cfd study of the flow in the vicinity of a subsea pipeline khalid m. saqr, mohamed saber, amr a....
TRANSCRIPT
CFD Study of the Flow in the Vicinity of a Subsea Pipeline
Khalid M. Saqr, Mohamed Saber, Amr A. Hassan, Mohamed A. Kotb
College of Engineering and TechnologyArab Academy for Science, Technology and Maritime
Transport1029 Abu Qir, Alexandria – EGYPT
1. Problem outlines
• Subsea pipelines are subjected to hydrodynamic stresses due to marine currents
• These stresses may rupture the pipeline and cause financial losses and environmental hazards.
• There is a demand to improve the methods used to protect subsea pipelines from hydrodynamic stresses
• This paper presents a comparison between two protection methods.
1. Problem Outlines
• Current protection methods– Trenching/Burying the pipeline into seabed.– Concrete weight coating.– Concrete mattress adding.– Rock dumping (covering).
1. Problem Outlines
• The proposed double barrier method
Pipeline
Barrier
Seabed
2. Methodology: Physical Model
• Computational Fluid Dynamics (CFD) model
Trenching method
Double barrier method
a
b
α ranges from 0.1 to 0.75
XY
L
HU
Trench in seabed
b
a
Pipeline
Barrier
Seabedb
a
2. Methodology: CFD Approach
A survey of relevant literature showed that the current approaches involve:
1. Two and three dimensional models
2. Finite volume framework
3. RANS turbulence models
2. Methodology: Governing Equations
• Continuity: (1)
• Momentum: (2)
• Reynolds stress closure: (3)
• Turbulence models:– k – ε model
Turbulence kinetic energy
(4)
0
i
i
x
U
ijji
ijijj
Sxx
PuuUU
x 2
ijijTij kSuu
3
212
2Sk
xxkU
x Tjk
T
ji
i
2. Methodology: Governing Equations
turbulence dissipation rate
(5)
– Eddy viscosity Cμ = 0.09
– Realizable k-ε model
(6)
k
CSk
Cxx
Ux T
j
T
ji
i
2
22
1
kCSC
xxU
x j
T
ji
i
2
21
5
,43.0max1
C
kUAA s
0
1C ijijijijSSU *
2kCT
WAs 6arccos
3
1cos6 3
8
S
SSSW kijkij
i
j
j
iij x
u
x
u
2
1
2. Methodology: Governing Equations
– k-ω turbulence model
– SST k-ω turbulence model
A hybrid model which applies the standard k-ε model in the near wall region and k-ω in the main stream region
kUx
kxx
kx
U ii
ijj
Tji
i**
2
ij
ijj
Tji
i Uxkxxx
U
kT
9
5
40
3
100
9* 2
1*
2. Methodology: CFD Model Reliability Check
Elementary computational model
Different grid resolutions
Compare flow field obtained by different grids
Predictions agree ?
Refine grid resolution
NO
Select the optimum grid
YES
VERIFICATION
Test turbulence model
Best agreementwith measurements ?
NO
Change model
Select best turbulence model
Optimize numerical scheme
VALIDATION
Final Computational Model
2. Methodology: Validation
•CFD Model Validation
Comparison between CFD predicted pressure coefficient using four turbulence models and experimental measurements of [9] on the pipe wall.
2
2
1U
PPC p
3. Results: Flow structureα = 0
0.0 0.3 0.6 0.9 1.2 1.5
α = 0 0o
90o
180o
270o
Flowdirection
Figure 5. Contours of normalized velocity magnitude and vectors over a bare pipe
Flow structure of the bare pipe
3. Results: Flow structureα = 0.1
α = 0.25
α = 0.5
0.0 0.3 0.6 0.9 1.2 1.5
α = 0.25
α = 0.1
α = 0.75
α = 0.5
α = 0.75
3. Results: α = 0.1
3. Results: α = 0.25
3. Results: α = 0.5
3. Results: α = 0.5
4. Conclusions
1. It can be concluded that the double barrier method is a prospective alternative to trenching at small aspect ratios.
2. With the difficulties faced during the trenching process, especially when the pipeline route passes a rocky terrain, the double barrier method appears as an efficient and reliable alternative.
3. The present work also reveals that the low-Reynolds number turbulence models (k-ω) performs better than the high-Reynolds number models in the present problem.
4. With proper construction of the non-uniform grid, a number of cells as small as 3×104 can be sufficient to produce accurate results.