The effect of failure criteria on risk-based inspection planning of offshore wind support structures

Offshore Wind Turbine (OWT) support structures are subjected to harsh deterioration mechanisms due to the combined action of wind loading, sea induced load actions and corrosive environment. Fatigue failure becomes a key failure mode for offshore wind structures, as they experience considerable number of stress cycles (more than 10 million cycles per year). Fatigue failure can be assessed through fatigue assessment approaches. However, such assessments possess various uncertainties which may be quantified and updated through findings from in-service inspections. Since, offshore maintenance actions incur significant costs, an optimal maintenance strategy which balances the maintenance efforts against the risk of failure is desired. Based on pre-posterior decision theory, a risk-informed maintenance optimization can be performed to define the optimal maintenance strategy and support the decision maker(s). Within the risk maintenance optimization scheme, the probabilistic deterioration model is updated based on the inspection outcomes. Several fracture mechanics models have been used in the literature to estimate the deterioration of the structure containing flaws. Although, a through-thickness failure criterion is commonly used in the literature as the failure criteria, a Failure Assessment Diagram (FAD) approach has been receiving increasingly attention, as well. This investigation examines the effect of the selected fracture mechanics models and failure criteria on the optimal maintenance strategy. Moreover, the obtained maintenance strategies corresponding to different fracture mechanics models are compared for a tubular joint case study structure.