Liquefied Natural Gas

Liquefied natural gas is natural gas that has been purified and refrigerated until it condenses into a clear, colourless liquid at around minus 162 degrees Celsius at atmospheric pressure. The point of liquefaction is volume: as a liquid the gas takes up about 1/600th of the space it occupies as a vapour, which is what makes it practical to store and transport large quantities over long distances where a pipeline is impossible or uneconomic. LNG is produced at a liquefaction plant, shipped by sea, and returned to gas at a regasification terminal near the market.

LNG is carried by purpose-built LNG carriers whose cargo containment systems keep the liquid cold and contain it safely. The two dominant designs are membrane tanks, which line the hull with a thin metal membrane backed by insulation, and the spherical Moss-type tanks that give some carriers their distinctive rounded profile. However well insulated, the tanks cannot be kept perfectly cold, so a small amount of cargo continually evaporates as 'boil-off' gas. Rather than waste it, modern ships burn this boil-off as fuel in dual-fuel engines, which is one reason LNG carriers are among the most fuel-efficient large ships afloat.

Beyond being a cargo, LNG has become an important marine fuel in its own right. Burning it produces far less sulphur oxide and particulate matter than conventional heavy fuel oil and lower nitrogen oxides and carbon dioxide, so a growing number of ships across many segments are being built or converted to run on LNG to meet tightening emissions rules. Handling LNG demands strict safety precautions because of its extreme cold, which can embrittle steel and injure people, and because its vapour is flammable, so training, gas detection and cargo-handling procedures are rigorous.