Erosion mechanism for submarine pipeline on graded sediments using a coupled PORO-CFD-DEM model

To investigate the meso-mechanism of scouring process, a numerical model that couples the porous medium model, computational fluid dynamics and discrete elements method (PORO-CFD-DEM model) is established and validated. Two pipeline conditions, suspended and partially buried, are considered. The porous medium is used to replace most of the discrete elements, significantly reducing the computational requirements. Two particle generation methods, namely fictious grid method (FGM) and breath-first search method (BFM), are proposed, and three kinds of particle samples are generated with different gradations: Gaussian, gap and continuous. Detailed analyses of flow, fluid-particle interaction and sediment motion is conducted with an increasing inlet velocity. When the pipeline is suspended, the seepage flow is induced by the gap flow beneath the pipeline, with the pressure dropping along the seepage path. Around partially buried pipeline, the seepage flow is much weaker and the pressure concentration appears. The seepage force fluctuates, and the loose bed is disturbed, leading to randomly collapses. Different particle movement patterns, including the jumping, suspended transportation and the formation of sand slope, are captured during the simulations. It’s confirmed that the PORO-CFD-DEM model can capture the meso-movement of the particles and provides insights into the theory of sediment transportation.