Abstract:There is strategic demand in China to open up space for deep-sea aquaculture and develop related large-scale base-station equipment and technology. Research and development of deep-sea aquaculture cages will greatly promote the spatial expansion and production-mode transformation of aquaculture in China and improve the utilization capacity of deep-sea and fishery resources. Generally, large cages located in deep water are more vulnerable to strong currents, high waves, and typhoons than cages inshore. It is critical to analyze the hydrodynamic characteristics of such cages because of their rocking and drifting motions under heavy marine environmental loads. In this study, a numerical model of a semi-submersible cage under wave action was established based on the finite element method, and the mooring line tension and motion of the cage were calculated by simulations. The cage was modeled using the 3D modeling software Creo Elements/Pro. After model development, it was imported into ANSYS for pre-processing. The simplified cage structure model was imported into the ANSYS Workbench to divide the mesh; the hydrodynamic response and other data related to the cage structure could then be analyzed by establishing a wet surface model. The water depth calculated by numerical simulation in this study was 120 m. The cage adopts a 4-point anchoring form, and the length of the anchor rope is 360 m. The cage can reach a semi-submersible working state by adjusting the bottom pontoon. To compare the dynamic response characteristics of the cage under different draft conditions, three different working conditions were set for the semi-submersible cage: no-load (draft depth of 8.4 m), half-load (draft depth of 33.6 m), and full-load (draft depth of 43.2 m). First, the accuracy of the numerical model was verified by comparing the computer simulation values with the physical flume test values. Then, the dynamic responses of the semi-submersible cage were studied under three ballast conditions. Finally, the calculation results of the mooring line tension, heave, surge, and pitch of the cage were analyzed and compared under different wave conditions. The results show that the calculated values were in agreement with experimental values, and the relative error between them was approximately 5%. This demonstrates that the numerical model can be used to analyze the hydrodynamic characteristics of the semi-submersible deep-sea cage. When the wave height is the same, there is no obvious correlation between the mooring line tension of the cage and the wave period. On the other hand, when the wave period is constant, the mooring line tension on both sides increases with an increase in wave height. Additionally, the heave, surge, and pitch of the cage were all positively correlated with the wave height. With an increase in the draft of the semi-submersible cage, the heave, surge, and pitch values of the cage decreased. The maximum heave and surge values of the cage are 12.67 m and 10.59 m, respectively, under the three ballast conditions, and the maximum pitch value of the cage under no-load conditions is less than 15°. Based on the above work, it can be observed that the semi-submersible cage structure has good stability. Moreover, the results can provide a theoretical reference for the design of marine cages in China.
