Abstract:Shallow sea net enclosure aquaculture (NEA) is a new ecological aquaculture model in which piles, nets, ropes, and other structures are the main components. The breeding space extends from the sea surface to the seabed and includes the natural shoreline. The general aquaculture area of NEA is approximately dozens of hectares, and the effective aquaculture water volume can be up to several million cubic meters. The vast aquaculture space provides a sufficient range of activities for farmed fish, and the quality of farmed fish is significantly better than that produced by other traditional farming modes. In recent years, more than ten NEAs have been built across the country, all of which have achieved good farming results. However, according to the field survey, almost all modern NEA facilities have different levels of damage on the net, which causes huge economic losses. After this analysis and summary, it was found that the main problem lies in the net panel. The lack of effective structural safety theory and design specifications was found to be a key factor. In this study, a numerical model of the NEA was established using the lumped mass point method. The forces on the lumped mass point of the net included gravity, buoyancy, twine force, horizontal drag, and horizontal inertia. StokesⅢorder wave theory was used to calculate water particle velocity and acceleration. Newton's second law equation was used to establish the equation of motion, and Euler´s method was used to solve it. Finally, MATLAB was used to visualize the results. In the direct connection mode, the left and right edges of the twine were tied directly to the piles, which is the common connection method. In the indirect connection mode, the left and right edges of the net were not directly tied to the piles, but were connected to the piles through horizontal ropes. In this mode, the twine was not directly connected to the piles; therefore, it can transfer the force of the rope to the twine, thereby reducing the force on the edge of the rope and protecting the net. In this study, the effect of the pile and net connection method on the maximum displacement of the net panel, maximum force of the twine and rope, and force of the joint point under the action of wave loads were investigated. The results showed that the displacement of the net panel increased with the wave height. When the wave height was less than 4 m, the maximum displacement at the wave crest and wave trough were almost the same. When the wave height was greater than 4 m, the contact area of the net panel with the wave gradually decreased at the moment of a wave trough. In the same situation, the wave force acting on the net panel was reduced. In short, as the wave moves from crest to trough, the height of the net above the water surface increases. When the wave height is 4 m, the distance from the highest point of the net to the wave trough is 4 m. When the wave height is 5 m, the distance from the highest point of the net to the wave trough is 4.5 m; when the wave height is 6 m, the distance from the highest point of the net to the wave trough is 5 m. And when the wave height was 6 m, the net is 5 m above the water surface. At the moment of the wave trough, the overall net panel was driven to move backward, and the force that can act on the net panel decreased; thus, the displacement was reduced. The indirectly connected net was not connected to the piles, resulting in all the forces at the edges being borne by the ropes. In the direct connection method, the force was shared between the twine and the rope. This caused the maximum displacement of the indirect connection to be larger than that of the direct connection. In addition, the overall movement trajectory of the net panel was asymmetrical. According to the law of wave motion, the contact area between the net and seawater differs at different times under the condition of water discharge. Consequently, the forces acting on it and the maximum displacement are different. Under the same wave height, the maximum force positions of the twine and rope were both at the connecting channel steel of the vertical ropes. The maximum force of the indirect connection was lower than that of the direct connection. The maximum force of the joint point was within 0~1 m below the horizontal plane, and the maximum force at the joint point of the indirect connection was less than that of the direct connection. During the actual construction process, special reinforcement measures should be taken for the maximum-force parts of the twine, rope, and joint points. Such construction can effectively reduce the risk of breaking the rope and twine, and avoid unnecessary construction work caused by blind reinforcement. As the wave height increased, the force of the horizontal twine increased; the force of the twine in the indirect connection method was less than that in the direct connection method. The findings of this study can be used to determine the mode of connection and safety of an NEA system.
