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LNG

LNG - Liquefied Natural Gas is a gas that is naturally in the earth. Mainly LNG contains Methane, but also contains Ethane, Propane, Butane etc. About 95% of all LNG are transported in pipelines from the gas fields to shore, for example, gas pipes from the oil fields in the North Sea and down to Italy and Spain. Gas carriers transport the remaining 5%. When LNG is transported on gas carriers, the ROB and boil off from the cargo is utilised as fuel for propulsion of the vessel. Cargo cooling plants for large LNG carriers are very large and expensive, and they will use a lot of energy. Small LNG carriers have cargo-cooling plants, and can also be utilised for LPG transportation.

The sea transport of LNG is from the Persian Gulf and Indonesia to Japan, Korea and from the Mediterranean to Northwest Europe and the East Coast of USA and from Alaska to the Far East.

LNG is used for energy purposes and in the petro-chemical industry.

Related Terms

HEEL

The amount of liquid cargo retained in a cargo tank at the end of discharge. It is used to maintain the cargo tanks cooled down during ballast voyages by re-circulating through the sprayers. On LPG ships such cooling down is carried out through the reliquefaction plant and on LNG ships by using the spray pumps.

LNGC

LNG Carrier - an ocean-going ship specially constructed to carry LNG in tanks at 160 C. Current average carrying capacity of LNGs is 125,000 cubic metres. Many LNGCs presently under construction or on order are in the 210,000 215,000 cubic metre range.

LD COMPRESSOR

A compressor which handles the LNG boil-off vapour for fuelling the boiler on an LNG carrier.

LOW DUTY COMPRESSOR

A compressor which handles the LNG boil-off vapour for fuelling the boiler on an LNG carrier.

LNG CARRIER

Liquefied natural gas carrier.
These gas carriers are special as they are designed for loading gas at atmospheric pressure with a temperature down to -163 deg C. Fully refrigerated LNG carriers are either built with independent tanks type B Moss-Rosenberg patent with spherical tanks or French patents that utilises membrane tanks.

Spherical tanks of Moss Rosenberg patent are built in aluminium. French patents with membrane tanks are built either in stainless steel, 9% nickel steel or ferronickel steel that have a 36% nickel content. Common for all these steel types is that they have a thermal expansion coefficient close to 0. These gas carriers are built from 20000 m3 to 125000 m3. The largest LNG carriers are, at all times, contracted on basis of long cargo contracts over about 25 years. This is because these tankers are very expensive to build, and are designed for LNG trade. The LNG tankers compete with gas transportation in pipelines on shore, and the sea transport amount to about 5% of the total LNG transport.

These tankers are special in that the vapour boil off from the cargo is utilised as fuel to the vessels propulsion. For the large LNG tanker, the vapour boil off is between 0,18% to 0,25% of the cargo capacity per 24 hours. It is possible to produce cargo-cooling plants for the large LNG tankers, but to cool 125000 m3 LNG about 6000 kW/h is required. This indicates that this is too expensive, and it is more appropriate to utilise the vapour boil off for propulsion. The smaller LNG tankers, on the other hand, have a cargo cooling plant, and they transport some in LPG/LNG/LEG trade.

LNG carriers have a special procedure for cooling the cargo tanks before loading, which is specified in the tanker's operation manuals and certificates. The tankers are equipped with a spray plant where Methane is pumped into the tank's spray line (perforated lines), which is installed inside the cargo tank. Understandably, one must cool the cargo tanks a considerable amount of degrees to be ready to load. One must never begin to load a cargo tank before there is -136 deg C in the middle of the tank, or by the tank's equator.

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AIRLOCK

A separation area used to maintain adjacent areas at a pressure differential. For example, the airlock to an electric motor room on a gas carrier is used to maintain pressure segregation between a gas-dangerous zone on the open deck and the gas-safe motor room which is pressurized.

CERTIFICATE OF FITNESS

A certificate issued by a flag administration confirming that the structure, equipment, fittings, arrangements and materials used in the construction of a gas carrier are in compliance with the relevant Gas Code. Such certification may be issued on behalf of the administration by an approved classification society.

GAS-FREE CONDITION

Gas-free condition describes the full gas-freeing process carried out in order to achieve a safe atmosphere. It therefore includes two distinct operations: Inerting and Aeration. (Note: — In some gas trades the expression ‘Gas-free’ is used to denote a tank which is just Inerted. Some gas carrier operations can stop at this stage; for example prior to special dry dockings or cargo grade changes).

SUBMERGED PUMP

A type of centrifugal cargo pump commonly installed on gas carriers and in terminals in the bottom of a cargo tank. It comprises a drive motor, impeller and bearings totally submerged by the cargo when the tank contains bulk liquid.

GAS CARRIER

A ship designed for the transport of liquefied gases.
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