A probabilistic framework for offshore wind turbine loss assessment

During their operational life, offshore wind turbines (OWTs) may be exposed to severe storms, resulting in extreme wind and wave loading acting on the OWT structure (and substructure). This paper proposes quantifying financial losses associated with an OWT exposed to extreme wind and waves using a probabilistic risk modelling framework, as a first step towards evaluating offshore wind farm (OWF) resilience. The proposed modelling framework includes a number of novel elements: 1) the development of site-specific fragility relationships (i.e., likelihood of different levels of damage experienced by an OWT over a range of hazard intensities), properly accounting for uncertainties in both structural capacity and demands; 2) the implementation of a closed-form technique, based on a combinatorial system reliability approach, to assess failure consequences (e.g., financial loss) for both structural and non-structural components; 3) a coherent treatment of epistemic uncertainties across the framework (e.g., sampling variability in fragility estimation), providing loss results accounting for uncertainty of estimation. These aspects can allow for more informative comparisons of various design solutions in terms of structural fragility and risk and/or for an improved evaluation of probabilistic losses for decision making. An illustrative application to two case-study sites is presented as a simplified walk-through of the calculation steps in the proposed framework, discussing possible outcomes. For instance, the results from the illustrative application indicate that the structural components play an important role in the overall risk profile of an OWT, but this depends on the site-specific wind and wave conditions. The calculation of losses provides a foundation from which a more detailed assessment of OWT and OWF resilience could be developed.