Support structures of Offshore Wind Energy Conversion Systems(OWECS) are exposed to combined loading from wind and waves. Therefore, the fatigue damage evaluation is an important part in the design phase of offshore wind farms. Different methods for calculation of actions on offshore support structures at sites in North and Baltic Sea are presented. Traditional concepts for fatigue design vary from deterministic methods, which have been adopted from experiences in the design of oil and gas platforms, to calculations in the time domain, which are applied to the actual generation of multi megawatt onshore wind energy converters. As results show, time domain approaches should be used preferably.For braced structures the fatigue strength of tubular joints is determined by local approaches. Besides the structural stress approach, sophisticated concepts like the notch stress approach can be used for fatigue assessment. A concluding fracture mechanics analysis supports both material selection and risk management.
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