Numerical investigation of pre-tension effects in mooring systems on global responses of gravity-type fish farms

This study numerically investigates the pre-tension effects of the mooring systems on the global responses of gravity-type fish farms under static and dynamic conditions. A comprehensive numerical method is developed by integrating hydrodynamic models, including the Morison model and irregular wave modelling, with structural models comprising a six-degree-of-freedom (6-DOF) system and a modified extended position-based dynamics (XPBD) algorithm. The proposed method is validated using published experimental data for a net cage in uniform flow and a vertical net in regular waves. Results show that under a static condition without waves and currents, the mooring chains in the single fish cage system fully detach from the seabed, and the fish cage becomes fully submerged when the pre-tension ratio exceeds 0.023. Under a dynamic condition of an ultimate limit state, the mooring tensions significantly increase while the floating collar's and sinker tube's surge, heave, and pitch are dramatically reduced with the increasing pre-tension ratio. Higher pre-tension ratios also lead to reduced fluctuations in mooring line forces. However, when the pre-tension ratio exceeds 0.01, mooring chains may fully detach from the seabed. A comparison between a single fish cage and a 4 × 2 multi-cage fish farm under the same pre-tension ratio of 0.01 is performed. It reveals that reinforced mooring lines should be strategically placed in the direction of strong waves and currents to enhance the stability and safety of the multi-cage fish farm system. These results highlight the importance of carefully selecting the mooring system's pretension to prevent anchor failures under various sea states.