Extreme response analysis of a floating offshore wind turbine and Fatigue analysis of the mooring lines
Master thesis
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https://hdl.handle.net/11250/3146831Utgivelsesdato
2024Metadata
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Sammendrag
In this study, we evaluate the design of a floating offshore wind turbine mooring system forintermediate water depths of 60 meters. We also analyzed extreme load cases and fatigue analysisof the mooring lines. The mooring system proposed for this case is a 3-segment chain-rope-chainconfiguration, with polyester rope as the intermediate rope segment. The mooring line's bottomsegment 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 abuoy to prevent collision with the seabed. To model the nonlinearity of the polyester rope, we usedthe Syrope method recommended by DNV. This method determines the dynamic stiffness of themooring line. The environmental conditions at the Sørlige Nordsjø II (SN II) site, located 140 kmoff the coast of Norway, were analyzed using a joint distribution model from previous studies todetermine significant wave height, wave period, and wind speeds.We proposed five different mooring configurations to be tested and compared. The resultssuggest 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 maximumloads, standard deviation of the load, and hence fatigue damage. This makes MS1 the best choiceamong 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) wereidentified for detailed analysis. The results indicate that DLC61 is more critical for the design thanDLC16 due to higher extreme tension and larger platform surge displacement.To evaluate fatigue, a maximum dissimilarity algorithm (MDA) was first implemented toselect representative sea states from a large database of generated sea states. This method resulted12in a manageable number of simulations while trying to represent all of the sea conditions. Fatigueanalysis revealed that the chain segment at the fairlead should be the main focus of the fatigueanalysis as recommended by API which gives a lower estimated fatigue life. The fatigue life forthe 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 offthe coast of Norway in 60-meter water depths for wind farm development with a focus on mooringline design and fatigue analysis. This study finds the MS1 to be the best of the five proposedmooring configurations and the extreme response analysis of the mooring line suggests that evenat the 50-year return period sea state (DLC61) the extreme mooring line tension reaches around35% of the MBS. The fatigue analysis also suggests that the chain segment at the fairlead has asuitable estimated life of 280 years (safety factor of 1). These results suggest that MS1 can be areliable design for the development of a floating offshore wind farm at the proposed site.