North West Shelf Shipping

Ship Technical Information

A Moss Rosenberg design LNG vessel passes in front of a membrane style vessel.
A Moss Rosenberg design LNG vessel passes in front of a membrane style vessel.

LNG has been safely and successfully carried by sea for over 40 years, accumulating in over 4,000 voyages and 100 million miles of laden sea passage. The world LNG fleet has grown from 120 vessels in 2000 to over 300 in 2008 as LNG becomes an attractive cleaner energy source.

LNG is carried at -163 degrees centigrade in specially engineered cryogenic containment systems and has a specific gravity of approximately 0.46. The cargo tanks act like giant thermos flasks and during transit, as the cargo warms very slightly, boil off gas is generated usually at the rate of about 0.12% - 0.15% /day. This boil off gas is fed into the ship's propulsion plant and used as fuel, thus reducing emissions.

In addition to insulation, a process of 'auto-refrigeration' is used to ensure the LNG is kept cold. This involves storing the LNG as a boiling cryogen - that is it is stored at its boiling point. As the LNG heats up, vapour boils off and the heat generated from the change from liquid to gas cools the remaining LNG. Because the insulation is very efficient, only a small amount of boil off is necessary to maintain the temperature in the LNG tanks.

Shore loading arms connecting to the LNG carrier's manifold. Liquid cargo and Vapour return lines are connected. The loading and unloading operations form a closed loop so that vapours are not vented to atmosphere.
Shore loading arms connecting to the LNG carrier's manifold. Liquid cargo and Vapour return lines are connected.
The loading and unloading operations form a closed loop so that vapours are not vented to atmosphere.