## 5 Conclusions  
In this paper, a new model based on the OC3-Hywind spar to support a 5-MW wind turbine is presented. The clump masses are attached to the catenary mooring system to further improve its wave-resistance ability. Following many calculations and analyses of the new models of different weight of clump masses and different position in the previous period, the influence of the mass and position of the clump masses on the response motion of the platform was determined. In addition, the change of the position of the clump masses was found to have an obvious influence on the response motion of the spar platform. Similarly, an increase in the mass of the clump masses will increase the heave of the spar platform and reduce the surge of the platform, but the impact on the surge is very small. Therefore, in this paper, the weight of the clump masses remained unchanged, with only the position of the clump masses changed to obtain five new models. The response motions of the five OC3 CM models and the original OC3 model under six load cases were calculated by the codes. Also, the free decay experiment was performed to verify the accuracy of the codes. From the above analysis and discussion, several important conclusions can be drawn, as follows:  
1. Through the comparison of the free decay test, it can be concluded that the code developed in-house is accurate, so it can be used for subsequent research on OC3 offshore wind turbines. 2. The OC3 CM model can significantly reduce the surge of the spar platform at wind and wave loads, and the clump masses are more effective when they are mounted closer to the sea bed. However, it must be pointed out that the clump masses cannot be located too close to the anchor point, otherwise they will make the connection node touch the seabed. The best model is the OC3 CM model P5, which can reduce the value of the surge by about 30.89% under a regular wave and reduce the peak amplitude of surge by about 38.4% under an irregular wave. 3. The OC3 CM model has no significant effect on the heave of the spar platform under wind and regular wave loads; however, after adding the clump masses, the platform will settle about 1.3 m, but the impact is negligible compared to its drought depth of 120 m. In addition, the OC3 CM model P5 induces a small change in the peak pitch amplitude, while the OC3 CM model P5 can reduce the peak pitch amplitude and the peak heave amplitude by about 20% and 19%, respectively, in the wind and irregular wave load. Figure 17: Fatigue loads of the mooring line in different load cases. 4. The clump masses will increase the fairlead 1 tension by about 30% and increase the fairlead 2 and 3 tensions by about 24.5%. However, the position of the clump masses and the changes of wind and wave loads indicate no significant effect on the fairlead tension. The segment of the mooring line from the connection node to the vessel node is obviously straightened by the clump masses. This will increase the tension of the mooring line and should, therefore, limit the motion of the platform. The affected frequency range makes a difference to the fatigue loads on the moorings, but the regularity is not obvious. At the same time, the fatigue loads of the OC3 CM model P5 and P2 are relatively small. 5. This paper only intends to provide a way to reduce the response motions, regardless of its increased cost. In subsequent studies, the cost will be taken into consideration and the platform structure as well as the mooring system will be changed simultaneously. These changes are expected to make the OC3-Hywind have better performance under different environmental conditions and better economic benefits. Conceptualization, Z.L. and Y.T. ; methodology, Y.T. ; software, Y.T. ; validation, Y.T. ; formal analysis, Y.T. ; writing--original draft preparation, Y.T. ; writing--review and editing, Y.T. and Z.L. ; supervision, Z.L. ; guidance, Z.L. ; coding, W.W. and G.Q.