Generic framework for the reliability assessment of ageing offshore wind turbine jacket support structures.

Wind Europe annual report for 2019 shows that a significant amount of installed offshore wind turbines will reach their design service life in the next decade. Most of these structures will remain in service if granted permission for life extension. Life extension remains a complex decision given the limited experience in the offshore wind industry, and so structural integrity assessment of ageing structures is seen as a potential upcoming challenge. While the existing guidelines provide a general process for assessment, it is crucial to have robust offshore wind-specific guidelines by including relevant concepts and notation of the wider structural integrity assessment and models that can adequately account for the time-dependent degradation mechanism more accurately. Offshore wind turbine (OWT) support structures are exposed to harsh marine environments with considerable uncertainties in the environmental loads and soil properties, making structural integrity assessment difficult. Hence, reliability assessment is seen as the most suitable approach to methodically account for these uncertainties. In this thesis, a generic framework for assessing the reliability of ageing OWT jacket support structures is developed based on a non-intrusive formulation. A parametric finite element analysis (FEA) model of a typical OWT jacket support structure was developed incorporating operational and environmental load and soil-structure interactions in order to map its response under varying input conditions appropriately. The results from several FEA simulations have been analysed through multivariate regression, deriving performance functions and formulation of relevant limit states. for this assessment, five limit states were considered: deflection, buckling, vibration, ultimate and fatigue limit states. The reliability index under each limit state is then calculated using the first-order reliability method (FORM). The developed reliability assessment framework has been applied to the NREL 5MW OWT OC4 jacket design to determine the reliability of critical components of the structure. The results of this reliability assessment show that, for the given stochastic conditions, the structural components of the OWT jacket support structure are found to be within acceptable reliability levels as defined in DNV-OS-J101 Offshore wind turbine design standards. A robust inspection, maintenance and repair (IMR) plan, adequately executed, proved to boost the remaining life of the structure, thus, making life extension of ageing offshore structures more economical for owners of wind turbines and operators. The validity and applicability of the framework to the OWT industry were reviewed, and recommendations were made on the next steps required for the deployment of this framework in the offshore wind energy sector.