Reliability and up-scaling of offshore wind turbines

The structural response of the main components of offshore wind turbines (OWTs) is considerably sensitive to amplification as their excitation frequencies approach the natural frequency of the structure. Furthermore, uncertainties present in the loading conditions, soil, and structural properties highly influence the dynamic response of the OWT. This issue is critical in up-scaled wind turbines due to the reduction of the natural frequency. In most cases, the cost of the structure reaches around 30% of the entire OWT because conservative design approaches are employed to ensure its reliability. As a result, this study aims to address the following research question: Are reliability considerations essential in the upscaling of offshore wind turbines? The specific aims are to (1) determine the distribution and impact of the design parameters, (2) predict the structural reliability in fatigue, and (3) apply an integrated design approach in the upscaling. An analytical model to determine the fatigue life of the structure considering the soil-structure interaction under 15 different loading conditions was developed. A sensitivity scheme using two global analysis was developed to consistently establish the more and less important design parameters. Also, a systematic uncertainty quantification (UQ) scheme was employed to model the uncertainties of model input parameters based on their available information. The UQ framework used reliability analysis with an accurate surrogate model to consistently determine the probability of failure of the structure based on the fatigue limit state design criterion. Finally, to analyze the design space and provide more integral design solutions for the up-scaling of offshore wind turbines the Environmental-Based Design approach was applied. The results show high sensitivity for the wind speed and turbulence intensity, and moderate sensitivity for parameters usually considered as deterministic values in design standards. Additionally, it is shown that applying systematic UQ produces a better approximation and efficiency of the fatigue life under uncertainty and more accurate estimations of the structural reliability. Regarding the upscaling of OWT, the application of the EBD was found to enhance the development of more innovative and holistic designs for large wind turbines. Consequently, more reliable, and robust structural designs for the next generation of large offshore wind turbines may be achieved.