Fracture Mechanics Based Fatigue Assessment for a Spar-Type Floating Wind Turbine

Floating wind turbines (FWTs) are exposed to dynamic and cyclic environmental loads during their service life. Fatigue assessment has become an important aspect in the design phase of a FWT. A fracture mechanics (FM) based fatigue assessment was performed for the 12 points around the tower base of a 5 MW floating wind turbine supported on a spar platform. The aligned wind and waves are selected as environmental conditions for the fatigue assessment. The stress ranges on the wind turbine tower base are achieved through a rainflow counting method based on the results from the time-domain analysis using the FAST software. A comparison between fatigue lives predicted by the FM and S-N curves based approaches is made. The impact of the variation of initial crack depth, critical crack depth and stress concentration factors (SCFs) on the ratio of the fatigue life predicted by two approaches is investigated. The study shows that the fatigue life predicted by the FM based approach is more conservative than that predicted by the S-N curves based approach and also the fatigue life is highly sensitive to the material constant of Paris Law C and SCFs.