Within this paper calculations in the frequency domain are discussed for the fatigue evaluation of support structures of offshore wind energy converters (OWECs) under wave loads. Different possibilities of modeling the dynamic behavior of support structures of OWECs are discussed and restrictions of simplified conventional methods are shown. Also the different damage prediction methods are compared and an extension of often used methods is introduced to take S-N-curves with varying slopes into account.
An increasing number of wind energy converters has recently been installed in offshore environments. This development leads to the demand of combining design concepts for wind energy converters and offshore structures. Within the research projects with the short titles "GIGAWIND" and "GIGAWIND plus", which are funded by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety of Germany, the authors are working on different aspects of this question.
Concepts for the fatigue design of offshore structures use deterministic design methods or calculations in the frequency as well as in the time domain. A hybrid time-frequency domain fatigue analysis can be used to significantly reduce the amount of calculation time for the support structure under wave loading.
The calculations in frequency domain as well as the hybrid analysis technique result in a description of the stress distribution for each considered sea state in the frequency domain. It is therefore necessary to predict the damage of structural details from this distribution. Within this paper several approaches from the literature are compared with respect to realistic conditions of the sea environment. In accordance to design guides provided by Germanischer Lloyd (2004) or NORSOK (1998) the calculations should consider S-N-curves with varying slopes. Such varying slopes can not be integrated in all of the existing approaches. Therefore a possible extension to cover such S-N-curves presented and discussed.