An integrated nonlinear wind-waves model for offshorewind turbines

This thesis 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. First, a two-dimensional fully nonlinear wave simulator is developed. The transient nonlinear free surface problem is formulated assuming the potential theory and a higher-order boundary element method (HOBEM) is implemented to discretize Laplace's equation. For temporal evolution a second-order Taylor series expansion is used. The code, after validation with experimental data, is successfully adopted to simulate overturning plunging breakers which give rise to dangerous impact loads when they break against wind turbine substructures. The impact force is quanti ed by means of an analytical model and the total hydrodynamic action is nally obtained by adding the impulsive term to the drag and inertial ones. In the second main core of the thesis, emphasis is placed on the random nature of the waves. Indeed, a global simulation framework embedding the numerical wave simulator into a more general stochastic environment is developed. Namely, rst a linear irregular sea is generated by the spectral approach, then, only on critical space-time sub-domains, the fully nonlinear solver is invoked for a more re ned simulation. The space-time sub-domains are de ned as a wind turbine near eld (space) times a time interval when wave impacts are expected (time). Such a domain decomposition approach permits systematically accounting for dangerous effects on the structural response, which would be totally missed by adopting linear or weakly nonlinear wave theories alone, without penalizing the computational effort normally required. At the end of the work the attention is moved to the consequences that the proposed model would have in the quantification of the structural risk.