Dynamic Response Analysis Of Gravity Cage Under Waves And Current

Marine fishery culture is playing an increasingly important role in human life,and will be a very efficient agricultural technology in the future.However,the environmental pollution in shallow sea area is becoming more and more serious due to overfishing,which makes marine fishery resources face tremendous pressure,forcing aquaculture industry to shift from offshore to deep sea.Deep sea has many advantages compared with offshore sea,but the environment conditions of deep sea are more open and harsh than offshore sea.When the cage is exposed to harsh marine environment,the small-scale structures of the cage will suffering from severe dynamic response under hydrodynamic loads caused by waves and current.The hydrodynamic performance mechanism of cage is more complex,so the safety and reliability evaluation of the cage is very important.According to the above requirements,the dynamic responses of gravity cage under waves and current are studied in this paper.The buoyancy distribution method is improved,the calculation conditions are enriched.At the same time,the transfer function between waves and heave motion and the response spectrum of the floater are deduced,and the different effects caused by different wave spectrums on heave motion of the floater are confirmed.It is found that the heave motion of the floater under irregular waves satisfies the Gauss distribution after time-frequency transformation.By combining different wave spectrums with bending stress formulas,the results of the most probable maximum normalized bending stress distribution on the floater are extended from 0 to arbitrary value of the circular frequency,which revise the original calculation method and enrich the calculation conditions.The main contents of this article are as follows:The buoyancy distribution method is improved.The equivalent parameters of numerical calculation models of the floater and cage are calculated and analyzed.The equivalent formulas of floater and net are given.Examples show that the heave motion of floater and cage can be simulated better by the improved buoyancy distribution method,and the calculation stability is high,which ensures the correctness of subsequent calculations.The structural dynamic responses of the floater and cage under regular waves and current is numerically simulated.In particular,the important influence of the twine-property on motion pattern of the net in ABAQUS is discussed.At the same time,the motion pattern of the cage is compared with the experiment,which proves that the equivalent cage structure can better simulate the motion pattern of the net.The time-history motion curves of the single floater and the whole cage under irregular waves are studied and discussed.The first nine modes of the single floater obtained by ABAQUS and MATLAB are compared.The transfer function between waves and floater and the response spectrums of the floater are deduced.The probability distribution of the floater under irregular wave is found after time-frequency transformation.By combining different wave spectrums with bending stress formulas,the results of the most probable maximum normalized bending stress distribution on the floater are extended from the circular frequency of 0 to the circular frequency of arbitrary value.The original calculation results are corrected.It is found that the maximum response of bending stress appears at the midpoint of the floater on the wave propagation direction.