Significant cost benefits can be realised by LNG transporters by switching from traditional steam turbine propulsion plant to reliable and fuel-efficient electronically-controlled MAN B&W Diesel ME low speed diesel engines when used in conjunction with reliquefaction of the cargo’s boil-off gas.

Every steam turbine-powered LNG carrier loses more than US$ 3.6 million annually because of the relatively low efficiency (around 30%) of the propulsion plant. In addition, the environment also suffers from unnecessarily excessive CO₂ emissions.

Heavy fuel-burning low speed two-stroke diesel engines with efficiencies of around 50% (in combination with reliquefaction of the boil-off gas) offer significant economic benefits for those trades where loss (consumption of cargo) is not accepted and delivery of the full amount of LNG is valued. A reduction in CO₂ emissions of around 30% compared with steam propulsion should also be appreciated as environmental controls tighten worldwide.

Another factor favouring a switch to diesel propulsion is that manning steam-powered tonnage will become more difficult as steam-qualified engineering staff decline. Moreover, shipyards have much more experience of installing diesel engines than steam plant, and only a limited number of companies are now able to supply large marine steam turbines and boilers.

MAN B&W Diesel have prepared a technical paper (see below) in which the future direction of LNG carrier projects is discussed. The key points include:

• Electronically-controlled ME engines – approved for service by leading shipowners in other sensitive trades. These are particularly attractive for LNG ships, whose lifetime could be 40 years. 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, eliminate 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 apparent heavier operating propeller.
  
  
• Shipboard reliquefaction technology. This has matured considerably in recent years, tapping wide experience from land-based and LPG carrier installations.
  
  The 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.