Structural reliability assessment of deteriorating offshore wind turbine monopiles for extreme limit states

Offshore wind turbines are exposed to various deterioration mechanisms in harsh marine environment. Steel corrosion in sea-water is one of the major and most expensive challenges for their long-time structural integrity. This degradation introduces significant uncertainty in predicting structural performance over time, making structural reliability analysis and uncertainty quantification necessary for safe and cost-effective design, maintenance and lifeextension strategies. In this study, a bi-modal corrosion model is utilized to estimate corrosion-induced material loss in offshore wind turbine monopiles and to model uncertainty for assessing their structural reliability. The model is able to capture variability and uncertainty in corrosion loss, which makes it suitable for reliability analysis under realistic conditions. Furthermore, the model is calibrated based on seawater temperature, allowing the consideration of temperature/climate effects on corrosion loss and structural reliability of deteriorating monopile foundations supporting offshore wind turbines. Structural reliability of both pristine and deteriorated components which have been designed using safety factors provided by IEC 61400-1 ed. 4 are estimated for several extreme load cases. For each load case, reliability is evaluated for two representative mean annual sea-water temperature conditions corresponding to different wind farm locations. The reduction in reliability level due to corrosion in the final year of the planned lifetime of OWTs, which can be used for life extension studies, is also quantified. Results illustrate that corrosion loss can result in up to 18 % reduction in the reliability level of the considered OWT monopile. The results contribute to a more comprehensive understanding of how environmental factors and material degradation affect structural reliability and performance, supporting site-specific risk-informed decision-making for inspection, maintenance, and life-extension strategies in offshore wind energy infrastructure.

Keywords: Target reliability, Extreme load, Reliability of deteriorated structures, Offshore wind turbine deterioration, Steel corrosion.