Elastic Response And Fatigue Analysis Of Floating Collar Of Net Cage In The Open Sea

With the improvement of people's living standards,the demand for marine aquatic products is also increasing.In order to meet the demand for fishery products,developing aquacultural fishery resources become an inevitable choice.In recent years,the development of traditional marine fishery resources has been around coastal areas.The breeding and fishing of fishery resources have been affected by many factors such as marine pollution.Morever,overfishing and excessive exploitation decrease the available offshore marine fishery resources.The volume of fishery aquaculture capture thus gradually declines.To expand the marine fishery resources,protect the coastal ecological environment and expand the development of marine fisheries space,the deep-sea farming has become the world's fishery trend.The net cage farming in the open sear is the most popular way in the marine aquaculture.The development of the aquacultural fish farm has been issued in developed countries such as Norway and the United States.However,the development of fish cage industry in China is still backward and few related research is conduct.Based on the key projects of the National Natural Science Foundation of China and the project of the Youth Fund,this study carried out the hydrodynamic and safety fatigue analysis of the net cage.In this paper,the elastic response,stress distribution and fatigue characteristics of the floating system are analysed by the numerical simulation.The main contents of the research are as follows:(1)The elastic response of the floating structure using curved beam method is analysed.The elastic response analysis model of the floating collars was established by the modal superposition and curved beam methods.The rigid motions of the net cage and floating collars was analysed by Newton's second law.The physical model experiment and numerical results verified the model of floating collar motions and deformations.The influence of different wave incidence angles on the deformation of the floating collar,and the maximum position of the deformation of the floating frame is analysed.The response of the floating frame in frequency domain under random wave is analysed.The transfer function,coherence and phase difference between the plane deformation and outer deformation and waves were calculated.(2)The stress distribution of the floating system jointly using the hydrodynamic and finite element model are studied.Through the finite element model and the hydrodynamic model,the iteration to calculate the stress of the floating frame is established.The mathematical model was verified by comparing the experimental results of the model with numerical results.The effects of the stress and deformations on the floating collar such as netting,anchorage are analysed.Deformations and stress distribution of the single-point-mooring and multiple-point-mooring net cage systems in currents are evaluated.(3)Based on the time-domain and frequency-domain method,fatigue life of floating collars is estimated.In time-domain method,a simplified fatigue time domain analysis model is established to calculate the fatigue damage and life of the floting system based on the linear fatigue damage theory and the SN curve of the material.The influence of wave height and period on the fatigue lifetime is analysed.The effects of the material SN curve,wave directions,mooring line arrangement and extreme sea state on the fatigue life of the floating frame were studied in details.In frequency-domain method,the frequency-domain fatigue analysis model of the floating system is established by using the Weibull,Rayleigh,Gamma and general extreme value functions.The short-term and long-term distributions of the stress range are fitted by difference density probability functions.Based on the short-term and long-term distribution of the stress range,the fatigue lifetime are estimated.