The US and UK last week announced plans to develop enormous floating offshore wind turbines that can be deployed in much deeper waters and further out to sea.

By being freed of the towers that would usually tether offshore wind turbines to the seabed, floating turbines are not restricted to the usual 60-meter (200-foot) depth limit, and instead can be located in waters several hundred meters deep. Not only does this increase the areas of sea and ocean that can be harvested for wind, it grants access to faster and more reliable winds. And because wind turbines can be located further from the coast, they can be positioned beyond the sight of inland communities that may be less than impressed by the insertion of a wind farm into their scenic coastal view.

Last week Energy Ministers from the world's 23 largest economies met in London at the Clean Energy Ministerial co-chaired by US and UK Energy Secretaries Steven Chu and Edward Davey. In a statement issued ahead of the event, Davey announced that both the US and UK would be "making funding available," and described the two countries as being "determined to work together to capitalise on this shared intent."

Though couched in rather vague language, the statement does go into more detail as to the funding available. The UK's Energy Technologies Institute (ETI) is currently commissioning a £25 million ($41 million) floating demonstrator project. Selected participants will be invited to submit concepts of a floating wind turbine between 5 and 7 megawatts (MW) capacity. The winner will be asked to produce a working prototype to be installed by 2016. In the meantime, the ETI is narrowing down sites that could host the demonstrator. The additional waters opened up by floating wind technology allow access to hundreds of of gigawatts of additional wind resources. The UK's offshore wind potential has been calculated at 2200GW, which is thought to be about a third of Europe's total.

Meanwhile, the US Department of Energy has announced a $180 million "funding opportunity" for four offshore wind demonstration projects, though none is explicitly allocated for floating wind technology. Despite an estimated 4150GW of offshore wind potential, the US has no offshore wind infrastructure as yet, and floating technology may further the cause if NIMBYists can be persuaded that turbines will be placed out of sight and earshot.

Though floating wind turbines do exist, the cautious research-led approach adopted by the US and UK makes sense when you look at the scale of the task they've taken on. Wind turbines over 5MW in capacity are enormous: Enercon's 7.5MW E-126 inland turbine has a rotor diameter of 126 meters (413 feet).

To date, the largest floating wind turbine that has been installed is Norway's 2.3MW Hywind which has a rotor diameter of 82 meters (270 feet). The fact that its development cost is $62 million raises questions about whether the investment earmarked by the US and the UK will be enough to produce a stable floating turbine with more than double that capacity.

The form that the potential floating turbines will take hasn't been decided, but the most likely design is that individual turbines will effectively form vertical spar buoys, each moored by several catenary cables weighted at the ends. This would effectively be an enlarged version of the system used to moor the Hywind turbines—which actually employs additional 60-ton weights at the middle of each catenary cable in order to provide extra tension. Indeed, the next stage of the Hywind project hopes to apply the principle to a 5MW turbine in waters 320 meters (1,000 feet) deep—Norway has large-scale floating wind ambitions too.

An alternative means of mooring would be to adopt a tension-leg system, commonly used on offshore oil platforms since the 1980s. Something like an inverted pendulum, a tension-leg system would see a turbine mounted on a highly buoyant platform with steel tendons stretching vertically down to the sea bed. Research by MIT and the National Renewable Energy Laboratory suggests that 5MW turbines should be feasible at depths of up to 200 meters (650 feet).

The idea of multiturbine pontoons has additionally been, er, floated, but there are fundamental concerns about the cost of such systems, as well as their ability to withstand the extreme forces floating wind turbines would face during extreme weather.

Offshore wind plans are by no means restricted to the US and Europe. The Fukushima Recovery Floating Wind Farm Pilot Project confirmed by the Japanese government last month will initially consist of a 2MW turbine, to be joined by two 7MW turbines by 2016. And by 2020 the Marubeni Corporation plans to build a 1GW floating wind installation off Japan's northern coast, partly funded by the country's tsunami recovery budget. If it goes ahead, the scale of the wind farm would eclipse any sort of offshore wind farm currently in existence or under construction, though much bigger installations are in development, including a 9GW offshore farm at Dogger Bank off England's east coast.