.7.5 Final Meshing

4.7.5.1 Rectangular Collar

Figure 4-67 Rectangular Collar, Final Meshing

4.7.5.2 Triangular Collar

Figure 4-68 Triangular Collar, Final Meshing

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4.7.5.3 Rounded Collar

Figure 4-69 Rounded Collar, Final Meshing

4.7.5.4 Helical Wire (Full Wire)

Figure 4-70 Helical Wire (Full Wire), Final Meshing

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4.7.5.5 Helical Wire (Half Wire)

Figure 4-71 Helical Wire (Half Wire), Final Meshing

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4.8 ANSYS Scour Prevention Model Parameters and Boundary Conditions

4.8.1 UDF

The UDF implemented in the Scour Prevention Models was the same UDF that was implemented

in the original 3D Model. The code for the UDF can be found in Appendix 10.4.

4.8.2 Setup Options

The Setup Options used for the Scour Prevention Models were the same as the Setup Options

used in the original 3D Model. The options can be found in Section 4.3.4.

4.8.3 Fluid Properties

The Fluid Properties used in the Scour Prevention Models were the same as the fluid properties

used in the original 3D model. The properties can be found in Section 4.3.2.

4.8.4 Inlet Velocity

The inlet velocity range used in the Scour Prevention Models was not the same as seen in the

original 3D Model. For the Scour Prevention Models, only an inlet speed of 0.5m/s was tested on

all models. This was deemed to provide adequate results from which to determine if a certain

scour prevention model was effective at reducing the effects of scour. Given the time constraints

for the project, the time demand of running 11 different inlet speeds on 5 different models was

and analysing different scour ranges in each of those simulations was too much.

4.8.5 ANSYS Model Used

The ANSYS Models used in the Scour Prevention Models was the same as the ANSYS Models used

in the original 3D Model. The models can be found in Section 4.3.5.

4.8.6 Zero Shear on Walls

The Zero Shear condition in the Scour Prevention Models was applied to the same walls as the 3D

Model. These walls can be seen in Section 4.3.6.

Methodology, Determining if Scour will occur Mark Donnelly-Orr

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4.9 Determining if Scour will occur

4.9.1 Stresses on Seabed

The shear stress on the seabed was the vital value from which to determine if sediment

movement, and hence scouring, will occur. The shear stresses on the seabed were taken from the

converged simulations in ANSYS Fluent, with the seabed Wall Shear Values being the values of

interest.

As discussed in the Section 3.2.9, by knowing the shear stress acting on the seabed, it can be

determined if scouring will occur. Utilising the Shields Diagram (discussed in the Section 3.2.9.3),

it can be determined whether sediment movement will occur or not. But given the variance in the

sediment size in Dublin Bay, and the varying seabed shear stresses as a result of the varying tidal

currents, an arbitrary shear stress value can’t be chosen from which to conclude if scour will

occur or not. A range can be given for the varying parameters, i.e. for sediment movement to

occur with a bed shear stress of 0.2 Pa, the sediment size must be less than 0.5mm.

Using the Shields Diagram, and the accompanying Boundary Reynolds Number and the Critical

Shields Stress Equations, it could be determined what stresses are required for sediment

movement to occur.

As discussed in Section 3.2.9.3, the Boundary Reynolds Number and the Critical Shields Stress

Equations are as follows:

Critical Shields Stress:

𝜃𝑐 = 𝜏𝑜

(𝜌𝑠 − 𝜌𝑤)𝑔𝐷

Boundary Reynolds Number:

𝑅𝑒∗ = 𝑈∗𝐷

𝑣

And using the following parameters in the equations

Unit Parameter Value

𝝉𝒐 Seabed Shear Stress Varying from 0 to 3.5 Pa

𝝆𝒔 Density of Seabed Sediment 2082 kg/m3

𝝆𝒘 Density of Fluid 1023.07 kg/m3

𝒈 Gravity 9.81 m/s2

𝑫 Sediment Particle Diameter Varying from 0.2-0.8 mm

𝑼∗ Shear Velocity Depends on Seabed Shear Stress

𝒗 Kinematic Viscosity 0.00000144 m2/s

Table 4-18 Parameters for Shields Equations

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By varying the theoretical shear stress from 0 – 3.5Pa, in increments of 0.1Pa, over a range of

sediment sizes from 0.0002 – 0.0008m, in increments of 0.0001, the following Figure 4-72 was

obtained.

Figure 4-72 Shields Diagram with Various Sediment Sizes

It shows how the different increments have an effect on the interception of the Shields

Parameter. The interception point was then calculated so that the Critical Shields Stress and

Boundary Reynolds Number were known at the point where scouring would start to occur. By

rearranging the Critical Shields Stress equation or Boundary Reynold Number, the actual seabed

shear stress required for sediment movement to occur could be calculated and are shown in

Table 4-19.

Rearranged equations:

Rearranged Critical Shields Stress:

𝜏𝑜 = 𝜃𝑐(𝜌𝑠 − 𝜌𝑤)𝑔𝐷

Rearranged Boundary Reynolds Number:

0.01

0.1

1

10

0.1 1 10 100 1000

Cri

tica

l Sh

ield

s St

ress

Boundary Reynolds Number

Shields Diagram with Various Sediment Sizes

0.0002 0.0003 0.0004 0.0005

0.0006 0.0007 0.0008 Shields Curve

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𝜏𝑜 = (𝑅𝑒 × 𝑣

𝐷)

2

× 𝜌𝑤

Sediment Size

(m)

Interception Boundary Reynolds

Interception Critical Shields

Stress (Pa)

Rearranged Critical Shields Stress Equation (Pa)

Rearranged Boundary Reynolds Number

Equation (Pa)

0.0002 1.677190926 0.073862047 0.153457315 0.149188501

0.0003 2.610284133 0.052795496 0.164533521 0.160606601

0.0004 3.602438402 0.042178921 0.175263594 0.172069334

0.0005 4.727657641 0.036784322 0.191059671 0.18966289

0.0006 6.014845694 0.034405259 0.214443228 0.213194901

0.0007 7.375859443 0.032718084 0.237915185 0.235537205

0.0008 8.969339551 0.032348947 0.268835359 0.266668238

Table 4-19 Interception Points and Rearranged Equations

As can be seen in Table 4-19 above, the two rearranged equations give almost identical answers.

Taking the average of these two values, the seabed shear stress required to cause scouring to

occur for those sediment sizes can be obtained and is shown in the table below.

Sediment Size (m) Critical Seabed Shear Stress (Pa)

0.0002 0.151322908

0.0003 0.162570061

0.0004 0.173666464

0.0005 0.19036128

0.0006 0.213819065

0.0007 0.236726195

0.0008 0.267751798

Table 4-20 Critical Seabed Shear Stress required for Sediment Movement of various Sediment

Sizes

With Table 4-20, depending on which sediment size is most prominent around a certain turbine

pylon, the user will know what seabed shear stress is required for sediment movement, and

hence scouring, to occur.

A detailed sample calculation of the above calculations can be found in Appendix 10.5.

4.9.2 Streamlines

Another method of verifying whether scour will occur, not the extent or magnitude of it, but the

confirmation that the fluid flow is flowing as expected, is to investigate the streamlines behind

the pile. By visualising these streamlines and implementing them in ANSYS CFD-Post, it is possible

to confirm the nature of the fluid flow. In order to implement the streamlines, a source plane had

to be created in ANSYS CFD-Post, as shown in Figure 4-73, and Figure 4-74 below.

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Figure 4-73, Streamlines Source Plane, Close Up Isometric View

Figure 4-74 Streamlines Source Plane, Source Points

The details of the plane are:

Width: 5m

Height: 10m

The plane is in line with the YZ plane and has a X-coordinate value of 2.5m (at the edge of the

pile)

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There are 200 evenly spaced seed points on this plane, seen in Figure 4-74, that have a forward

and backward direction. The forward-backward direction means that from streamline will be

shown of the particle not only as it travels downstream, but where it has flown from in order to

reach the seed points. The reason the forward-backward direction was chosen in order to capture

the flow around the pile as well. If a source plan was placed upstream of the pile, then the details

of the trailing vortices were not captured. For this reason, the source plane was placed

downstream of the pile.

4.9.3 y-Velocity Component

An additional method of verifying whether scour will occur and that it is occurring where

expected is to investigate the y-component of the fluid flow velocity. Again, using ANSYS CFD-

Post, contours can be applied behind the pile that show the y-component of the fluid flow, and it

can be seen if the fluid flow is impacting on the seabed floor.

The source plane for the contours is shown below in Figure 4-75, and was applied on the

symmetry wall.

Figure 4-75 y-Velocity Component Source Plane, Isometric View

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5 Results

5.1 3D Model

5.1.1 Scour Regions of 3D Model

As mentioned in Section 4.9.1, the seabed shear stresses limits being applied to the simulations

can be seen in Table 4-20. In order to understand the extent of the scour regions, these limits

must be applied to the simulations. As the critical shear stress required for sediment movement

varies with sediment size, and the shear stress on the seabed varies with current speed, different

scour progressions can be represented.

5.1.1.1 Varying Current Speeds

In Section 5.1.1.1, the inlet current speed is varied. The current speeds shown are 0.1m/s, 0.2m/s,

0.25m/s, 0.3m/s, 0.35m/s, 0.4m/s, 0.45m/s, 0.5m/s, 0.55m/s, 0.6m/s, 0.7m/s, 0.9m/s, 1.1m/s,

1.3m/s, and 1.42m/s, from Figure 5-1 to Figure 5-15. The sediment size being tested is kept

constant at 0.0005m; this was the chosen sediment size as it was the average sediment size in

Dublin Bay. At this sediment size, the critical seabed shear stress required for scouring is

0.1903Pa. With this value, the wall shear results on the seabed can be limited to this range, i.e.

from 0 Pa to 0.1903Pa, so that any region above 0.1903Pa would be highlighted as red and will

give an easy visual understanding of where scouring is occurring.

This technique was applied to every sediment size being tested, and aside from the 0.005m

sediment size case being shown below, all the results can be found in Appendix 10.7.1

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Figure 5-1 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0005m

Figure 5-2 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0005m

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Figure 5-3 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0005m

Figure 5-4 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0005m

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Figure 5-5 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0005m

Figure 5-6 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0005m

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Figure 5-7 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0005m

Figure 5-8 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0005m

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Figure 5-9 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0005m

Figure 5-10 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0005m

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Figure 5-11 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0005m

Figure 5-12 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0005m

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Figure 5-13 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0005m

Figure 5-14 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0005m

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Figure 5-15 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0005m

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5.1.1.2 Varying Sediment Sizes

In Section 5.1.1.2, the sediment size is varied while the inlet current speed is kept constant. The

current speed being shown is 0.5m/s, while the sediment sizes being shown are from 0.0002m,

0.0003m, 0.0004m, 0.0005m, 0.0006m, 0.0007m, and 0.0008m, from Figure 5-16 to Figure 5-22 .

For each case with varying sediment size, the limit on the wall shear range results was set to the

values found in Table 4-20.

This technique was applied to every inlet current speed being tested, and aside from the 0.5m/s

inlet current case shown below, all the results can be found in Appendix 10.7.2

Figure 5-16 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0002m

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Figure 5-17 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0003m

Figure 5-18 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0004m

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Figure 5-19 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0005m

Figure 5-20 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0006m

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Figure 5-21 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0007m

Figure 5-22 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0008m

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5.1.2 Streamlines of 3D Models

The streamlines of the 3D Model are shown in this section. Three inlet current speeds are shown

in order to give the reader an understanding of how the streamlines vary with increasing current

speed, but it was deemed unnecessary to include the streamlines for every inlet current speed

simulated. The three inlet current speeds shown are 0.2m/s, 0.7m/s, and 1.42m/s.

An isometric view of the streamlines is shown for each case, but to aid the reader to visualise the

streamlines, a full range of views of the streamlines can be found in Appendix 10.8. The views

shown in the Appendix are listed below.

1. Isometric

2. X+ (Viewing Downstream)

3. X- (Viewing Upstream)

4. Y+ (Viewing from Below)

5. Y- (Viewing from Above)

6. Z+ (Viewing from the Left)

7. Z- (Viewing from the Right)

Section 4.9.2 shows the location of the streamline source plane and the details of the streamlines.

5.1.2.1 0.2m/s Streamlines

Figure 5-23 Streamlines for 0.2m/s current, Isometric View

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5.1.2.2 0.7m/s Streamlines

Figure 5-24 Streamlines for 0.7m/s current, Isometric View

5.1.2.3 1.42m/s Streamlines

Figure 5-25 Streamlines for 1.42m/s current, Isometric View

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5.1.3 y-Velocity Component of 3D Model

The y-velocity component behind the pile is shown in this section. Section 4.9.3 shows the

location of the y-velocity source plane and the details of the y-component.

5.1.3.1 0.2m/s y-Velocity Component

Figure 5-26 y-Velocity Component, 0.2m/s Current, Symmetry Wall Plane

5.1.3.2 0.7m/s y-Velocity Component

Figure 5-27 y-Velocity Component, 0.7m/s Current, Symmetry Wall Plane

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5.1.3.3 1.42m/s y-Velocity Component

Figure 5-28 y-Velocity Component, 1.42m/s Current, Symmetry Wall Plane

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5.2 Scour Prevention Models

5.2.1 Scour Regions of Scour Prevention Devices

Similar to Section 5.1.1, the following Figures show the extent of scouring around the various

designs. In order for comparability, all the models were run with a 0.5m/s inlet current speed. The

limit of the range being checked in the Figures is 0.2677Pa, i.e. the critical shear stress for a

sediment size of 0.0008m. This limit was applied to all the scour prevention models.

