Research Of Ultimate Longitudinal Capacity Of LNG Carrier

With the globalization of world economy, Trade and Communication among countries in this world become more and more frequent, about 70% of which are experienced by means of shipping which play a very important role in general transportation. Large ships carrying energy resource draw more attention in varied kinds of ship. LNG Carrier to be researched in this thesis belongs to this category. With the increasing demand of energy the development of LNG Carrier is towards large-scale. But something is wrong with the development procedure of LNG Carrier, the main reason is that accidents are more likely to happen is their operation with the development of LNG Carrier towards large-scale and if accidents happened the ocean pollution and compensation may is much higher than the value of the vessel. So it is very interesting for the safety of shipping to perform the ultimate capacity research of LNG Carrier. Ultimate Capacity is a very important aspect of LNG Carrier's strength. The traditional criteria for safety design is based on safety factor method characterized by minimum sectional modulus satisfying ultimate capacity, meanwhile checking very local component strength. These criteria are all based on the linear elasticity theory, so we cannot really assess the collapse capacity of ship hull, then it is hard to decide the true safety of ship structure. So it is essential to research the ultimate capacity of LNG Carrier and perform analysis of parameter on it. It helps to figure out the real safety margin, rationally and fully utilize the material, lighten the dead weight of ship, reduce the cost and increase the load capacity of ship. Now, There are main four kinds of method to calculate the longitudinal strength of ship hull-empirical formulas, nonlinear finite element method, idealized structural unit method and simplified Smith method. Based on the analysis of the characteristics, weakness and strength of very kinds of methods, we incline to choose simplified Smith method when performing calculation of the longitudinal strength of a 138000m3 LNG.. Simplified Smith method is a relative simple and effective method for the calculation of longitudinal strength of ship hull. The main idea of this method is to divide the whole section into many little structural units and the capacity of whole section is equal to the summation of the one of individual structural unit which function individually. The characteristics of these structure units can be derived by means of theoretical analysis, analytical derivation, finite element analysis and practical experiments. In the procedure of analysis, we employ the incremental load method. In any level of load, we obtain the capacity of individual structural unit and then integrate the multiply of unit's capacity and its distance to instantaneous neutral axis of loading. Iterative continue till a maximum bending moment is reached. To predict the post-ultimate capacity of ship hull, the iterative procedure can continue. So we can obtain not only the ultimate capacity of ship hull but also its post-ultimate capacity behavior. HULLST and ULS both are computer program system for calculation and analysis of large ship hull's ultimate capacity based on the simplified smith method. The former was developed by Yao who is the professor in Oscar National University in the last century of 90, and latter is a general user interface based calculation software developed by the author aiming to adopt the derivation of structural units based upon non linear FEM and solve the inconvenience of use of HULLST and inapplicability to practical engineering using object-oriented C++ programming under Windows OS environment. Then the ultimate capacity of 138000m3LNG Carrier is calculated and analyzed using ULS and HULLST,many useful data and graph is obtained. Furthermore, compare the solution of both programs and draw a conclusion that the ULS is accurate, effective and easy-to-use. Finally a LNG Carrier structural design method based on ultimate state is presented Finally, the thesis perform the analysis of influence of yielding stress of material and residual stress induced by the welding on the LNG Carrier's ultimate capacity. It is seen that the influence of these two factors on ultimate strength of LNG Carrier is obvious, but it's ultimate capacity does not increase with the increase of yielding of material in a linearrate. The increment of ultimate capacity lags behind the increment of yielding stress of material。Residual stress induced by the welding make a small influence to the LNG Carrier's ultimate capacity. The influence to LNG Carrier's ultimate capacity under hogging is larger than the one to it's ultimate capacity under sagging. So it is suggested that residual stress induced should be diminished in the fabrication of LNG Carrier.