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.

LNG containment insulation is the layered barrier that limits heat leaking into the cargo, and Invar is a nickel-iron alloy used in one membrane design because it barely contracts when the cargo cools it.

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.

Reliquefaction is the process of cooling boil-off gas back into liquid and returning it to the cargo tanks, and subcooling is the partial version that chills the vapour enough to reduce cargo loss without eliminating it.

Steam turbine propulsion drives an LNG carrier by raising steam in boilers that can burn cargo boil-off gas, fuel oil, or any mixture of the two, and turning a turbine geared to a single propeller shaft.

Dual-fuel diesel electric propulsion uses medium-speed engines burning either boil-off gas or liquid fuel to generate electricity, which drives electric propulsion motors rather than a directly coupled shaft.

ME-GI and X-DF are the two families of low-speed two-stroke marine engine burning natural gas, differing in whether gas is injected at high pressure late in the cycle or admitted at low pressure and premixed.

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.

Cooldown is the controlled chilling of warm cargo tanks by spraying LNG into them, and gassing up is the preceding step of replacing inert gas with methane vapour so no carbon dioxide remains to freeze.

Custody transfer is the measurement determining how much energy passed between seller and buyer at loading or discharge, calculated from tank level, liquid temperature and density, and a laboratory analysis of the cargo composition.

Ship-to-ship transfer is the movement of LNG directly between two vessels moored alongside one another, used for transshipment, replenishing floating storage, and breaking a full cargo into smaller parcels.

The ship-shore interface is everything that must be compatible between a vessel and a terminal before LNG can be transferred, from manifold height and spacing to the linked emergency shutdown systems on both sides.

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.

An LNG freight rate is the daily cost of hiring a carrier, assessed by shipbrokers for standard routes and vessel types, and it swings violently because fleet supply cannot respond within the time demand changes.

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.

Crewing an LNG carrier requires officers holding STCW gas tanker endorsements earned through both training and documented service on gas carriers, a qualification pool that expands more slowly than the fleet it has to staff.

LNG bunkering is the supply of liquefied natural gas as fuel to ships rather than as cargo, delivered by bunker vessel, road tanker or shore pipeline, and governed by its own codes and standards.