5.2.1.1 Basic Model

Figure 5-29 Seabed Shear Stress, Basic Model, Current Speed: 0.5m/s, Sediment Size: 0.0008m

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5.2.1.2 Rectangular Collar

Figure 5-30 Seabed Shear Stress, Rectangular Collar, Current Speed: 0.5m/s, Sediment Size:

0.0008m

5.2.1.3 Triangular Collar

Figure 5-31 Seabed Shear Stress, Triangular Collar, Current Speed: 0.5m/s, Sediment Size:

0.0008m

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5.2.1.4 Rounded Collar

Figure 5-32 Seabed Shear Stress, Rounded Collar, Current Speed: 0.5m/s, Sediment Size:

0.0008m

5.2.1.5 Helical Wire (Half Wire)

Figure 5-33 Seabed Shear Stress, Helical Wire (Half Wire), Current Speed: 0.5m/s, Sediment

Size: 0.0008m

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5.2.1.6 Helical Wire (Full Wire)

Figure 5-34 Seabed Shear Stress, Helical Wire (Full Wire), Current Speed: 0.5m/s, Sediment Size:

0.0008m

5.2.2 Streamlines of Scour Prevention Devices

The streamlines of the Scour Prevention Models are shown in this section. The only inlet current

speed shown is 0.5m/s as this was the only inlet speed the Scour Prevention Models were tested

with. This was deemed adequate as it was the effect of various scour prevention devices that was

of interest, not the extent to which they work with various inlet current speeds.

An isometric view of the streamlines is shown for each case, but to aid the reader to visualise the

streamlines, a full range of views of the streamlines can be found in Appendix 10.8. The views

shown in the Appendix are listed below.

1. Isometric

2. X+ (Viewing Downstream)

3. X- (Viewing Upstream)

4. Y+ (Viewing from Below)

5. Y- (Viewing from Above)

6. Z+ (Viewing from the Left)

7. Z- (Viewing from the Right)

Section 4.9.2 shows the location of the streamline source plane and the details of the streamlines.

Results, Scour Prevention Models Mark Donnelly-Orr

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5.2.2.1 Basic Model Streamlines

Figure 5-35 Streamlines for Basic Model, 0.5m/s current, Isometric View

5.2.2.2 Rectangular Collar Model Streamlines

Figure 5-36 Streamlines for Rectangular Collar Model, 0.5m/s current, Isometric View

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5.2.2.3 Triangular Collar Model Streamlines

Figure 5-37 Streamlines for Triangular Collar Model, 0.5m/s current, Isometric View

5.2.2.4 Rounded Collar Model Streamlines

Figure 5-38 Streamlines for Rounded Collar Model, 0.5m/s current, Isometric View

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5.2.2.5 Helical Wire (Half Wire) Model Streamlines

Figure 5-39 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Isometric View

5.2.2.6 Helical Wire (Full Wire) Model Streamlines

Figure 5-40 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Isometric View

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5.2.3 y-Velocity Components of Scour Prevention Devices

The y-velocity component behind the pile in the Scour Prevention Models is shown in this section.

Section 4.9.3 shows the location of the y-velocity source plane and the details of the y-

component.

5.2.3.1 Basic Model y-Velocity Component

Figure 5-41 y-Velocity Component, Basic Model, 0.5m/s Current, Symmetry Wall Plane

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5.2.3.2 Rectangular Collar y-Velocity Component

Figure 5-42 y-Velocity Component, Rectangular Collar Model, 0.5m/s Current, Symmetry Wall

Plane

5.2.3.3 Triangular Collar y-Velocity Component

Figure 5-43 y-Velocity Component, Triangular Collar Model, 0.5m/s Current, Symmetry Wall

Plane

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5.2.3.4 Rounded Collar y-Velocity Component

Figure 5-44 y-Velocity Component, Rounded Collar Model, 0.5m/s Current, Symmetry Wall

Plane

5.2.3.5 Helical Wire (Half Wire) y-Velocity Component

Figure 5-45 y-Velocity Component, Helical Wire (Half Wire) Collar Model, 0.5m/s Current,

Symmetry Wall Plane

Discussion, 3D Model Mark Donnelly-Orr

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5.2.3.6 Helical Wire (Full Wire) y-Velocity Component

Figure 5-46 y-Velocity Component, Helical Wire (Full Wire) Model, 0.5m/s Current, Symmetry

Wall Plane

6 Discussion

6.1 3D Model

There are numerous points that need to be discussed about the 3D Model implemented for this

report. The following sections focus on certain aspects of discussion for the 3D Model.

The results from Section 5.1.1 show that scouring will occur once an inlet current speed of

approximately 0.225m/s to 0.275m/s, depending on the sediment size, is reached, after which the

extent of the scouring region increases in size as the inlet speed is increased until an inlet current

speed of 0.4m/s to 0.6m/s, depending on sediment size, is reached, after which a live-bed

scenario is shown to occur. This is an important understanding as it gives a range at which scour

will occur and the form of scour that is likely to occur that is easy to identify.

6.1.1 Scour Region Shape

A central area of discussion is whether the scour regions predicted are reliable in their prediction

of where scour will actually occur. While there are many reports on the effects of scouring in

literature, most of these reports are based on morphological models that predict the depth of the

scour after certain time durations. The model developed for this project does not have this ability

Discussion, 3D Model Mark Donnelly-Orr

154

to capture this data and is purely a steady solution. [42] shows the stress amplification on the

seabed around a pile from both a steady numerical model and an experimental setup, shown in

Figure 6-1. By manipulating the data from the 3D Models and applying certain conditions, a

comparable figure can be created.

Figure 6-1 Seabed Shear Stress Amplification. (a) Numerical Model Results, Published Study

[42]. (b) Experimental Results, Published Study [86].

In Figure 6-2 below, the CFD results from the 3D Model have been manipulated in order to obtain

a comparable figure to Figure 6-1. The amount of contours was changed to 7 and the limit of the

seabed shear stress was set to 0.7Pa, so that the stress amplification would be 7 and hence

comparable to Figure 6-1.

Note: the image was reflected on the X-Y plane in order to make the Figures more comparable.

The ranges shown are shear stress amplification which is related to the equation 𝛼𝜏 =|𝜏𝑜|

𝜏∞, where

τ∞ is equal to 0.1Pa for the given CFD model and τo varies depending on the area of interest. So 7

in Figure 6-2 represents an area where the seabed has a shear stress of 0.7Pa etc.

Figure 6-2 Seabed Shear Stress Amplification, CFD Results, Current Study

Discussion, 3D Model Mark Donnelly-Orr

155

It can be seen that Figure 6-1 (a) and Figure 6-2 are similar in their depiction of the seabed shear

stress amplification. The magnitudes of the amplification are closely related, with an amplification

of 7 being the maximum in the region close to the pile wall. The contour shapes are also very

similar between two CFD models, but there is a slight indent in the current study CFD results

contours close to the pile wall when compared to the numerical model results from [42]. The

region of this amplification is slightly further around the pile in the current study CFD results as

opposed to the numerical model results from [42]. This is likely a result of the separation point

occurring further around the pile, but overall the agreement of the results from the current study

CFD and the numerical model from [42] is reasonable.

When Figure 6-2 is compared with Figure 6-1 (b), again there are strong similarities, but some

differences. The amplification values are comparable, but the shape and the location of the

contours are somewhat different. Similar to Figure 6-1 (a), the regions of amplification from the

experimental data are closer to the upstream face of the pile compared to the current study CFD

results. The shape of the contour regions are also different, with the experimental contours

showing the maximum region of shear stress to be disjointed from the pile wall as opposed to the

numerical model results and the current study CFD results.

Overall, the stress amplification results show the regions where scour is likely to start occurring

initially and where it will spread to. These amplification shapes are the shapes expected

regardless of what magnitude of shear stress is acting on the seabed. This helps understand

where scour will occur at low current speed, i.e. up to 0.5m/s, and the areas affected. This in turn

helps to understand when the occurrence of scour becomes a substantial issue and cannot be

ignored in terms of the structural integrity damage is causes to the wind turbine pylon.

The following sections look at various aspect of the scour region in more detail.

6.1.1.1 Theoretical Calculations

The scour region predictions are based on the limits calculated from the Shields Equations, seen

in Section 3.2.9.3. While the Shields diagram has been used right up to present by many hydraulic

engineers [88], this does not necessarily mean that it can be trusted and used for the seabed

shear stress limits calculations. Ideally other sources of threshold shear stress would be used,

such as the updated Shields Diagrams created by [88, 92], but given the longevity of the original

Shields Diagram and its use in the real world, and the time constraints of writing this report, the

Shields Diagram was deemed adequate in determining the shear stress limits. Another factor that

made the Shields Diagram trustworthy was the amount of variables it took into consideration, 7

Discussion, 3D Model Mark Donnelly-Orr

156

in total, meaning it incorporated a lot of different factors into the equations used for calculating

the threshold shear stresses required for sediment movement.

Another aspect of the theoretical calculations that needs to be considered is whether the shear

stress threshold limits calculated give realistic results. The range calculated in this current study

ranges from 0.15Pa to 0.26Pa, while [55] predicts a range from 0.2Pa to 1.2Pa for non-cohesive

sand-size sediment to be entrained and [104] predicts a range of 0.1695Pa to 0.39Pa for sediment

sizes ranging from 0.0002m to 0.0008m. So it can be seen that the range calculated in the current

study are in keeping with ranges calculated in other literature. A direct comparison of different

ranges is difficult to achieve given the number of variables that apply to each unique situation.

But given the ranges found in other literature, the range calculated in this report is deemed

accurate.

A source of error in the shear stress threshold calculations was the averaging of the rearranged

Critical Shields Stress and Boundary Reynold Number Equations. When the interception points

was calculated from Figure 4-72 and inputted into the rearranged equations, the threshold shear

stress was calculated, but both equations produced values for the threshold shear stress. There

was a discrepancy of approximately 0.003Pa between the two results, with the rearranged Critical

Shields Stress Equation giving the higher value. The average of the two results was taken as the

threshold shear stress, instead of either one by itself. But given the small difference between the

two results, this was deemed appropriate.

A limitation of the critical shear stress calculations and of the report itself was the sediment

density used in the critical shear stress calculations. Only one value of density was used in the

calculations, and was done under the assumption that all the sand in Dublin Bay had the same

density, which is a large assumption to make given the varying bedrock and sands, see Figure 3-9,

found in Dublin Bay. The density used was 2082kg/m3, but other sources stated that the density

of wet sand can vary from 1922 kg/m3 to 2082 kg/m3, depending on the consistency of the sand

and whether there is gravel in it, or if it is packed or loose. The limits were calculated for three

different densities, shown in Appendix 10.6, and ideally these limits would have been applied to

the 3D Model in the same manner as seen in Section 5.1.1 and Appendix 10.7, but it was found

the change in the scour region prediction was negligible, and the resulting extra figures needed

unnecessary.

6.1.1.2 Varying Current Speeds

The effect of the increasing inlet current speeds is obvious to see when looking from Figure 5-1 to

Figure 5-15, with the scour region increasing as the current speed increase. It can be seen that the

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scour region increases from the region of maximum seabed shear stress amplification, identified

in Figure 6-2, to the areas of lower seabed shear stress amplification, and having the least effect

on the regions identified as lower seabed shear stress amplification. This supports the project

hypothesis that scouring will occur at the base of wind turbine pylon in Dublin Bay.

It can be seen that the critical change, from live-bed to clear-water scenario, comes between the

inlet speeds of 0.4m/s and 0.6m/s, after which live-bed movement is deemed to occur, regardless

of pile vicinity. There is a sliver of seabed, in the wake of the pile, where the effects of scouring

are limited, for inlet speeds above 0.7m/s, but given the thinness of this sliver, it can be assumed

that sediment movement would flow over the region from the regions of sediment movement.

These observations are based mainly on the figures in Section 5.1.1.1 which only look at a

sediment size of 0.0005m, but the observations hold true when the figures from Appendix 10.7.1,

which show the other sediment sizes, are investigated.

6.1.1.3 Varying Sediment Sizes

The effects of sediment size on the effects of scouring can clearly be seen from Figure 5-16 to

Figure 5-22. With a reduction on the amount of scouring when a larger sediment size is

considered. This makes sense, as the bigger the sediment size, the heavier the sediment particle,

and hence it will require more fluid force to initiate movement. The range of sediment sizes

investigated change the movement from clear-water to a live-bed scenario, with live-bed scour

occurring when a small sediment size is investigated and clear-water scour occurring when a large

sediment size is investigated. This mainly occurs because of the constant current speed, 0.5m/s,

that is shown in the results in Section 5.1.1.2. 0.5m/s is a critical flow rate at which the sediment

movement is transitioning from clear-water to live-bed, so any change in sediment size is likely to

have a significant impact on the characterisation of the movement type. Looking at the various

flow rates investigated in Appendix 10.7.2, it can be seen that for a higher flow rates, i.e. above

0.7m/s, that the sediment size has little impact on the scouring region, whereas for lower flow

rates, i.e. below 0.4m/s, the sediment size has a substantial impact on the scouring regions.

It is hard to draw conclusions from these pictures as each case specifically only looks at a single

sediment size around a wind turbine pylon. Realistically, there will be a mixture of sediment sizes

around each wind turbine pylon, there may be cases where one particular sediment size is

prominent around a turbine, but it is impossible to tell without going out into Dublin Bay and

checking all the sand around each turbine, and given the likelihood of live-bed scenario occurring

frequently, the sediment size will constantly vary around the wind turbine pylons. A conservative

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158

approach would be to only look at the smallest sediment size expected in Dublin Bay, 0.0002m,

and use this as the basis of our conclusions.

