Effect of seepage flow on sediment incipient motion around a free spanning pipeline

Sediment incipient motion is the first step of the whole process of sediment transport. However, previous numerical works simplified the seabed surface as a type of impermeable and rigid boundary, and ignored the effect of seepage flow on the mobility of bed particles. In this paper, to reveal the physics behind sediment incipient motion around a free spanning pipeline, an integrated numerical model, coupling the SST (Shear-Stress Transport) turbulence model with the porous seabed model, was proposed. Numerical studies showed that with the periodic formation and shedding of vortices around the pipeline, both the oscillatory and residual excess pore-pressures developed within the seabed. In some cases, the vertical gradient of excess pore-pressure (seepage force) had a significant impact on the mobility of bed particles around the pipeline. It was found that lower saturation degree and seabed permeability would remarkably increase both the oscillatory and residual seepage forces, and thus enhance bed particle mobility. While for a sandy seabed with smaller soil shear modulus, the oscillatory seepage force was only slightly reduced, but a high residual seepage force would be generated with time. This could lead to obvious decrease in the submerged weight of bed particles, making them more easily dragged away from the seabed.