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South Korean shipbuilder Daewoo Shipbuilding & Marine Engineering (DSME) has completed its first LNG carrier equipped with a full reliquefication system.
The ship in question is the 180,000 m3 LNG Schneeweisschen, the first LNGC built by the yard with dual-fuel, two-stroke engines developed by Winterthur Gas & Diesel (WinGD). WinGD terms these propulsion units its Generation X dual-fuel (X-DF) engines.
DSME developed a partial reliquefication system (PRS) several years ago for use with LNG carriers propelled by M-type, electronically controlled, gas-injection (ME-GI) engines developed by MAN Turbo & Diesel.
Low-pressure X-DF and high-pressure ME-GI engines are both dual-fuel, two-stroke units that have gained favour for gas-powered vessels in recent years due to their high efficiency and low operating costs. Natural cargo boil-off gas (NBOG) rates on LNG carriers provide more fuel than these engines require when operating at lower speeds.
DSME has developed its full reliquefication engineering package – which it terms its methane refrigeration system-full reliquefication (MRS-F) design – as a complement to its PRS concept. The shipbuilder points out that with the MRS-F technology it has optimised the reliquefication option for both X-DF and ME-GI LNG carriers.
The MRS-F full reliquefication system works in tandem with the LNG carrier’s fuel gas supply system and is comprised of a PRS and an independent MRS cycle. Based on the reverse Brayton cycle, the MRS refrigeration system uses NBOG as a refrigerant. Because a redundancy fuel gas compressor is used as a refrigerant compressor in X-DF applications, there is no need for additional refrigerant compressors.
DSME explains that by combining the PRS and MRS technologies, full reliquefication of the LNG carrier’s BOG is possible for all operating scenarios, including the at-anchor state. The MRS only needs to come into play at ship speeds below 15 knots as the PRS covers any reliquefication requirements that may arise at higher ship speeds.
By obviating the need to bring the ship’s gas combustion unit into play, irrespective of vessel speed, MRS-F systems enable reductions in operating costs to be achieved. It is claimed that savings of up to US$7.2M over the life of the vessel can be made.