The world's first floating natural gas platform is to be built by Royal Dutch Shell, opening up vast new areas of the deep seabed for gas exploration.

The massive platform, nearly half a kilometre long, will be the biggest floating offshore drilling structure in the world, weighing in at about 600,000 tonnes – equivalent to six aircraft carriers – and staffed by 110 people at a time. Five times more steel will be used in its construction than went into the Sydney Harbour Bridge. Shell would not say how much it is expected to cost, but the total cost of exploiting the company's Australian off-shore oil fields, where it will be used, is likely to exceed $30bn.

It will take about five years to build, and is not expected to be fully operational before 2017.

Floating offshore gas platforms could be used to explore areas of the globe previously too remote for drilling. Companies are racing to discover offshore resources in deep water, as the world's readily available stores of onshore and close-to-shore oil and gas have already been snapped up. Advances in technology and melting sea ice are also helping to allow oil and gas exploration in sensitive parts of the globe, such as the Arctic, where a scramble to claim the undersea resources is now under way.

Shell has no such plans yet, and will moor its new platform 200km out to sea off the coast of Australia at the Prelude gas field. The size of the Shell platform means it can only be used on large gas fields, as it would not be economically viable on smaller fields.

"Our innovative FLNG technology will allow us to develop offshore gasfields that otherwise would be too costly to develop," said Malcolm Brinded, executive director, of Shell's upstream international business. "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 [and] help accelerate the development of gas resources."

He said the company was seeking to develop more floating platform projects.

Ann Pickard, country chair of Shell in Australia, said the technology would be "a game-changer for the energy industry".

Liquefied natural gas is a growing market as it is easier to transport. It is shrunk by about 600 times in the cooling process and can be transported before being turned back into a gas and used for power generation or heating, though it can also be used as a road fuel in specially adapted vehicles.

The floating platform, which Shell has now started to design in detail, will be built in South Korea. It will take gas from the Prelude field and liquefy it to -162C (-260F) on board, from where it will be removed by tankers and shipped to the rapidly growing LNG markets in Asia. Previously, gas had to be piped to onshore facilities to be liquefied.

The facility would be designed to withstand even the most severe cyclones, Shell said.