numerical modeling of seabed gouging by ice masses - and
TRANSCRIPT
MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Numerical Modeling of Seabed Gouging by IceMasses
and soil-pipe interaction
Hossein Fadaifard, MSc John Tassoulas, Ph.D.
Department of Civil, Architectural, and Environmental EngineeringThe University of Texas at Austin
February 14, 2013
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Outline
MotivationSeabed scourSeabed scour Pipe Interaction
Numerical ModelingFluid-Structure Interaction (FSI)Rigid Cylinder Penetration
Numerical Examples and RemarksRidge Scour
AppendixRidge
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Arctic Ocean
• Home to large untappedreserves
• 13% oil reserves [1]• 30% gas reserves [1]
• Marine pipelines fortransportation of fluids
• Install on seabed• Trench and/or embed
into seabed• Less susceptible to
man-made hazards• Susceptible to seabed
gouging by ice masses
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Seabed scour
• Ice features drifting in Arcticenvironment.
• Come in contact withseabed in shallower waters.
• Scour the seabed for severalkilometers.
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Seabed scour
• Scour seabed and remoldthe seabed surface.
• Limited informationavailable about actualprocess.
• Move at speeds of 0.1 m/s.
• Scour deformation occurs inundrained condition.
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Seabed scour
• Gouge depths typically rarelyexceeding 1m in depth.
• Canadian Beaufort Sea(1970s): 2.5m [4]
• Canadian Beaufort Sea(1995): 0.3m [2]
• Grand Banks (2004):0.3m [3]
• Inherently a 3-dimensionalproblem.
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Seabed Scour and Pipe Interaction
• Trench and embed pipelinesto prevent contact with iceridges.
• Fill trench back with infill.
• Deeper trenches moreexpensive.
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Seabed Scour and Pipe Interaction
• Indirect transfer of forces topipeline
• Concern about the safety ofpipes.
• Study behavior of pipesunder extreme loading dueto ridges.
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Classical Approach: Soil-Structure Interaction (SSI)
Seabed scour modeling:
• Soil modeled as a porous medium.• Accurate model for soil.• Includes load-history dependency behavior of soil.
• Large deformations require re-meshing.• Computationally expensive.• Solution projection between meshes deteriorates nonlinear
convergence.• Difficult to parallelize.
• Requires solving a nonlinear contact problem.
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Current Approach: Fluid-Structure-Object Interaction
• Model soil as a highly viscous non-Newtonian fluid with a “yield” stress.• Herschel-Bulkley model used to approximate soil behavior.
σf = 2µf (γ) ε− pI, (1)
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Pipe Penetration
Rigid cylinder, w/ streamlines Rigid cylinder, w.o streamlines
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Ridge Scour
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Ridge Scour
Case I
X
Y
20 40 60 80 100
10
20
30 t = 134.85
X
Y
20 40 60 80 100
10
20
30 t = 1
X
Y
20 40 60 80 100
10
20
30 t = 100
X
Y
20 40 60 80 100
10
20
30 t = 50.55
Case II
X
Y
20 40 60 80 1000
10
20
30 t = 1
X
Y
20 40 60 80 1000
10
20
30 t = 100.05
X
Y
20 40 60 80 1000
10
20
30 t = 134.85
X
Y
20 40 60 80 1000
10
20
30 t = 50.5
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Seabed scour
fixed pipe
-10000
-5000
0
5000
10000
15000
20000
25000
-50 -40 -30 -20 -10 0 10
For
ce a
ctin
g on
pip
e (N
/m)
Distance from center of ridge to center of pipe (m)
Forces acting on pipe vs. relative distance of ridge center to pipe
F1F2
||F||
• Extreme cases:• Pipe artificially fixed in
place.• Pipe artificially allowed to
freely “float”.
• Pipe allowed to displace,attached to spring.
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Seabed scour – “floating” pipe
floating pipe
-0.5
0
0.5
1
1.5
2
2.5
-50 -48 -46 -44 -42 -40
For
ce a
ctin
g on
pip
e (N
/m)
Distance from center of ridge to center of pipe (m)
Forces acting on pipe vs. relative distance of ridge center to pipe
F1F2
||F||
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
-50 -48 -46 -44 -42 -40
Def
lect
ion
of p
ipe
(m)
Distance from center of ridge to center of pipe (m)
Pipe deflection vs. relative distance of ridge center to pipe
∆ x1∆ x2
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Seabed scour – Pipe with artificial spring
Pipe with spring
-10000
-5000
0
5000
10000
15000
20000
-50 -40 -30 -20 -10 0 10 20
For
ce a
ctin
g on
pip
e (N
/m)
Distance from center of ridge to center of pipe (m)
Forces acting on pipe vs. relative distance of ridge center to pipe
F1F2
||F||
-0.01
-0.005
0
0.005
0.01
0.015
0.02
-50 -40 -30 -20 -10 0 10 20
Def
lect
ion
of p
ipe
(m)
Distance from center of ridge to center of pipe (m)
Pipe deflection vs. relative distance of ridge center to pipe
∆ x1∆ x2
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
3-dimensional Scour (without pipe)
3d Scour
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Concluding Comments
• Approximating soil behavior using Herschel-Bulkley modelpromising.
• Problem is very computationally demanding.• ∼ 36 hr for a typical 2D run on a single core.• Projected run time of 5-10 days for 3D analysis on TACC (16
cores).
• Currently working on parametric studies.
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Sources I
D. L. Gautier, K. J. Bird, R. R. Charpentier, A. Grantz, D. W. Houseknecht,T. R. Klett, J. K. Moore, T. E.and Pitman, C. J. Schenk, J. H. Schuenemeyer,K. Sorensen, M. E. Tennyson, Z. C. Valin, and C. J. Wandrey.Assessment of undiscovered oil and gas in the arctic.Science, 324:1175 – 1179, 2009.
Arnaud Hequette, Marc Desrosiers, and Peter W. Barnes.Sea ice scouring on the inner shelf of the southeastern canadian beaufort sea.Marine Geology, 128(3-4):201 – 219, 1995.
Tony King, Ryan Phillips, John Barrett, and Gary Sonnichsen.Probabilistic pipeline burial analysis for protection against ice scour.Cold Regions Science and Technology, 59(1):58 – 64, 2009.
P. Wadhams.Ice in the Ocean.CRC PressINC, 2000.
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MotivationNumerical Modeling
Numerical Examples and RemarksAppendix
The University of Texas at Austin
Application: Ridge scour
Table 1: Properties used for preliminary runs of ridge scour
Scour depth 1 m.Ridge base width 10 m.Ridge speed 0.2 m/s.Attack angle 30.5 deg.pipe diameter 24 in.Yield stress 1765 Pa.Yield strain-rate 0.024 1/ssoil mass density: 1400 kg/m3water mass density: 1000 kg/m3water dyn. viscosity: 1e-3 kg/m.s
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