Fatigue reliability and calibration of fatigue design factors of wave energy converters

Target reliability levels, which are chosen dependent on the consequences in case of structural collapse, are used in this paper to calibrate partial safety factors for structural details of wave energy converters (WECs). The consequences in case of structural failure are similar for WECs and offshore wind turbines (no fatalities, low environmental pollution). Therefore, it can be assumed that the target reliability levels for WEC applications can be overtaken from offshore wind turbine studies. The partial safety factors cannot be directly overtaken from offshore wind turbines because the load characteristics are different. WECs mainly focus on wave loads where for offshore wind turbine the wind loads are most dominating. Fatigue failure is an important failure mode for offshore structures. The scope of this paper is to present appropriate Fatigue Design Factors (FDF), which are also called Design Fatigue Factors (DFF), for steel substructures of WECs. A reliability-based approach is used and a probabilistic model including design and limit state equation is established. For modelling fatigue, the SN-curve approach as well as fracture mechanics are used. Furthermore, the influence of inspections is considered in order to extend and maintain a certain target safety level. This paper uses the Wavestar prototype located at Hanstholm (DK) as case study in order to calibrate FDFs for welded and bolted details in steel structures of an offshore bottom-fixed WEC with hydraulic floaters.