London, May 22 (IANS) A nearly half-kilometre long floating rig will be the world's heaviest and largest, weighing 600,000 tonnes or as much as much as six aircraft carriers.

 
 Shell announced plans to build a giant floating gas refinery, named the Prelude, to cost $10 billion.
 
 The outsize rig will allow gas extraction by drilling rigs and platforms far out at sea to be transferred to the ship for processing, doing away with the need for
 pipelines to a land based refinery.
 
 It means the company will be able to drill for gas in new fields hundreds of kilometres from the nearest land and will first be used in the company's Prelude oil field 200 km from the coast of West Australia, reports the Daily Mail.
 
 The floating refinery is the next logical development for oil companies as new supplies of oil and gas get more remote and demand continues to increase.
 
 The company tried to ally environmental concerns, saying the ship - which will be 488 metres long and weigh 260,000 tons unladen and 600,000 tons when filled with gas - had been designed to stay at sea in a Force 5 hurricane.
 
 Gas will be extracted from the ground by separate production platforms and then piped to the ship, where it will be liquified by cooling it to minus 162C. The process reduces the volume of the gas by 600 times, the report said.
 
 The liquified gas can then be transferred to tankers which can ship it anywhere in the world.
 
 A South Korean shipyard is due to start building the vessel using five times more steel than was used to build the Sydney Harbour Bridge.
 
 It should be finished in 2017.
 
 The ship will be moored at the Prelude gas field for 25 years where it is expected to produce the equivalent of 110,000 barrels of oil in gas a day.
 
 Shell's Malcolm Brinded said: "Our decision to go ahead with this project is a true breakthrough for the LNG industry, giving it a significant boost to help meet the world's growing demand for the cleanest-burning fossil fuel."