## 4 Numerical Results
### Further Discussions  
To examine the effects of some structural parameters (mooring line stiffness EA and static friction coefficient \(C_{B}\)) on the obtained conclusions, the floating wind turbine dynamic responses were further analyzed with different EA (i.e., 350 \(\times\) 10\({}^{6}\), 370 \(\times\) 10\({}^{6}\), 384.243 \(\times\) 10\({}^{6}\), 400 \(\times\) 10\({}^{6}\), and 420 \(\times\) 10\({}^{6}\)) and different \(C_{B}\) (i.e., 0.1, 0.15, 0.2, 0.25, and 0.3) when under regular and irregular waves. Regular wave case C1 and irregular wave case C6 were adopted because the wind turbine moves most violently when under these cases. The mooring system configuration T3 was adopted because T3 performed best and is recommended in this study. The simulation results with different mooring line stiffness (EA) are presented in Figures 16 and 17. It can be directly seen that with the change of mooring line stiffness, the wind turbine movements exhibited little difference. The surge movement changed more obviously; however, the maximum difference between them was smaller than 5%, indicating that mooring line stiffness has a negligible effect on wind turbine movements. A possible explanation for this is that mooring line stiffness is  
Figure 15: Frequency\(-\)amplitude curve and first peak amplitude in C6: (**a**) and (**b**) surge; (**c**) and (**d**) pitch; (**e**) and (**f**) yaw.  
great, resulting in little elastic stretch of the mooring lines. Although mooring line stiffness changes, the elastic stretch changes slightly. Thus, mooring line stiffness has a negligible effect on the wind turbine's movement and the conclusions obtained before are generic for different EA.  
Figures 18 and 19 show wind turbine movements with different static friction coefficients (\(C_{B}\)) when under regular and irregular wave cases C1 and C6. From Figures 18 and 19, it can be seen that as the static friction coefficient changed, the wind turbine movements exhibited little difference between them, similar to that of mooring line stiffness. In particular, the surge movement changed more obviously than the pitch and yaw movements, maybe resulting from the fact that the surge movement amplitude is larger than that of pitch and yaw. A possible explanation is that a spar-type floating wind turbine uses a gravity-based mooring system, which is moored mainly by mooring line gravity but not seabed friction force. Hence, seabed friction force contributes only a small portion of the total mooring system force. Further, the static friction coefficient has negligible effects on the wind turbine movements and the conclusions obtained before are generic for the different static friction coefficients.  
Figure 16: Wind turbine surge, pitch, and yaw movements with different mooring line stiffnesses when under regular wave case C1: (**a,b**) surge movement; (**c,d**) pitch movement; (**e.f**) yaw movement.  
Figure 17: Wind turbine surge, pitch, and yaw movements with different mooring line stiffnesses when under irregular wave case C6: (**a**,**b**) surge movement; (**c**,**d**) pitch movement; (**e**,**f**) yaw movement.  
Figure 18: Wind turbine surge, pitch, and yaw movements with different static friction coefficients when under regular wave case C1: (**a**,**b**) surge movement; (**c**,**d**) pitch movement; (**e**,**f**) yaw movement.