Composites fatigue analysis of the 15 MW offshore reference wind turbine blade in BECAS and OpenFAST

Wind energy is becoming one of the best renewable energy options. In recent years, offshore wind turbines have been developed, moving away from the coast, searching for a higher quality wind resource with a lower environmental impact. However, the greater the distance from the coast, the deeper the sea depth, making the cost of fixed foundations prohibitively expensive. It is in this context that floating wind turbines have been developed. The design and simulation stages are of great importance to reduce their maintenance to a minimum, given their great distance from the coast, and to guarantee maximum service life. In this thesis, composites fatigue theory is applied to calculate the service life of the blades for the 15 MW offshore reference wind turbine, developed by the International Energy Agency, operating on a semi-submersible platform, under a realistic set of load cases. Firstly, an overview of wind energy in the world is presented, with a special focus on both fixed and floating offshore energy. Trends in recent years and expectations for the years to come are analysed in the first chapter. The thesis continues by introducing in the second chapter the theoretical basis of fatigue, with special attention to its application to wind turbines. Once the theoretical foundations have been laid, the 15 MW offshore reference wind turbine, its blades and the semi-submersible platform are presented in the third chapter, continuing with the methodology applied in the work, set out in the fourth chapter. The following chapters develop this methodology. The environmental conditions and load cases are described to represent a realistic case of power production on a specific location, and the wind turbine is simulated under these conditions with the help of OpenFAST as the simulation tool. The results of the simulations are presented in chapter five. For the fatigue analysis, the simulation results are post-processed with the help of the software BECAS, to obtain the stress cycle at multiple points of various blade cross-sections and finally compute the service life of the blade. The analysis and results are presented in chapter six. The final section presents the conclusions of the thesis and the final results are discussed.