A fully nonlinear wave model to account for breaking wave impact loads on offshore wind turbines

This paper presents a numerical model capable of simulating offshore wind turbines exposed to extreme loading conditions. External condition-based extreme responses are reproduced by coupling a fully nonlinear wave kinematic solver with a hydro-aero-elastic simulator. The transient nonlinear free surface problem of water waves is formulated assuming the potential theory and a higher-order boundary element method (HOBEM) is used to discretize Laplace’s equation. For temporal evolution a second-order Taylor series expansion is implemented. The code is successfully adopted to simulate overturning plunging breakers, which give rise to dangerous impact loads when they break against wind turbine substructures. Emphasis is also placed on the development of a global simulation framework that aims at embedding the wave simulator into a more general stochastic environment. Indeed, first a linear irregular sea is generated by a spectral approach, then only on critical sub-domains, where wave impacts are expected, the fully nonlinear solver is invoked for a more refined simulation. This permits to systematically account for dangerous effects on the structural response (which would be missed by adopting linear or weakly nonlinear wave theories alone) without penalizing the computational effort.