Thermal design of LNG cabin on sea transports

The LNG (Liquefied Natural Gas) cabin is a key component in the maritime transport of -163°C LNG. The cryo-insulation performance of cabin directly affects the security of LNG transport greatly. Based on a 10000m 3 LNG ship, two structural schemes of the LNG cabin are certain, which include their materials, supports and parameters. One is two single cylinder with 5000 m 3 , the other is a vertical cylinder with 10000 m 3 . With the safety requirements of cryopreservation of -163°C LNG, it calculates the heat leakage of two schemes. Lastly, the temperature field distributions of two types of LNG cabins with full load are simulated by 3d mathematic models by finite element software. KEYWORD: LNG; cargo tank; heat-insulating layer; thermal design International Conference on Industrial Technology and Management Science (ITMS 2015) © 2015. The authors Published by Atlantis Press 1542 Table. 1 Parameters of 5000m 3 single cylinder cabin Diameter of cabin Length of cabin Volume of cabin Vertical gap Length of ship Width of ship 15m 33.5m 5036.365m 3 5m 120m 20m Nowadays, the cabin is usually made of low temperature aluminum-killed steel, 9% nickel steel, and Cr-Ni austenitic. Since stress of aluminum alloy is the minimum among three materials, it is not suitable for cabin for its large thickness wall. The parameters of the other two materials are shown in Table 2. Table.2 Comparison of two materials Material Allowable stress Minimum design pressure Weight of cabin 9% nickel steel 213Mpa 3.5 Mpa 211T Cr-Ni austenitic 150 Mpa 2.74 Mpa 292T With low hardness, moderate intensity, plasticity, toughness, good mechanical properties, good welding performance, Cr-Ni austenitic is more suitable for LNG cabin. To prevent heat leakage, LNG cabin is arranged on the other surface with 300mm polystyrene insulation materials, through special adhesive adhesion. To protect layer polyethylene foam, 0.5mm galvanized steel sheet is on the most outer layer. In the second scheme, vertical cylindrical cabin structure includes outer cabin and inner cabin. A cylinder cabin with a bottom plate consist the inner cabin, with 9% nickel steel. A vault, cylinder cabin and floor board consist the outer cabin, with carbon steel for it is not contact LNG directly. The structure and parameters of 10000m 3 vertical cylinder cabin are shown in Table 3. Table.3 Parameters of vertical cylinder cabin Item Filler Geometric volume Effective volume Evaporation rate Material Diameter Height Inner tank LNG 11200m 3 10404 m 3 0.046% 9% nickel steel 24m 15 m Outer tank Nitrogen, pearly sane ---Q345R 28 m 17 m Insulation structure of vertical cylinder cabin includes top cabin, bulkhead and bilge. The top cabin is made of low carbon steel, concrete and 9% nickel steel. To prevent thermal deformation, 9% nickel steel and expanded perlites consist of bulkheads with a layer of elastic mat inside. There is a ring bottom around the insulation beam in bilge, for bearing the weight of cabin and LNG, contacting heat transfer and hull. Perlite concrete is used as thermal insulation in vertical cylinder cabin, while foam glass brick is used in bilge heart part. The parameters of insulation materials in vertical cylinder cabin are shown in Table 4. Table.4 Parameters of insulation materials in vertical cylinder cabin Item Vault Ceiling Inner cabin Bottom Material Low carbon steel Concr ete 9% nickel steel Glass fiber 9% nickel steel Expaned perlite Foam glass Sand 9% nickel steel Concret e Thermal concuctivity W (m·K) 11.6 0.70 10.2 0.038 10.2 0.05 0.049 0.269 10.2 0.7 Thickness mm 6.8 350 5 1200 6 1000 430 300 16.7 100 4 SUPPORTING STRUCTURE AND HEAT LEAKAGE Single cylinder cabin is horizontal, supported by two bearings. One of support is mobile, for avoiding gravity by bending and additional stress. The support is a double-layer compression wood filled with epoxy mortar pad for insulation. Assuming environment temperature is 40°C, heat leakage of single cylinder cabin is calculated from formula (1).