A numerical investigation of erosion around offshore pipelines

Abstract

In this paper, a numerical investigation is carried out to model sea bed erosion around offshore pipelines deployed at the shoreline. The open-source CFD model REEF3D is used for the numerical modeling. The model solves the Reynolds-averaged Navier-Stokes equations to calculate the flow hydrodynamics. Turbulence around the pipeline under the wave action is calculated using the k − ω model. The convective terms of the governing equations are discretized using the fifth-order finite difference WENO scheme. The free surface is captured with the level set method. The morphological evolution of the erosion process is calculated based on the simulated hydrodynamics. A fully coupled hydrodynamics-sediment model is used in the present study. For a more realistic capturing of the sea bed deformation, the modified critical bed shear stress on a sloping bed together with a sand slide algorithm is implemented in the model. The sea bed elevations are captured based on the Exner formula. The numerical model is validated against the experimental data for pipeline erosion under the influence of waves. A good agreement between experimental data and simulated results is observed. The numerical model is then utilised to simulate the erosion around offshore pipelines placed on different sea bed materials namely silt, coarse sand and very coarse sand. The temporal variation of the maximum erosion beneath the pipeline, erosion extent and the magnitude for different sea bed materials are presented and discussed. It is found that the silt sea bed offers more sea bed stability compared to the very coarse sand sea bed. Furthermore, the effect of the Keulegan-Carpenter (KC) number on the maximum erosion beneath the pipeline is also investigated. It is observed that the maximum erosion beneath the pipeline increases with KC number.