Study On The C-Type Independent Tank Bearing Structure Design Of The Small And Medium LNG Carrier

The small and medium LNG carrier is a new concept having emerged in LNG carrier development history. Its structure and equipment are quite different from the large-scale LNG carrier. The design and manufacture of the large-scale LNG carrier was very mature. However, there was little report about the design and manufacture of the small and medium LNG carrier in the other countries except for Mini-LNG Ship Series which was developed and built by the Norway Skaugen corporation. In China, the development of such ship is in its infancy. C-type independent tanks system is generally used in this kind of ship. The structure design of the bearings is very important for the design of cargo tank area. The following issues were solved in this paper.The locations selection and the structure calculation of the bearings. The locations of the bearings are directly related to the stress of the cargo tank. Two basic criterion of the location selection of bearings are given in the paper. The first one is the stress of container at the selected bearing location should be well-distributed and the maximum stress of container shouldn't exceed the material's allowable stress. The second one is the selected bearing location must be on the strong framework of ship. Because there is no special rules about the bearings design, the existed Rules for Classification of Sea-going Steel Ships is researched and referenced for the bearings structure design, and the preliminary structure design plan is given.The FE strength analysis of the bearings structure. While the ship is sailing at the sea, the static and dynamic loads act on the bearings simultaneously. So the structure stress of the bearings will be very complicated. Therefore the stress distribution of the bearings is directly related to the reliability and security of this type of LNG carrier. The type and the function of the C-shape independent tank's bearings of the small and medium LNG carrier are described in detail in this paper. Combining with the Rules for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk of the China Classification Society, the force distribution function of the bearing's bearing-load area is deduced in different working condition including in still water, with heave only, with rolling only, and with heave and rolling simultaneously. The finite element model of the fixed bearings of a 6400m3 LNG carrier is created. The loads required by the rules are imposed on the model with loads superposed method as to the specified working condition. The stress results are obtained and analyzed, and the dangerous working condition and the dangerous position are found. Referring to the stress distribution, structure improvement measures are provided. So this paper is meaningful for structure strength analysis and structure design of the similar bearings.The Low-temperature materials's distribution. Because the container is filled with the low-temperature liquid and was supported by the bearings, there will be temperature gradient on the bearings. Temperature gradient will cause thermal stress and affect the material properties. In order to get the temperature field layout of mini-scale LNG ship hull structure near the bearing of cargo tank, a finite element thermal analysis using ANSYS software was taken. Meanwhile combining with the allowable design temperature of materials, the suitable materials distribution is given, which can avoid the material's low-temperature break. It is considered that the hull structure near the bearings of the cargo tank is surrounded by the low-temperature liquid cargo, seawater and air, and achieves heat balance by convective heat transfer and heat conduction. Some simplified method of FE thermal model and the location of convection load were given, too. An instance of the analysis is given in this paper. This method gives a meaningful attempt for such works about materials distribution.