Influence of Suction Dredging on the Failure Mechanism of Sandy Submarine Slopes: Revisited With a Coupled Numerical Approach

Abstract

The installation of shallow foundation systems for offshore wind turbines like gravity foundations requires the excavation of the weak top soil of the seabed to place the structure on more stable ground. This excavation can be done through suction dredging resulting in a pit. Different slope angles of this pit can be realized using this technique. As the failure mechanisms of artificial submarine slopes using suction dredging are barely investigated, relatively small final slope angles of max. 10 degree are reached to guarantee stability. Nevertheless, small-scale experiments show that submarine slopes with overcritical slope inclinations can be stable for a while when prepared with suction dredging. Steeper inclinations would significantly reduce the disturbance of the marine fauna and the amount of sand to be removed and therefore meet both economic and ecological interests. The investigations of the failure mechanism in the submarine slope during suction dredging are carried out with a coupled Euler-Lagrange approach, namely the combination of the Computational Fluid Dynamics (CFD) and the Discrete Element Method (DEM). This method enables the computation of particle-particle as well as the fluid-particle interaction forces and hence their influence on the investigated submarine slope behavior. The calculations are carried out with the open source software package CFDEM® coupling, which combines the discrete element code LIGGGHTS® with CFD solvers based on OpenFOAM®. Additionally, small scale model tests of suction dredging of sandy submarine slopes are carried out. The displacement of the soil grains is monitored with a high-speed camera. To take into account effects of contractancy and dilatancy, a loosely and a densely packed sand are investigated and the influence of the packing density on the failure mechanism is evaluated. The experimentally gained results will be compared to the numerical ones to evaluate the capability of the coupled CFD-DEM method to depict the failure behavior of submarine slopes during suction dredging.