Design Methodology for the Evaluation of Fatigue Life of the Connection Point for the Mooring System of a Semi-Submersible Floating Offshore Wind Turbine Platform

The design of the station-keeping system for floating offshore wind turbines (FOWT) plays an important role in the development of wind energy projects due to the complexities associated with the applied loads and the system’s non-linear behavior. This paper seeks to propose a suitable structural design for the connection point of the mooring chain for the DeepCwind OC4 semi-submersible platform. The fairlead tension of the catenary mooring lines and the dynamic behavior of the platform were analyzed using the time domain simulation tool Open Fast. The designed structure was evaluated for fatigue damage under normal load conditions to ensure a reliable and safe operation of the connection point. The effect of turbulent wind, irregular waves, currents, marine growth, and the control system of the turbine was considered to determine the fairlead tension of the mooring lines. The design of the structure corresponding to the support of the mooring line was evaluated under fatigue loads. It was observed that the definition of a suitable wave scatter diagram, and an adequate selection of the S-N curve, both play a very important role in the results obtained for the fatigue damage; therefore, it is important to perform a good set of assumptions if these input data are not available from measurements on-site or from suitable experiments. The use of environmental data like the North Atlantic wave scatter diagram, as is usual for ship design, can yield an over-estimated structural design.