Revisiting the Mechanics of Onset of Scour Below Subsea Pipelines in Steady Currents

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This paper summarizes the results of a series of experiments performed in a recirculating flume (namely the mini O-tube) to investigate the onset of scour below pipelines in steady currents. The experiments were performed on model seabed with two different lengths, to assess the effects of sediment supply on the potential for onset of scour. In each experiment the flow velocity and pipeline embedment were recorded continuously. These measurements have been compared with an empirical formula proposed by Sumer et al. (2001) which predicts the onset velocity of a pipeline partially buried in a flat seabed. In general, present experimental results show that when the upstream sediment supply is limited, onset of scour may happen due to piping at a velocity well below that predicted by the empirical formula, but following significant local scouring around the pipe. This result, although only preliminary, suggests the potential for tunnel erosion beneath deeply embedded pipelines in the field where the sediment supply upstream of the pipe may be interrupted by rock outcrops or seabed slopes. A series of numerical simulations have also been conducted to investigate the dynamic pressure difference between upstream and downstream of the pipeline prior to onset of scour. The numerical results show that despite local scour significantly alters the surrounding seabed for a deeply embedded pipe, the pressure gradient across the pipe is similar in magnitude to that measured for a flat seabed and is of sufficient size to cause piping.