Offshore Wind Turbines (OWTs) have increased in popularity because of numerous technological advancements in the sector of renewable energy. With such an increase in the number of installments, there is a need for multi-hazard vulnerability assessment of the OWT at a given site. Because of hurricane and tsunami loads pertaining to the offshore site, structural failure analysis with respect to wind and wave loads at the critical section of the OWT is required. The existing simulation methods for the failure assessment are computationally expensive and require many simulations to estimate the multi-hazard behavior. The goal of this thesis was to study multi-hazard behavior of the monopile OWTs. The multi-hazard vulnerability analysis was performed for a 5 MW OWT simulation model developed by National Renewable Energy Laboratory (NREL). Specifically, FAST v8, a simulator developed by NREL was used to perform the simulations for the coupled dynamic response of the structure. In pursuit of the goal, this thesis also aims at developing two surrogate models to estimate the response of OWTs for risk assessment at the low computational cost. Surrogate models replace the traditionally used tedious nonlinear aero-hydrodynamic simulations without loss of accuracy and less computational effort. The surrogate models created were Stepwise Multiple Linear Regression (SMLR) and second order polynomial Response Surface Metamodels (RSMs). Results from each of the models were compared with the observed FAST responses. It was concluded that the RSM model capable of representing nonlinear behavior of the response offered more accurate results with less computational effort when compared to SMLR. Then, vulnerability analysis performed for multi-hazard loadings revealed flexural failure was the most critical failure at multi-hazard loading scenarios among others, including deflection and shear. More rigorous analysis accounting for the variation in both structural and multi-hazard loading parameters was performed. The result emphasized on the importance of considering uncertainties in structural and loading parameters to improve structural reliability of monopile OWTs.
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