Coupled stability of offshore wind monopile foundations and submarine slopes

Abstract

In response to the current research gap concerning the existing of monopiles in submarine slope areas, this thesis conducts a study through finite element numerical simulation. The key methodologies employed include using a modified Mohr-Coulomb model with strain accumulation that considers the effects of high cycle accumulation strain for describing the mechanical behaviour of drained seabed sand, utilising the shear strength reduction method to determine the safety factor of submarine slopes, and employing a soil-pile interaction model to describe the contact behaviour between the seabed and the monopile. The study uses the Abaqus package to solve for multi-field coupling in porous media. The research primarily considers three main topics: the coupling effects of monopiles and submarine slopes under long-period cyclic loads and, under extreme storm load conditions, and a novel failure surface prediction method for submarine slopes according to seabed topography and finite element method. The study finds that the installed monopiles can act to prevent slope sliding, resulting in a slight increase in the overall safety factor of the slope. However, under high cycle accumulation strain and the influence of storms, an area around the monopile characterised by a funnel shape may experience abnormally high pore pressures and strain softening, leading to a reduction in the overall safety factor of the slope. Although the displacement of the monopile caused by storms or accumulation strain remains within a small range, the occurrence of a landslide could suddenly increase the displacement of the monopile, causing instability or even collapse of the monopile and tower structures. Considering the complex submarine topographical features potentially encountered in practice, a new finite element-based model for predicting potential failure surfaces of submarine slopes is proposed. This model demonstrates that the installed monopiles but before loading does not significantly affect the location of potential failure surfaces of submarine slopes, but it does influence the depth and safety factor of these failure surfaces.