The IGC rules and CCS’s Rules for Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (hereinafter referred to as the Bulk Liquid Rules) require the gas carriers to be constructed with protection of double hulls and double bottoms. Special liquid cargo tanks shall be arranged, with a cargo containment system, including the primary panel and the secondary panel, the ancillary insulation and space between panels, and the adjacent structures to support these members when necessary. Five cargo containment systems are defined for LNG carriers, among which the GTT membrane type and Moss spherical tank type are most common.

1. Conch cargo containment system (A-type independent liquid cargo tank)

"Methane Princess" and its sister ship "Methane Progress", built in 1964, are the first batch of dedicated LNG carriers. They are equipped with 5083 aluminum alloy prismatic independent liquid cargo tanks. The tanks are covered with the insulation materials, such as Plywood and Balsa Wood, which are called Conch cargo containment system. Such liquid cargo tanks are classified into the self-supported A-type independent liquid cargo tanks defined by IGC rules. Conch has high reliability, and "Methane Princess" has been in operation for more than 30 years and was not retired until 1997. In recent years, Norwegian LNG New Technologies (LNT) has also invented a brand-new A-type independent liquid cargo tank LNTA-BOX？, which has been applied to small and medium-sized LNG carriers.

Figure 1 Five Kinds of Cargo Containment Systems

Figure 2 Insulation of A-type Independent Liquid Containment System

Figure 3 A-type Independent Liquid Cargo Tank LNTA-BOX？

2. GTT membrane cargo containment system

At the end of 1960s, French Gas Transport Company (GT) and French Gas Technology Company (TGZ) developed GT NO and TGZ Mark membrane LNG cargo containment systems respectively, which are the predecessors of GTT NO96 and Mark- III membrane tanks.

GTT NO96 is a non-self-supported liquid cargo tank system, of which the main and secondary panels are welded with invar alloy membrane with 36% nickel composition. The thinnest lock part is only 0.5mm thick, and the column plate is 0.7mm thick . Invar alloy has extremely low thermal expansion coefficient and low thermal conductivity, capable of keeping a fixed size in a large temperature range. During operation, the invar membrane is tightly attached to the plywood insulation box, and the force exerted by liquid cargo on the membrane is directly transmitted to the hull structure of the cargo tank through the insulation box. The insulation box has some strength, and the interior is filled with perlite or glass wool insulation material for insulation. The latest one adopts polyurethane foam board for insulation, further reducing the evaporation rate, and the system is adopted on large LNG carriers manufactured in China.

Figure 4 N096 Super + Containment System

Figure 5 GTT NO96

GTT Mark-III is also a non-self-supported liquid cargo tank system. The primary panel is made of 1.2mm thick 304L stainless steel membrane with grooved ribs. The grooved ribs in longitudinal and transverse directions can absorb the deformation of stainless steel membrane caused by expansion and  contraction. The  secondary panel  is  a three-in-one composite material bonded with two layers of glass fiber cloth and one layer of aluminum foil, and the insulating material is reinforced polyurethane foam .

GTT membrane-type LNG carrier has the advantages such as small main dimension, high utilization rate of tank capacity, flat deck, small wind area and small blind area of bridge view under the same cargo capacity, but it also has loading restriction . That is, the liquid level must be lower than 10% or higher than 70% to reduce the impact caused by liquid cargo sloshing and ensure the stability of the whole ship. On the basis of mature design of NO96 and Mark III, GTT has continuously developed better and more efficient membrane tank types,  such  as NO96L03/L03 +, N096 Super+, MaxN096, Max, Mark-III Flex, Mark V and GTT NEXT1. The latest generation of LNG carrier built by Hudong-Zhonghua Shipyard has adopted N096 Super+ membrane tank, while that by Dalian Shipbuilding Industry Co. , Ltd. and Jiangnan Shipyard (Group) Co ., Ltd. have adopted Mark-III Flex membrane tank .

Figure 6 Mark-III Containment System

Figure 7 GTT Mark-III Type

ensure the stability of the whole ship. On the basis of mature design of NO96 and Mark III, GTT has continuously developed better and more efficient membrane tank types,  such  as NO96L03/L03 +, N096 Super+, MaxN096, Max, Mark-III Flex, Mark V and GTT NEXT1. The latest generation of LNG carrier built by Hudong-Zhonghua Shipyard has adopted N096 Super+ membrane tank, while that by Dalian Shipbuilding Industry Co. , Ltd. and Jiangnan Shipyard (Group) Co ., Ltd. have adopted Mark-III Flex membrane tank .

