The Study Of Fluid-structure Interaction Of The Flotation Ring Of A Gravity-type Fish Cage

The demands of fishery products are growing up with the increasing world population. And the people frequently enjoy themselves in the offshore.Then the development of the offshore aquaculture is faced with stagnation.It is unallowable that the offshore farming brings serious pollution.Therefore the aquaculture must be put into the open sea because of high quality of water,few recreations and high efficiency for the draff in the open ocean. Open ocean aquaculture has been put in schedule,and open ocean fish cage is becoming an important technique.The flotation ring is a primary loading-bearing component of a gravity-type fish cage,which supports the whole fish cage.Its safety is closely related to the feasibility of fish cage.And large deformations of a flotation ring have been observed in the field scale.Therefore it is necessary to study the hydroelasticity of the flotation ring.The aim of the dissertation is to analyze the elastic deformation of a circular ring subjected to water waves,coupled with its rigid-body motions.There are seven parts in the dissertation.Firstly,the author introduces the background and development of the deep-water fish cage.Secondly,the numerical model of a flotation ring is proposed,and the equations governing its six degrees of freedom motions are built up according to the theory of rigid body kinemics.In the third chapter the deformation equations of the ring are given by virtue of the curved beam theory.The in-plane and out-of-plane deformations of the ring are represented according to the modal superposition method,and they are applied to the governing equations.Then it becomes easy to solve the governing equations.For the cross-sectional diameter of the ring is much less than wave length,the wave forces are presented by Morison fomula.Because the acceleration of the ring is included in Morison's formula,the deformation equations of the ring are coupled with its motion equations.Then it is necessary to validate the above governing equations before they are used to solve the hydroelasticity of the ring.In the fifth chapter the good correlation of the maximum positive displacements for the surge and heave of the ring is obtained through the comparison between the predicted results and model experimental data.The shape of the out-of-plane and in-plane deformed ring predicted from the numerical method agrees well with the results from the commercial software ANSYS.So the numerical method is feasible to analyze the elastic deformations of the ring subjected to water waves.And then some discussions of hydroelasticity of the ring are carried out,including the effect of the wave direction,the configuration of mooring cables and the dimension of the ring.Also the responses of the deformations and motions of the ring with wave period are considered.For the further realization the deformations,motions and mooring cable forces are analyzes under the poor wave conditions.Finally the conclusion is given,and prospects of open ocean fish cage are discussed.