The Analysis On The Longitudinal Ultimate Carrying Capacity Of Membrance LNG Carrier

The LNG (Liquefied Nature Gas) Carrier is a special kind of ship to transport nature gas under one hundred and sixty-three Centigrade degrees below zero. It has a very important status in the global economy. The LNG Carrier has a difficult technical problem, That is how to make it sail safely under the so low temperature and avoid the structure having brittle failure. This requests the carrier must have enough structural strength. It has big possibility that the carrier is destroyed because of not having enough longitudinal structural strength. So it is very important to evaluate the residual safety and research the longitudinal ultimate carrying capacity of LNG Carrier.The ultimate bending moment is a very important factor of Ship's longitudinal ultimate carrying capacity. By taking account the failure mode of yielding, buckling and their combination, as well as the non-liner interactions between structural members, the ultimate bending moment is defined by the moment corresponding to the point with zero slope on the moment-curvature curve of the ship hull. The definition has considered as the most reasonable so far. The curve will be used in this paper.The nonlinear FEM was applied to calculate the longitudinal ultimate strength with the ANSYS software. In this paper,two method are introduced: the increment method to load four points pure bending moment and Multi-Point Constraint. Use the two methods to calculate the longitudinal ultimate strength of the Nishihara's box girder. In the course, the method to choose load, gridding density and the size of load step are discussed. The results show good agreement with the test, the two methods are proved. Then, the methods are applied to calculate the cabins of the Membrance LNG Carrier. The moment-curvature curve is drawn under the sagging and hogging condition and the destructive process is described. The influences of the main parameters on the longitudinal ultimate strength are studied, including initial deformation, welding residual stress, thickness of plates and yielding stress.After the research, it tells us that the nonlinear FEM is a tried method to calculate the longitudinal ultimate carrying capacity by taking account the non-liner interactions. When the longitudinal ultimate carrying capacity is used in the structural design, the residual safety between the initial and ultimate bending moment must be considered. The influence is small of the initial deformation and the welding residual stress on the longitudinal ultimate strength, but the influences to the plasticity can not be ignored.