With contracts underway for an entirely new generation of very large LNG carriers of 200,000m3 and above, ABS is providing guidance to owners and shipyards on a range of issues that will affect the selection of the propulsion system for these advanced vessels. Direct drive, slow speed diesel plants, coupled with an on-board liquefaction plant to handle the cargo boil-off, or 4 stroke medium speed diesel electric propulsion, or gas turbine with diesel electric drive appear to offer the greatest operational efficiencies for the new designs of large LNG carriers. These alternatives are more economical and offer greater overall efficiency with an added advantage of providing greater flexibility and redundancy, according to ABS.
Although slow or medium speed diesel engines have been selected for some of the recent LNG carriers, owners are also considering and specifying the dual fuel installation option that uses both gas boil-off and ordinary bunkers. Variations of the dual fuel arrangements include: diesel engine or gas turbine driven generators with one propulsion shafting system and a liquefaction plant; diesel engine or gas turbine driven generators with two propulsion shafting systems and a liquefaction plant; and diesel engine or gas turbine driven generators with two azimuth thrusters and a liquefaction plant. To date, slow speed diesel with re-liquefaction plant as well as a gas combustion unit, and medium speed dual fuel diesel with gas combustion units, are the preferred options for the new large LNG carriers recently ordered in Korea.
A dual fuel diesel-electric system uses forced boil-off from the cargo tanks as the primary fuel and marine diesel oil as back-up fuel. The arrangement can also be adapted to current LNG carrier designs. Furthermore, shipbuilders and engine designers that are proponents of dual fuel systems point out that a gas-electric propulsion plant is more compact than the traditional steam turbine plant used for LNG carriers, increasing cargo capacity within the same dimensioned hull.It would appear that gas turbine with simple and combined cycles using heat recovery units to drive steam turbo alternators are another alternative being explored. Industry is currently developing the fuel gas systems for these gas turbine options.
However, a diesel plant also raises other issues that need to be addressed, particularly with vibration. ABS researchers are conducting detailed investigation into the effects of diesel engine vibration on membrane systems. The critical issue appears to be the interaction between the structural resonance that is excited by the diesel engine and the separate resonance that is created within the membrane containment system interacting with LNG.
ABS has issued a Guide for Design and Installation of Dual Fuel Engines providing technical guidance for the various machinery arrangements. The Guide focuses on a double or single wall piping system for the gas fuel lines in way of the machinery spaces. Further Guidelines for the equipment used in association with alternative propulsion systems are being developed. These include the re-liquefaction units and gas combustion units, also known as thermal oxidizers.