6.1.2 Streamlines

Figure 5-23, Figure 5-24, and Figure 5-25 show an isometric view of the streamlines around the

pile. Appendix 10.8 contains figures from all major angles to aid the reader visualise the

streamlines. It can be seen that there is no significant difference in the streamlines for the range

of flow rates shown, indicating that the flow rates in between will exhibit the same streamline

shapes. There are two prominent vortices that form in the downstream of the pile; one travelling

in an upwards direction, and one travelling in a downwards direction. The upward downstream

vortex is an unexpected result and is discussed in detail in Section 6.3 as to the significance of it.

Looking at Figure 10-197, Figure 10-204, and Figure 10-211, it can be seen that the downwards

vortex has an inner vortex. The significance of this inner vortex is not understood and upon

investigation of the literature no explanation could be found.

It can be seen that the downstream vortices diverge strongly from the centre of the pile heading

in an upwards and downwards direction. This means that a lot of the energy in the downstream

flow is not impacting upon the seabed, resulting in a reduction of the seabed shear stress

amplitude. Again this is discussed in more detail in Section 6.3. The vortices also diverge slightly

from the symmetry plane, again the significance of this is not understood, but it likely explains

why the sliver of region exhibiting no scouring in the wake of the pile occurs. As the vortices

sweep down along the symmetry plane and along the seabed before rotating back up away from

the seabed, this results in a region where the fluid particles in the vortices are heading up away

from the seabed and not down onto the seabed, resulting in a region of reduced seabed shear

stress.

Both the upwards and downward downstream vortices that form have large radii initially before

gradually becoming smaller and smaller before dispelling and merging with the steady down flow.

This is a result of the fluid dissipating the energy gained from the turbulence caused by the pile. It

can be seen in Figure 10-199, Figure 10-206, and Figure 10-213 that the downwards vortex is

stronger than the upwards vortex. This is assumed as the downward vortex has more streamlines.

The assumption is based on the fact that the streamlines coming from the source plane are

evenly distributed, therefore if more streamlines are in the downwards vortex, it is a result of the

flow traveling in that direction. Conversely the upwards vortex has a longer reach in the x-

direction before dissipating, this extra length is likely due to the zero shear condition on the top

Discussion, 3D Model Mark Donnelly-Orr

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surface, whereas the downward vortex has a boundary layer to quickly dissipate the streamlines

energy.

Investigating the streamlines flowing around the pile wall before separation occurs, and just in

front of the pile, best seen in Figure 10-200, Figure 10-207, and Figure 10-214, further

confirmations can be made about the flow regime. It can be seen that as the streamlines

approach the front of the upstream pile wall, they dip down slightly before physical contact; this

is a result of the stagnation pressure gradient form on the pile wall from the oncoming flow. The

stagnation pressure causes a secondary flow to generate towards the seabed, before developing

into the horseshoe vortices, which flow down and around the pile wall, which is also depicted in

the figures mentioned. This phenomenon is explained in more detail in Section 3.2.1. The fact the

streamlines are capturing this phenomenon and the development of the horseshoe vortices is a

good indication that the 3D Model is capturing the detail required to make conclusions.

Overall the streamlines do not directly support the project hypothesis, but rather aid the reader

in visualising the flow regime around the pile, and show that features of the flow regime are

being captured.

6.1.3 y-Velocity

The y-velocity component of the contours again aids the reader in visualising the flow regime, and

is closely linked to the streamlines seen in the previous Section. In Figure 5-26, Figure 5-27, and

Figure 5-28 the y-component of the velocity are shown. The red contours indicate areas where

the fluid flow is traveling in an upwards direction and the blue indicates areas where the fluid

flow is traveling in a downwards direction. The contours seen in these Figures compare very well

with what was seen in the streamlines in the previous section, with the upward and downward

vortices forming in the downstream behind the pile.

A limitation of using the y-velocity contour plot to visually aid the reader is the location of the

source plane. Using the symmetry plane as a source gives good comparison with the streamline

direction, but if the plane is offset on the z-axis and is stop in a region on the other side of the

vortices, then the y-velocity is flipped behind the pile, i.e. there is a downwards y-velocity in the

upper half of the flow field and an upwards y-velocity in the lower half of the flow field behind

the pile.

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6.2 Clear-Water/Live-Bed Criterion

From the results seen in Section 5.1, it is possible to determine when the seabed sediment

movement transitions from a clear-water to a live-bed scenario. Table 6-1 depicts what the

estimated threshold velocity for live-bed initialisation for the different sediment sizes being

analysed.

Sediment Size (m) Threshold Velocity (m/s)

0.0002 0.4

0.0003 0.45

0.0004 0.45

0.0005 0.5

0.0006 0.55

0.0007 0.55

0.0008 0.6

Table 6-1 Threshold Velocity for live-bed initialisation for different sediment sizes

These threshold velocities were determined based on the figures in Appendix 10.7.1. The exact

moment of live-bed scouring was very difficult to determine and Table 6-1 is a best approximate.

Additional simulations were run with inlet speeds of 0.45m/s and 0.55m/s in order to better

determine the threshold velocity for sediment sizes 0.0003, 0.0004, 0.0006, and 0.0007m, as they

feel in the range between 0.4m/s and 0.5m/s, and 0.5m/ and 0.6m/s. Ideally simulations would

be run with 0.01m/s increments over these ranges, but given time constraints, 0.5m/s increments

were deemed adequate.

As explained in Section 3.2.6, live-bed scour occurs when the scour hole is continually supplied

with sediment by the approach flow. Therefore the effects of scouring should only occur up to the

threshold velocity, after which the depth of the scour hole should be filled with sediment from

the seabed until an equilibrium depth is reached, as depicted by Figure 3-29. This helps support

the understanding of the hypothesis, that clear-water scour will occur in the near wake vicinity

and will reach an equilibrium depth once live-bed scour occurs. This would give the assumption

that the scour holes will be a permanent feature of the wind turbine pylon bases, as sediment is

never being deposited, it is being scoured out, under clear-water criteria, or it is being continually

scoured and filled, under live-bed scenario. But the Kish and Bray Banks are there for a reason,

they are self-nourishing banks, with sediment being constantly shifted onto the banks. This is

likely a result of the geographical layout of Dublin Bay and the surrounding coastal region acting

as a giant vortex with sediment simply being shifted around the bay and surrounding coastal

regions.

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161

So the sediment dynamics of the Kish and Bray Banks, upon which the Dublin Array is being built,

is deemed to be as follows:

1. At flow rates below the threshold velocity clear-water scouring will occur, creating holes

and depressions around the wind turbine pylon bases. The depth of these holes will

increase from a minimum at low flow rates to a maximum at the threshold velocity flow

rate. But if the flow rates drop back down, it is thought that the scour holes will fill up to

the previous equilibrium depth experienced at that flow rate due to the self-nourishing

aspect of the sand banks. The self-nourishing rate is not known, and will vary depending

on flow rates and other factors

2. If the flow rates increases above the estimated threshold velocity, the scour holes and

depressions will decrease in depth (as shown in Figure 3-29) before again reaching an

equilibrium depth. Given the entire bed is in motion at these flowrates any self-

nourishing aspect of the sand banks is simply being scoured out of the holes.

3. Once the flow rates drop below the threshold velocity the scour holes will fill up again

because of the self-nourishing bank to the equilibrium depth experienced before the flow

rates went above the threshold velocity. This equilibrium depth will be proportional to

the flow rate being experienced, i.e. if the flow rate is just below the threshold velocity,

then the equilibrium depth will be quite big, but as the flow rates drop, the self-

nourishing aspect of the banks will fill the scour holes more and more, but it is not

expected that the scour holes will ever be fully filled up to the level experienced when the

wind turbine pylons were not installed, unless long periods of low rates is experienced in

Dublin Bay.

Note: the maximum scour depth that will be experienced around the wind turbine pylons,

regardless of the self-nourishing aspect, will be at the flow rate when the transition from clear-

water to live-bed scouring occurs.

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6.3 Downstream Vortices

Figure 5-23, Figure 5-24, and Figure 5-25 shows an unexpected result.

Looking at any of the literature covered in the Section 3 about the flow around piles, there is not

any information on the flow regime downstream of the pile, only that lee-wake vortices are

expected. The direction of the downstream flow is not specified, whether a downwards or

upwards motion is expected. Investigating the literature, there aren’t any sources of numerical

investigations into the flow regime around a cylindrical pile in the same circumstances as this

project, i.e. a vertical cylindrical pile attached to a floor and extending to the fluid boundary.

Many papers are available on numerical and experimental investigation of flows around finite

length piles, such as [105-109].

While these papers give an understanding of the expected flow regime behind a finite length

cylindrical pile, the flow regime will be drastically different when the pile is not finite length, but is

extended to the fluid boundary, i.e. to the sea surface in the current project.

There are many experimental sources of offshore wind turbine base tests, but they are generally

based on the effects of the flow regime impact on the seabed and turbine structure, and do not

investigate or attempt to visualise the downwind fluid regime or the vortices that form as a result

of the obstruction of the pile.

The issue that arises is that in the ANSYS simulations run for the various current speeds, it can be

seen from Figure 5-23, Figure 5-24, and Figure 5-25 that there is a large amount of upwards

travelling vortices that sweep up to the sea surface. This was unexpected as a zero shear

condition was placed on the upper boundary that replicates the sea surface. With this zero shear

condition in place, there wasn’t any expected reduction in the flow velocity that would cause the

vortices to be pulled up towards the sea surface. When this is compared to the simulations from

[106, 108], it can be seen that there is a substantial difference in the downstream flow regimes.

The two scenarios can’t be compared directly given the simulations in [106, 108] look at a finite

length pile while the current project looks at a pile that extends to the fluid boundary.

The downstream flow expected around a finite circular pile on a flat plate was theoretically

visualise in [110] from experimental data and is shown in Figure 6-3. The critical part of this

comparison is the trailing vortices that form from the top of the pile and arch downwards as

opposed to the upward vortices that are occurring in the 3D Models. This flow regime was also

numerically simulated in [108] and Figure 6-4 shows the trail vortices that form from the top of

the pile. Similar results will be shown from a replicated ANSYS simulation as explained below.

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163

Figure 6-3 Overview of flow around a Wall Mounted Cylindrical Pile, [110]

Figure 6-4 Mean Arched Vortices visualised by streamlines [108]

In order to confirm the downstream fluid regime is accurate in the current project simulations,

the finite length pile simulations and experiment seen in [105-110] were replicated in the ANSYS

software, while keeping the same parameters and boundary conditions that were being used in

the original 3D Model simulations done in the current project. By investigating the fluid flow of

this replicated simulation and comparing them to the simulations seen in [106, 108] conclusions

can be made as to whether the simulation parameters implemented in the original simulations

are correct or not and that the upwards downstream vortices seen in the original 3D Models are

not an anomaly but rather a feature of the fluid boundary , i.e. the sea surface.

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164

The model created to replicate the simulations in [106, 108] was an extension of the original

model, all the dimensions were the same but the height of the model was extended to 30m while

the cylindrical pile was kept at 10m tall and a basic meshing was used. All the details of this

replicated ANSYS model can be found in the Appendix 10.11. A detailed mesh was not

implemented as the general flow regime of the model was of interest and extensive accuracy was

not needed. The meshing implemented was deemed to be adequate. The same ANSYS

parameters and boundary conditions implemented in the 3D Models were implemented in the

Finite Length Pile Model.

Chosen results are shown below but the full set of results showing the streamlines can be found

in Appendix 10.11. Figure 6-5 is from the replicated ANSYS simulation and is purposefully

manipulated so that it is as comparable as possible with Figure 6-4 from [108]. It can be seen that

the two figures are very similar in terms of the flow regime and streamlines which shows that the

ANSYS parameters and boundary conditions implemented are not causing any discernible issues

to arise that would give rise to the upwards vortices that occur in the original 3D Model. Leading

to the conclusion that the upward vortices seen in the original 3D Model are a feature of the

downstream fluid flow. Figure 6-6 further shows the downward trailing vortices that form as a

result of the finite length pile.

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Figure 6-5 Mean Arched Vortices visualised by streamlines, Finite length Pile Model

Figure 6-6 Downward Trailing Vortices visualised by Streamlines, Finite length Pile Model, Side

View

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6.4 Scour Prevention Devices

The main finding from the development of the scour prevention devices is that they cause

significant flow change around the pile. This isn’t necessarily beneficial or detrimental in regards

to the effects the flow change has the scouring that will occur around the wind turbine pylon

base. There was a substantial limitation in the analysis of the scour prevention devices that arises

as only one current speed was simulated, 0.5m/s, and only one sediment size, 0.0005m, was

considered in the analysis. This could prove to be a considerable limitation because since the flow

regimes are being changed so substantially around the scour prevention devices, any increase or

decrease in flow rates could lead to significantly different flow regimes then calculated at 0.5m/s.

But given the threshold velocity for a live-bed situation is around 0.5m/s, the scour prevention

models should give results that allow for a critical assessment of the devices. And given the time

constraints of the project and the computational limits, the number of simulations that could be

run was limited.

While the effects of the devices on flow can be visualised and discussed, it was not possible to

find literature on numerical investigation into scour prevention devices, similar to the designs in

the current study, which were performed without a morphological seabed. In these papers, the

streamlines were not shown, hindering the current study’s ability to confirm the streamlines

obtained in the Scour Prevention Devices Models.

