## 5 Conclusions  
This study proposes a novel mooring system with six mooring lines for a spar-type floating offshore wind turbine. Three and six fairleads were utilized in the developed mooring system. When there were three fairleads, the effects of the included angle between the two mooring lines in the same group on mooring system performance were examined by varying the included angle to 20\({}^{\circ}\), 30\({}^{\circ}\), and 40\({}^{\circ}\). When there were six fairleads, the mooring lines were uniformly connected around the spar-type platform. FAST was employed to simulate the wind and wave interaction of the floating wind turbine. Wind turbine surge, pitch, and yaw movements were analyzed in time and frequency domains to quantitatively examine the developed mooring system performance. The following conclusions are drawn:  
1. The developed mooring system with six mooring lines can effectively reduce wind turbine movements. The mooring system with three fairleads performs better than that with six fairleads;
2. The developed mooring system with six mooring lines and three fairleads is recommended in real application. In this system, the included angle between two mooring lines in a same group should be 40\({}^{\circ}\);
3. When under regular waves, the recommended mooring system can effectively reduce wind turbine surge, pitch, and yaw movement by 39.51%, 6.8%, and 12.34%, respectively, compared with the original mooring system;
4. When under irregular waves, the recommended mooring system can effectively reduce wind turbine surge, pitch, and yaw movement by 56.08%, 25%, and 47.5%, respectively, compared with the original mooring system;  
Figure 19: Wind turbine surge, pitch, and yaw movements with different static friction coefficients when under irregular wave case C6: (**a**,**b**) surge movement; (**c**,**d**) pitch movement; (**e**,**f**) yaw movement.  
* 5. Mooring line stiffness and the static friction coefficient between the seabed and mooring line have negligible effects on wind turbine movements. These conclusions are generic for differences in mooring line stiffness and static friction coefficients.  
This study proposed a novel mooring system with six mooring lines for spar-type 5 MW floating offshore wind turbines, and its performance was examined. However, its performance on other spar-type floating wind turbines (e.g., 10 MW wind turbine) is not clear at present, and the question of whether wind turbine size affects the developed mooring system's performance is not solved. Hence, in future work, the developed mooring system will be applied for different spar-type wind turbines to check whether the obtained results are generic. At present, the authors think that the developed mooring system can effectively reduce floating wind turbine movements, and can provide reference for engineers and researchers.  
Conceptualization, C.W., S.L.; methodology, C.W., S.L.; software, C.W., S.L. and Y.T., S.L.; validation, Y.T., S.L. and Y.Y.; formal analysis, Y.T., S.L.; investigation, S.L.; resources, S.L.; data curation, S.L.; writing--original draft preparation, Y.T., S.L.; writing--review and editing, Y.Y.; visualization, Y.Y.; supervision, S.L.; project administration, Y.T., S.L.; funding acquisition, S.L. All authors have read and agreed to the published version of the manuscript.  
This work is supported by research grants from the National Key R&D Program of China under Grant 2019 YFB1504402.  
Not applicable.  
The authors declare no conflict of interest.