Wind energy is becoming one of the best renewable energy options. In recent years, offshore wind turbines have been developed, moving away from the coast, searching for a higher quality wind resource with a lower environmental impact. However, the greater the distance from the coast, the deeper the sea depth, making the cost of fixed foundations prohibitively expensive. It is in this context that floating wind turbines have been developed. The design and simulation stages are of great importance to reduce their maintenance to a minimum, given their great distance from the coast, and to guarantee maximum service life. In this thesis, composites fatigue theory is applied to calculate the service life of the blades for the 15 MW offshore reference wind turbine, developed by the International Energy Agency, operating on a semi-submersible platform, under a realistic set of load cases.