An LNG CARRIER is a tank ship designed for transporting liquefied natural gas (LNG). As the LNG market grows rapidly, the fleet of LNG carriers continues to experience tremendous growth.
The first ship Methane Princess was taken into operation in 1964 and remained in operation until it was scrapped in 1998. Until the end of 2005 a total of 203 vessels have been built, of which 193 are still in service.
At the moment there is a boom in the fleet, with a total of more than 140 vessels on order at the world's shipyards. Today the majority of the new ships under construction are in the size of 120,000 m³ to 140,000 m³. But there are orders for ships with capacity up to 260,000 m³. As of 6 March 2010, there are 337 LNG ships engaged in the deepsea movement of LNG.
The following is based on a typical set-up of tanks, which is to have from 4 to 6 tanks all along the centre line of the vessel. Surrounding the tanks is a combination of ballast tanks, cofferdams and voids. These areas give the vessel a double-hull type design.
Inside the tank there are normally three pumps of the submerged-motor type. There are two main cargo pumps which are used in cargo discharge operations and a much smaller pump which is the spray pump. The spray pump is used for either pumping out liquid LNG to be used as fuel via a vaporizer, or for cooling down cargo tanks. It can also be used for "stripping'" out the last of the cargo in discharge operations. All of these pumps are contained within what is known as the pump tower which hangs from the top of the tank and runs the entire depth of the tank. The pump tower also contains the tank gauging system and the tank filling line all of which come to almost the bottom of the tank.
In Membrane type vessels there is also an empty pipe with a spring-loaded foot valve that can be opened by weight or pressure. This is the emergency pump tower. In the event both main cargo pumps fail the top can be removed from this pipe and an emergency cargo pump lowered down to the bottom of the pipe. The top is replaced on the column and then the pump is allowed to push down on the foot valve and open it. The cargo can then be pumped out.
All the cargo pumps discharge into a common pipe which runs along the deck of the vessel. It branches off to either side of the vessel to the cargo manifolds which are used for loading or discharging.
All the cargo tank vapour spaces are linked via a vapour header which runs parallel to the cargo header. This also has connections to the sides of the ship next to the loading and discharging manifolds.