THREE-DIMENSIONAL FLOW AROUND A SPAN SHOULDER OF A PIPELINE/CABLE: INSIGHT FROM EXPERIMENTS COMBINED WITH NUMERICAL MODELLING

Current-induced three-dimensional (3-D) scour below offshore pipelines and cables (P/Cs) is a concern in both the offshore oil and gas industry and the offshore renewable energy industry. This 3-D scour phenomenon can lead to elongation of free spans and control the time scale of self-burial. Thus, much experimental research has focused on span elongation due to 3-D scour. However, due to limitations in measurement techniques, especially in accurately assessing the flow structure in the gap, the scour propagation mechanism is not well understood. In this study, scour propagation below a pipe in steady current was investigated experimentally, and the 3-D flow around a span shoulder was further studied using a 3-D numerical RANS model. The experimental results and numerical simulations show that the scour propagation may be attributed to two processes: erosion induced by 3-D flow on the upstream side of the span shoulder, and spanwise avalanching (or sand sliding) towards the scour hole on the downstream side of the span shoulder. The numerical results indicate that the flow is highly three-dimensional at the scour front; a spiral flow forms upstream side of the scour front with the maximum spanwise flow velocity reaching 0.32 (U) over bar (a) under the pipe.