Extreme response analysis of a floating offshore wind turbine and Fatigue analysis of the mooring lines

Abstract

In this study, we evaluate the design of a floating offshore wind turbine mooring system for intermediate water depths of 60 meters. We also analyzed extreme load cases and fatigue analysis of the mooring lines. The mooring system proposed for this case is a 3-segment chain-rope-chain configuration, with polyester rope as the intermediate rope segment. The mooring line's bottom segment and middle segment lengths were modified to reach 10% of the mean breaking strength (MBS) pretension in the polyester segment. The bottom part of the polyester was attached to a buoy to prevent collision with the seabed. To model the nonlinearity of the polyester rope, we used the Syrope method recommended by DNV. This method determines the dynamic stiffness of the mooring line. The environmental conditions at the Sørlige Nordsjø II (SN II) site, located 140 km off the coast of Norway, were analyzed using a joint distribution model from previous studies to determine significant wave height, wave period, and wind speeds. We proposed five different mooring configurations to be tested and compared. The results suggest that while the mooring system 5 (MS5) configuration provides better platform stability, the MS1 configuration is the preferable design due to its reduction of mooring line maximum loads, standard deviation of the load, and hence fatigue damage. This makes MS1 the best choice among the proposed line configurations for further analysis. Extreme load cases (DLC16 at 25 m/s wind speed and DLC61 at 43 m/s wind speed) were identified for detailed analysis. The results indicate that DLC61 is more critical for the design than DLC16 due to higher extreme tension and larger platform surge displacement. To evaluate fatigue, a maximum dissimilarity algorithm (MDA) was first implemented to select representative sea states from a large database of generated sea states. This method resulted12 in a manageable number of simulations while trying to represent all of the sea conditions. Fatigue analysis revealed that the chain segment at the fairlead should be the main focus of the fatigue analysis as recommended by API which gives a lower estimated fatigue life. The fatigue life for the chain at the fairlead was estimated to be around 280 years with a safety factor of 1. In conclusion, this study tries to evaluate the mooring system design for the proposed site off the coast of Norway in 60-meter water depths for wind farm development with a focus on mooring line design and fatigue analysis. This study finds the MS1 to be the best of the five proposed mooring configurations and the extreme response analysis of the mooring line suggests that even at the 50-year return period sea state (DLC61) the extreme mooring line tension reaches around 35% of the MBS. The fatigue analysis also suggests that the chain segment at the fairlead has a suitable estimated life of 280 years (safety factor of 1). These results suggest that MS1 can be a reliable design for the development of a floating offshore wind farm at the proposed site.