The initial design of a 12-MW floating offshore wind turbine (FOWT) was made by the University of Ulsan (UOU) based on the 5-MW offshore wind turbine of the National Renewable Energy Laboratory (NREL) using the law of similarity. The tower design was checked through the eigenfrequency and fatigue strength analysis according to the GL guideline of tower design conditions. The direct expansion of the 5 MW wind turbine support structure caused a resonance problem of the tower of the 12-MW UOU FOWT and the tower length was adjusted to avoid the 3P resonance. Wind turbines are required to have a design life of more than 20 years and shall be designed to endure both ultimate and fatigue loads experienced during the design life. The platform pitch motion of FOWTs due to combined wave and wind loading may result severely in both fore-aft forces and moments at the base of the tower. In this study, we used the simplified fatigue analysis, which is generally applied when considering safety margins by stress to predict the fatigue life of tower. In order to calculate the fatigue load, the Markov matrix was constructed by using the cycle counting method to determine range, average value, and cycle number of loads from peak and valley values of actual load histories simulated by FAST v8 of the tower base. The predicted fatigue life at the tower base was follow by S/N curves for welded steel structures and it was calculated by the Palmgren-Miner’s rule.

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