## 3 Results and Analysis
### Stress Response Characteristics of Floating Collar  
Under the action of regular waves, the largest deformation is found at point D (as shown in Figure 16) of the SPM-GDW cage's floating collar [26]. In this section, point D is selected as a potentially hazardous point on the floating collar for analysis of its stress response characteristics. Figure 17 shows the time history curves of the stress at point D of the cage's floating collar under irregular waves and currents at different flow velocities. Table 5 gives the corresponding statistical results of the stress peaks at point D of the floating collar. It is found that with the increase in flow velocity, the peak stress on the floating collar increases. As the flow velocity increases from 0 m/s to 1.24 m/s, the peak stress on the floating collar increases by 122.69%. In addition, the yield stress of the floating collar was 26 MPa and the maximum stress of the floating collar under the maximum current velocity was 14.92 MPa, So the safety factor is 1.74, which proved that the selection of material and size for floating collar was also reasonable.  
Figure 16: Different junctions of floating collar [26].  
Figure 15: Spectral density of mooring line tension under irregular wave and current.  
Figure 18 shows the relationship between the stress on the cage's floating collar and the wave elevation during the time interval of 1100 s to 1250 s under irregular waves and currents. It is observed that there is a correlation between the wave height and the peak stress on the floating collar under the action of irregular waves and currents. The peak stress occurs almost simultaneously at the peak of wave elevation. However, the stress on the floating collar exhibits high-frequency response characteristics while satisfying the trend of wave frequency response. This high-frequency response may contribute to the dynamic coupling between the net and the floating collar under the action of waves and currents. This high-frequency response may also lead to fatigue failure of the floating collar, which should be paid attention to in the design.  
Figure 19 shows the spectral density curves of the stress on the floating collar under the action of irregular waves and currents. It is found that the stress spectral density curves at point D of the cage's floating collar exhibit similar patterns to the spectral density curves of the mooring line tension shown in Figure 15. The flow velocity has a significant impact on the peak values of the stress spectral density of the cage's floating collar. As the flow velocity increases to 1.24 m/s, the peak of the stress spectral density curve increases significantly, and the peak of the spectral density corresponds to the peak of the wave input spectrum.  
Figure 17: Time domain curves of floating collar stress of cage under irregular wave and current. (a) U = 0 m/s; (b) U = 0.62 m/s; (c) U = 1.24 m/s.  
Figure 18: Comparison between floating collar stress and wave surface elevation under irregular wave and current. (a) U = 0 m/s; (b) U = 0.62 m/s; (c) U = 1.24 m/s.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Case** & **Peak Value of Stress/MPa** \\ \hline Case 1 & 6.70 \\ Case 2 & 10.78 \\ Case 3 & 14.92 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Statistics for floating collar stress under irregular wave and current.