Numerical simulation of violent breaking wave impacts on a moored offshore wind turbine foundation over nonuniform topography

Abstract

Breaking wave impact on a moored offshore wind turbine foundation over a variable-depth seabed is considered in time domain, based on the fully nonlinear potential theory. An adiabatic model is used to simulate the variation in air cavity volume and pressure imposed on the dynamic boundary condition of the inner free surface. The whole impact process is solved using the dual coordinate system, where a local stretched coordinate system is adopted to determine both the shape and location of the initial impact zone, while the global coordinate system is applied to track further development of both the upper and lower jets. A higher-order boundary element method is introduced to establish water integral equations of the main fluid domain and the thin jet, which are combined together with the match conditions of pressure and velocity on the interface. By employing auxiliary functions, the temporal derivative of velocity potential is accurately obtained to decouple the mutual dependence of body and fluid motions. A specified global coordinate-based finite element method is used to derive the dynamic equilibrium equation of the mooring line element. Through the hinged condition at the fairlead location, the motion equations of the wind turbine and its mooring system are simultaneously solved using the Newton-Raphson iterative method. Extensive simulations are performed and discussed for the free surface profile, pressure distribution, body motion, and air cavity features. The wind turbine moored at the seabed with larger slope was found to experience relatively higher air cavity pressure and achieve larger rotational speed, smaller horizontal, and vertical speed.