Comparative structural reliability analysis of monopile-supported offshore wind turbines: considering the effect of flexible foundation and simulation time on long-term extreme response extrapolation

This paper deals with the development of semi-probabilistic design formats for ultimate limit state design of monopile-supported Offshore Wind Turbines. The effect of the flexible foundation and simulation time on long-term extreme response extrapolation and their model uncertainties from structural reliability analysis perspective are addressed. The benchmarked NREL 5-MW monopile OWT, designed according to IEC 61400-3-1, is considered as a case study. Time-invariant structural reliability analysis is conducted for this case to illustrate the variation in implied safety level for various characteristic wave-to-wind moment ratio while considering the uncertainties in simulation time and foundation’s flexibility on the extreme response extrapolation. The results indicate that the definition of bias in extreme response extrapolation due to simulation length can have an important effect on the implied safety level. The effect of foundation’s flexibility model on the implied safety level is found to be marginal for this specific case, however, a systematic investigation is still required to document this effect. In general, structural reliability analyses indicate that the implied safety levels in current design standards of IEC-1400-3-1 should properly be harmonized to account for such uncertainties.