深水养殖网箱浮架变形模拟及结构改进设计

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

由高密度聚乙烯材料制作的网箱浮架在波浪流冲击下具有明显的变形,过大变形甚至会导致浮架失效破坏。该研究介绍了一种用于模拟波流作用下网箱系统受力变形的有限元计算模型,并利用前人的研究结果对数值模型进行了验证。在此基础上,以中国南海区常用规格80 m周长的圆形网箱为研究对象,设定27种大浪强流条件,对各种规则波和水流组合条件下深水网箱浮架动力变形进行了数值模拟,给出了不同波流工况下浮架最大变形位置处的应变模拟结果。结果表明,浮架变形随着波高或流速的增加而增大,但随着波浪周期的增加变化很小,在波高7m、周期9s、流速1.5m/s条件下浮架发生屈服破坏的风险极大。为降低高海况下浮架局部屈服破坏风险,对浮架结构进行了改进,科学提出了一种能够有效增强网箱浮架结构安全的技术措施。

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.