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LNG carrier
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An LNG carrier is a ship designed for transporting liquefied natural gas (LNG). As the LNG market grows rapidly, the fleet of LNG carriers continues to experience tremendous growth.
first ship Methane Princess was taken into operation in 1964 and remained in operation until it was scrapped in 1998.
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Encyclopedia
An LNG carrier is a ship designed for transporting liquefied natural gas (LNG). As the LNG market grows rapidly, the fleet of LNG carriers continues to experience tremendous growth.
History
The first ship Methane Princess was taken into operation in 1964 and remained in operation until it was scrapped in 1998. Until the end of 2005 a total of 203 vessels has been built, of which 193 are still in service.
New building
At the moment there is a boom in the fleet, with a total of more than 140 vessels on order at the world's shipyards. Today the majority of the new ships under construction are in the size of 120,000 m³ to 140,000 m³. But there are orders for ships with capacity up to 260,000 m³. As of 29 December 2008, there are 300 LNG ships engaged in the deepsea movement of LNG.
Containment systems
Today there are four containment systems in use for newbuildings. 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 (GT&T).
There is a trend towards the use of the two different membrane types instead of the self supporting storage systems. This is most likely due to the fact that 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 maybe be considered in the future for offshore storage where bad weather will be a significant factor.
When transporting natural gas the gas is cooled down to approximately -163 degrees Celsius where it will condense to a liquid kept at atmospheric pressure. The tanks onboard the LNG carriers function as giant thermoses where the liquid will be kept cold during storage. No insulation is perfect, however, and so the liquid is constantly boiling during the voyage.
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, 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.
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.
Reliquification and Boil Off
The Liquid Methane is carried at around -160 deg C.
Because of the design of the tanks there is very little boil off. (Boil off is the gas being released from the liquid).
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 can be lost.
The Methane 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 reliquification plants to be fitted to vessels, thus allowing the boil off to be reliquified and returned to the tanks. Because of this the vessels' operators and builders have been able to contemplate more efficient Slow Speed Diesel engines. Previously most LNG carriers have been steam turbine powered. Exceptions are the LNG carrier Havfru (built as Venator 1973) originally had dual fuel diesel engine, and the sistership Century (built as Lucian 1974) was built with dual fuel gas turbine before being converted to diesel engine in 1982. Now there are being built LNG-carriers with dual fuel diesel-electric propulsion.
See also
External links
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