LNG carrier
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An LNG carrier is a ship designed for transporting liquefied natural gas. As the LNG market is growing rapidly in the present decade, the fleet of LNG carriers is also growing rapidly.
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[edit] History
In 1914, Godfrey Cabot patented a barge to carry liquid gas, demonstrating that waterborne transportation was technically feasible. It was not until 1959, however that the Methane Pioneer, a converted cargo ship, was used to carry LNG between Lake Charles, Louisiana and the UK [1].
The first purpose-built ship, called the Methane Princess, went into operation in 1964 and remained in operation until 1998 when it was scrapped. To the end of 2005 a total of 203 vessels had been built and only 10 of them had yet been scrapped.
Mid decade, there is a boom in the size of the LNG fleet. The Gas Carrier Register indicates that there were more than 140 vessels on order at the world's ship yards in late 2005. Today the majority of the new vessels are in the size range of 120,000 m3 to 140,000 m3, but there are orders for ships with capacities of up to 200,000 m3.
[edit] Containment systems
In order to transport natural gas, it is cooled to approximately -163 degrees Celsius where it condenses to a liquid at atmospheric pressure shrinking to approximately 1/600 of its original volume with a density of 420 to 490 kg/m3. The tanks onboard LNG carriers function, in effect, as big thermos containers wherein the liquid remains boiling for the duration of voyage. Some gas is removed to prevent a gradual buildup in pressure; this is known as Boil Off Gas (BOG). The latent heat of vapourization required to turn a small amount of LNG from a liquid to a gas is what keeps the remaining liquid cooled.
Recently, designs have been developed for pressurized transport systems as well, to be called pressurized natural gas (PNG) carriers, although none have yet been constructed [2].
At present, there are four containment systems in use for new ships. Two of the designs are of the self-supporting type. The other two are of the membrane type which are patented designs owned by Gaz Transport and Technigaz (GT&T). The trend is toward the membrane instead of the self-supporting types, most likely due to lower construction costs.
[edit] Moss tanks
This design is owned by the Norwegian company Moss Maritime and it is a spherical aluminum tank. It was developed in 1971 by Kvaerner. This is a self supporting type.
[edit] IHI prismatic
Ishikawajima-Harima Heavy Industries has developed a self supporting tank type. This tank type is very similar to the ones used on the first ship, Methane Princess. The tank is made of aluminum.
[edit] TGZ Mark III
This design was developed by Technigaz and it is of the membrane type. The first membrane consists of stainless steel with 'waffles' to absorb the thermal contraction when the tank is cooled down. Then below that there is a plywood skin which has anchor strips fixed to it and the primary membrane is welded to this. Below the plywood there is expanded foam insulation which then is stuck to the secondary membrane which in known as Triplex (Two layers of Fiberglass with a layer of tinfoil in the middle. Then below the triplex is another layer of insulation which is boned to the final layer of plywood. This plywood is then bonded to the ships inner hull.
The space between the primary and secondary membranes is called the interbarrier space and is kept filled with nitrogen at low pressure. Then the space between the secondary and the inner hull is called the insulation space and this is also kept nitrogen filled at a slightly higher pressure then the interbarrier space.
While loading these spaces cooldown and the nitrogen contracts meaning that more nitrogen must be added to maintain pressure. Once the ship has discharged and the tank begins to warm up the nitrogen expands and the pressure increases meaning that the nitrogen must be vented.
Both the interbarrier and insulation space are constantly monitored for both hydrocarbon levels and pressure of N2 to maintain a safe and healthy tank
[edit] 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 constructed of plywood boxes filled with Perlite, a lightweight insulating material.
[edit] Propulsion
LNG carriers are unique in that the large majority of them are propelled by steam turbines, with new ships still being built with this propulsion method. This is because the simplest way of handling the boil off gas (BOG) is to burn it in the ships' boilers, creating enough steam to propel the ship when supplemented with additional gas from the cargo tanks. Diesel engines have largely replaced steam turbines in all other ship types, but until recently diesel engines adapted to run on BOG have not been widely utilised, even though the technology has been around since the early 1980's. However, the rapid expansion of the LNG fleet has meant that in the first decade of the 21st century there is a shortage of sea going personnel qualified to operate steam turbine ships. High prices for LNG are also driving the quest to maximise the yield from the transported cargo. Modified diesel engines burn less gas than steam turbines due to greater fuel efficiency. Combined cycle systems have also been implemented, with COGAS (COmbined Gas And Steam) electric propulsion arrangements having thermal efficiencies close to or greater than diesel engine systems. In this arrangement, the gas is burnt in a gas turbine and the waste heat from the gas turbine used to generate steam to run a supplementary steam turbine. However, recent developments have enabled the boil off gas to be re-liquified and returned to the cargo tanks, allowing conventional diesel engine propulsion systems to be utilised. All this has meant that coming into the 21st century the last refuge for steam ships could eventually disappear.
[edit] See also
[edit] External links
- Marine Transportation of LNG (Pdf)
- Society of International Gas Tankers and Terminal Operators
- Transportation of LNG in Small LNG carriers
[edit] Sources
- The Gas Carrier Register
- Hamworthy LNG Reliquefication website viewed 30th June 2006
