These ships are designed and constructed to carry natural gas or petrol gas in the liquified form. They are highly specialized vessels and require more technical skills and knowledge to construct these vessels. The vessels are built with flush decks and the superstructure built aft of the ship. The superstructure hosts the bridge and accommodation areas. The tanks in which the compressed natural gas is loaded is built into the flush decks but special materials are used to insulate the tanks from the hull. This is because the hull is made of steel and will become brittle and eventually break if it comes in contact with the natural gas. The gas is stored at a temperature of -160degree Celsius so at that temperature any metal in contact with the gas will become brittle and the entire structure of the ship will collapse. The propulsion system used in LNG ships is the steam turbine. The steam turbine operates with a boiler which supplies superheated steam at high pressure that impacts on the turbine blades that cause the blades to rotate. The resulting mechanical energy is transmitted through a series of gears and a shaft to the propeller shaft. The rotation of the propeller produces the thrust that propels the ship forward. The steam turbines are operated by boilers which supply superheated steam at high pressure. The boilers are fired by Boil- off gas( BOG). The LNG ships are exclusively propelled by steam turbines because they utilize BOG which is produced by excess pressure from the tanks. The excess gas is used to fire the boiler to produce steam that powers the ship. If the BOG is not sufficient to fire the boiler then liquid fuel (Heavy Fuel Oil) is used.