Analysis Of Hydrodynamic Characteristics Of Gravity Cage Mooring Under Extreme Waves

With the improvement of the living conditions of the Chinese people and the increasing demand for high-quality protein,the development of marine pastures has become increasingly urgent.However,due to overfishing and unreasonable development near the coastline in recent decades,environmental pollution in shallow water areas has become more serious,which makes mariculture gradually develop to the deep sea.In order to have a better aquaculture environment and higher-quality aquaculture products,China is vigorously developing deep-water aquaculture tools,and more and more deep-water cage equipment is put into use.Currently,deep-water gravity cages are widely used.Deepwater cages are often subjected to harsher sea conditions than offshore.As a high-input,high-risk,and high-output aquaculture equipment,the safety of the cage mooring system is related to the entire marine aquaculture system.At present,Chinese deep-water cage equipment mainly relies on similar products imported from abroad,with few independent research and development,and lack of theoretic support for the cage design.Therefore,studying the hydrodynamic characteristics of the cage mooring system in the marine dynamic environment,especially under the action of extreme waves,has important scientific significance and practical value for the design and layout of the cage mooring system.To this end,this article first uses OrcaFlex application programming interface and programming software MATLAB to establish a gravity cage mooring system.In order to verify the reliability of the numerical model,a physical model of the gravity cage is established according to similar criteria,and the experiment was carried out in the wave-making pool of the Coastal and Offshore Engineering Laboratory of South China University of Technology.The test was conducted with the wave-making pool of the offshore engineering laboratory.The test results show that the tension values of the front and rear moorings in the numerical simulation are basically consistent with those in the physical experiment under the regular wave condition.Qinzhou Bay in the northern part of the South China Sea is less polluted by biologic residues,which is suitable for the development of marine fisheries.In order to further consider the safety performance of the cage mooring system under extreme waves,the hydrodynamic numerical study was carried out for the gravity cage with a depth of 20 meters outside Qinzhou Bay,Guangxi Province.The extreme survival sea state uses extreme wave conditions with a return period of 100 years.Considering the symmetry of the cage mooring system,the tension changes of the mooring lines between the wave directions 0° and 45° is analyzed representatively.The results show that when the wave direction is 0° which paralleling to the mooring system axis,the tension amplitude of the cable is smaller and more uniform than when the wave direction 45°,it reminds that the mooring arrangement should be parallel to the mooring system axis.At the same time,the load difference of the freak wave on the cage mooring system under the same sea conditions is analyzed.The freak wave is generated according to the phase modulation method in MATLAB and can occur at a specified time and location.Then,the wave surface duration curve is imported into OrcaFlex for calculating the motion response.The results show that the heave and wave force of the mooring system change dramatically at the moment of freak wave occured,and the tensions of the mooring lines increase instantaneously,which is extremely unfavorable to the normal operation of the cage.And reliable suggestions for strengthening the mooring lines are given according to the calculation results.Finally,in order to explore the relationship between the time frequency response and wave flow of the mooring system under wave flow mixing,considering the limitations of Fourier transformation in processing non-smooth signals,that is,the change of signal frequency over time cannot be derived.This paper uses wavelet transform as the signal time-frequency.The generalized Morse wavelet is chosen as the mother wavelet due to its analytic and flexible timefrequency resolution.Firstly,the dynamic response of the cage mooring system under two conditions of different wave speeds and wave angles is calculated,and then fast Fourier transform and generalized Morse transform are used to analysis the mooring line tension in time-frequency.The results show that under different current rates,the greater the current rate,the greater the spectral amplitude,the frequency in the high-frequency region is affected by waves,and the frequency in the low-frequency region is affected by the current.Under different wave angles,the larger the angle between wave and current,the more we can see from the time-frequency transformation of mooring line tensions about the difference between the frequencies of the waves and currents.The frequency spectrum after fast Fourier transform shows the change of signal components in frequency,and the time-frequency diagram after generalized Morse wavelet transform shows the change of signal frequency with time.The research methods and results of this paper can provide a certain reference for the scientific arrangement of the gravity cage mooring system,and provide a preliminary research foundation for the large-scale operation of large deep-water gravity cages.