Øyvind Hagen/StatoilHydro

FAR out to sea, the wind blows faster than it does near the coast. A turbine placed there would thus generate more power than its inshore or onshore cousins. But attempts to build power plants in such places have foundered because the water is generally too deep to attach a traditional turbine's tower to the seabed.

One way round this would be to put the turbine on a floating platform, tethered with cables to the seabed. And that is what StatoilHydro, a Norwegian energy company, and Siemens, a German engineering firm, are doing. In the next few days the first of their floating offshore turbines should be connected to Norway's electricity grid. Once that has been done, it will start a two-year test period generating about 1 megawatt of electricity—enough to supply 1,600 households.

The Hywind is the first large turbine to be deployed in water more than 30 metres deep. The depth at prototype's location, 10 kilometres (six miles) south west of Karmoy, is 220 metres. But the turbine is designed to operate in water up to 700 metres deep, meaning it could be put anywhere in the North Sea. Three steel cables running to the seabed prevent it from floating away.

It is an impressive sight. Its three blades have a total span of 82 metres and, together with the tower that supports them, weigh 234 tonnes. That makes the Hywind about the same size as a large traditional offshore turbine delivering comparable power. The tower sits on a conical steel buoy that extends 100 metres below sea level and has a diameter of 8.3 metres at its base and 6 metres at its top. The buoy contains 300 tonnes of concrete ballast that stops the structure toppling over by placing its centre of gravity well below the water's surface.

That is necessary because, even though it is tethered, the motion of the sea causes the tower to sway slowly from side to side. This swaying places stress on the structure, and that has to be compensated for by a computer system that tweaks the pitch of the rotor blades to keep them facing in the right direction as the tower rocks and rolls to the rhythm of the waves. That both improves power production and minimises the strain on the blades and the tower. The software which controls this process is able to learn how to do the job better by measuring the success of previous changes to the rotor angle and using that information to fine-tune future attempts to dampen wave-induced movement.

If all works well, the potential is huge. Henrik Stiesdal of Siemens's windpower business unit reckons the whole of Europe could be powered using offshore wind, but that competition for space near the coast will make this difficult to achieve if only inshore sites are available. Siting turbines within view of coastlines causes conflicts with shipping, the armed forces, fishermen and conservationists. But floating turbines moored far out to sea could avoid such problems. As with so many things, out of sight is out of mind. That, plus the higher wind speeds that mean that a deep-water turbine could generate four times as much power as a shallow-water one, make the sort of technology that the Hywind is pioneering an attractive idea.

One obvious drawback is that connecting deep-water turbines to the electrical grid will be expensive. But the biggest expense—the one that will make or break far-offshore wind power—will probably be maintenance. In deep seas, it will not be possible to use repair vessels that can jack themselves up on the seabed for stability, like the machines that repair shallow-water turbines. Instead maintenance will only be possible in good weather. If the Hywind turbine turns out to need frequent repairs, the cost of leaving it idle while waiting for fair weather, and of ferrying people and equipment to and fro, will outweigh the gains from generating more power. But if all goes according to plan, and the turbine does not need such ministrations, it would put wind in the sails of far-offshore power generation.