Earlier LNG carriers were mainly been driven by steam turbines, and the boil-off gas from the LNG cargo was used as fuel. Wärtsilä then introduced its Hamsworthy reliquefaction system, a solution to liquefy the boil-off gas and return the LNG back to the cargo tanks; the company is now on its third generation of reliquefaction systems.

In January, Wärtsilä announced that a series of four LNG carriers being built by the Hudong-Zhongua Shipbuilding yard in China will feature Wärtsilä LNG reliquefaction plants and gas handling systems. The reliquefaction plant will reliquefy 70% of the boil-off gas from the ships’ LNG cargo and return it to the cargo tanks, while the balance is fed by the gas handling system to the engines for propulsion purposes.

HHI’s newly developed gas treatment system, comprising a MAN B&W ME-GI engine; a BOG high-pressure compressor; a BOG re-liquefaction system; an LNG fuel gas supply system (FGSS); and Hyundai integrated gas supply system (Hi-GAS) can re-liquefy 100% of the BOG and re-store it in the LNG tank which can later be used as the ship’s fuel, HHI said.

According to MAN, various factors determine the rate of the boil-off gas evaporation. However, boil-off gas is estimated to equal about 80-90% in laden voyage, and about 40-50% in ballast voyage, of the energy needed for the LNG vessel at full power. Full power is defined as a voyage speed of 19-21 knots. This speed has been accepted in the market as the most optimal speed for LNG carriers.

Running on gas, an LNG carrier equipped with the HHI system can save up to 50% of the fuel cost in comparison with a standard vessel using heavy fuel oil (HFO) or marine gas oil (MGO), and produce 92, 80 and 23 percent less emissions of SO_x, NO_x and CO₂, respectively, HHI said.

Equipped with this new gas treatment system, two 176,000 m³ LNG carriers HHI is building for Knutsen, Norway are scheduled to be delivered in 2016.