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NUMERICAL SIMULATION OF WAVE-INDUCED

SCOUR AND BACKFILLING BENEATH

SUBMARINE PIPELINES

David R. Fuhrman

1

, Cuneyt Baykal

1

, B. Mutlu

Sumer

1

, Niels G. Jacobsen

2

, Jørgen Fredsøe

1

1

Technical University of Denmark

2

Deltares

Outline

Model description

Simulation of wave-induced scour

Simulation of wave-induced backfilling

Conclusions

Model description

Fully-coupled sediMorph model (OpenFOAM)

Jacobsen & Fredsøe (2014)

Hydrodynamic model

Incompressible Reynolds-Averaged Navier-Stokes (RANS) equations

Two-equation

k

w turbulence closure (Wilcox 2006)

Sediment transport model

Engelund-

Fredsøe bedload transport method

Turbulent-diffusion equation for suspended sediment concentration

Engelund-

Fredsøe reference concentration

Reference level:

b

=3.5

d

(similar to Liang & Cheng 2005)

Model description (2)

Morphological model

Sediment continuity (Exner) equation

h

t

1

1

n

q

Bi

x i

D

E

,

D

w s

u

3

c b

,

i

1 , 2

E

T

c

x

3

x

3

b

Instantaneous bed updating (no smoothing)

Sandslide model of Roulund et al. (2005)

Model description (3)

Model domain

40

D

x 10

D

Frictionless rigid lid at top boundary

Wave conditions (oscillatory flow) introduced at left boundary

Outline

Model description

Simulation of wave-induced scour

Simulation of wave-induced backfilling

Conclusions

Wave-induced scour

Conditions chosen to match selected experimental conditions (Sumer

& Fredsøe 1990, Fredsøe et al. 1992)

Pipeline diameter:

D

= 3 cm

, grain diameter

d

= 0.19 mm

Warm-up period (morphology off) of

10

T

KC=U m

T w

/D

5.6

11

15

19.6

21.1

25.3

30

T (s)

1.10

1.22

2.50

3.00

2.64

3.51

3.50

U m

(m/s)

0.153

0.240

0.177

0.196

0.239

0.216

0.257

q

max

0.13

0.19

0.10

0.092

0.12

0.094

0.11

Scour

(

KC

= 5.6

)

t

*

g

s

1

d

3

D

2

t

S

D

0 .

1

KC

Scour

(

KC

= 30

)

Summary of scour profiles

Summary of scour time series

Scour

(

KC

= 11

)

Features develop matching natural vortex ripple length

(Brøker 1985):

D

1 .

2

a

,

1 .

2

2

KC a

2

U m

2 .

1

/

T w

Summary of scour results

Equilibrium scour:

Time scale:

T

*

t

'* max

0

S

max

S dt

'*

S

max

Outline

Model description

Simulation of wave-induced scour

Simulation of wave-induced backfilling

Conclusions

Backfilling (

KC

= 30 to

5.6

)

Backfilling time series

• to

KC

= 5.6:

To

KC

= 15:

Backfilling profiles

Backfilling to

KC

= 5.6:

Scour with

KC

= 5.6:

Backfilling summary

Equilibrium scour:

Time scale:

T

*

t

*

 max

0

S

S

0

S

min

S

min

dt

*

Remarks on practical application

Utilize the time scalings:

t

*

g

s

1

d

3

D

2

t

,

T

*

1

50 q

5 3

Taking the grain size as already full scale, model and full scale morphological times may be related as:

t

full

t

model



D

full

D

model



2

 q full q model



5 3

Combining simulated scour/backfilling times give:

t

* model

15

t

full

O

1 day

Outline

Model description

Simulation of wave-induced scour

Simulation of wave-induced backfilling

Conclusions

Conclusions

Fully-coupled RANS model for simulating scour processes beneath submarine pipelines

Hydrodynamics + turbulence description

Sediment transport (bed and suspended load) + Bed morphology

Accurate simulation of pipeline scour depths as well as scour time scales

Model predicts backfilling to profiles governed by new wave climate

Consistent with experimental findings

! (Fredsøe et al. 1992)

Results published in:

Fuhrman, D.R., Baykal, C., Sumer, B.M., Jacobsen, N.G. & Fredsøe, J. (2014) Numerical simulation of wave-induced scour and backfilling processes beneat submarine pipelines. Coast. Eng. 94 , 10-22.

Animation of tsunami-induced scour

ASTARTE EU FP7 project: www.astarte-project.eu

NUMERICAL SIMULATION OF WAVE-INDUCED

SCOUR AND BACKFILLING BENEATH

SUBMARINE PIPELINES

David R. Fuhrman

1

, Cuneyt Baykal

1

, B. Mutlu

Sumer

1

, Niels G. Jacobsen

2

, Jørgen Fredsøe

1

1

Technical University of Denmark

2

Deltares

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