On Geoje Island, off the coast of South Korea, as many as 5,000 workers have been building the largest vessel ever constructed. With a deck the size of seven football fields and containing three times as much steel as the Golden Gate Bridge, the Prelude will spend years anchored above a natural gas field off Australia, pumping fuel from under the seabed and turning it into a liquid that can be shipped to Asian customers. Led by Royal Dutch Shell, the project could transform the global gas industry.