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Gard News 189, February/April 2008


The AL GATTARA and the TEMBEK – the largest LNG vessels in the world – have recently been delivered to OSG Nakilat. They are the first of four for OSG Nakilat and join a world fleet of about 240 LNG vessels.

With a cargo-carrying capacity of 216,200 m3, the so-called Q-Flex design makes these vessels 40 per cent larger than the standard 150,000 m3 LNG carrier. They have a length overall of 315 m, breadth of 50 m, depth of 27 m with a summer draft of 12.5 m. But size is not the only tradition these DNV-classed carriers have broken.

Innovation includes propulsion by two licence-built MAN-B&W 25,320 BHP slow-speed diesels driving twin shafts/propellers (with two rudders independent of each other) with a 19.5 knot service speed. Innovative because in the past during their voyages LNG carriers typically used boiled-off gas from the cargo tanks in boilers to produce steam to power the steam turbines. Although the amount of BOG (boiled off gas) was relatively small compared to the overall cargo, nevertheless the BOG was sufficient to power the vessel. But researchers determined that economies of scale of large LNG carriers, coupled with preservation of the valuable LNG cargo in the tanks to maximise delivery quantities, warranted alternative propulsion to steam turbine and thus the use of fuel-efficient slow speed diesels.


Membrane cryogenic cargo tank for LNG carried at below -162°C on Q-Flex class LNG vessel.

But BOG occurs as part and parcel of carrying LNG at -162ºC in heavily insulated membrane containment system cargo tanks, and the loss of BOG creates both an economic and an environmental issue. Re-liquefaction, a system widely used for gas handling on land, was seen to be the solution, but new re-liquefaction technology was required for application on a sea-going vessel. The AL GATTARA and the TEMBEK are among the first LNG carriers to have a re-liquefaction system. Although the system is very complex, in part because of requirements for large amounts of electricity and for absolute reliability and safety, simply put the system removes heat from the BOG and then returns the LNG back into the cargo tanks. The innovative solution and new technology involved careful supervision by the OSG Gas site team of 17 persons at each building yard.

Crews of about 28 will operate AL GATTARA and TEMBEK, which have berths for cadets who will train on these innovative vessels. It is expected that AL GATTARA and TEMBEK (along with a fleet of LNG new buildings) will carry LNG from the State of Qatar’s North Field (said to have about 15 per cent of the world’s proven natural gas reserves) to markets around the world.

 

Gard News 189, February 2008/April 2008

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Gard News is published quarterly by Gard AS, Arendal, Norway.