Design and hydrodynamic response of the gravity-type liftable cage under wave conditions

A gravity-type net cage has always been considered as one of the most important facilities in deep-sea mariculture, due to its simple structural pattern and high cost-effectiveness. However, the harsh marine environment in the deep sea has posed a serious threat to the safety of the cage facilities. Especially in the South China Sea, typhoon weather often occurs. Furthermore, conventional gravity-type net cages cannot fully meet the large-scale and serious deep-sea aquaculture in recent years. Therefore, the safe and reliable net cage is often required in the deep-sea mariculture industry. This study aims to present a gravity-type liftable net cage. The function was also designed to submerge its main body into the sea before the typhoon arrives, in order to avoid the impact of extreme sea conditions, particularly for the safety of the cage facilities and farmed fish. Specifically, the steel-truss structure was used to replace the HDPE floating collars in a conventional cage. The steel ballast pontoons were arranged to connect the truss structure. Thus, this structure was utilized to control the ballast water for the cage lifting. The ballast water was used to control the inflation and deflation pipelines that connected to the pontoons. Once the gas in the pontoons was released, the seawater was injected into the pontoons to decrease their buoyancy for the cage diving. Conversely, when the pontoons were inflated, the seawater was released to increase the pontoons' buoyancy for the cage's rising. In addition, the integrated grid-type mooring system was used to replace the distributed mooring system in the conventional cage. A systematic investigation was made to evaluate the hydrodynamic properties of the liftable net cage. The physical model test was then conducted on the cage under floating and submerged states. The scale ratio of the net cage's main body was 1:30 during the test. The variable scale similarity criterion was used to simulate the net system. The net with real-scale net twine was then selected in the test. A comparison was finally made on the cage with floating and submerged states under the different wave conditions, from the perspectives of the motion response and mooring force. The test results showed that the total average mooring force of the cage was reduced by more than 20% under the submerged state, compared with the normal floating state. The maximum was reduced by approximately 63.22%. Moreover, the wave, front, rear, and back wave side mooring forces were reduced by 66.54%, 60.55%, 64.97% and 58.39%, respectively. The liftable net cage successfully lowers the mooring lines in order to enhance its mooring safety by submerging itself in the water. Additionally, the average heave motion amplitude response of the cage under the submerged state decreased by more than 20%, compared with the normal floating state. The maximum decrease was about 48.50%. In terms of the pitch motion amplitude, the maximum decrease of the cage was 58.41%. As such, the liftable cage effectively improved the structural safety and stability by diving into water. Besides, the connecting ropes were arranged to control the stability of the cage, because the surge motion amplitude of the cage under submerged state was much larger than that under floating condition. To sum up, the gravity-type liftable cage can be expected to apply to the extreme sea conditions in deep-sea aquaculture. The finding can also provide necessary technical data to further optimize the gravity-type liftable cage.