Fatigue Life Evaluation for Mooring Line and Tower of Floating Wind Turbine

In this study, fatigue lives of mooring line and tower of a floating wind turbine structure were investigated. A SPAR type wind turbine with three catenary mooring lines, a rotor-supporting tower, and three blades is considered as an example structure. Mooring line tension and tower base stress are calculated through a time-domain coupled analysis. Boundary element method (BEM) and convolution are applied to floating body formulation. The mooring lines, tower, and blades are formulated with catenary elements, elastic beam elements, and aerodynamic elastic rotating beam elements, respectively, using finite element method (FEM). By analyzing the mooring line tensions with fatigue curves according to API-RP-2SK and statistical methods such as simple summation (SS), combined spectrum (CS), and combined spectrum with dual narrow band (CSDN), fatigue life of a mooring line is obtained. Fatigue life of a tower is obtained by analyzing the tower base stress with fatigue curves according to API-RP-2A-WSD and the statistical methods SS, CS, and CSDN. From the numerical results, fatigue features of mooring line and supporting tower are investigated and compared. Through an analysis of fatigue life by changing breaking strength of mooring line, the effect of breaking strength on fatigue life variation is also discussed.