Dynamic Analysis of Three Alternative Mooring Systems for a Semi-Submersible Wind Turbine in Intermediate Water Depth

Three forms of the mooring system in 60 m water depth are proposed for semi-submersible with partially inclined columns (SPIC) concept floating wind turbine (FWT). One is a simple form with only catenary lines, and the other two are hybrid forms including clump weights. The clumps are attached to the suspended section for Hybrid form1 and the bottom section for Hybrid form2. Hybrid form2 achieves the smallest line length and chain weight. Three alternative proposals can be evaluated through mooring line characteristics, dynamic responses, utilization factors, and simple cost analysis. Hybrid form2 allows for smallest pretension, and largest stiffness and nonlinearity only at large offsets. Under operational conditions, the mean surge for Hybrid form1 and Hybrid form2 is similar, but the fairlead tension is significantly smaller for Hybrid form2. Under the survival condition, the clumps of Hybrid form2 are lifted up and put down, leading to small mean offsets of FWT but large wave-frequency components of line tension. Among the three forms of the mooring system, the Hybrid form2 can limit the FWT to the smallest offset range while also controlling the mean mooring line tension to a level similar to the other two forms. Under normal working conditions and accidental conditions with single line broken, the maximal surge motions of FWT under the restraint of three mooring systems all meet the design requirements. The mooring line strength of the three mooring systems meets the requirements in ultimate limit state (ULS) and accidental limit state (ALS) analyses. Among them, the utilization coefficient of Hybrid form2 is closest to 1, demonstrating its best economic performance.