Deformation simulation and structural improvement design for floating collar of deep-water aquaculture net cage

Abstract: Due to various environmental issues and coastal zone resource conflicts in recent years, cage aquaculture has been increasingly developed in more exposed sea water where the water depth exceeds 20 m and the water quality is much better than the one close to the shore line. As an important part of deep-water net cage, the floating collar made of high-density polyethylene (HDPE) may undergo great deformation resulting from strong winds, waves and current. Site observations have confirmed that the deformation of larger net cage in open sea is more severe than that of smaller cage. The floating collar may deform to a large extent that normal functionality is disabled, causing heavy economic loss to farming enterprises. Therefore, the aim of this study is to analyze the dynamic deformation of the floating collar of a net cage under the combined influence of waves and current in order to help improve the design to increase the bearing capacity of a net cage. In this study, a numerical model was established through a commercial program, Orcaflex, and applied to simulate the dynamic behavior of a floating net cage in waves and current. The program is able to perform dynamic analysis of a wide range of offshore systems, typically including boundary conditions such as vessels, buoys, etc., as well as finite element modeling of line structures. The whole cage system, composed of a floating collar, fishing net and mooring lines, was modeled by a combination of line elements, three and six degree of freedom buoys. For the surface collar, lines were connected by six degree of freedom buoys, which transferred rotational and translational motion to represent the bending stiffness of the material. For the fishing net and mooring lines connecting the collar, line segments were connected by three degree of freedom buoys with no moment contributions, considering that the bending stiffness was insignificant, which can be set to zero. To validate the numerical model, we considered a case based on the motions of the net cage under regular waves and current, from which we calculated and compared it with the simulated results. The comparison of the results indicated a good agreement. Afterwards, the numerical model was used to simulate the dynamic deformation of the floating collar of a net cage with a circumference of 80 m, which was connected by an eight-point mooring system. This type of net cage is used extensively in the South China Sea. Regular waves with different wave heights (5, 6, or 7 m), wave periods (9, 11, or 13 s), and current velocities (0.6, 1.0, or 1.5 m/s) were set as the sea conditions for the calculations. We considered the von Mises strain and stress at the mooring line connection point on the floating collar to represent the deformation of the floating collar. The results showed that deformation occurred when the collar was exposed to sea loads, and became greater as the wave height or current velocity increased. However, the maximum strain on the collar did not change significantly as the wave period increased, which indicated that the effect of the wave period on collar deformation was small. When the net cage was subjected to the sea loads with waves (height 7 m and period 9 s) and current velocity of 1.5 m/s, the maximum strain on the floating collar was equal to 2.47%, which was the largest among all of the waves and current conditions. The largest value for the von Mises stress was very close to the yield stress of 24 MPa, which may increase the likelihood of plastic deformation. Therefore, to reduce the risk of plastic deformation, we designed a casing pipe measuring 0.5 m in length with an outer diameter of 0.355 m and a thickness of 15 mm. The casing pipes were installed and fixed at each position of the mooring line connection point on the floating collar. The deformation results demonstrated that the maximum strain on the cage collar with casing pipes was much smaller than that without casing pipes, and thus using casing pipes can greatly reduce the local deformation of the cage collar to ensure the structural safety in severe sea conditions.