Float my turbine

Meeting global climate change goals will among other things require a dramatic/radical shift away from fossil fuels and increased support for renewable energy technologies. In the near term wind must play a significant role. It is the one renewable technology that can be deployed economically at both utility and community scales today.

That is true for land-based wind energy not necessarily for offshore wind. However, the offshore wind resource is vast and in the United States is the only option that a number of east coast states have for meeting their ambitious clean energy goals. In order to take full advantage of this potentially inexhaustible clean resource, offshore wind farms will have to eventually be located farther from shore and operate reliably and economically there.

That means that eventually they will probably have to float. (GW)

Floating challenge for offshore wind turbine

By Jorn Madslien
BBC News
September 8, 2009

Project manager Sjur Bratland of StatoilHydro explains how the Hywind works

Heading offshore in the Rygercruise catamaran, it is a journey into uncharted territory.

Not in terms of sea charts - the Norwegian energy giant StatoilHydro knows these waters well, having spent the past 30 years drilling for oil and gas here - but in terms of technology.

Statoil has constructed the world's first full-scale floating wind turbine a couple of hours by catamaran from the oil town Stavanger, in the hope that one day vast wind farms could be constructed far offshore in water depths of up to 700m.

Standing firm as the catamaran rolls in the waves, Sjur Bratland is optimistic with regards to the technology's potential.

Yet, having spent the past two years listening to a whispering crowd of nay-sayers, he wants proof. He wants to see for himself that the turbine can cope with winter storms that whip the North Sea into a froth and winds that rip roofs off houses.

And he wants to be sure that the supply industries can deliver the right turbines, supply ships and so on.

Fortunately, Mr Bratland is in charge of the Hywind project, and he has been given some 400m Norwegian kroner ($66m; 35m) to play with by Statoil, with the government injecting a further 59m kroner.

Trouble shooting

As the turbine slowly emerges through the mist, the first impression is how stable it is.

As the catamaran moves closer, the turbine stands as firm as if it was pinned to the seabed, the way conventional offshore turbines are, its 65m tubular carbon steel tower and slowly rotating 80m diameter blades, together stretching 100m above the sea.

The most interesting aspects of the turbine can be found in the depths of the sea, where a 100m long steel cylinder weighing 3,000 tonnes thanks to its ballast of water and rocks is anchored to the sea-bed with mooring lines that can hold the structure at depths of up to 700m.

So-called slack anchors are used, allowing the structure to move with the seas. In fact, in spite of its apparent sturdiness, the 138 tonne turbine is constantly moving.

Everything below the water line is "known technology from the oil and gas industry", where StatoilHydro has 30 years of experience from its extensive offshore operations around the world, Mr Bratland explains.

"Actually, the really tricky thing is to apply this technology in a completely new setting in a new industry," he says. "That has been a challenge."

Deap seas

The Hywind turbine will be tested over a two-year period, at the end of which Mr Bratland hopes to have found proof that offshore wind farms can be built, and that they are economically viable - perhaps even competitive with conventional offshore wind.

Turbines pinned to the sea-bed are relatively cheap in water depths of up to 25m, when the basic monopole foundation can be used. At greater depths of up to 50m, the tubular turbine towers will need gravity bases and stronger steel structures that push the price up.

Statoil's floating turbine requires the waters to be at least 120m deep, though beyond that the sea is the limit - literally.

"But first, we'll need to look for smarter solutions that make it commercially viable," says Mr Bratland.

Better and cheaper turbines

Above sea level, the offshore structure has been bolted together with a conventional offshore turbine of the sort used for near-shore wind farms that are bolted to the sea-bed.

Ease of entry for maintenance is a seemingly trivial yet major headache for Mr Bratland.

Though the seas are relatively calm, the waves are nevertheless too large for a safe mooring against the base of the structure to enable the BBC team to climb the 17m steel ladders up to the service deck.

At a more fundamental level, turbine technology must be developed specifically for far offshore conditions, he insists.

On land, turbines are getting taller all the time to accommodate ever larger blades, explains Mr Bratland.

For the purposes of floating wind farms, such turbines are not light enough, they are too tall and the rotor blades are too small, according to Mr Bratland's early assessments, made even before the 2.3 megawatt turbine has started delivering electricity through the sea-bed cable that connects it to the Norwegian national grid.

He wants turbine manufacturers to produce lower turbines, to take advantage of winds blowing strongly and steadily close to the surface of the sea. Closer to land, the wind lifts to enter the beaches and climb over the cliffs.

Lower turbines should, together with clever design and material selection, help reduce the turbines' weight, and thus their need for ballast, which in turn should reduce sub-sea costs.

At the same time, Mr Bratland wants the power output of each turbine to be raised to perhaps 6MW, and he wants it all to be cheaper than it is today.

This could be possible as there is tremendous potential for economies of scale, both in terms of mass production, which reduces the cost of each turbine, and in terms of the construction and management of vast wind farms offshore where there are few limits on available acreages.

Moreover, onshore production of floating turbines that are then towed offshore to be anchored in place should prove cheaper than the weather-restricted and costly off-shore construction of sea-bed turbines.

Grid cables transporting the power to shore are neither expensive - in the greater scheme of things - nor technologically complicated to roll out, so this is a minor part of the project, according to Mr Bratland.

"Still, we have some way to go," says Mr Bratland.

"Today's solutions are too costly as the margins are so much slimmer in this area than in the oil and gas industry."