Advances in fatigue design of circumferential welds in offshore wind turbine monopile support structures

The increasing trend in size and thickness of offshore wind turbine monopile support structures necessitates the need for continued research to refine fatigue design methodologies by ensuring both structural integrity and cost-effectiveness. This study investigates fatigue life predictions at the circumferential welds in monopile foundations by performing a detailed analysis of the SLIC fatigue data, obtained from 50 mm thick as-welded specimens, and incorporating realistic weld geometries in the evaluations. Moreover, comparisons are drawn between the monopile-specific D curve introduced in the new edition of the DNV-RP-C203 standard, the generic D curve, and the processed SLIC data. The results show that, at a reference thickness of 25 mm, the new monopile-specific D curve mitigates the over-conservatism of the generic D curve, which is designed for a wide range of applications, resulting in 3.3 times increase in fatigue life at 107 cycles. Furthermore, the results from sensitivity analyses performed by accounting for variations in weld geometry demonstrate the critical importance of precise weld geometry characterisation in evaluating S-N fatigue data. Additionally, the findings from this research underscore the critical role of residual stresses in the fatigue design and life assessment of in-service monopiles. The advancements presented in this study contribute to better understanding of the fatigue life in large thickness monopile support structures. The findings from this study not only aid in re-evaluating the fatigue life of aging offshore wind turbine monopiles but also contribute to designing longer-lasting foundations for future installations.