Submarine pipelines have played a significant role in coastal, marine, and offshore engineering for transporting offshore oil and gas to shore. Over the last few decades, identifying the cause of pipeline failure has been a top interest for many researchers. Local scour is reported to be one of the main causes that threaten the safety of subsea pipelines. A great number of experimental and numerical studies have been performed to investigate local scour around subsea pipelines. Oscillatory and steady flows are commonly used to model flow near the seafloor induced by the motion of waves and currents, respectively. However, the studies of local scour around combined steady and oscillatory flow are limited. In this study, local scour around subsea pipelines under combined steady and oscillatory flow is investigated using the numerical method. The numerical model comprises Reynolds-averaged Navier-Stokes flow model, k-ω turbulence model, sediment transport model, and seabed evolution model. The effects of the combined flow ratio of the steady to oscillatory flow velocities on the scour are quantified and discussed.
Over the last few decades, local scour below submarine pipelines has been studied extensively, because it has strong potential for causing pipeline failure in the ocean environment. If a pipeline is exposed to the flow or shallowly buried, the difference between the upstream and downstream pressure of the pipeline causes the onset of scour (Sumer et al., 2001; Zang et al., 2009, 2021). After the onset of scour, the increased flow velocity through the small gap between the pipeline and erodible bed leads to strong amplification of shear stress on the bed, which is the acting force for sediment transport and scour. Local scour problems have drawn many researchers to perform extensive experimental and numerical studies over the years.