Dynamic Responses And Fatigue Analysis Of Gravity Cage Floating Structures In Current And Waves

With the development of the society and the increasing of the population around the word, more and more marine products are needed in the market. Due to the coastal bay aquaculture environment has been destructed seriously, resulting in decrease of the fish quality and increase mortality. So growing aquaculture cage will be an inevitable trend and the size of the cage is increasing gradually. However, the sea environment in deepwater is harsh, which leads new challenges to the safety of fish cage and the aquaculture facilities.In our country, the early development and widespread use of the deepwater gravity cage. The floating structure is mainly responsible for the resistance to waves and current, for providing the buoyancy of entire cage and for supporting effective farming volume of nets system. The fish cage will suffer severe dynamic response under the action of hydrodynamic load. The safety and reliability of the floating structure will determine the success of the whole fish cage. Thus, to study dynamic response for the floating structure of the fish cage under the wave and current loads is very important and necessary.In this paper, the numerical model of the floating structure was built based on finite element method, and a series of physical model test was conducted to validate the numerical model. Then the study of deformation characteristics and the stress distribution of the cage floating structure under the wave and current action, and the fatigue analysis of the floating structure.The paper includes the following sections.The first chapter is a preface, including the meaning and background of the study and main types of open ocean cages. Then the progress of in the fish cage study and content of the paper is also given.In the second chapter, introduce the finite element analysis method and the analysis process of ANSYS. And the basic theory of finite element dynamic response is introduced and the solving method is given. Then discuss the inertia force coefficient and drag coefficient in the Morison equation. The SHELL element in ANSYS is used to simulate the floating collar and the numerical model of the floating structure is established. Finally the physical model test on stretching of ring specimen was conducted to validate the deformation.In the third chapter, the numerical simulation is performed to analyze the dynamic response for the floating structure under the action of current. The deformation and stress characteristics of the floating structure under different flow direction and flow velocity were investigated. Also the deformation of the pipe which supported the metal net under static load is given.In the forth chapter, the floating structure exposed to wave is investigated by numerical model, and the deformation response and stress characteristics of floating structure under combined wave and current are analyzed. At the same time, the fatigue behavior of the floating structure under random wave is analyzed. In addition, the fatigue damage and fatigue life of the floating structure under the different wave periods and height is also analyzed by using the rainflow counting method, the S-N curve and the Miner theory.At the end of this paper, the achievements are summarized and the prospects of open ocean aquaculture are also given.