Time-domain fatigue assessment for blade root bolts of floating offshore wind turbine (FOWT)

A methodology for time-domain fatigue assessment for preloaded blade root bolts floating offshore wind turbine (FOWT) is proposed in the paper. The stress responses of the blade root bolts are calculated by finite element (FE) method in terms of the time history of environmental loading on the blade root joint where the wind, wave, and current are taken into account. Stress ranges on blade root bolts and the corresponding cycles are achieved by the rain-flow counting method. Fatigue life prediction for blade root bolts is then conducted with use of S-N curves and Palmgren-Miner's (PM) rule. During the fatigue assessment, the mean stress effects and the influence of different design and installation factors on fatigue strength of blade root bolts are considered. A fatigue assessment for blade root bolts of a NERL 5 MW Spar type FOWT is performed to demonstrate the capability of the proposed methodology. Fatigue lives of the blade root bolts with different design and installation factors are predicted and the impact of bolt preload, bolt size and thickness of connecting flange on the fatigue strength of blade root bolts is discussed.