Probabilistic Assessment of the Effect of Bolt Pre-Load Loss Over Time in Offshore Wind Turbine Bolted Ring-Flanges Using a Gaussian Process Surrogate Model

Achieving and maintaining a suitable level of bolt pre-load is critical to ensure structural reliability under the Fatigue Limit State for bolted ring-flanges in offshore wind turbine structures. Bolt pre-load is likely to vary over lifetime, with re-tensioning applied if relaxation exceeds design guideline allowance. An approach to assess the influence of varying bolt pre-load may be useful in the operational context. Recent work has demonstrated the suitability of a Gaussian Process surrogate model to emulate Finite Element Method structural simulations models of bolted ring-flanges, with computational efficiency gains. In this paper we predict cumulative fatigue damage in bolts over time, given uncertainty in bolt pre-load estimation, using a Gaussian Process surrogate model. We perform Structural Reliability Analysis to deliver approximations of annual Probability of Failure and the Reliability Index, under the Fatigue Limit State. Our approximations are compared to targets defined in relevant design standards. Furthermore, we incorporate observations, and maintenance actions, in updating the Structural Reliability Analysis during operation, and suggest practical applications of this method to inform inspection and maintenance practices.