As discussed in Section 6.5, the meshing of the scour prevention devices were different then that

used in the 3D Model that is proven to give dependable answers in Section 6.6. In order to form a

basis on the trustworthiness of the meshing developed for the scour prevention devices, a basic

model was created. This basic model was the same geometry as used in the 3D Model, but used

the same volume meshing that was used in the Scour Prevention Models. By comparing the

solution of this basic solution with the solution for the 3D Model run under the same conditions

and parameters, conclusions can be made about the quality of the meshing used in the Scour

Prevention Models. Comparing Figure 5-29 and Figure 5-19, it can be seen that there are strong

similarities between the two models and that it can be concluded that the meshing developed for

the Scour Prevention Models are dependable and will provide reliable results. There are some

meshing issues in the results, and these can be seen in the form of irregular scour prediction

regions, this is discussed in more detail in Section 6.5.

While the meshing developed for the Scour Prevention Models was deemed adequate, the two

helical wire models had a reduced inflation layer, which was necessary due to the complex

geometry of the wires. Therefore the flow regime of the two helical wires must be carefully

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167

considered before any conclusions made. Another limiting factor of the conclusions made about

the helical wire models is the fact that the wires are not actually wires, as seen in Figure 4-62 and

Figure 4-64. The wires were fileted on the contact with the pile because of the difficulty of

implementing an inflation layer without the fileting. So when conclusions are made about the

helical wire model, consideration must be given.

6.4.1 Scour Region

The scour regions on the seabed predicted clearly show that the there is a substantial difference

with the scour prevention devices added onto the pile. The shear stresses on the devices

themselves were not shown as it is presumed that the devices installed would be able to

withstand the stresses caused by the fluid flow. Additionally, the scope of this report does not

include investigating the forces occurring on the pile nor the scour prevention devices, but on the

seabed.

It can be seen that the three collars investigated, seen in Figure 5-30, Figure 5-31, and Figure 5-32

have a relatively similar scour regions to each other and that the two helical wire models, seen in

Figure 5-33 and Figure 5-34 have similar scour regions to each other. When the three collars are

compared to the basic model, it can be seen that there is a larger region where scouring is

deemed to occur, not so much in front of the collar, but in the wake of the collar. Of the three

collars tested, the triangular collar has the smallest region of scour prediction, indicating that the

triangular collar is the most effective of reducing the impact of scouring, although the scour

region predicted is still larger than that seen in the basic model. The triangular collar model, as do

the other collar models, has a reduced shear stress in the far wake field near the symmetry line

compared to the basic model, indicating that the turbulence caused by the collars is beneficial at

mitigating scour in the far field wake.

The height of the collars from the seabed could be an important factor. This aspect of the collars

wasn’t investigated at all. If the collars were tall enough, particularly the rectangular collar, then

the collar would essentially act as a pile itself, with a larger diameter then the pile on which it is

fitted. Ideally the collars fitted should be as flush to the seabed as possible to reduce the amount

of flow being pushed around it and to maximise the reduction in scour region.

The scour region on the helical wire models is substantially different then that seen in the basic

model. This is mainly due to the turbulence that is caused by the wires as the fluid flow travels

around the pile. The scour regions in the helical wire models are very similar to each other; the

only difference is that the half wire model has a large region of increased shear stress in the far

downstream along the symmetry line. The scour regions in the near vicinity wake of the piles are

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168

almost identical, with the full wire model being slightly reduced in size. The shapes of the scour

regions in the near wake vicinity are smoother compared to the near wake scour region in the

basic model and the extent of the scour region shape is not as far from the pile as it is in the basic

model, indicating the helical wires keep the disturbed flow close to the pile wall, or push the flow

up and away from the seabed, which is the desired effect of the scour prevention devices

designed. Experimentally, [80] calculated that the reduction in scour depth using a helical wire

was a reduction of 46.3% indicating that the while the region affect is reduced, the ultimate scour

depth will also be reduced, which was not calculated in the Scour Prevention Models.

There is an element of vortex shedding in the scour regions predicted by the helical wires. This is

seen in the flick at the end of the scour region, and the variance in the shear stress values on the

seabed as the flow travels downstream.

6.4.2 Streamlines

The streamlines of the Scour Prevention Models give a good indication of the expected

turbulence in the wake of the pile, and how the devices are altering the flow regime. By

investigating and comparing the streamlines for the different devices, conclusions can be made

about the various scour prevention devices. Ideally, the devices will cause an alteration in the

flow that will mitigate the effects of scouring.

The streamlines in the basic model, seen in Figure 5-35, when compared with the streamlines

from the 3D Model, seen in Figure 5-24, show that the two models are not very comparable as

the streamlines are considerable different. While the general streamline shapes are consistent,

some features were missing or were unexpected. The distinction between the upwards and

downwards downstream vortices seen in the 3D Model is not seen in the Basic Model, which

appears to be more chaotic. There is some semblance of the upwards and downwards

downstream vortices in the Basic Model, but the downwards vortex dissipates abruptly while the

3D Model downwards vortex continues much further downstream. The orientation of the

downstream vortex is different in the Basic Model, seeming to rotate about the Z-axis orientation,

as opposed to the 3D Model which rotates about the X-axis orientation. The upwards vortex is

also different in the Basic Model, covering a larger area, again rotating about the z-axis

orientation, as oppose to the X-axis orientation seen in the 3D Model. Similar to the downwards

vortex, the Basic Model upwards vortex appears to dissipate abruptly. These points raised

indicate that the flow around the pile in the Basic Model is significantly different to the 3D Model,

reducing the strength of the conclusions made about the effectiveness of the Scour Prevention

Models.

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169

The Rectangular Collar Model has a significant effect on the downwards downstream vortex,

effectively stopping it from forming, with the majority of the vortex occurring in the immediate

wake of the pile above the collar, seen in Figure 10-227. The upwards downstream vortex is still

forming, but is more similar to the upwards vortex seen in the Basic Model as oppose to the

upwards vortex in the 3D Model. The important effect of the rectangular collar is the reduction in

the downward downstream vortex, which causes scour in the downstream. As discussed earlier in

this Section, the height of the collar could have a significant impact on the scour region, and if it

were more flush to the seabed, it can be assumed that the model would produce results very

similar to the basic model, but importantly, the collar would take the impact of the shear stress

occurring and reducing the area of scouring, as shown in Figure 3-39 and Figure 3-40.

The Triangular Collar Model streamlines are vastly different to the streamlines seen in the Basic

Model, with a large downwards downstream forming, rotating about the Z-axis orientation, seen

in Figure 10-234. The height of the vortex is unexpected, reaching up to half the piles height, as

opposed to approximately a quarter of the pile height in the Basic Model. Despite this large

vortex, the smallest scour region predicted for the collar models was on the Triangular Collar

Model, so while the downwards vortex is large, it can be concluded that the energy in it dispersed

over the large area, reducing its intensity on the seafloor. The upwards downstream vortex is

more similar to the upwards vortex seen in the 3D Model.

The Rounded Collar Model is similar in some aspects to the Triangular Collar Model. The upwards

downstream vortex is very similar, while the downwards downstream vortex reaches the same

height, half way up the pile, and covers the same area, but with less streamlines and hence less

magnitude. The majority of the downwards downstream vortex streamlines are close to the

seabed as seen in the Basic Model, indicating that there will be more shear stress on the seabed,

and this is proven when looking at the scour region of the Rounded Collar Model compared to the

other collar models, it has a larger scour region.

The two Helical Wire Model streamlines are similar to each other with some slight variation,

showing that the half wires and full wires will have different effects on the flow regime. The main

effect of the wires is to form a third downstream vortex. In addition to the downwards and

upwards downstream vortex, a vortex forms off the middle of the pile, more predominantly in the

Half Wire Model then the Full Wire Model, and travels downstream. The shapes of the upwards

and downwards vortices overall, in the Half Wire Model, are more similar to the streamlines

found in the 3D Model, with the addition of the middle vortex. The shapes of the upwards and

middle vortices in the Full Wire Model, are more chaotic, with the two vortices mingling together

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in the downstream and moving upwards. The downwards downstream vortices in both Helical

Wire Models indicate that there should be a significant amount of shear stress on the seabed, and

this is confirmed in Figure 5-33 and Figure 5-34, but while the intensity of the scour region is high,

it is not spread over a large area or gradually fades out, the scour regions stops abruptly.

Overall, the streamlines of the Scour Prevention Device Models were important in understanding

the shape and extent of the scour regions, and gave an indication of how the devices were

altering the flow.

6.4.3 y-Velocity

The y-velocity contours gave an extra visualisation of the flow regime behind the pile and a better

understanding of the magnitudes of velocity expected. This was possible because the legend of

the y-velocity contour Figures was limited to the maximum velocity found over all the Scour

Prevention Device Models. By doing this, the reader can compare the y-velocity of each of the

devices and see which devices had the greatest impact on the downstream flow regime. An

aspect of the y-velocity figures that must be considered before an analysis can be made is the

location of the source plane. This aspect is discussed in Section 6.1.3.

Comparing the Basic Model y-velocity, seen in Figure 5-41, with the y-velocity from the 3D Model,

seen in Figure 5-27, it can be seen that the similarities are limited. The magnitude of colour of the

two figures should be ignored as the legends are not the same, but the shape of the contours

should be compared. The distinction between the upwards and downwards vortices is obvious in

the 3D Model and split approximately just below the half height of the pile, whereas in the Basic

Model, the separation between the upwards and downwards vortices occurs at approximately a

fifth of the way up the pile from the seabed. Again this reduces the confidence in the conclusions

made from the Scour Prevention Models.

The Rectangular Collar Model shows strong upwards velocity while having almost no downwards

velocity. It can be seen that the largest upwards velocity occurs just off the corner of the collar,

beside where the intense short downward vortex was shown, in Figure 10-227. The y-velocity

shows that the collar is effective at taking the force, and associated shear stress, of the fluid flow

and pushing it up and away from the seabed. This leads to the conclusion that scouring occurring

around the pile and collar on the seabed is a result of the fluid flowing around the front of the

collar, which can be seen in Figure 10-228 by the streamlines. Again this confirms that a flush

rectangular collar would perform better than a tall collar, which essentially acts as a larger pile.

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The y-velocity of the Triangular Collar Model, seen in Figure 5-43, confirms the findings from the

streamlines, that the downwards downstream vortex is very large and reaches up to over half the

pile, indicating there will be a reduced region of scour.

The y-velocity contours of the Rounded Collar Model, seen in Figure 5-44, show and unexpected

result, that there is a region of upwards velocity near the seabed and at the top of the pile and a

region of downwards velocity in the middle of the pile. This unexpected occurrence could be

explained by the source plane location, that the downwards downstream vortices are near the

seabed, but at the symmetry plane, the vortex has an upwards motion as it travels around the

vortex circle. This shows that the potential flaw of the location of the source plane highlighted in

Section 6.1.3.

The y-velocity contours of the two Helical Wire Models, seen in Figure 5-45 and Figure 5-46, show

that, similar to the streamlines, there is a significant difference between the Full Wire and Half

Wire Models. The Half Wire Model shows a region of downwards velocity behind the lower half

of the pile and an upwards velocity behind the upper half of the pile. The upwards vortices have

an area of almost zero y-velocity, indicating that the wires are disturbing the flow and causing

regions of downwards flow in a region of upwards flow. The same feature does not occur in the

downwards y-velocity in the lower half of the pile, which is where it would be most effective in

reducing the flow on the seabed and hence the scour region. The Full Wire Model does not show

the same features as the Half Wire Model, but instead has an upwards and downwards y-velocity

with the two diverging from half way up the pile. The small region of intense upwards velocity

seen immediately behind the pile at the half way point was unexpected, but could be show that

the wires are causing turbulence and pushing the flow up, which is ideal for reducing scour.

6.4.4 Scour Prevention Device Logistical Factors

As discussed in the above Sections, the different scouring prevention devices have different

effects on the flow regime around the pile. While the different aspects of the altered flow regime

can be investigated and examined, the actual devices themselves should be evaluated. Many

factors must be considered before a conclusion on which devices is best suited can be made.

6.4.4.1 Manufacturability

The manufacturability of the devices is a critical factor that must be consider. The collars would

be simple enough to manufacture as they can essentially be made as extensions of the wind

turbine base pylon, although this would require cooperation with the manufacturer of the wind

turbines themselves and a modification made to the existing designs. If it is not possible to

Discussion, Scour Prevention Devices Mark Donnelly-Orr

172

incorporate the manufacturing of the collars, then it will still be relatively simple to construct the

collars out of steel panels.

While the idea is simple for the helical wire, and the manufacturing of a steel wire is relatively

simple, the wire must be 0.5m thick if it is to be most effective at reducing scour, which will

obviously be very difficult and expensive to make.

6.4.4.2 Implementation & Installation

The installation of the collars will be non-existent if is they are prefabricated onto the wind

turbine structures; if they are to be retrofitted onto a wind turbine, then there are two possible

methods. Either the collar is made in two halves and sunk down to the base of the wind turbine

and joined together via hyperbaric welding. Or as the foundation is being installed, the collar is

slipped over the top and slid down to the base. Either method is dangerous and not without risk.

The installation of the helical wire would be simpler. Only two anchor points need to be installed

onto the wind turbine wall for the wire, this would likely be achieved by both welding, for the

upper anchor most likely over the seawater surface, and hyperbaric welding, for the anchor that

will be at the base of the pylon. If the wire is 0.5m thick, then sturdy anchors will be needed hold

the weight of the wire, which will be significant.

6.4.4.3 Durability

The durability of the collars will be another important factor. Dublin Array will not want to have

to constantly reinstalled scour prevention devices if they are prone to wear. Ideally a device will

have the same lifetime as the wind turbines themselves in order to reduce the likelihood of the

devices having to be replaced. The collars will likely have a long lifetime as there is not much

dynamic impact on them. Similarly for the helical wires, there is not much dynamic force on them

underwater, but if the wire does wrap around the pile over the seawater surface, then it will likely

be exposed to the impact of waves, which could have severe impacts over the number of years.

