Numerical Simulation Of Two-Dimensional Local Scour Around Submarine Pipelines Using Two-Phase Flow Model

This study represents a numerical investigation of the scour phenomenon around submarine pipelines using SedFoam-a two-phase flow model for sediment transport implemented in the open source Computational Fluid Dynamics(CFD)toolbox OpenFOAM.The solver SedFoam is a newly developed two-phase flow model for sediment transport applications and has already provided a comprehensive two-phase flow sediment transport modeling framework to the scientific community.The two-phase k-? turbulence model was used to perform the simulations of turbulent flow.Both of the fluid and sediment phases were described through the Navier-Stokes equations.Comparison between the present numerical results and previous experimental measurements confirms that the numerical model successfully predicts the bed profile and scour depth during the scouring processes.The stream-wise lengths of the scour hole were slightly larger for the present numerical results while the maximum scour depths were similar to previous experimental measurements.After validations,the present numerical model was employed to simulate the two dimensional scouring around the submarine pipeline laid on an erodible bed.The numerical simulations were performed considering two primary cases,1.scouring under a single pipeline and 2.scouring under two identical pipelines in tandem arrangements.The effect of the embedded ratio(e/D,e=vertical distance between the bottom of the pipeline and original bed level and D=pipeline diameter)and the median grain size on scouring process below the single pipeline were numerically investigated.It is shown that the pipeline embedded ratio,e/D,has significant influence on the scouring propagation velocity.The maximum scour depth decreases with the increment of embedded ratio when the median particle diameter is 0.25mm.But when the particle size is increased,the scour depth first increases and then decreases with the maximum value found at e/D=0.25.Two pipelines of identical diameter in tandem arrangements were also considered for this study with different lateral gaps between the two pipelines.The lateral gap ratio,defined as g/D(g=lateral gap between two pipelines),ranges from 0.5 to 2 with an interval of 0.5.The effect of the embedded ratio has been investigated for tandem pipelines as well.For two pipeline cases the existence of downstream pipelines immensely affects the scouring processes below upstream pipeline and shows some major discrepancies to that of the single pipeline.When the pipelines are laid on bed(e/D=0),the maximum scour depth below the upstream pipeline is smaller comparing to that of the single pipeline while almost no scouring found below the downstream pipeline irrespective of lateral gap between the two pipelines.It is observed that a sand berm which was responsible for the lower scour depth below the downstream pipeline developed between two pipelines at the early stage of the scouring processes.The scouring processes below downstream pipeline did not expedite until the sand berm completely disappeared,and then,after the sand berm disappeared the two sets of pipeline acted as an extended pipeline.When embedded ratio was e/D=0 and 0.25,the scouring depth below both upstream and downstream pipelines found lower at g/D=0.5.But when the embedded ratio increased,the lowest scouring depth below both pipelines was achieved at g/D=2.0.No scouring was found in case of partially buried pipelines for both single and double pipeline cases irrespective of sand grain size and lateral gap ratios between the two identical pipelines.