Numerical modelling of pipeline scour under the combined action of waves and current with free-surface capturing

Understanding the pipeline scour processes under the combined action of waves and current is essential for adequate scour protection measures. This paper presents the numerical modelling of pipeline scour under the codirectional combined action of waves and current including the prediction of the unsteady free surface. The numerical modelling is performed with the open-source CFD model REEF3D. The model solves the Reynolds-Averaged Navier-Stokes (BANS) equations together with the k - omega turbulence model and a sediment transport algorithm. In order to ensure the accuracy of the flow field, the model is thoroughly validated for the hydrodynamics of co-directional combined waves and current. Results for the velocity profiles show a good agreement with the experimental observations. The model is then applied to simulate the different case scenarios of the scour under waves alone, current alone, and combined waves and current. The numerical results for scour below the pipeline show good agreement with the experimental data which confirms the applicability of the model to study pipeline scour under the combined action of waves and current. A series of simulations are run for different values of the non-dimensional parameter for the combined waves and current U-cm varying between 0 and 1.0 for the given KC number. The results demonstrate the correlation between values of U-cm and (1) the flow field below the pipeline, (2) the scour depth, and (3) the temporal variation of the scouring process. The findings highlight the variation of the maximum scour depth below the pipeline with U-cm.