Energy giant Shell has unveiled plans to build the world's biggest ship - a mega-tanker the length of more than four football pitches laid end to end.

The 480m mega-tanker will house a liquefied natural gas plant so it can process gas as it floats over offshore fields.

Known as the Prelude FLNG Project, the 75m-wide vessel will be able to "withstand a one-in-10,000-year tropical storm", according to Shell.

It will displace 600,000 tonnes and carry 50,000 tonnes of equipment.

The carrier will be used to develop Shell's Prelude and Concerto gas discoveries, located in the Browse Basin, about 200km off the northwest coast of Western Australia.

Shell said the technology will reduce both project costs and the environmental footprint of an LNG development.

This is because floating LNG can unlock "stranded" gas reserves that have previously been considered uneconomic to develop because of they are small or far from shore.

It can also place liquefaction facilities very close to, or directly over, offshore gas fields, doing away with the need for long-distance offshore gas pipelines or extensive onshore infrastructure.

The development of the ship is still subject to environmental and production approvals - as well as Shell's final decision on whether to go ahead with the investment.