Boil-off gas from the cargo on board LNG carriers has for decades been used to fuel steam plant propulsion, but LNG carriers with alternative propulsion are now being constructed.

Diesel engine and reliquefaction plant are now being introduced as a new solution and the first full-size reliquefaction plant is currently being installed. Ordinary fuel oil will be used for propulsion, while the boil-off gas will be reliquefied and fed back into the cargo tanks. No part of the LNG cargo will be consumed during the voyage.

The first QFlex LNG carrier, with a cargo capacity of approximately 216,000 m3, will be delivered from Hyundai Heavy Industries (HHI) in Korea to Overseas Shipholding Group, Inc in late 2007. This ship will trade between Qatar and the US/UK. Upon delivery, it will be the first or second in a series of LNG carriers that are larger than any ever delivered, and it will have the first full-scale reliquefaction plant on board.

The cargo on an LNG carrier is kept in a liquid state at minus 163 degrees Celsius. Gas naturally evaporates due to the cargo boiling during transportation.

LNG carriers have up to now used this boil-off gas from the cargo as fuel in boilers (for steam turbines). This is claimed to be a costly solution. The cargo to be delivered to the customer has been less than the volume loaded. In addition, the efficiency of the steam engines that burn LNG is significantly lower than that of the diesel engines that will replace them.

For the new QFlex LNG carrier, the cost reduction may be as high as USD 5 million a year.

The first installed reliquefaction plant on board the QFlex LNG carriers has been developed and delivered by Hamworthy and approved and certified by DNV.

Facts about Hamworthy
Hamworthy is a leading designer, developer and manufacturer of advanced marine fluid handling systems for ships and offshore oil and gas facilities. The company specialises in innovative and often highly customised designs for marine gas handling, pumping and wastewater systems.