An LNG carrier is a ship designed for transporting liquefied natural gas. As the LNG market is growing rapidly in the present decade, the fleet of LNG carriers is also growing rapidly.
In order to transport natural gas, it is cooled to approximately -163 degrees Celsius where it condenses to a liquid at atmospheric pressure shrinking to approximately 1/600 of its original volume with a density of 420 to 490 kg/m3. The tanks onboard LNG carriers function, in effect, as big thermos containers wherein the liquid remains boiling for the duration of voyage. Some gas is removed to prevent a gradual buildup in pressure; this is known as Boil Off Gas (BOG). The latent heat of vapourization required to turn a small amount of LNG from a liquid to a gas is what keeps the remaining liquid cooled.
Recently, designs have been developed for pressurized transport systems as well, to be called pressurized natural gas (PNG) carriers, although none have yet been constructed.
At present, there are four containment systems in use for new ships. Two of the designs are of the self-supporting type. The other two are of the membrane type which are patented designs owned by Gaz Transport and Technigaz (GT&T). The trend is toward the membrane instead of the self-supporting types, most likely due to lower construction costs.