This could arise if the wire losses tension over time and becomes slack, then the impact of the

waves could bang the wire into the wind turbine wall and with the small area of impact of the

wire, this could cause structural integrity of the wind turbine wall.

Rust is a critical factor that must be considered given the harshness of the marine environment.

Since the collars will ultimately be constantly submerged in water, rusting will occur at a slower

rate than the helical wires which will likely be exposed to both seawater and air which accelerates

the effects of rust dramatically. Even if the devices were made of galvanised steel, rusting will still

occur but at a slower rate.

Discussion, Scour Prevention Devices Mark Donnelly-Orr

173

6.4.4.4 Cost

The cost of the devices will be small when compared to the estimated cost of a single wind

turbine being installed in the Dublin Array, which will cost roughly €11,720,000 to €12,410,000

[111]. As the occurrence of scour is predicted to occur in the Dublin Array, with it comes the

potential of structural integrity to the wind turbine base that could either collapse or cause the

wind turbine to be shut down in order to assess the damage done to the base, reducing the

effectiveness of the Dublin Array. So the utilisation of a scour prevention device should be

strongly considered and given the cost of scour prevention devices is minimal compared to the

wind turbine structure, the cost is not a glaring issue.

6.4.5 Best Choice

If a scour prevention device had to be chosen from the analysis of the scour prevention devices

developed for this report, it is deemed that the triangular collar would be the most suitable

option for a number of reasons. Based solely on the numerical analysis, the triangular collar

performed the best in terms of reducing the scouring region compared to the other collars and

effectively reduced the scour region in the far field wake. The rectangular collar is also promising,

but given the height of the model tested, but not perform well, but experimental data from [83]

shows that a flush rectangular collar is quite effective against scour in steady current conditions.

The helical wires were deemed to be unpractical given the 0.5m diameter required for optimal

scour reduction efficiency, but the inclusion of smaller diameter wires would not only be practical

to manufacture, install, and maintain, but a relatively cheap endeavour with numerous wires

being used, and could be used in conjunction with an installed collar. A combination of helical

wires and a collar could prove to be very effective at reducing the impact of scouring in the near

wake vicinity of a wind turbine pylon.

Discussion, Meshing Mark Donnelly-Orr

174

6.5 Meshing

There are numerous aspects of the meshing that need to be discussed in order to achieve a full

understanding of the final meshing developed.

Looking at results figures in Section 5.1.1, there are irregularities that occur in the shear stress

prediction that are indicated in Figure 6-7 by the red ellipse. These occur in all the 3D Models and

are a result of the change in meshing element sizing due to the volume meshing, transitioning

from larger elemental size to smaller element size in the blocks closer to the pile. The effect of

this is particularly seen in Figure 5-34. This is a potential source of error in the results as this

region and likely the region outside of these lines as well, but given the interest of this report is

around the pile and the downstream, it is not a substantial error source. It may make the model

more difficult to converge, but given the convergence criteria were reached, it can be concluded

that the irregularities are acceptable.

Figure 6-7 Meshing Irregularities

The meshing scheme developed was found to be very comparable with the meshing schemes

developed in the literature for various numerical investigations of flow around piles and the

effects of scouring, examples of these meshing schemes can be found on the following papers,

[42, 97, 99-101], which all use a hex-dominant, grid-like meshing scheme with a bias towards the

pile and an inflation on the pile wall and seabed.

Discussion, Meshing Mark Donnelly-Orr

175

In terms of the amount of iterations needed to reach the convergence criteria, the k-ε models

generally needed 1200 iterations while the SST Transition model generally needed an additional

2150 iteration to reach the convergence criteria. The time needed to run both models took

approximately 6 hours.

The convergence criteria for the 3D Model was set to 1x10-5. This will have a significant impact on

the quality of the results calculated, the lower the convergence criteria, the more accurate the

solution will be, but it will also be harder to reach said convergence criteria and a finer mesh will

be required. While if the convergence criteria is set higher, then the solution will not be as

accurate and it’ll be easier for the model to reach the convergence criteria. But a convergence

criterion of 1x10-5 is generally the accepted standard in the industry so by achieving convergence

on the 3D Models with this criterion shows a good meshing standard was reached.

The meshing on the Scour Prevention Models if different to that implemented in the 3D Model.

This was due to the time constraints and limitations and the fact that the purpose of this report is

not to design a scour prevention device, but to investigate scouring at the base of offshore wind

turbine pylons. What the meshing developed for the Scour Prevention Models allowed for was a

basic understanding of the various devices and their ability to reduce the impact of scouring and

it was deem adequate enough to achieve this understanding.

Discussion, Mesh Validation Mark Donnelly-Orr

176

6.6 Mesh Validation

The mesh validation methods implemented in Section 4.5.1 do not support the hypothesis of the

project directly, but they are a critical part of the project as they provide confidence in the ability

of mesh to capture the details of the fluid flow. If the mesh validations are comparable with that

found in literature, then it can be assumed that the meshing developed is accurately capturing

the flow details, and hence solid conclusions can be made from the results obtained from the

simulations. This is a critical procedure to the overall success of the current study as the entire

project was conducted through numerical simulations without any experimental comparison,

apart from examples found in literature.

A limitation that may have an impact on the quality of the meshing results is the velocity profile

used in the current study compared to that in the literature which is being used for the mesh

validation methods. The velocity profile used in the current study is a power law velocity profile

and, as explained in Section 3.1.5, this was deemed appropriate for the marine environment

being replicated. The velocity profile used in the numerical model from [42]is a Van Driest velocity

profile which is very similar in shape to the power law, but differences between the two could

lead to different pressure distribution formations seen in the mesh validation methods.

Another limitation in the mesh validation comparisons is the fluid properties used in the

numerical simulations. The density of water used in the current study was higher, given it was

seawater, then may have been used in the numerical model in [42]which only states that water is

the fluid used. This means there is potentially a difference of 23.1kg/m3 in the fluid used in the

numerical models in the current study, which uses a fluid density of 1023.07kg/m3, and the

numerical study of [42], which assuming water as the fluid has a density of 999.97kg/m3. This

could have a considerable effect on the wall shear distributions as there is more force behind the

fluid flow in the numerical model of the current study.

The mesh validation weren’t preformed on the scour prevention devices as the devices

themselves alter the flow, compared to that of an unaltered pile, and hence the pressure and wall

shear distribution will inherently be different and won’t be comparable to sources in the

literature. Investigation of the literature found no sources of mesh validation for the developed

scour prevention models, given the unique geometry of the devices developed in the current

study this was not surprising, nor any methods of mesh validation for scouring prevention devices

developed in other literature sources such as devices developed in [72-74, 76-79, 81-85]. Given

the meshing implemented in the scour prevention models is somewhat similar to the meshing

implemented in the 3D Models, a certain amount of confidence can be placed on the results of

Discussion, Mesh Validation Mark Donnelly-Orr

177

the scour prevention models. But given the reduction in the inflation layer on some of the scour

prevention models, the conclusions made from the scour prevention models should be cautiously

considered.

The number of sample data points used in all the mesh validation techniques was 100, taken

along the various data lines. This was considered to be an adequate amount in order to obtain the

detail required to make conclusions about the mesh quality.

The data taken from the published study where digitally extracted using the PlotDigitizer

Software. While the extraction was performed as accurately as possible, there will be some slight

irregularities in the extracted data that is plotted in data comparison figures. This is a slight

limitation but one that is not deemed to be substantial.

The following sections discuss the quality and merits of the various mesh validation techniques.

6.6.1 Coefficient of Pressure Distribution around Pile Wall

The coefficient of pressure distributions around the pile wall, seen in Figure 4-50, show

reasonably good comparability and that the close wall meshing and inflation layer developed

around the pile wall is accurately capturing the fluid data. The experimental data from the

published study does differ slightly with a greater negative coefficient of pressure occurring

further around the pile then predicted in the CFD data from the current study and the published

study. But overall the shape of the coefficient of pressure distribution is acceptable.

The amount of literature that showed the coefficient of pressure distribution for models using a

RANS solution method where limited and [37] shows the coefficient of pressure distribution for

different solution methods, seen in Figure 4-41. It can be seen that while there are differences

between different numerical models and experiment data, the overall shape of the pressure

distribution is very similar in each case, as it is in Figure 4-50, giving validation to the mesh

developed. A reason for the experimental data being slightly off trend, while showing similarities

to the CFD data, is due to a higher Reynolds number being used, 1,200,000 as opposed to

1,000,000 being used in the CFD data.

A limitation of this mesh validation method is the simulation used for obtaining the data. The

coefficient of pressure distribution obtained from the current study was taken from a simulation

that had been modified, as explained in Section 4.5.1.1, so while the data is comparable, it can be

expected that there will be a different coefficient of pressure distribution in the 3D Model. It is

not expected that the distribution will be significantly different.

Discussion, Mesh Validation Mark Donnelly-Orr

178

6.6.2 Coefficient of Pressure Distribution along Upstream Pile Wall

The coefficient of pressure distributions along the upstream pile wall, seen in Figure 4-51, shows

very good comparability, again confirming that the near wall meshing is accurately capturing the

fluid data and is refined enough. The spike at the base of the pile is the result of the horseshoe

vortices forming while the gradual reduction in the pressure distribution travelling down the pile

occurs because of the pressure gradient forming, as discussed in Section 3.2.1. The initial spike in

the coefficient of pressure captured at the base of the pile is promising and shows that the mesh

is capturing the details of the fluid flow at the base of the pile which is critical as this is where

scour will occur. If accurate coefficients of pressure are being shown in this region, then it can be

expected that the wall shear on the seabed is also accurate.

A limitation of this mesh validation method is the simulation used to obtain the data struggled to

converge, in order to achieve convergence, the convergence criteria was lowered to 1x10-3. This

will reduce the accuracy of the data captured, but given the similar distribution plots, it was deem

that the lowering of the convergence criteria did not have a significant effect on the solution.

6.6.3 Coefficient of Pressure Distribution along Upstream Symmetry Line

The coefficient of pressure distributions along the upstream symmetry line, seen in Figure 4-52,

show very good comparability, confirming that the near wall mesh and inflation layer on the

seabed is capturing the fluid data. The current study CFD data starts at a higher pressure

distribution initially before gradually coming into agreement with the other data sources roughly

2.5m in front of the pile wall. The dip in pressure distribution that is captured is a result of the

horseshoe vortices forming and impacting on the seabed. The capturing of the dip is a good

indicator that the horseshoe vortices are forming around the pile, which is a good indicator that

scouring is likely to occur, as the vortices induce the shear stress on the seabed that causes

scouring to occur.

The dip predicted from the current study CFD data indicates that there is a substantial drop in the

pressure distribution roughly 2m in front of the pile wall, more so then predicted by the published

studies. Importantly the predicted dips in the pressure distributions occur that the same distance

in front of the pile, 2m. The different magnitudes predicted are likely a result of a combination of

different model parameters implemented in the various data sources

A limitation of this mesh validation method is the simulation used to obtain the data struggled to

converge, in order to achieve convergence, the convergence criteria was lowered to 1x10-3. This

will reduce the accuracy of the data captured, but given the similar distribution plots, it was deem

that the lowering of the convergence criteria did not have a significant effect on the solution.

Discussion, Mesh Validation Mark Donnelly-Orr

179

6.6.4 Wall Shear Distribution along Upstream Symmetry Line

The wall shear distribution along the upstream symmetry line, seen in Figure 4-53, show very

good comparability, again confirming that the near wall meshing and inflation layer on the seabed

are capturing the fluid data. Similar to the pressure distribution along the same data line, the

current study wall shear distribution data starts at a higher value then the published study data

before coming into agreement roughly 2.5m in front of the pile. The dip that follows is again a

result of the horseshoe vortices forming in front of the piles. The horseshoe vortices push back

from the pile wall, causing a negative wall shear in the x-direction. The extent of the vortices

reach to the distance when the wall shear crosses the zero wall shear line and become negative,

roughly 2.25m in front of the pile.

The dip from the current study follows the experimental data obtained from [42] much more

closely than the CFD data obtained from[42]. The shape is not followed very closely, but

acceptably so, while the magnitude of the wall shear distribution is matched very well.

A limitation of this mesh validation method is the simulation used to obtain the data struggled to

converge, in order to achieve convergence, the convergence criteria was lowered to 1x10-3. This

will reduce the accuracy of the data captured, but given the similar distribution plots, it was deem

that the lowering of the convergence criteria did not have a significant effect on the solution.

6.6.5 Boundary Layer Formation

Looking at Figure 4-54 and Figure 4-55, it can be seen that the CFD boundary layer formation

conforms well to the UDF boundary layer formation plot. Although the CFD plot is appears very

linear and not smooth, it can be seen that the data points of the CFD plot all fall on the UDF plot,

showing that at the relative heights, the CFD velocity is that expected from the UDF code. This

behaviour is seen in the boundary layer formation regardless of whether the flow rate is slow or

fast, as seen between Figure 4-54 and Figure 4-55.

The reason behind the linear nature of the CFD plot, while it could be deemed a slight limitation,

is due to the inflation layer and the large element size in the freestream, far from the pile wall.

The reason the data for the boundary layer plot being so far away from the pile was so that the

boundary layer could fully form without any impact from the flow disturbance caused by the pile.

Because of the inflation layer, there are initially many points for the CFD plot, but after 0.1m from

the seabed, the height of the inflation layer, the element size increases to 3.5m, so not many data

points were available for the plot. But it can be seen that those points that the fluid velocity was

as implemented by the UDF code. The linear nature of the plot makes the boundary layer

Discussion, Mesh Validation Mark Donnelly-Orr

180

formation graph look incorrect, but upon investigation, the CFD plot is actually realistically

accurate.

