Combined wind and wave loads need to be considered when designing Floating Offshore Wind Turbines (FOWTs). The design of FOWT gets more complicated as the non-linear coupling effect, particularly, the aerodynamic loads which are dependent upon the wave-induced motions. A spectral method that can take account of the non-linear coupling effect is used. The coupling effect may usually be considered by using a direct time-domain method which takes account of the combined loads. Fatigue damage of structural details then evaluated by using Miner's rule and rain-flow counting over the direct time-domain results. Considering the non-linear properties of the aerodynamic loads, the stress transfer function is no more useful, because the response is non-linear and coupled. Then, a large number of simulations are needed to evaluate the fatigue damage during the lifetime. Otherwise, a spectral approach may be used for evaluating the fatigue damage by taking account of the non-linear coupling effect. A semi-submersible FOWT is considered and the fatigue damage at three different locations is evaluated. An in-house time-domain code is used to analyze its behavior. For the spectral approach, the variance of the stress and the bandwidth parameter plays a key role. The results show the variance of the response under combined load is not given by a simple sum of those under the wind-only condition and wave-only condition due to the non-linear coupling effect. A formula to correct the variance of the simple sum as a function of the wind speed and wave height is proposed.
Fatigue strength is one of the most important issues to be considered when designing an offshore structure including Floating Offshore Wind Turbine (FOWT). Generally, the fatigue damage analysis of an offshore structure is to be performed in time domain. Rain-Flow Counting (RFC) method developed by Endo et al (1974) is used to calculate the stress range and the number of a cycle from time history data of stress, then the fatigue damage is calculated by using S-N curve based on Palmgren-Miner's law.