Site-specific ultimate limit state fragility of offshore wind turbines on monopile substructures

Assessing the risk posed by severe storms to offshore wind turbines (OWTs) is a challenging task. Stochastic environmental conditions represent the main source of variable loading; consequently, a high level of uncertainty is associated with assessing structural demands on OWT structures. Failure of any of the primary structural components implies both complete loss of the OWT and loss of earnings associated with production stoppage. In this paper, we propose the use of a probabilistic risk modelling framework to assess the structural risk posed by extreme weather conditions to OWTs. To achieve this, fragility functions are developed for OWTs on monopile foundations exposed to extreme metocean conditions using dynamic aero-elastic simulations. Structural fragility represents a key component of any probabilistic risk model and expresses the likelihood of different levels of damage experienced by an OWT over a range of wind and wave hazard intensities. We compare the effect of various modelling and analysis choices on the obtained fragility functions and investigate potential interdependencies between failure modes of OWT structural components. Results from this study highlight how different assumptions affect the estimated structural performance and the resulting structural fragility of a case-study OWT. We apply the proposed framework to two case-study sites, one in the USA East Coast and one in the North Sea, discussing possible outcomes of the proposed framework.