A vertical two-dimensional numerical model is established for simulating wave-induced scour below pipelines. The fluid motion due to waves is approximated by a sinusoidally oscillatory flow. The twodimensional Reynolds-averaged Navier-Stokes equations are solved with a k-ω turbulence model closure. The suspended sediment transport is described by a convection-diffusion equation of sediment concentration with prescribed boundary conditions. The bed load is calculated using an empirical formula. Time-dependent bed profiles are updated based on the period-averaged sediment mass conservation equation. The numerical model is validated against the experimental data available in literature.
Scour below a pipeline has been investigated both experimentally and numerically in the past decades. Most numerical studies reported so far have been on local scour in steady currents. Studies on local scour below offshore pipeline in waves are rare. Local scour below pipelines subjected to steady currents has been understood relatively well. The experimental observations showed that local scour development below pipeline in steady currents can be classified into two stages (Mao, 1986; Sumer and Fredsøe, 2002). In the early stage of the scour development there is no vortex shedding behind the pipeline. The jet flow through the gap between the pipeline and the bed produces high bed shear stress. The sand directly below the pipeline is scoured away and deposited just behind the pipe, forming a sand dune behind the pipeline. This process of scour is called tunnel scouring (Sumer and Fredsøe 2002). As the scour depth under the pipe increases to a certain value, the sand dune behind the pipeline is pushed downstream gradually and its height decreases during the process of migration downstream. When the gap between the pipe and bed becomes large enough and the sand dune is far away from the pipe, the vortex shedding occurs.