Moving LNG by sea

An LNG carrier is a ship built around insulated cargo tanks that hold liquefied gas at minus 162 degrees, with an engine able to burn the cargo vapour that inevitably boils off during the voyage.

Cargo containment is the system that holds LNG at minus 162 degrees inside a ship, and it comes in two families: membranes, which are thin barriers supported by the hull, and free-standing tanks, which carry their own load.

A free-standing LNG tank is one that supports its own weight and cargo loads rather than transferring them to the ship's hull, the two commercial designs being the Moss spherical tank and IHI's prismatic SPB tank.

Boil-off is the small fraction of an LNG cargo that evaporates each day as heat leaks into the tanks, and it is what keeps the rest of the cargo cold.

LNG carriers fall into recognised size classes running from small-scale vessels under 30,000 cubic metres, through the 174,000 cubic metre conventional standard, to the 266,000 cubic metre Q-Max ships built for Qatar.

An ice-class LNG carrier is a vessel strengthened to operate independently in sea ice, the highest commercial grade being Arc7 ships able to break ice astern, built to serve Arctic export projects along the Northern Sea Route.

The LNG voyage cycle is the round trip of a laden leg carrying cargo and a ballast leg returning empty, with a small heel of LNG retained throughout to keep the tanks cold enough to load again on arrival.

LNG chartering is the hire of a carrier, either for a single voyage at a spot rate or for a period under a time charter at a daily rate, with the charterer directing the vessel and the owner supplying and crewing it.

LNG carriers are built by a very small group of shipyards, concentrated in South Korea, China and Japan, because the cargo containment work demands skills and facilities that take years to develop.