3. Moss spherical tank cargo containment system (B-type independent liquid cargo tank)

In  the  early  1970s,  Moss  Rosenburg  from  Norway developed the Moss spherical tank cargo containment system . The liquid cargo tank is a spherical tank, which is independent of the hull and can support the cargo load by itself. Such tank belongs to the self-sustaining B-type independent liquid cargo tank defined by IGC rules, and it is equipped with drip tray as part of the secondary panel. The tank body is made of 9% nickel alloy steel or 5083 aluminum alloy, and supported by a vertical cylindrical skirt structure outside the equatorial ring . The connection between the skirt structure and the equatorial ring of the spherical tank is welded by connectors with special cross sections, which can absorb the deformation or displacement caused by expansion and contraction or  shaking  of the  hull and  tank  body . The outside of the spherical tank is covered with insulating materials such as polyester ammonia foam, phenolic foam or polystyrene foam .

Figure 8 Moss Rosenburg

Figure 9 Schematic Diagram of IHI-SPB Liquid Tank

Moss spherical tank can be built  separately and  finally  assembled to  the  hull  structure . Hence, the construction period  is  shorter  than that  of  membrane - type LNG carrier, and the tank is easy to be inspected and maintained. At the same time, the spherical design eliminates the influence of sloshing and can load liquid cargo at any liquid level. Mitsubishi Heavy Industries of Japan has made overall optimization on the basis of traditional Moss, and developed a new generation of more efficient Moss LNG carrier, which has been put into use .

Figure 10 Lifting of Liquid Cargo Tank

Figure 11 KC-1 Membrane Cargo Containment System

4. IHI SPB Prismatic cargo containment system (B-type independent liquid cargo tank)

In the early 1990s, Ishikawajima-Harima Heavy Industries of Japan successfully developed SPB independent liquid cargo tank. It can be known from its name that the tank belongs to SPB self-sustaining B-type independent liquid cargo tank defined by IGC rules, with prismatic liquid tank. The liquid tank is made of 5083 aluminum alloy, and insulated by polyurethane foam. It has the advantages such as high space utilization rate, no sloshing influence, good visual field of cab, and easy inspection and repair. The development of IHI SPB made Japan the first country outside Europe to have a patent for LNG cargo containment system.

5. Cargo containment system of C-type independent liquid cargo tank

According to the IGC rules, C-type independent liquid cargo tank is pressure-resistant and self-supported, meeting the pressure vessel standard. The tank body is made of low-temperature resistant materials, generally into the cylindrical horizontal tank or spherical tank with uniform shape and almost no internal structure, which is covered with the insulating materials. Compared with other cargo containment systems, it is less difficult to build and is widely used on the small and medium-sized LNG carriers. The first domestic 30,000m3 LNG carrier delivered by Jiangnan Shipyard in 2015 used C-type independent liquid cargo tank (German TGE cargo containment system), which is also the largest C-type LNG carrier in the world.

Figure 12 Composition of KC-1 Membrane Cargo Containment System

6. KC-1 membrane cargo containment system of South Korea

As a major country in the construction of LNG carriers, South Korea always hopes to have its own patented cargo containment system. After years of research and development, KOREA GAS CORPORATION (KOGAS), Daewoo Shipbuilding & Marine Engineering (DSME), Hyundai Heavy Industries (HHI) and Samsung Heavy Industries (SHI) jointly developed South Korean KC-1 membrane tank in 2014, which has been used on two LNG carriers. The cargo containment system is developed based on the onshore LNG storage tank technology, which minimizes the dimensions of the hull and the primary panel. The primary and secondary panels are made of 1.5mm thick 304L stainless steel with grooved ribs, and polyurethane foam is used for insulation. Its structure is simpler than GTT membrane design, which effectively reduces the cost. Obviously, in order to ensure the safety of LNG transportation, any newly-designed low-temperature containment system must keep improving and pass the test of time, so as to win the trust of the market.