There are basically two types of liquefied gas carrier:

One to carry natural gas such as butane (LNG – liquefied natural gas). LNG is carried by reducing the temperature to -160° C when it is carried in fast well insulated and expensive ships or by converting it (expensive process) into methyl alcohol and carrying it in conventional tankers. The economic advantages for each depends on the distance the gas has to be carried. The latter method is better for long distances and it is also safer.

One to carry Liquefied Petroleum Gas (LPG). LPG can be carried at higher temperatures than LNG and is also, in some cases, liquefied by pressure or a combination of both. LPG carriers are usually smaller than LNG carriers.

Design features.

• All LNG carriers must have double skins. Tanks must be constructed of material that will not become brittle at very low temperatures ie. aluminium, nickel, etc.
• Each tank must be insulated from the others and the ship’s hull to protect the mild steel of the hull from becoming brittle at very low temperatures and to reduce the cargo loss due to “boil off”.
• These tankers are possibly the only steam turbine ships still being built as the boil off can be used in the boilers to power the ship.
• There are two main types of tank construction – Independent or free standing tanks (spherical/prismatic) and “membrane” tanks (rectangular tanks without sharp corners).
• The tanks which carry the liquefied gas within the outer hull must be adequately supported. Very strict safety precautions both with respect to the construction and operation of the vessels.

Inside a membrane tank.

LNG carrier Cross River.