Numerical modeling of local scour around a subsea pipeline on cohesive seabed under steady currents

The present study introduces a numerical model for simulating local scour around a subsea pipeline on a cohesive seabed, representing a pioneering endeavor in the development of such a numerical model. The flow dynamics around the pipeline are solved using Reynolds-Averaged Navier-Stokes (RANS) equations with the SST k-omega turbulent model. Sediment movement on cohesive seabed is assessed through erosion rates, determined by an empirical formula derived from experimental data, rather than sediment transport rates typically used for non-cohesive sediment. Three different erosion rate formulas-linear, power law, and stochastic-are investigated in the simulation of the scour process. The numerical simulations are validated using laboratory data with artificial and marine sediments, demonstrating a satisfactory level of agreement in predicting scour bed profiles. Nevertheless, it is recommended to utilize the erosion rate formula that is calibrated to the specific sediment type based on experimental data for precise numerical prediction of scour process. The study reveals that the equilibrium scour depth is approximately a linear function of the bed shear stress, and the nondimensional time scale is inversely proportional to the bed shear stress within the specified range. Subsequently, an examination of the scale effect on the scour process with cohesive sediment around a pipeline is conducted. The results indicate that the nondimensional equilibrium scour depth marginally increases with larger pipeline diameter on cohesive seabed, contrary to the observed trend for non-cohesive sediments.