Numerical modeling of local scour and forces for submarine pipeline under surface waves

A two-dimensional numerical model is developed to predict local scour around submarine pipelines induced by the orbital fluid motion under surface water waves. Instead of being simplified to oscillatory flow, the wave motion is modeled using a fully nonlinear wave model. The numerical model is based on the two-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations with a Shear-Stress Transport (SST) k-ω turbulence closure. Both suspended load and bed load sediment transportations are considered. The moving boundaries of free surface and the evolution of seabed due to local scour are tracked using the Arbitrary Lagrangian-Eulerian (ALE) method. The Streamline Upwind Petrov-Galerkin Finite Element Method (SUPG-FEM) is used to discretize the governing equations. The numerical model is validated against the benchmarks of linear and nonlinear wave propagations and their interactions with submerged structures as well as local scour around submarine pipeline in steady current. Comparisons between the numerical results and available theoretical, numerical and experimental data show satisfactory agreements. The proposed numerical model is then used to investigate the nonlinear wave-induced local scour around pipelines placed flat and sloping seabed. The effects of wave height and wave period on local scour and wave forces on the pipelines are examined. The numerical investigations suggest the necessity of utilizing the free surface wave model rather than the simplified oscillatory flow model for the problem of local scour around submarine pipelines in case of large amplitude nonlinear waves and pipelines over uneven seabed.