Fatigue Crack Growth Life And Reliability Prediction Of New Type LNG Tank

LNG ship is an important tool for liquefied natural gas transportation.Fatigue is one of the most significant failure modes of ship structures.Independent type B tank is the main liquid cargo maintenance system of LNG ship,and the reliability of which is a direct impact on the personal and property safety of the ship.The traditional fatigue analysis method is based on S-N curve and linear cumulative damage theory and has been recommended by major classification societies and widely used in engineering practice.However,the traditional fatigue analysis method doesn't take the effects of initial defects and load sequence into consideration.As for independent type B tank,the yield strength of high strength steel plates used in constructing is exceeded 400 MPa which is the maximum yield strength permitted by the fatigue rules of classification societies.As the consequence,the fatigue assessment results based on the conventional fatigue analysis method may be unreliable.As a more reasonable theory,a fatigue assessment method based on crack propagation has attracted increasing attention of researchers,and a number of classification societies have proposed primary rules based on fracture mechanics.In this paper,the crack propagation life prection of the LNG tank and its reliability analysis based on neural network combined with Monte Carlo methods have been done.The proposed procedure could provide references for the structural design and optimization of the tank.The main contents and conclusions of this article are as follows:(1)The background of this study and the references reviewing of LNG ship fatigue assessment and fatigue reliability were firstly made.As for the key issues such as crack propagation rate model,the fracture parameters and failure assessment,two crack propagation models which are commonly used in engineering were introduced,concludes the stress intensity factor empirical formula of some typical structures,then because of the current empirical formulas are only suitable for type I stress intensity factor,this paper calculate the type II and type III stress intensity factor of three dimensional surface crack with finite element analysis.Finally,this paper makes a detailed introduction of failure assessment diagram technique.(2)A deeply study of DNV rules about independent type B tank was carried out.A finite element model of target tank was built.According to the DNV rules,this paper concludes the fatigue load cases and calculates the fatigue load,then builds a simplified fatigue load spectrum.Finally,calculate a typical welded joint's fatigue life with Paris crack growth formula.The fatigue life prediction results meet the design requirements.(3)Introducing several reliability analysis methods,and trying to combine crack propagation life calculation with reliability analysis.Proposing a fatigue reliability calculate method which is based on neural network and Monte Carlo method.Then taking a typical welded joint of the tank as research subjects,verify the prediction accuracy of neural network firstly,then based on this neural network,calculate the fatigue failure probability.Finally,verify the accuracy of this method by compared with other reliability calculation method.