Four very large LNG carriers destined for transport of gas from Qatar to the UK have been ordered at DSME of Korea.

Each vessel with a carrying capacity of 210.000 m3 LNG will feature 2 x 6S70ME-C engines designed by MAN B&W Diesel and be delivered during 2007-08.

Furthermore, four sets of MAN B&W Holeby 7L32/40 GenSets will be featured per vessel.

Each vessel has been specified with reliquefication plants to process the boil-off gas occurring between the loading point in Qatar and their destination in the United Kingdom.

Says Ole Grøne, MAN B&W Diesel Vice President Two-stroke Sales and Marketing, "Significant cost benefits can be realised by LNG transporters by switching from a traditional steam turbine propulsion plant to reliable and fuel-efficient diesel engines and, in particular for large vessels, to electronically-controlled MAN B&W Diesel ME low speed diesel engines, when used in conjunction with reliquefaction of the cargo’s boil-off gas."

MAN B&W Diesel’s electronically-controlled ME engines are particularly attractive for LNG ships, whose lifetime could be 40 years. The great flexibility for fuel economy and emissions optimisation is inherent in the design and the control system software can be updated routinely as required.

A pair of ME-type low speed engines, arranged to drive fixed pitch propellers via clutches, eliminates any concern over propulsion plant reliability and redundancy. Mobility is always ensured and maintenance can be done in port.

In the unlikely event that operation with only one engine should become necessary, the ME engine renders itself perfectly suitable to accept the apparently heavier operating propeller.

Shipboard reliquefaction technology has matured considerably in recent years, tapping wide experience from land-based installations and LPG carrier installations. Among others Hamworthy KSE’s patented Moss RS closed nitrogen cycle system for reliquefying boil-off gas offers a solution for pumping LNG back to the cargo tanks and hence the opportunity to deliver more cargo to the buyers.

MAN B&W Diesel has produced a paper that thoroughly examines operating costs and additional income from the transport and sale of LNG. The paper, entitled; 'LNG Carrier Propulsion by ME Engines and Reliquefaction', analyses fuel oil, lube oil and maintenance costs for both propulsion and electrical power generation under various operating conditions on a comparison basis for a proposed diesel-based solution against a steam plant. The evaluation shows that substantial economic benefits can be gained from diesel propulsion in conjunction with gas reliquefaction over the steam turbine-powered option, simply because of the big difference in thermal efficiencies while the first cost of the various propulsion systems is virtually the same.