Assessment and Analysis of Accidental and Ultimate Load Cases in the Design of Floating Offshore Wind Turbines

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. Faults in wind turbines can arise in different and diverse components, such as the pitch control system, the gearbox, electrical generator, etc. Mathematical models either focus on the dynamic behaviour and energy generation prediction at healthy states or the analysis of faults and reliability aspects. However, the dynamic behaviour and energy generation capabilities are hardly separable from the faults and reliability issues. Therefore, a health-informed mathematical model that is able to predict, both the energy generation under different failure situations and the wear of a specific component due to the dynamic behaviour, is highly valuable. This work is focused on a coupled non-linear aero-hydro-servo-elastic mathematical model for two different floating offshore wind turbines, the IEA 15 MW offshore reference wind turbine and the NREL 5MW reference wind turbine, where several fault cases are intentionally implemented. This mathematical mode is evaluated over a wide range of environmental conditions based on correlated wind and wave data from a specific location. Firstly, an overview of wind energy in the world is presented, with a special focus on floating offshore energy. Trends in recent years and expectations for the years to come are analyzed in the first chapter. The thesis continues by introducing in the second chapter a literature review of the current ”state of the art”. Once the theoretical foundations have been laid, the two systems with the corresponding platforms are presented in the third chapter, continuing with the methodology applied in the work, set out in the fourth chapter. In the following chapter, the environmental conditions and failure scenarios are described to represent a realistic case of power production on a specific location, and the wind turbines are simulated under these conditions with the help of OpenFAST as simulation tool. The project aims to investigate both accidental loads, which occur as a direct result of an accident or exceptional circumstances, and ultimate loads, related to extreme environmental conditions or unforeseen events. The results of the simulations are presented, focusing, for each variable of interest, on maximum, minimum value and standard deviation. Hence, the impact of the different failures is evaluated and, eventually, final annual energy generation is estimated based on failure statistics, comparing the realistic results with the ideal estimations where failure do not occur. The final section presents the conclusions of the thesis and the final results are discussed.