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 Double barrier method α ranges from 0.1 to 0.75

6. 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

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 viscosityCμ = 0.09 –Realizable k-ε model (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. 2.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.9

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

18. 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 3×10 4 can be sufficient to produce accurate results.