Generic framework for reliability assessment of offshore wind turbine jacket support structures under stochastic and time dependent variables

Offshore wind turbines (OWTs) are exposed to harsh marine environments with considerable uncertainties in the environmental loads and soil properties, constituting their integrity assessment a challenging task and qualifying reliability assessment as the most suitable approach in order to systematically account for these uncertainties. In this work, a generic framework for the reliability assessment of OWT jacket support structures is developed based on a non-intrusive formulation. More specifically, a parametric FEA (finite element analysis) model of a typical OWT jacket support structures is developed incorporating load and soil-structure interaction, in order to map its response under varying input conditions. The results from a number of deterministic FEA simulations are post-processed through multivariate regression, deriving performance functions for relevant limit states. For this analysis five limit states are considered, i.e. buckling, deflection, fatigue, frequency and ultimate limit states. The reliability index under each limit state is then calculated using FORM (first order reliability method) to allow calculation of low probability values. The proposed framework has been applied to the NREL 5 MW OWT OC4 jacket to assess the reliability of critical components of the structure. The results of the reliability assessment indicate that, for the considered stochastic conditions, the structural components of the jacket structure are found to be within acceptable reliability levels. The proposed framework, which can be applied in various complex engineering systems, has demonstrated to be capable of effectively assessing the reliability of OWT jacket structures and can be further applied to optimize jacket structures on the basis of reliability.