Structural Reliability Analysis and Robust Design of Offshore Wind Turbine Support Structures
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The purpose of this master thesis is to develop and evaluate a novel approach for performing structural reliability analysis and robust design of offshore wind turbine support structures. Offshore wind turbines are structures especially prone to uncertainties due to its exposure to environmental conditions. Therefore, the models that take into account probabilistic descriptions of variability are more natural approaches. In this sense, a structural reliability analysis and a robust design are two complementary approaches that allow for incorporating uncertainty and randomness in the design process.To achieve the objective of this study, it has been developed a simplified model of a monopile structure that allows for doing the necessary calculations efficiently. The simplified model is implemented as a time-domain simulation in MATLAB and allows to solve the transient dynamics of a beam. In parallel, the rotor loads are obtained from rotor simulations in FEDEM Windpower software. Uncertainty is studied in the following system parameters: aerodynamic damping, soil stiffness and cross section of the support structure. Simple probability distributions are used in order to describe the uncertainty on these parameters. The basis of the research is a probability analysis based on several approaches. First, it is raised an individual analysis for each system parameter and wind speed. Secondly, the way used to add the aerodynamic damping in the simplified model has given rise to two different analyses. The effect of the soil stiffness and the structure cross section is investigated through a correlated and uncorrelated sampling of both variables. Finally, a combined probability analysis is performed in order to observe the effect of analysing the three system parameters at the same time. Additionally, the reference offshore wind turbine is also analysed in order to enable evaluate the effects of the other analyses. The probabilistic analysis is performed with a variable number of samples and the displacements at the tower top and the bending moments at the mud-line of the structure are obtained as a result. These outputs are used to evaluate the equivalent fatigue damage of the support structure.The wind turbine considered is the 5-MW NREL, with the reference support structure defined in the OC3 project.The results show that the response of the proposed simplified model is significantly different depending on the system parameter. The soil stiffness and the aerodynamic damping have shown a weak effect on the response of the structure. The reason could be an underestimation of the uncertainties associated to these parameters. Conversely, the structure cross section has shown a greater impact, probably due to dynamic amplifications linked to resonance problems. The uncorrelated analysis of the soil stiffness and the structure cross section has shown the greater effects on the response of the structure, given that the sampling approach of this case leads to a greater scattering of the system parameters and, consequently, of the results.