| LNG ship independent tanks have gradually become a mainstream cargo tank.The tank is one of the important structures of the LNG ship,and meeting the fatigue strength is the basic requirement.Fatigue calculation methods based on fracture mechanics have relatively comprehensive considerations,and the fatigue life assessment of structures is more realistic,and the forecast results are easy to test.This method is also widely used.In this paper,a B type independent tank of LNG ship is taken as the research object,which mainly includes the following three aspects: The series calculation of stress intensity factor of Surface Cracks at weld toe of longitudinal stiffener joints and the calculation formula of the corresponding stress intensity factor are obtained;The detailed calculation process of the fatigue life calculation method based on fracture mechanics is carried out and the fatigue life prediction of key joint in the independent tank of LNG ship is carried out.The main factors affecting the fatigue life of B type tank structure are analyzed.The main work of this paper is as follows:(1)Introduce the research background and significance of this topic.The research status of LNG ship,stress intensity factor and fatigue load spectrum is summarized.(2)Introduce the basic principle of fatigue life prediction based on crack propagation,including the calculation of stress intensity factor of crack;fatigue crack growth rate model;the solution process of fatigue load in LNG ship calculation process and the principle and method of failure criterion;and a fatigue life calculation flow chart is given.(3)Analyze the structural form of the longitudinal stiffener joints of the tank in detail,and the finite element models of the joint with different surface cracks were established.The calculation results show that the specific shape of the weld toe and the web size have a larger effect on the calculation of the stress intensity factor.A series of finite element models are established for these two factors and the corresponding stress intensity factors are calculated.Based on the results analysis,new empirical formulas have been proposed and the accuracy is verified.(4)The fatigue load calculation of the key joint in the B type independent tank of the LNG ship is analyzed.According to the DNV specification,the fatigue load and specific calculation formulas that the LNG ship needs to consider the actual calculation are given.The load conditions and the construction method of the fatigue load spectrum that the ship needs to consider during operation are analyzed.(5)The load response analysis of a B type tank of a LNG ship is carried out,the fatigue hotspot was determined and the fatigue life of the key joint of the tank was predicted.(6)Analyze the main factors affecting the life of the structure.The effect of the fatigue load spectrum of different working conditions on the fatigue life is discussed in detail.The effect of the specific shape and size of the initial defect on the life is analyzed.The calculation results of the two crack propagation models are compared.Finally,summary and prospect of this paper have been given. |
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