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Numerical Modelling of Onset Conditions of Scour below Offshore Pipeline in Steady Currents

A numerical model is developed to predict the onset condition of local scour below a partially embedded offshore pipeline subject to steady currents. The pressure difference over the pipeline induced by steady currents is calculated by solving Reynolds Averaged Navier-Stokes (RANS) equations with a k-ω turbulence model closure. The onset condition for scour is defined as the moment when the seepage flow pressure gradient at any point in the seabed blow the pipeline exceeds the floatation gradient of the seabed soil. In this numerical model, the average seepage pressure gradient along the buried pipe surface is employed to evaluate the onset condition. It is found that numerical results agree well with experimental data available in literature. The effects of flow Reynolds number, incoming flow boundary layer thickness, water depth and embedment on pressure drop coefficient are examined.