Study On Structural Strength And Fatigue Analyses Of Platform Net Cage

In response to the rapid growth of human demand for fishery resources,aquaculture has developed rapidly.Aquaculture has increased to 46% of the world’s total seafood consumption(Food and Agriculture Organization of the United Nations,2018).Corresponding aquaculture cages are widely used.The structural safety of the cage is closely related to the property safety of fishermen.The net cage must be able to operate safely under higher sea state and more complex marine environment conditions.The failure risk of net cage structures increases,which brings challenges to the safety and durability evaluation of cage structure.Under the long-term wave and current loads,the cage structure of marine aquaculture has the risk of fatigue damage.As the basis of fatigue assessment,firstly,the stress of the cage is analyzed based on the finite element model and hydrodynamic model.Then,the fatigue life is calculated using Palmgren-Miner linear cumulative damage theory and spectral-based fatigue analysis.In this paper,taking the platform cage as an example,based on the spectral-based fatigue analysis and finite element method,combined with the hydrodynamic model,the wave fatigue analysis of the floating system is carried out.In the numerical simulation analysis,the accurate calculation of sea current and wave load is especially important which is the key to the accurate calculation of stress and deformation of the cage structure.In the numerical simulation,the net cage plate and shell structure is simulated by surface element model,and the thin rod structure group is simulated by Morrison model.In this thesis,the structural design of the platform net cage,the equivalent numerical simulation of net,the hydrodynamic load calculation,the overall yield and buckling strength check,the fatigue strength assessment and structural optimization of typical nodes of the overall net cage are carried out.The main research contents are as follows:(1)The structural design of the platform net cage was carried out,the main dimensions of the overall net cage were determined according to the net cage design principles,and the main bone materials as well as the plate thickness of the net cage were checked and determined according to the specifications.(2)In order to simulate the hydrodynamic performance of the net,the equivalent is carried out according to the equal projected area of the net.The effects of different net clothing diameters and different drag force coefficients on the hydrodynamic load are discussed.The control load is screened,and its maximum response value is obtained as the design wave load for strength calculation.(3)Based on the design wave for hydrodynamic load calculation the screened design wave is applied to the overall platform type net box,the overall finite element simulation analysis is performed,yielding and buckling strength checks are performed,and the location of stress concentration is clarified,and all nodes in the overall net box that are easy to fatigue are selected as typical nodes.(4)Fatigue assessment is carried out on the selected typical nodes,and fatigue assessment is performed based on the hot spot stress S-N curve and the linear cumulative damage model formulation based on the Palmgren-Miner rule for spectral analysis,life prediction,and structural optimization of node locations that do not meet the requirements.