Page 67 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

5.2 LNG

LNG is a gas transport product. The gas, primarily methane (CH 4), is
converted to liquid form for ease of storage or transport. It is produced close
to the production facilities in a LNG liquefaction pant, stored, transported in
cryogenic tanks on an LNG carrier and delivered to an LNG regasification
terminal for storage and delivery to a pipeline system.

Melkøya LNG Plant with LNG Carrier Arctic Princess Photo: StatoilHydro

LNG carriers are used when the transport distance does not justify the cost
of a pipeline. The main drawback is the cost of the liquefaction, calculated as
how much of the total energy content of the gas is used for liquefaction.
About 6% of energy content is used to produce LNG in a large modern plant,
due to overall thermal efficiency. More than 10% could be consumed with
smaller, gas turbine-driven trains. This compares to losses of about 0, 6-1,
and 0 % per 1000 km of transport distance for large pipeline systems.

The LNG feedstock comes from a gas plant as outlined above. It must
satisfy sales gas specifications. Ethane, propane and butane all have
freezing points of less than -180°C and can be part of the LNG, but the
concentration of methane is generally above 90%. Some NGLs are also
needed as refrigerant for the cryogenic process.

5.2.1 LNG liquefaction
LNG processes are generally patented by large engineering, oil and gas
companies, but are generally based on a two or three stage cooling process.
A three stage liquefaction plant is shown in this simplified figure:

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