Transcript CFD Study of the Flow in the Vicinity of a Subsea
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 Technology Arab Academy for Science, Technology and Maritime Transport 1029 Abu Qir, Alexandria – EGYPT [email protected]
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
U
• Computational Fluid Dynamics (CFD) model L Trench in seabed b Y X
b a
α
ranges from 0.1 to 0.75
b a
Trenching method
Pipeline Barrier Seabed a
Double barrier method
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: • Momentum:
U i
x i
0
x j
U j U i
u j
u i
P
x i
x j
2
S ij
• Reynolds stress closure:
u j
u i
2
T S ij
1 2 3
k
ij
• Turbulence models: –
k – ε model Turbulence kinetic energy
x i
i
x j
T
k
x j k
T S
2 (1) (2) (3) (4)
2.
Methodology: Governing Equations turbulence dissipation rate
x i
i
x j
T
x j
C
1 – Eddy viscosity
T
C
k
2
k
T S
2
C
2
k
2 C μ = 0.09
– Realizable
k ε
model
x i
i
x j
T
x j
C
1
S
C
2
k
2 (5) (6)
C
1 max 0 .
43 , 5
A s
6 cos C 1 3 arccos
A
0
W
1
k A s U
8
S ij
S jk S ki U
*
S
3
S ij S ij
ij
ij
ij
1 2
u i
x j
u
x i j
2.
Methodology: Governing Equations –
k ω turbulence model U i U i
x i
x i k
x j
x j
*
T
x j
T
k
ij
x j
x i U i
*
k
k ij
x j U i
2
T
k
5 9 3 40 * 9 100 * 1 2 –
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
2. Methodology: CFD Model Reliability Check Elementary computational model
VERIFICATION
Different grid resolutions Refine grid resolution Compare flow field obtained by different grids NO Predictions agree ?
YES Select the optimum grid
VALIDATION
Test turbulence model Change model NO Best agreement with measurements ?
Select best turbulence model Optimize numerical scheme 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.
C p
P
P
1 2
U
2
3. Results: Flow structure
α = 0 0.0
0.3
0.6
0.9
1.2
1.5
180 o Flow direction 90 o 270 o α = 0 0 o Figure 5. Contours of normalized velocity magnitude and vectors over a bare pipe Flow structure of the bare pipe
3. Results: Flow structure
α = 0.1
α = 0.1
α = 0.25
α = 0.25
α = 0.5
α = 0.5
α = 0.75
0.0
0.3
0.6
0.9
1.2
1.5
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 problem. ω) performs better than the high-Reynolds number models in the present 4. With proper construction of the non-uniform grid, a number of cells as small as 3 ×10 4 can be sufficient to produce accurate results.