Figure 4-56 and Figure 4-57 show the velocity contour plot of the 3D Model fluid domain, which

give another visual demonstration of the boundary layer formation growing from the inlet before

reaching a steady state. This, along with the CFD and UDF plots, is further confirmation the

boundary layer formation is being implemented, adding to the confidence of the solutions

produced by the 3D Model.

Section 6.6.6 explains that this linear plot is actually the capturing of the viscous sublayer

6.6.6 Viscous Sublayer

As mentioned in the section above, the irregularities in the CFD boundary layer formation plot is

due to the viscous sublayer. As discussed in the Section 3.1.5, the viscous sublayer is a region

close to the seabed where the velocity profile is essentially laminar. This phenomenon is known

as the Law of the Wall. Figure 3-16 in Section 3.1.5 (with a logarithmic x-axis) shows the

phenomenon, where the blue lines shows the linear velocity profile, the brown line represents

the power law (log law) velocity profile, and the red line indicates the actual velocity profile. As

seen, within the viscous sublayer, the velocity profile is essentially linear, following the linear

profile before transitioning to the log law profile as the distance is sufficiently far enough away

from the seabed.

When Figure 4-54 and Figure 4-55 shown in Section 4.5.2 have their X-axis converted to a

logarithmic scale, as seen below in Figure 6-8 (which has an additional linear velocity profile

inserted in order to show the linear velocity gradient in the viscous sublayer), it can be seen that

the CFD velocity profile follows the trend shown in Figure 3-16 in Section 3.1.5. The blue CFD

velocity data line follows the linear velocity profile (green dashed line) while it is in the viscous

sublayer region, before jumping to the power law velocity profile (red dashed line) when it has

left the viscous sublayer, and following it relatively accurately.

By proving that the near wall meshing and inflation layer is capturing the details in the viscous

sublayer, and proving that it is actually there, the meshing can be deemed to be capturing the

fluid data to a high degree.

Discussion, Mesh Validation Mark Donnelly-Orr

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Figure 6-8 Comparison of the CFD Velocity Profile results with theoretical Linear Velocity profile

and the Velocity profile developed in the UDF code

6.6.7 Mesh Validation Summary

Six methods of mesh validation were used to show the quality of the mesh and all six methods

show that the meshing developed for the current study is producing very strong similarities with

the data found in the literature. This is an excellent result and proves that the results from the

simulations will be accurate and that solid conclusions can be made from them.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.01 0.1 1 10

Inle

t V

elo

city

(m

/s)

Height above Seabed (m)

Viscous Sublayer, 1.1m/s

CFD Velocity Profile UDF Velocity Profile Linear Velocity Profile

Discussion, Meshing Independence Mark Donnelly-Orr

182

6.7 Meshing Independence

The meshing independence tests performed do not support the hypothesis of the project directly,

but they allow the mesh to be improved and validated which in turn allow for a more accurate

solution and hence better conclusions. Although a point is reached in the meshing independence

tests where decreasing the element size does not alter the results, this is not always the final

mesh chosen. It was deemed that meshing independence occurs at around 4,000,000 elements in

the 3D Model, but due to computational and time constraints, it was deemed that an element

count of 2,500,000 was a good compromise that would allow for dependable results while being

within the necessary computational limits.

There was one source of error in the meshing independence tests, when the number of elements

was increased towards 5,900,000, an irregularity occurred in front of the pile on the seabed,

shown in Figure 6-9 below. This was unexpected as the model fully converged, and no

understanding could be determined to explain the phenomenon. The result of this irregularity can

be seen in Figure 4-31 when Point 3 sharply drops as it is encompassed by the irregularity. The

irregularity can also be seen forming in the meshing independence test containing 3,800,000

elements, seen in Figure 4-35. Irrespective of this irregularity, the other monitoring points used in

the meshing independence show good consistency and don’t show any irregularities.

Figure 6-9 Meshing Independence Test Irregularity

Recommendations Mark Donnelly-Orr

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7 Recommendations

In order to fully understand the effects scouring will have on the Dublin Array installation on the

Kish and Bray Banks, and the effects the scour prevention devices will have on mitigating the

impact of scouring, the following aspects deserve further research.

7.1 3D Model Improvement

Various CFD turbulence models should be utilised in order to assess the change the

models had on the flow around the wind turbine pylon. The current study only utilises a

RANS based turbulence model, the k-ε model, and a laminar-turbulent transition model,

the SST Transition model. Due to the computational limits, this was deemed adequate for

the purpose of this report, but a comparison of other models, such as a LES, DES, or DNS

model, could prove very informative.

The 3D Model simulations developed in the current study were not run with a mesh that

was independent. A meshing count of roughly 4,000,000 elements was deemed necessary

to achieve meshing independent, but given the computational limits of the facilities at

hand, the meshing count was 2,500,000 used. To achieve full confidence in the meshing,

all the 3D Models simulations developed should be run with a mesh of over 4,000,000

elements.

Volume meshing should be removed and a denser, more uniform, mesh applied to the

whole flow domain. Doing so should result in a smoother seabed shear stress prediction

which, as discussed in Section 6.5, is not the result due to the volume meshing

implemented.

The 3D Model simulations should not be run with symmetry applied down the middle of

the flow regime. While the results from the current study are not affected by the

application of the symmetry and, given the computational limits, the symmetry

application saved a vast amount of time, a full seabed shear stress result would be

beneficial for analysis purposes.

7.2 Scour Prevention Models Improvement

There are many areas of interest that must be addressed or further investigated before the scour

models developed in the current study are validated.

A more comprehensive meshing is essential for the models created. While the meshing

developed was adequate to perform a basic analysis of the effects of the devices, solid

conclusions could not be made.

Recommendations Mark Donnelly-Orr

184

If a more comprehensive mesh is developed, then a meshing independence test must be

performed on the model in order to determine the final meshing required.

The final mesh developed will need to be validated in order to assess the quality of the

results being obtained. This would be a difficult process as the devices designed are

unique in terms of their dimensions, proportions, and orientation. So while a mesh

validation technique could be implemented, to find literature to compare to could prove

very difficult.

The orientation of the flow onto the helical wire models is another area not addressed in

the current study. Given that helical wires are double treaded, the flow dynamics will only

vary for 180° around the pile, some numerous angles should be checked over this range.

This couldn’t be done in the current study because of the symmetry being applied to the

model, the wires do not continue wrapping down the pile after hitting the symmetry

plane, but rather are mirrored and so start wrapping up the pile after the symmetry

plane.

The devices themselves should be further developed and different aspects investigated,

such as various diameters, heights, thread angles, thread diameters, or number of

threads, in order to obtain a better understanding of how the various factors affect the

scour prevention ability of the device.

7.3 General Improvements

There are areas of the current study that don’t fully capture all the dynamics that are actually

occurring in those particular areas.

The effects of waves on the extent of scouring were not considered. Given some of the

wind turbine pylons may be exposed to low water heights at extreme low tides, the

effects of waves should not be ignored.

The range of scour depth expected around the wind turbine pylon bases should be

predicted in order to form on understanding of what is to be expected. This would

require a transient morphological seabed to be implemented in the simulations.

The self-nourishing aspect of the Kish and Bray Banks should be predicted or quantified in

order to give an understanding of the seabed dynamics of the banks and the predicted

filling of the scour holes.

A full year analysis should be conducted to understand the effect seasonal changes have

on the input parameters of the 3D Models. Any changes in temperature or salinity will

change the density and viscosity values of the water, which in turn could have an effect

on the extent of scouring.

Conclusion Mark Donnelly-Orr

185

8 Conclusion

The primary purpose of this report was to determine if scouring will occur and to what extent at

different sea current speeds. Through a combination of numerical and theoretical means this was

achieved and it was found that clear-water scour will occur from a current speed ranging from

0.225m/s, for the smallest sediment size found in Dublin Bay, to 0.275m/s, for the largest

sediment size. The scouring is predicted to initially occur against the pile approximately 60°

around from the direction of the current flow. The extent of scour experienced around the wind

turbine pylon base will increase in area and depth as the sea current speed increases. At a current

speed of 0.4m/s, for small sediment sizes, or 0.6m/s, for large sediment sizes, the threshold

velocity for live-bed sediment initialisation is reached and the entire seabed will transition from a

clear-water scour criterion to a live-bed scour criterion. At this current speed, the deepest scour

depth is expected. At any current speeds above the threshold velocity, the entire seabed will

experience live-bed scour, the scour depth will reduce slightly before reaching an equilibrium

depth at higher current speeds.

The consistency of the meshing validation techniques with the literature shows that the fluid flow

regime predicted is reliable. The theoretical results for the shear stress required for sediment

movement are in keeping with ranges found in the literature. The combination of these factors

allows the assumption to be made that the results calculated are accurate and trustworthy, and

hence that the conclusions made are reliable.

The secondary purpose of this report is to determine the scouring mitigation effects of various

scour prevention devices that could be installed on the wind turbine pylons. This was achieved,

but to unsatisfactory confidence level. The meshing used in the scour prevention device models

was not as accurate as that used in the original 3D model used for the primary objectives. While

reasonably realistic results were predicted, and some conclusions made, it was hard to make a

solid choice on which design was more effective due to a series of anomalies across the devices

investigated. The collars investigated effectively increased the size of the scour region, and the

two helical wire models, while providing slightly dissimilar results, showed an effective method of

causing turbulence in the flow regime which will reduce the momentum of the horseshoe vortices

that form and will, inherently, reduce the extent of the scour region.

References Mark Donnelly-Orr

186

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Appendices, A - Folk’s Classification System Mark Donnelly-Orr

190

10 Appendices

10.1 A - Folk’s Classification System

The Folk Classification Scheme [28] is a system of grain-size nomenclature of terrigenous

sediments and sedimentary rocks, wherein fifteen major textural groups are defined on the ratios

of gravel, sand, silt, and clay. Further subdivision of each class is based on the median diameter of

each size fraction present.

The basis of the classification is a triangular diagram on which are plotted the proportions of

gravel (material coarser than 2 mm.), sand (material between 0.0625 and 2 mm.), and mud (here

defined as all material finer than 0.0625mm, i.e. silt plus clay), as shown in Figure 10-1 below.

Letters refer to textural names shown in Figure 10-2. Fields are defined by the percentage of

gravel (shown on the left "leg" of the triangle) and the ratio of sand to mud (shown on the base).

[28]

Figure 10-1 The 15 major textural groups [28]

Appendices, A - Folk’s Classification System Mark Donnelly-Orr

191

Figure 10-2 Textural Names of Classifications seen in Figure 82 [28]

The bottom of the Triangular diagram, as seen in Figure 10-1, can be expanded into a sand-silt-

clay triangle, shown below in Figure 10-3. Letters refer to textural names shown in Table 10-1

below.

Appendices, A - Folk’s Classification System Mark Donnelly-Orr

192

Figure 10-3 Expansion of the bottom tier of Figure 82 [28]

Letters Textural Name

M1 Silty Sand

M2 Muddy Sand

M3 Clayey Sand

N1 Sandy Silt

N2 Sandy Mud

N3 Sandy Clay

O1 Silt

O2 Mud

O3 Clay

Table 10-1 Textural Names of Classification seen in Figure 84 [28]

Appendices, B - Definition of Phi Mark Donnelly-Orr

193

10.2 B - Definition of Phi

Phi is defined as:

𝑃ℎ𝑖 = − log2 𝑆

Where:

S = Particle Size

This relationship is shown in Figure 10-4 below.

Figure 10-4 Particle Size shown in Phi and mm, and related to the Wentworth and Folk's

Classification Schemes [27]

Appendices, C - y+ Definition Mark Donnelly-Orr

194

10.3 C - y+ Definition

The y+ value is a non-dimensional wall distance for a wall bounded flow and is defined by:

𝑦+ =𝑢∗𝑦

𝑣

Where:

𝑢∗ = friction velocity at the nearest wall

y = is the distance to the nearest wall

v = local kinematic viscosity of the fluid

Appendices, D - UDF Code Mark Donnelly-Orr

195

10.4 D - UDF Code

The purpose of this UDF code is to provide a velocity gradient profile at the inlet to model the

seabed boundary layer using a power law.

#include "udf.h" DEFINE_PROFILE(inlet_x_velocity, thread, position) {

real x[ND_ND]; real y; // Vertical Distance face_t f; begin_f_loop(f, thread) {

F_CENTROID(x,f,thread); y = x[1]; F_PROFILE(f, thread, position) = 1.42*pow((y/10),0.143);

} end_f_loop(f, thread)

}

Appendices, E - Seabed Shear Stress Calculations Mark Donnelly-Orr

196

10.5 E - Seabed Shear Stress Calculations

The following is a sample calculation of how one of the data lines was obtained in Figure 4-72.

The method was repeated for every different sediment size.

To plot the data lines needed, values for both the Critical Shields Stress and the Boundary

Reynold Number had to be calculated for a specific seabed shear stress. A certain seabed shear

stress was chosen and applied to the two equations, and then the two values were used to plot a

point on the Shields Diagram. If this point fell above the Shields Curve, then sediment movement

was deemed to occur. If it fell below the Shields Curve, then sediment movement wasn’t deemed

to occur.

As a reminder:

Critical Shields Stress:

𝜃𝑐 = 𝜏𝑜

(𝜌𝑠 − 𝜌𝑤)𝑔𝐷

Boundary Reynolds Number:

𝑅𝑒∗ = 𝑈∗𝐷

𝑣

Where:

τo = Bed Shear Stress (Pa)

ρs = Density of seabed sediment (kg/m3)

ρw = Density of fluid (kg/m3)

g = Gravity (m/s2)

D = sediment particle diameter (m)

U* = √𝜏𝑜

𝜌𝑤 (m/s)

v = Kinematic Viscosity (m2/s)

The following variables were used for the calculations.

