LNG Containment

Today there are four containment systems in use for new build vessels. Two of the designs are of the self supporting type, while the other two are of the membrane type and today the patents are owned by Gaz Transport & Technigaz (GTT).

There is a trend towards the use of the two different membrane types instead of the self supporting storage systems. This is most likely because prismatic membrane tanks utilize the hull shape more efficiently and thus have less void space between the cargo-tanks and ballast tanks. As a result of this, Moss-type design compared to a membrane design of equal capacity will be far more expensive to transit the Suez Canal. However, self-supporting tanks are more robust and have greater resistance to sloshing forces, and will possibly be considered in the future for offshore storage where bad weather will be a significant factor.

Moss tanks
This design is owned by the Norwegian company Moss Maritime and it is a spherical tank.

IHI
Ishikawajima-Harima Heavy Industries has developed the SPB, or Self supporting Prismatic type B tank. This tank type is very similar to the ones used on the first ship, MV Methane Princess. Only two vessels currently have the SPB containment system.

TGZ Mark III
This design is originally by Technigaz and it is of the membrane type. The membrane consists of stainless steel with 'waffles' to absorb the thermal contraction when the tank is cooled down. The primary barrier, made of corrugated stainless steel of about 1.2 mm thickness is the one in direct contact with the cargo liquid (or vapour in empty tank condition). This is followed by a primary insulation which in turn is covered by a secondary barrier made of a material called "triplex" which is basically a metal foil sandwiched between glasswool sheets and compressed together. This is again covered by a secondary insulation which in turn is supported by the ship's hull structure from the outside. So, going from the inside of the tank outwards, we have :

LNG
Primary barrier of 1.2 mm thick corrugated/waffled Stainless Steel
Primary Insulation (also called the interbarrier space)
Secondary barrier of triplex membrane
Secondary Insulation (also called the insulation space)
Ship's hull structure.

GT96
This is Gaz Transport's tank design. The tanks consists of a primary and secondary thin membrane made of the material Invar which has almost no thermal contraction. The insulation is made out of plywood boxes filled with perlite and continuously flushed with nitrogen gas. The integrity of both membranes is permanently monitored by detection of hydrocarbon in the nitrogen. An evolution is proposed by NG2, with the replacement of nitrogen by argon as the flushed inert and insulation gas. Argon has a better insulation power as nitrogen, which could save 10% of boil-off gas.

CS1
This Combined System NUMBER ONE is well described in this document: http://www.witherbyseamanship.com/pages/product/product.asp
However, the success of this CS1 has been very short in time, since now it appears to be a problematic design.

Reliquefaction and Boil Off
In order to facilitate transport, natural gas is cooled down to approximately -163 degrees Celsius at atmospheric pressure, at which point the gas condenses to a liquid. The tanks on-board an LNG carrier effectively function as giant thermoses to keep the liquid gas cold during storage. No insulation is perfect, however, and so the liquid is constantly boiling during the voyage.

According to WGI, on a typical voyage an estimated 0.1% - 0.25% of the cargo converts to gas each day, depending on the efficiency of the insulation and the roughness of the voyage. In a typical 20-day voyage, anywhere from 2% - 6% of the total volume of LNG originally loaded may be lost.

The gas produced in boil off is traditionally diverted to the engines and used as a fuel for the vessel. This can be 100% gas or a percentage gas and oil firing. Recent advances in technology have allowed reliquefication plants to be fitted to vessels, allowing the boil off to be reliquefied and returned to the tanks. Because of this the vessels' operators and builders have been able to contemplate the use of more efficient Slow-Speed Diesel engines (previously most LNG carriers have been steam turbine-powered). Exceptions are the LNG carrier Havfru (built as Venator in 1973), which originally had dual fuel diesel engines, and its sister-ship Century (built as Lucian in 1974), also built with dual fuel gas turbines before being converted to a diesel engine system in 1982. Vessels using dual or tri-fuel diesel electric propulsion systems are now in service.