Effect of scour erosion and riprap protection on horizontal bearing capacity and reliability of monopiles using FEM-BPNN-RSM coupled method

Abstract

Scour erosion around monopiles of offshore wind turbines (OWTs) can significantly reduce their horizontal bearing capacity and reliability. To combat scouring, riprap protection has been identified as a simple and effective method. This study aims to analyze the impact of different scour depths and riprap protection on the horizontal bearing capacity and reliability of monopile-supported OWTs, considering the interaction between the structure and the soil. In this research, we establish three-dimensional finite element models and propose a novel coupled analysis method, integrating the finite element method (FEM), back-propagation neural network (BPNN), and response surface method (RSM), to assess the bearing capacity reliability under various scour and protection conditions. This analysis incorporates extreme value data from 30 years of marine environmental conditions and employes an 8MW wind turbine as a case study. By utilizing the established response surface, we determine the failure probability and reliability index. The results demonstrate that the rotational center of the monopile shifts either downward or upward with increased scour depth or implementation of riprap protection. Furthermore, the findings reveal that riprap protection not only effectively mitigates the negative impact of scour on the bearing capacity of monopiles but also enhances their bearing capacity and reliability directly.