CFD Study of the Flow in the Vicinity of a Subsea Pipeline

1. 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 k.saqr@aast.edu

2. 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.

3. 1. Problem Outlines • Current protection methods • Trenching/Burying the pipeline into seabed. • Concrete weight coating. • Concrete mattress adding. • Rock dumping (covering).

4. 1. Problem Outlines • The proposed double barrier method

5. 2. Methodology: Physical Model • Computational Fluid Dynamics (CFD) model Trenching method αranges from 0.1 to 0.75 Double barrier method

6. 2. Methodology: CFD Approach • A survey of relevant literature showed that the current approaches involve: • Two and three dimensional models • Finite volume framework • RANS turbulence models

7. 2. Methodology: Governing Equations • Continuity: (1) • Momentum: (2) • Reynolds stress closure: (3) • Turbulence models: • k – ε model Turbulence kinetic energy (4)

8. 2. Methodology: Governing Equations turbulence dissipation rate (5) • Eddy viscosity Cμ = 0.09 • Realizable k-εmodel (6)

9. 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

10. Methodology: CFD Model Reliability Check

11. 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.

12. 3. Results: Flow structure Flow structure of the bare pipe

13. 3. Results: Flow structure

14. 3. Results: α = 0.1

15. 3. Results: α = 0.25

16. 3. Results: α = 0.5

17. 3. Results: α = 0.5

18. 4. Conclusions • It can be concluded that the double barrier method is a prospective alternative to trenching at small aspect ratios. • 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. • 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. • With proper construction of the non-uniform grid, a number of cells as small as 3×104 can be sufficient to produce accurate results.