LNG Carriers

Built more than 30 years apart, the similarities between "Methane Polar", delivered in 1969 by Kockums in Sweden, and "K Freezia", delivered in 2000 by Daewoo in Korea, are striking. While overall length has hardly increased, cargo capacity of the new vessel at 138,000 cubic meter is almost double that of the old design. Speed meanwhile has increased from 16.5 to 20.5 knots.

"Methane Polar" delivered in 1969 by Kockums in Malmö, Sweden "K Freezia" delivered in 2000 by Daewoo in Korea

LNG carriers have been used to transport liquified natural gas overseas on a commercial basis since the late 1960s. Nowadays, a fleet of about 130 vessels transports 5% of the world annual gas consumption from producer to consumer. Over the years, there have been many improvements in the designs, but the main propulsion system is still the same. In all other sectors of commercial shipping, the steam turbine has been replaced by much more efficient diesels, but LNG carriers stick with steam turbines. The main reason is for this is the steam turbine propulsion system's unique capability to running on two cheap fuels simultaneously: Heavy Fuel Oil and Boil-off Gas. This feat, combined with a very high reliability, ensured the survival of the steam turbine in spite of its very low thermal efficiency. Until now...

In 2002, Chantiers de l'Atlantique in France recieved the first order from Gaz de France for a 74,000 cubic meter diesel-electric driven LNG carrier. Diesel manufacturer Wärtsilä in Finland will deliver the dual-fuel diesel gensets. The choise to select diesels instead of the conventional steam turbine indicated that there are owners and/or charterers in the LNG shipping community who are willing to try new technology, which increases thermal efficiency of the propulsion plant. A few LNG carrier operators have indicated they are interested in a more fuel efficient vessel, but charterers so far have been unwilling to consider engaging anything other than steam turbine driven LNG carriers.

Due to the small size of the LNC carrier ordered at Chantiers de l'Atlantique, its power requirements are too low to lend itself for gas turbine drive. However, larger vessels, such as "K Freezia", with propulsion power requirements between 24 and 30 MW, are ideally suited for the use of aero-derivative gas turbines. It is these vessel that will be used in this review of alternative propulsion plants for LNG carriers.