Probabilistic Calibration of Fatigue Design Factors for Offshore Wind Turbine Support Structures

This paper describes a reliability-based approach to determine fatigue design factors (FDF) for offshore wind turbine support structures made of steel. The FDF values are calibrated to a specific reliability level and linked to a specific inspection and maintenance (I&M) strategy used for the considered offshore wind turbines in such a way that the specific uncertainties for the fatigue life are accounted in a rational manner. Similar approaches have been used for offshore oil & gas sub-structures, but the required reliability level for offshore wind turbines is generally lower and the fatigue loading is different. Based on representative stochastic models and response functions a set of FDF values are determined which depend on the degree of accuracy of the fatigue analysis and the consequences of fatigue failure of a critical detail. It is illustrated how the FDF values can be reduced if inspections are planned during the design life, and thereby the basis is available for selecting a cost-effective fatigue design. Further, the results can be used as basis for recommendations in standardization.