Scour holes/Scour protection: Effect on wave loads

EWEC 2007 MILANO

Erik Asp Hansen

Erik Damgaard Christensen

Outline of the presentation

Added mass for a vibrating cylinder(Related to dynamical analyses of the entire wind turbine)• Description of inertia and added mass• Three deflections of the cylinder• Effect on damping

Scour hole: effect on loadsscour protection: effect on loads(Better estimation of wave loads)• Method based on CFD

Two different tools used: WAMIT

WAMIT (“Wave analyses MIT”, it was developed at MIT)• A first order panel method• It solves diffraction and radiation problems for offshore structures• It can be used to analyse floating body motions• In this study the added mass is determined by WAMIT

Two different tools used: NS3

Navier-Stokes solver in 2D and 3D Finite volume approach Multi-block domain Structured grid in each block Free surface, Volume of Fluid (VOF) Forces on hydraulic structures

Two different tools used: NS3

Animation:

A comparison: A non-cylindrical structure

Regular waves:Wave period = 9.4 sWater depth = 25 mLarge diameter = 15 mSmall diameter = 5 m

Panels for WAMIT calculations

Inertia and added mass

Definition of inertia coefficeintCan be said to be a combination of Froude-Krylow force and added

mass.The Inertia coefficient for a large vertical cylinder:

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.5

1

1.5

2

2.5

3

D/L

CM

MacCamy Fuchs

dt

UdACf x

Mx

dt

Ud x

MC

Phase difference

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

-1.5

-1

-0.5

0

0.5

1

1.5

D/L

rad

MacCamy Fuchs

Why do we get a phase difference?

structure

dundisturbex dApinstead

dt

UdVol

The phase difference is partly due to:

effect of diffraction

phase difference and size of added mass

Added mass

td

Xd2

2

dt

dXACf aadd

1 Ma CC

2D cylinder in aninfinite large volume of waterCa is equal to oneand:

Three types of deflections

tT

sinXt,zX odef

2

tT

sinXh

zzt,zX o

beddef

2

tT

sinXh

zzt,zX o

beddef

22

2

Inertia and added mass

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

1

2

3

D/L

CM

D/h=0.4 Inertia Coefficient VS. AddedMass Coefficient

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.5

1

1.5

2

Added m

ass c

a

Inertia Coefficent for uniform oscillation

AddedMass Coefficient

uniform

linear

parabolic

The horizontal force

damping_wave

dt

XdDC

dt

UdDC

dt

XdU

dt

XdUDC½f

ax

M

xxD

2

222

damping_wave

dt

XdD

dt

Xd

dt

UdDC

dt

XdU

dt

XdUDC½f

xM

xxD

2

22

2

22

1 Ma CCOften: is used though not in general correct =>

CFD calculations

Effect of scour protection on wave load: TEST A

Calculated horizontal forces and overturning moment for water depth h=10m, wave period T=8s wave height H=5.0m,

Effect of scour protection on wave load: TEST B

Calculated horizontal forces and overturning moment for water depth h=10m wave period T=8s wave height H=6.0m

Effect of scour protection on wave load: TEST C

Calculated horizontal forces and overturning moment for water depth h=10m wave period T=8s wave height H=5.0m current velocity V=1m/s

Conclusions

• Based on the calculation presented, it can be concluded that for offshore wind turbines the added mass is not related to by the equation.

• This will influence the dynamics of the structure and might give wrong estimates of the frequencies

• The wave damping could have a major influence as well• The horizontal forces are affected by the presence of a scour

hole or a scour protection. • scour protection will reduce the horizontal forces • scour protection will reduce the horizontal forces • but reduce the overturning moment compared to the plane bed

situation• A scour hole will increase the horizontal forces • but reduce the overturning moment compared to the plane bed

situation