Numerical modelling of current induced scour under offshore pipelines

Local scour evolutions under a pipeline are simulated using a vertical two-dimensional numerical model. The pipe is originally residing on or above a flat sand bed and subject to steady currents. The numerical model computes the developments of the flow and sediment transport with the update of the bed-shape based on the mass balance of sediment. Governing equations are solved by finite difference method in a non-orthogonal boundary-fitted curvilinear coordinate system. The Wilcox kω model is used as the turbulence closure. Both suspended and bed loads of sediment are considered in the model. The computational grid is adapted to the new geometry by solving a Poisson equation after each bed update. A sand slide model is employed in the simulation of the morphological change. Different time steps are used to march the flow and bed-profile in the model to reduce the computational cost. Scour predictions are compared with independent laboratory measurements. The vortex shedding phenomenon is captured at a later stage of the scour development and the predicted scour profiles agree well with those measured independently.