Study On Hydrodynamics Of An Offshore Semi-submersible Aquaculture Platform

For the past few years,the scale of the aquaculture industry has developed rapidly as people’s living standards continue to improve and demand for fish protein increases.However,the overfishing and the saturated nearshore marine aquaculture space have prompted the development of the aquaculture industry from nearshore to deep sea.To adapt to the complex hydrodynamic environment in deep sea,including deep water,severe waves,and strong currents,a new semi-submersible aquaculture platform,composed of pontoons,columns and nets,comes into being.At present,there is a lack of basic research on the hydrodynamics of semi-submersible aquaculture platforms in currents and waves,including the inclination of semi-submersible aquaculture platforms in current conditions,the influence of flexible nets on the hydrodynamic response of semi-submersible aquaculture platforms,the sensitivity of the semi-submersible aquaculture platform to the wave frequency in wave conditions,the flow velocity distribution inside the aquaculture platform,and the assessment of the suitable flow region for the farmed fish inside the aquaculture platform.None of these scientific problems have been resolved.Therefore,to solve the above problems,this study uses physical model tests and numerical simulation methods to conduct a detailed and comprehensive analysis of the dynamic response of the semi-submersible aquaculture platform in currents and waves,the distribution characteristics of the flow field,and the suitable flow region for farmed fish.The main research contents are as follows:Firstly,the hydrodynamics of the semi-submersible aquaculture platform in currents and waves are studied based on the physical model test.For each part of the semi-submersible aquaculture platform: the main frame,the flexible net and the mooring system,the gravity similarity,the variable-scale nets gravity similarity and the elasticity similarity are used to scale and build the test model.Then,the variation law of the inclination angle and mooring force of the semi-submersible aquaculture platform in different flow velocity conditions and the influence of wave height,wave period and draught on the semi-submersible aquaculture platform in regular waves and irregular waves are further analyzed.In addition,the effects of flexible nets on the surge motion,heave motion,pitch motion and mooring force of the semisubmersible aquaculture platform are explored in detail.Secondly,a numerical model of the hydrodynamic response of the semi-submersible aquaculture platform in wave conditions is established.The potential flow theory is used to calculate the wave force acting on the main frame and the Morrison equation is applied to calculate the wave force acting on the net.Combined with the motion governing equation,the dynamic response of the semi-submersible aquaculture platform in waves is solved.The numerical simulation results are compared with the experimental data of the physical model to verify the reliability and accuracy of the numerical model.Based on the numerical model,the effects of wavelength,draught,mooring form and wave frequency on the semi-submersible aquaculture platform in regular waves and irregular waves conditions are further analyzed.Thirdly,a numerical model of the flow velocity distribution inside and around the semisubmersible aquaculture platform in currents is established.The calculations of flow velocity distribution inside and around the semi-submersible aquaculture platform and the drag force acting on the aquaculture platform are solved by the method of computational fluid dynamics and the theory of porous media.Based on this numerical model,we study the influence of these factors,such as incoming flow velocities and net solidity,on the flow velocity distribution inside and around the semi-submersible aquaculture platform.Further,we explore the low flow velocity region near the aquaculture platform,and the drag forces,including acting on the frame and nets,acting on the upstream side and the backstream side of the aquaculture platform.Finally,based on the numerical simulation of the internal flow field of the shielding device for the semi-submersible aquaculture platform and the flow resistance test of the farmed fish,an evaluation model of the fish suitability of the aquaculture facility is constructed.To provide a shelter for the farmed fish in aquaculture facilities in rapid current conditions,this paper studies a shielding device of the semi-submersible aquaculture platform and focuses on the effects of three parameters of the shielding device on the flow field,drag force and aquaculture volume,namely the height of the middle plane nets,the inclination angle of the net and the net solidity.Based on the research on the internal flow field of the shielding device,combined with the flow resistance test of farmed fish,the fish suitability evaluation model of the aquaculture facility is constructed,and the optimization scheme of the parameters of the shielding device with the adaptability of the farmed fish as the optimization goal is realized.Quantitative analysis and evaluation are carried out on the effective aquaculture space of farmed fish inside the shielding device.