Unit Parameter Value

𝝉𝒐 Seabed Shear Stress Varying from 0 to 3.5 Pa

𝝆𝒔 Density of Seabed Sediment 2082 kg/m3

𝝆𝒘 Density of Fluid 1023.07 kg/m3

𝒈 Gravity 9.81 m/s2

𝑫 Sediment Particle Diameter Varying from 0.2-0.8 mm

𝑼∗ Shear Velocity Depends on Seabed Shear Stress

𝒗 Kinematic Viscosity 0.00000144 m2/s

Table 10-2 Shear Stress Calculation Parameters

Appendices, E - Seabed Shear Stress Calculations Mark Donnelly-Orr

197

For the case of a sediment size of 0.0002m and using a theoretical Seabed Shear Stress of 0.1Pa:

𝑈∗ = √𝜏𝑜

𝜌𝑤= √

0.1

1023.07= 0.009886

𝑅𝑒∗ = 0.009886 × 0.0002

0.00000144= 1.373

𝜃𝑐 = 0.1

(2082 − 1023.07) × 9.81 × 0.0002= 0.04813

This process was repeated with a theoretical seabed shear stress increasing from 0 to 2.7Pa in

increments of 0.1Pa. The following Table 10-3 was obtained.

Bed Shear Stress U* Boundary Reynolds Number Critical Shields Stress

0 0 0 0

0.1 0.009886608 1.373140032 0.048131982

0.2 0.013981775 1.941913257 0.096263964

0.3 0.017124108 2.378348302 0.144395946

0.4 0.019773216 2.746280064 0.192527928

0.5 0.022107128 3.070434455 0.240659909

0.6 0.024217145 3.363492424 0.288791891

0.7 0.026157507 3.632987041 0.336923873

0.8 0.027963551 3.883826513 0.385055855

0.9 0.029659825 4.119420097 0.433187837

1 0.0312642 4.342250048 0.481319819

1.1 0.03279017 4.554190272 0.529451801

1.2 0.034248216 4.756696603 0.577583783

1.3 0.035646673 4.950926795 0.625715765

1.4 0.036992301 5.137819545 0.673847747

1.5 0.038290669 5.318148477 0.721979728

1.6 0.039546433 5.492560129 0.77011171

1.7 0.04076353 5.661601392 0.818243692

1.8 0.041945326 5.82573977 0.866375674

1.9 0.043094726 5.985378636 0.914507656

2 0.044214256 6.140868909 0.962639638

2.1 0.045306131 6.292518137 1.01077162

2.2 0.046372303 6.440597648 1.058903602

2.3 0.047414507 6.585348253 1.107035584

2.4 0.048434291 6.726984849 1.155167566

2.5 0.049433041 6.865700161 1.203299547

2.6 0.050412008 7.001667819 1.251431529

2.7 0.051372323 7.135044905 1.299563511

Table 10-3 Boundary Reynolds Number and Critical Shields Stress Calculations, 0.0002m

Appendices, E - Seabed Shear Stress Calculations Mark Donnelly-Orr

198

The Boundary Reynolds Number and the Critical Shield’s Stress were then plotted, seen in Figure

10-5, with the Boundary Reynolds Number being the X-axis value and the Critical Shields Stress

being the Y-axis. They were plotted against the Shields curve, seen in Figure 3-48.

Figure 10-5 Shields Diagram showing the data line for 0.0002m sediment vs. the Shields Curve

The interception point of these two lines is the point where sediment starts to occur for 0.0002m

sediments. By finding the Boundary Reynolds Number value and the Critical Shear Stress value at

the intersection point, the seabed shear stress could be found out. By rearranging the Critical

Shear Stress and the Boundary Reynold Number Equations in terms of the seabed shear stress,

the seabed shear stress required for sediment movement could be calculated for that particular

sediment size.

The equations are:

Rearranged Critical Shields Stress:

𝜏𝑜 = 𝜃𝑐(𝜌𝑠 − 𝜌𝑤)𝑔𝐷

Rearranged Boundary Reynolds Number:

0.01

0.1

1

10

0.1 1 10 100 1000

Cri

tica

l Sh

ield

s St

ress

Boundary Reynolds Number

Shields Diagram with Sediment Size 0.0002m

0.0002m Shields Curve

Appendices, E - Seabed Shear Stress Calculations Mark Donnelly-Orr

199

𝜏𝑜 = (𝑅𝑒 × 𝑣

𝐷)

2

× 𝜌𝑤

Using a Windows Excel function to find the intersection point, it was found the Critical Shields

Stress and Boundary Reynolds Number values were:

Interception Point: 0.0002m Sediment Size

Critical Shields Stress 0.07386

Boundary Reynolds Number 1.67719

Table 10-4 Interception Point for 0.0002m

Subbing these into the rearranged equations:

Critical Shields Stress

𝜏𝑜 = 1.67719(2082 − 1023.07) × 9.81 × 0.0002 = 0.15345 𝑃𝑎

Boundary Reynolds Number

𝜏𝑜 = (1.67719 × 0.00000144

0.0002)

2

× 1023.07 = 0.14918 𝑃𝑎

It can be seen that there is some slight variance between the two answers, which should be

identical. Taking the average of the two values, the seabed shear stress required for sediment

movement is found to be 0.15132 Pa for sediments of size 0.0002m.

Sediment Movement Threshold: 0.0003m

Rearranged Critical Shields Stress Equation 0.1534

Rearranged Boundary Reynolds Number Equation 0.1491

Average Value 0.1513

Table 10-5 Sediment Movement Threshold Values, 0.0002m

This process was repeated for every sediment size being investigate, i.e. 0.0002m to 0.0008m.

Figure 4-72 obtains all of its data points from the data lines calculated in this section for each

sediment size.

Appendices, F - Seabed Shear Stress Calculations, Various Sediment Sizes Mark Donnelly-Orr

200

10.6 F - Seabed Shear Stress Calculations, Various Sediment Sizes

The following tables show the Critical Seabed Shear Stresses required for sediment movement of

varying sediment sizes and various sediment densities.

10.6.1 Wet Packed Sand; Sediment Density: 2082kg/m3

Sediment Size (m) Critical Seabed Shear Stress (Pa)

0.0002 0.1513

0.0003 0.1625

0.0004 0.1736

0.0005 0.1903

0.0006 0.2138

0.0007 0.2367

0.0008 0.2677

Table 10-6 Critical Seabed Shear Stress required for Sediment Movement of various Sediment

Sizes, with a Sediment Density of 2082kg/m3

10.6.2 Sand, Water Filled; Sediment Density: 1922kg/m3

Sediment Size (m) Critical Seabed Shear Stress (Pa)

0.0002 0.1346

0.0003 0.1442

0.0004 0.1528

0.0005 0.1654

0.0006 0.1843

0.0007 0.2048

0.0008 0.2282

Table 10-7 Critical Seabed Shear Stress required for Sediment Movement of various Sediment

Sizes, with a Sediment Density of 1922kg/m3

10.6.3 Sand with Gravel, wet; Sediment Density: 2020kg/m3

Sediment Size (m) Critical Seabed Shear Stress (Pa)

0.0002 0.1449

0.0003 0.1555

0.0004 0.1657

0.0005 0.1807

0.0006 0.2030

0.0007 0.2237

0.0008 0.2522

Table 10-8 Critical Seabed Shear Stress required for Sediment Movement of various Sediment

Sizes, with a Sediment Density of 2020kg/m3

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10.7 G - Seabed Shear Stresses

10.7.1 Varying Current Speed

In Section 10.7.1, the inlet current speed is varied. The current speeds shown are 0.1m/s, 0.2m/s,

0.25m/s, 0.3m/s, 0.35m/s, 0.4m/s, 0.45m/s, 0.5m/s, 0.55m/s, 0.6m/s, 0.7m/s, 0.9m/s, 1.1m/s,

1.3m/s, and 1.42m/s. The sediment size being tested is kept constant for each case and ranges

from 0.0002m to 0.0008m (but excluding 0.0005m which is shown in Section 5.1.1.1).

For each sediment size, the relevant critical shear stress was taken from Table 4-20.With this

value, the wall shear results on the seabed can be limited to this range, so that any region above

the critical shear stress would be highlighted as red and will give an easy visual understanding of

where scouring is occurring.

This technique was applied to every sediment size being tested and are shown below, apart from

Sediment size 0.0005m, which is shown in Section 5.1.1.1.

10.7.1.1 Sediment Size: 0.0002m

Figure 10-6 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0002m

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Figure 10-7 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0002m

Figure 10-8 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0002m

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Figure 10-9 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0002m

Figure 10-10 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0002m

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Figure 10-11 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0002m

Figure 10-12 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0002m

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Figure 10-13 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0002m

Figure 10-14 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0002m

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Figure 10-15 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0002m

Figure 10-16 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0002m

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Figure 10-17 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0002m

Figure 10-18 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0002m

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Figure 10-19 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0002m

Figure 10-20 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0002m

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10.7.1.2 Sediment Size: 0.0003m

Figure 10-21 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0003m

Figure 10-22 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0003m

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Figure 10-23 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0003m

Figure 10-24 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0003m

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Figure 10-25 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0003m

Figure 10-26 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0003m

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Figure 10-27 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0003m

Figure 10-28 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0003m

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Figure 10-29 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0003m

Figure 10-30 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0003m

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Figure 10-31 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0003m

Figure 10-32 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0003m

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Figure 10-33 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0003m

Figure 10-34 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0003m

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Figure 10-35 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0003m

10.7.1.3 Sediment Size: 0.0004m

Figure 10-36 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0004m

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Figure 10-37 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0004m

Figure 10-38 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0004m

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Figure 10-39 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0004m

Figure 10-40 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0004m

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Figure 10-41 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0004m

Figure 10-42 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0004m

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Figure 10-43 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0004m

Figure 10-44 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0004m

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Figure 10-45 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0004m

Figure 10-46 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0004m

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Figure 10-47 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0004m

Figure 10-48 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0004m

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Figure 10-49 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0004m

Figure 10-50 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0004m

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10.7.1.4 Sediment Size: 0.0006m

Figure 10-51 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0006m

Figure 10-52 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0006m

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Figure 10-53 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0006m

Figure 10-54 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0006m

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Figure 10-55 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0006m

Figure 10-56 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0006m

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Figure 10-57 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0006m

Figure 10-58 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0006m

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Figure 10-59 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0006m

Figure 10-60 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0006m

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Figure 10-61 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0006m

Figure 10-62 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0006m

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Figure 10-63 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0006m

Figure 10-64 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0006m

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Figure 10-65 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0006m

10.7.1.5 Sediment Size: 0.0007m

Figure 10-66 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0007m

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Figure 10-67 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0007m

Figure 10-68 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0007m

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Figure 10-69 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0007m

Figure 10-70 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0007m

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Figure 10-71 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0007m

Figure 10-72 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0007m

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Figure 10-73 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0007m

Figure 10-74 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0007m

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Figure 10-75 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0007m

Figure 10-76 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0007m

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Figure 10-77 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0007m

Figure 10-78 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0007m

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Figure 10-79 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0007m

Figure 10-80 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0007m

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10.7.1.6 Sediment Size: 0.0008m

Figure 10-81 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0008m

Figure 10-82 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0008m

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Figure 10-83 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0008m

Figure 10-84 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0008m

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Figure 10-85 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0008m

Figure 10-86 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0008m

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Figure 10-87 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0008m

Figure 10-88 Seabed Shear Stresses, Current Speed: 0.5m/s, Sediment Size: 0.0008m

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Figure 10-89 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0008m

Figure 10-90 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0008m

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Figure 10-91 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0008m

Figure 10-92 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0008m

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Figure 10-93 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0008m

Figure 10-94 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0008m

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Figure 10-95 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0008m

10.7.2 Varying Sediment Size

In Section 10.7.2, the sediment size is varied while the inlet current speed is kept constant. The

current speeds being shown are 0.1m/s, 0.2m/s, 0.25m/s, 0.3m/s, 0.35m/s, 0.4m/s, 0.45m/s,

0.5m/s, 0.55m/s, 0.6m/s, 0.65m/s, 0.7m/s, 0.9m/s, 1.1m/s, 1.3m/s, and 1.42m/s, while the

sediment sizes being shown for each current speed are 0.0002m, 0.0003m, 0.0004m, 0.0005m,

0.0006m, 0.0007m, and 0.0008m. For each case with varying sediment size, the limit on the wall

shear range results was set to the values found in Table 4-20.

