## 3 Results and Analysis
### Mooring Tension Characteristics  
Figure 13 depicts the time history curves of the cage's mooring tension under irregular wave and current conditions at different flow velocities. Table 4 summarizes the peak values of the cage's mooring tension. It is found that the mooring tension increases with the increase in flow velocity. As the flow velocity increases from 0 m/s to 1.24 m/s, the  
Figure 11: Comparison between cage surge motion 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.  
Figure 12: Spectral density of cage motion under irregular wave and current. (**a**) Heave; (**b**) Surge.  
Figure 10: Comparison between cage heave motion 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.  
peak value of the mooring tension experiences a significant increase of 205.34%. This significant increase is attributed to the longitudinal displacement of the cage's main body caused by the current velocity, which leads to an increase in surge motion. Consequently, the mooring lines are tightened, resulting in changes in the mooring tension. In addition, the breaking strength of the mooring line was 153 kN, and the maximum tension of the mooring line under the maximum current velocity was 67 kN; thus, the safety factor is 2.19, which proved that the design of the mooring line of the novel cage can meet the safety requirements. The holding power coefficient of the AC-14 anchor used in the cage was about 10. Therefore, the maximum anchor holding force was 154.35 kN, which proved that the anchor design in this research was also reasonable. But it needs to be noted that some new types of anchors may be used for other sea areas with serious environmental conditions; for example, the novel caisson-plate gravity anchor might be an option [2].  
Figure 14 shows the time history curves of the cage's mooring forces and wave elevation under the action of irregular waves and currents during the time interval of 1100 s to 1250 s. It can be observed that there is a correlation between wave elevation and the tension in the mooring lines under the influence of irregular waves. Regardless of the flow velocity, the peak values of tension and wave elevation occur almost at the same moment.  
\begin{table}
\begin{tabular}{c c} \hline
**Case** & **Peak Value of Tension/kN** \\ \hline Case 1 & 22.10 \\ Case 2 & 42.40 \\ Case 3 & 67.48 \\ \hline \end{tabular}
\end{table}
Table 4: Statistics for cage mooring tension under irregular wave and current.  
Figure 14: Comparison between mooring tension 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.  
Figure 13: Time domain curves of cage mooring tension under irregular wave and current. (**a**) U = 0 m/s; (**b**) U = 0.62 m/s; (**c**) U = 1.24 m/s.  
As shown in Figure 15, spectral density plots of the time history curves of the mooring line tension at different flow velocities are derived from Figure 13. The spectral density curves are used to analyze the characteristics of the cage's mooring forces under the combined action of waves with different frequencies. As shown in Figure 15, it is found that the flow velocity has a significant impact on the peak values of the spectral density of the mooring line tension. With the increase of the flow velocity, the peak of the tension spectral density curve increases substantially, and the location of the maximum peak corresponds to the peak of the wave spectrum. Furthermore, under low-frequency loads, the flow velocity significantly affects the spectral density of the mooring tension, and its value increases with increasing flow velocity. In contrast, under high-frequency loads, the spectral density curve of the mooring line tension is nearly zero.