A floating vessel that is longer than the Empire State Building is high has taken to the water for the first time.The hull of Shell's Prelude was floated in South Korea, BBC reported.When fully built, Prelude will be the largest floating facility ever created, weighing more than 600,000 tonnes.It would be used to help in the production of natural gas from 2017, Shell said, and would operate for 25 years off Australia's north-west coast.The area has a yearly cyclone season from November to April, but Prelude has been designed to withstand such conditions. It is hoped the facility will be able to produce enough gas to power a city the size of Hong Kong.Despite appearances, Prelude cannot strictly be described as a ship as it needs to be towed to its destination rather than travelling under its own power.Shell has not disclosed how much the vessel will cost, but industry analysts told Reuters that it would be likely to command a price of between $10.8bn and $12.6bn (£6.6bn - £7.7bn).Not content with Prelude's record-breaking size, Shell said it had already started work on an even bigger facility. ANN.Az