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10.7.2.1 Current Speed: 0.1m/s

Figure 10-96 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0002m

Figure 10-97 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0003m

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Figure 10-98 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0004m

Figure 10-99 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0005m

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Figure 10-100 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0006m

Figure 10-101 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0007m

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Figure 10-102 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0008m

10.7.2.2 Current Speed: 0.2m/s

Figure 10-103 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0002m

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Figure 10-104 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0003m

Figure 10-105 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0004m

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Figure 10-106 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0005m

Figure 10-107 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0006m

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Figure 10-108 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0007m

Figure 10-109 Seabed Shear Stresses, Current Speed: 0.2m/s, Sediment Size: 0.0008m

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10.7.2.3 Current Speed: 0.25m/s

Figure 10-110 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0002m

Figure 10-111 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0003m

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Figure 10-112 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0004m

Figure 10-113 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0005m

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Figure 10-114 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0006m

Figure 10-115 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0007m

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Figure 10-116 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0008m

10.7.2.4 Current Speed: 0.3m/s

Figure 10-117 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0002m

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Figure 10-118 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0003m

Figure 10-119 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0004m

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Figure 10-120 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0005m

Figure 10-121 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0006m

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Figure 10-122 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0007m

Figure 10-123 Seabed Shear Stresses, Current Speed: 0.3m/s, Sediment Size: 0.0008m

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10.7.2.5 Current Speed: 0.35m/s

Figure 10-124 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0002m

Figure 10-125 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0003m

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Figure 10-126 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0004m

Figure 10-127 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0005m

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Figure 10-128 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0006m

Figure 10-129 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0007m

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Figure 10-130 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0008m

10.7.2.6 Current Speed: 0.4m/s

Figure 10-131 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0002m

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Figure 10-132 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0003m

Figure 10-133 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0004m

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Figure 10-134 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0005m

Figure 10-135 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0006m

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Figure 10-136 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0007m

Figure 10-137 Seabed Shear Stresses, Current Speed: 0.4m/s, Sediment Size: 0.0008m

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10.7.2.7 Current Speed: 0.45m/s

Figure 10-138 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0002m

Figure 10-139 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0003m

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Figure 10-140 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0004m

Figure 10-141 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0005m

Figure 10-142 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0006m

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Figure 10-143 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0007m

Figure 10-144 Seabed Shear Stresses, Current Speed: 0.45m/s, Sediment Size: 0.0008m

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10.7.2.8 Current Speed: 0.55m/s

Figure 10-145 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0002m

Figure 10-146 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0003m

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Figure 10-147 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0004m

Figure 10-148 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0005m

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Figure 10-149 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0006m

Figure 10-150 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0007m

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Figure 10-151 Seabed Shear Stresses, Current Speed: 0.55m/s, Sediment Size: 0.0008m

10.7.2.9 Current Speed: 0.6m/s

Figure 10-152 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0002m

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Figure 10-153 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0003m

Figure 10-154 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0004m

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Figure 10-155 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0005m

Figure 10-156 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0006m

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Figure 10-157 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0007m

Figure 10-158 Seabed Shear Stresses, Current Speed: 0.6m/s, Sediment Size: 0.0008m

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10.7.2.10 Current Speed: 0.7m/s

Figure 10-159 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0002m

Figure 10-160 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0003m

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Figure 10-161 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0004m

Figure 10-162 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0005m

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Figure 10-163 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0006m

Figure 10-164 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0007m

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Figure 10-165 Seabed Shear Stresses, Current Speed: 0.7m/s, Sediment Size: 0.0008m

10.7.2.11 Current Speed: 0.9m/s

Figure 10-166 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0002m

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Figure 10-167 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0003m

Figure 10-168 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0004m

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Figure 10-169 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0005m

Figure 10-170 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0006m

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Figure 10-171 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0007m

Figure 10-172 Seabed Shear Stresses, Current Speed: 0.9m/s, Sediment Size: 0.0008m

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10.7.2.12 Current Speed: 1.1m/s

Figure 10-173 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0002m

Figure 10-174 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0003m

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Figure 10-175 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0004m

Figure 10-176 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0005m

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Figure 10-177 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0006m

Figure 10-178 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0007m

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Figure 10-179 Seabed Shear Stresses, Current Speed: 1.1m/s, Sediment Size: 0.0008m

10.7.2.13 Current Speed: 1.3m/s

Figure 10-180 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0002m

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Figure 10-181 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0003m

Figure 10-182 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0004m

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Figure 10-183 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0005m

Figure 10-184 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0006m

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Figure 10-185 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0007m

Figure 10-186 Seabed Shear Stresses, Current Speed: 1.3m/s, Sediment Size: 0.0008m

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10.7.2.14 Current Speed: 1.42m/s

Figure 10-187 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0002m

Figure 10-188 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0003m

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Figure 10-189 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0004m

Figure 10-190 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0005m

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Figure 10-191 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0006m

Figure 10-192 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0007m

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Figure 10-193 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0008m

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10.8 H - 3D Model Streamlines

The views shown in this Section are listed below.

1. Isometric

2. X+ (Viewing Downstream)

3. X- (Viewing Upstream)

4. Y+ (Viewing from Below)

5. Y- (Viewing from Above)

6. Z+ (Viewing from the Left)

7. Z- (Viewing from the Right)

Section 4.9.2 shows the location of the streamline source plane and the details of the streamlines.

10.8.1 0.2m/s Streamlines

Figure 10-194 Streamlines for 0.2m/s current, Isometric View

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Figure 10-195 Streamlines for 0.2m/s current, X+ View

Figure 10-196 Streamlines for 0.2m/s current, X- View

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Figure 10-197 Streamlines for 0.2m/s current, Y+ View

Figure 10-198 Streamlines for 0.2m/s current, Y- View

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Figure 10-199 Streamlines for 0.2m/s current, Z+ View

Figure 10-200 Streamlines for 0.2m/s current, Z- View

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10.8.2 0.7m/s Current Speed

Figure 10-201 Streamlines for 0.7m/s current, Isometric View

Figure 10-202 Streamlines for 0.7m/s current, X+ View

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Figure 10-203 Streamlines for 0.7m/s current, X- View

Figure 10-204 Streamlines for 0.7m/s current, Y+ View

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Figure 10-205 Streamlines for 0.7m/s current, Y- View

Figure 10-206 Streamlines for 0.7m/s current, Z+ View

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Figure 10-207 Streamlines for 0.7m/s current, Z- View

10.8.3 1.42m/s Current Speed

Figure 10-208 Streamlines for 1.42m/s current, Isometric View

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Figure 10-209 Streamlines for 1.42m/s current, X+ View

Figure 10-210 Streamlines for 1.42m/s current, X- View

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Figure 10-211 Streamlines for 1.42m/s current, Y+ View

Figure 10-212 Streamlines for 1.42m/s current, Y- View

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Figure 10-213 Streamlines for 1.42m/s current, Z+ View

Figure 10-214 Streamlines for 1.42m/s current, Z- View

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10.9 I - Meshing Independence Results Tables

The Tables below shows the values obtained from the monitoring points in the Basic Model,

Meshing Independence Test 2, and Meshing Independence Test 3.

Basic Model: 498,188 Elements

Monitoring Point Pressure (Pa) Wall Shear (Pa) Velocity (m/s)

1 -146.6 5.36 -

2 -538.1 2.927 -

3 346.2 5.329 -

4 -113.7 1.437 -

5 -144.4 1.344 -

6 -169.4 0.1241 -

7 160.6 0.2342 -

8 160.6 0.291 -

9 -136.4 - 0.4423

10 -688.2 - 1.49

11 412.6 - 0.4354

12 -129.3 - 1.049

13 -165.5 - 1.063

14 -325.3 - 1.135

15 -211 - 0.09329

16 -1.661 - 0.8833

Table 10-9 Monitoring Points Values, Basic Model

Meshing Independence Test 2: 3,786,888 Elements

Monitoring Point Pressure (Pa) Wall Shear (Pa) Velocity (m/s)

1 -112.3 1.343 -

2 -587.6 3.863 -

3 345 6.062 -

4 -153.7 1.53 -

5 -173.2 1.447 -

6 -264 1.398 -

7 -187.2 0.989 -

8 -3.97 0.3282 -

9 137.5 - 0.2559

10 -730.6 - 1.523

11 418.2 - 0.4103

12 -146.9 - 1.107

13 -179.1 - 1.126

14 -224.2 - 0.7839

15 -172.3 - 0.2246

16 -3.984 - 0.9527

Table 10-10 Monitoring Points Values, Meshing Independence Test 2

Meshing Independence Test 3: 5,912,981 Elements

Monitoring Point Pressure (Pa) Wall Shear (Pa) Velocity (m/s)

1 -101.4 1.367 -

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2 -588.3 3.895 -

3 347 0.5806 -

4 -151.6 1.519 -

5 -168.1 1.501 -

6 -269.2 1.346 -

7 -179.7 1.129 -

8 1.541 0.5898 -

9 -132.9 - 0.2529

10 -736.5 - 1.541

11 422.1 - 0.4081

12 -144.6 - 1.106

13 -172.9 - 1.129

14 -210.7 - 0.786

15 -170.6 - 0.2246

16 1.548 - 0.9527

Table 10-11 Monitoring Points Values, Meshing Independence Test 3

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10.10 J - Scour Prevention Models Streamlines

The views shown in this Section are listed below.

8. Isometric

9. X+ (Viewing Downstream)

10. X- (Viewing Upstream)

11. Y+ (Viewing from Below)

12. Y- (Viewing from Above)

13. Z+ (Viewing from the Left)

14. Z- (Viewing from the Right)

Section 4.9.2 shows the location of the streamline source plane and the details of the streamlines.

10.10.1 Basic Model Streamlines

Figure 10-215 Streamlines for Basic Model, 0.5m/s current, Isometric View

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Figure 10-216 Streamlines for Basic Model, 0.5m/s current, X+ View

Figure 10-217 Streamlines for Basic Model, 0.5m/s current, X- View

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Figure 10-218 Streamlines for Basic Model, 0.5m/s current, Y+ View

Figure 10-219 Streamlines for Basic Model, 0.5m/s current, Y- View

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Figure 10-220 Streamlines for Basic Model, 0.5m/s current, Z+ View

Figure 10-221 Streamlines for Basic Model, 0.5m/s current, Z- View

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10.10.2 Rectangular Collar Model Streamlines

Figure 10-222 Streamlines for Rectangular Collar Model, 0.5m/s current, Isometric View

Figure 10-223 Streamlines for Rectangular Collar Model, 0.5m/s current, X+ View

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Figure 10-224 Streamlines for Rectangular Collar Model, 0.5m/s current, X- View

Figure 10-225 Streamlines for Rectangular Collar Model, 0.5m/s current, Y+ View

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Figure 10-226 Streamlines for Rectangular Collar Model, 0.5m/s current, Y- View

Figure 10-227 Streamlines for Rectangular Collar Model, 0.5m/s current, Z+ View

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Figure 10-228 Streamlines for Rectangular Collar Model, 0.5m/s current, Z- View

10.10.3 Triangular Collar Model Streamlines

Figure 10-229 Streamlines for Triangular Collar Model, 0.5m/s current, Isometric View

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Figure 10-230 Streamlines for Triangular Collar Model, 0.5m/s current, X+ View

Figure 10-231 Streamlines for Triangular Collar Model, 0.5m/s current, X- View

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Figure 10-232 Streamlines for Triangular Collar Model, 0.5m/s current, Y+ View

Figure 10-233 Streamlines for Triangular Collar Model, 0.5m/s current, Y- View

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Figure 10-234 Streamlines for Triangular Collar Model, 0.5m/s current, Z+ View

Figure 10-235 Streamlines for Triangular Collar Model, 0.5m/s current, Z- View

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10.10.4 Rounded Collar Model Streamlines

Figure 10-236 Streamlines for Rounded Collar Model, 0.5m/s current, Isometric View

Figure 10-237 Streamlines for Rounded Collar Model, 0.5m/s current, X+ View

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Figure 10-238 Streamlines for Rounded Collar Model, 0.5m/s current, X- View

Figure 10-239 Streamlines for Rounded Collar Model, 0.5m/s current, Y+ View

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Figure 10-240 Streamlines for Rounded Collar Model, 0.5m/s current, Y- View

Figure 10-241 Streamlines for Rounded Collar Model, 0.5m/s current, Z+ View

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Figure 10-242 Streamlines for Rounded Collar Model, 0.5m/s current, Z- View

10.10.5 Helical Wire (Half Wire) Model Streamlines

Figure 10-243 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Isometric View

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Figure 10-244 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, X+ View

Figure 10-245 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, X- View

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Figure 10-246 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Y+ View

Figure 10-247 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Y- View

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Figure 10-248 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Z+ View

Figure 10-249 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Z- View

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10.10.6 Helical Wire (Full Wire) Model Streamlines

Figure 10-250 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Isometric View

Figure 10-251 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, X+ View

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Figure 10-252 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, X- View

Figure 10-253 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Y+ View

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Figure 10-254 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Y- View

Figure 10-255 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Z+ View

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Figure 10-256 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Z- View

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10.11 K - Finite Length Pile Model Results

10.11.1 Model Geometry and Meshing

Figure 10-257 Finite Length Pile Model, Geometry, Reverse Isometric

Figure 10-258 Finite Length Pile Model, Geometry, Side View

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Figure 10-259 Finite Length Pile Model, Meshing, Reverse Isometric

Figure 10-260 Finite Length Pile Model, Meshing, Side View

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10.11.2 Finite Length Pile Model Streamlines

The views shown in this Section are listed below.

15. Isometric

16. X+ (Viewing Downstream)

17. X- (Viewing Upstream)

18. Y+ (Viewing from Below)

19. Y- (Viewing from Above)

20. Z+ (Viewing from the Left)

21. Z- (Viewing from the Right)

Section 4.9.2 shows the location of the streamline source plane and the details of the streamlines.

Figure 10-261 Streamlines for Finite Length Pile Model, 0.5m/s current, Isometric View

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Figure 10-262 Streamlines for Finite Length Pile Model, 0.5m/s current, X+ View

Figure 10-263 Streamlines for Finite Length Pile Model, 0.5m/s current, X- View

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Figure 10-264 Streamlines for Finite Length Pile Model, 0.5m/s current, Y+ View

Figure 10-265 Streamlines for Finite Length Pile Model, 0.5m/s current, Y- View

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Figure 10-266 Streamlines for Finite Length Pile Model, 0.5m/s current, Z+ View

Figure 10-267 Streamlines for Finite Length Pile Model, 0.5m/s current, Z- View