NUMERICAL SIMULATIONS OF THREE-DIMENSIONAL SCOUR BELOW SUBSEA PIPELINES/CABLES UNDER STEADY CURRENT CONDITIONS

Scoured induced free span is one of the major engineering hazards for submarine pipelines/cables (P/Cs) in offshore oil and gas industry and offshore renewable energy industry. Due to the lack of knowledge about scour propagation along spanwise direction, conservative and costly stabilization methods are usually adopted for span mitigation. To address the concern related to free spans, great efforts have been made to establish the prediction models for scour propagation rate, most of the research is based on laboratory experiments to propose semi-theoretical equations. However, due to the limitation of present measuring techniques, it is hard to physically measure the gap flow and bed shear stress distribution under the P/C models. Thus, the physical mechanism for scour propagation is still not well understood. As a complement to laboratory experiments, numerical simulations can reproduce the whole flow fields around a P/C undergoing three-dimensional scour, which makes it a great tool to understand the interaction between flow and evolution of erodible seabed. Thus, in the present study, a scour model is used to simulate the three-dimensional scour below a P/C under steady current conditions. Two scour propagation stages are observed, the different mechanisms for the two stages are explained using the bed shear stress amplification factor. The flow fields around the scour hole are also analyzed. The present results, combined with the available laboratory experiments [1,3], will give a better understanding about the mechanism of 3-D scour.