Effect of seepage flow on shields number around a fixed and sagging pipeline

It is known that the seepage force has a significant impact on the mobility of bed particles. To assess the effect of seepage force on the sediment transport around a suspended pipeline, a series of numerical tests have been carried out utilising the commercial software Comsol. In the simulation, the shear-stress transport turbulence model was used to simulate the upper turbulence flow field and the porous medium model was adopted to describe the dynamic response of the underlying seabed. Both the fluid flow and the seepage flow within the seabed were modelled. The effect of the vertical seepage force on the Shields number was investigated. The simulation results indicate that seepage plays a vital role in the process of tunnel scour, but becomes quite weak with the form of a scour hole beneath the pipeline. With an increase in scour depth, vortices shed from the pipe lead to a fluctuating fluid pressure, inducing significant upward seepage, and thus influencing the stability of bed particles. If sagging of the pipeline appears, the fluctuation of fluid pressure becomes weak and the seepage effect is minor.