Research On Fatigue Characteristic Of Membrance Tank LNG Ship Based On Simplified Procedures

Membrance tank LNG ship was one kind of the most advanced ship in the world.The LNG which the ship carried was preserved in low temperature circumstance. For the use of LNG ship and its special structure form , it was difficult to inspect LNG ship in the woking course . For this reason , there was extremely high requirement of its structure reliability in the course of design and construction. The structure objection which was allowed to occur was far lower than general ship. Its structure fatigue problem required more settlement than general ship .For the forty-years-long life of membrance tank LNG ship, the fatigue strength was especially important, compared with the general ship's life of twenty to twenty-five years .For this reason, it was especially necessary to analyze the fatigue strength of membrance tank LNG ship.At present, the methods of fatigue strength assessment which was applied by most ship classification society could be classed into two kinds, which was the simplified procedures and the directness calculation. In the simplified procedures , the calculation of fatigue load and stress was accomplished according to the formulas of rules. In the other way , the load could be caculated according to the formulas and the stress was got by FEA; In the directness calculation, the load was got by wave load software and the stress was got by FEA. Subsequently, the calculation of fatigue accumulated damage was completed by adopting the procedures of spectral analysis or design wave approach.The paper adopted the simplified procedures to calculate fatigue damage of the stiffen nodes of a 138000m3 membrance tank LNG ship .The wave moment, wave pressure , liquid cargo pressure were got by calculation according to experience formulas of rules . Applied the load to the three-dimensional FEA model which consisted of three cabins of middle ship and analyzed to get the general strength and stress distribution. According to the result of the FEA analysis, the stress range was caculated and its distributing of the whole life was assumed as Weibull distribution. The papers selected certain S-N curves according to the node form , caculated the fatigue cumulating damage of longitudinal stiffeners which were on a certain representative section plane, considering all kinds of component stress. From the results of analysis, the paper could draw some useful conclusions as follows. The fatigue cumulating damages of longitudinal stiffener nodes on upper deck and bilge boak were comparative in numerical value, which were larger than the fatigue cumulating damages of longitudinal stiffener nodes in shipboard. The shipboard of membrance tank LNG ship had good antifatigue performance. The calculation results of the paper could provide reference for future fatigue strength analysis and calculation. In the time of design , could refer to the results of fatigue strength analysis to improve the design of structure nodes and also ensured enough fatigue life of some hull structures which were effected by cyclic loading had always been an important concern in the operation of ships.