LNG has been safely and successfully carried by specialist ships for over 40 years. The worlds LNG fleet has grown to over 400 as LNG becomes an attractive cleaner energy source.
LNG is carried at about -160 degrees Celsius in specially engineered cryogenic containment systems and has a specific gravity of approximately 0.46. The cargo tanks are heavily insulated 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 compressed and warmed and then fed into the ship's boilers 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 warms, 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.