Onset of scour below a submarine pipeline under waves and current is studied experimentally and numerically in this paper. A series of laboratory tests were conduced in a mini O-tube (MOT) facility to investigate the onset conditions of scour below a pipeline subject to oscillatory flows superimposed on a steady current. The effect of parameter α, which is defined as α = Uc/(Uc+Uw), on the onset of scour under combined waves and current was investigated and a simple empirical formula for evaluating the criterion for onset of scour is presented. The numerical calculations were carried out to simulate the onset of scour using a calibrated numerical model. The pressure drop coefficient over a pipe under combined waves and current was studied. The effectiveness and accuracy of the empirical model was calibrated by the numerical results.

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