Submarine landslides are regarded as one of the major offshore geohazards that might affect offshore structures and associated facilities. The failed soil mass or debris that originate from a submarine landslide might travel hundreds of kilometres at a high speed and could affect offshore infrastructures. In the present study, numerical simulations are performed to investigate the velocity and run-out distance of the failed soil mass. The computational fluid dynamics (CFD) approach in ANSYS CFX is used for numerical simulation of the process, where the soft clay-rich sediments/debris are modelled as a non-Newtonian fluid. Large-deformation finite element (FE) simulations are also performed using the coupled Eulerian-Lagrangian (CEL) approach in Abaqus FE software. Similar to other large deformation FE analysis, Abaqus CEL and ANSYS CFX are computationally expensive. In offshore environments, the debris flows through water. The drag force resulting from water has a significant influence on the velocity of the debris and run-out distance. Modelling the downslope movement of an idealized soil block, it is shown that the pressure drag resulting from the pressure in front of the sliding block is the main source of the drag force. Progressive formation of additional shear planes, through localized plastic shear strain, might occur in the soil block during downslope displacements. A parametric study shows that the seabed slope angle and shear strength degradation due to undrained remoulding and/or water entrainment influence the failure patterns. For the cases analyzed, “flowslide” and “spread” type failures are obtained when the shear strength degradation of soil is considered. In terms of practical implications, the run-out distance will provide the information on whether an offshore structure will be affected by a failed soil mass resulting from a landslide. If so, the velocity will help to estimate the exerted force because it depends on the velocity of the moving soil block
