Ocean current

Spurred on by agreements to increase the proportion of renewables in the energy mix, and with the government’s recent announcement of accelerated grid connections, more onshore windfarms seem set to be built. Meanwhile, near-shore developments are also gathering pace.

But ironically, the one place where wind turbines cannot be built is the region where they are likely to have the most effect — far out at sea. Away from the land, winds are stronger and more reliable. Some 30 miles (50km) offshore in the North Sea, wind turbines can produce 30 per cent more power than in the shallower water 9 miles from land.

Also, deep water turbines are less likely to be near shipping routes and as most seabirds live near the shore, they will not be a hazard to wildlife. Not the least of their advantages is that they are not visible from the land so unlike onshore windfarms, they will not face opposition based on aesthetics.

But with the advantages come technical disadvantages. When the sea floor is 300m below the surface, how do you tether a wind turbine in place? And with the harsh conditions and large waves of a windy, open-sea environment, how do you ensure turbines survive?

Such is the demand for wind energy that several companies are trying to solve the problem. UK-based firm Blue H is working on a prototype based on a floating platform — similar to a small oil rig — which is tethered to the sea bed by three heavy cables. Earlier this year, it towed a non-generating prototype 11 miles off the Italian coast, where it is conducting trials.

Norwegian oil and gas giant Statoil-Hydro, working with Siemens, is also trialling prototypes with a design called Hywind where the mast, bearing a 2.3MW turbine, is bolted to a floating concrete buoy, again tethered to the sea bed by three cables.

But the most striking design comes from another Norwegian company called Sway. The Sway turbine, a 5MW unit, sits on top of a tower that continues as a submerged mast, fixed to the sea bed by a single tension leg —a metre-wide steel tube with a gravity anchor at the base. The mast, about 100m long below the water and 80m above it, contains up to 2500kg of gravel to act as ballast. This pushes the centre of gravity well below the centre of buoyancy, making the design stable and able to withstand the forces both of the high offshore winds and the weight of water moving past the mast due to tidal motion.

In theory, the tension leg can be anything up to 1,000m long, according to Michal Forland, the company’s chief financial officer. ‘But that wouldn’t make economic sense,’ he said. ‘So we say the optimal operational range is 100m to 400m deep.’ In the North Sea, this would allow the turbines to be placed around 20 miles offshore.

The Sway turbine differs from most conventional designs in that rather than facing into the wind, it has a downwind design — the wind comes from behind the turbine and blows past the mast before hitting the blades.

Two cable braces run from below the turbine to a pair of spars just above the waterline and back to the base of the mast underwater, at the point where it joins to the cable stay. These braces always face the wind, with the tension in the cables preventing the mast from bending.

Forland says the downwind arrangement is often seen as a weakness in wind turbine design but for Sway, it is a key advantage.

‘It allows us to build a tower which isn’t completely stiff and doesn’t have to stand precisely vertical all the time. That means that we can have a perfectly vertical turbine blade.’

Moreover, Forland claims the reinforcement from the cable stays allows the use of much larger turbines with twice the generating capacity of Sway’s competitors.

Most offshore windfarms are near the shore but deep water developments could produce considerably more power

In normal operation, with the wind putting a 50 tonne load onto the turbine, the tower leans at an angle of about 8° to the vertical. In this position, as Forland says, the turbine blades are vertical, which allows them to capture more of the wind’s power and operate efficiently.

‘Big gusts and waves push the mast away from its optimal position but we’re only talking about 3° maximum,’ he said. The tower is designed to withstand waves more than 30m high.

Because the cable stays must always be directly opposite the turbine blades there can be no twist in the mast-turbine assembly, unlike with conventional wind turbines where the turbine rotates on top of the mast to face into the wind. Instead, the Sway turbine has a clutch swivel rotor at the junction of mast and tension leg, and two universal joints at either end of the tension leg.

‘We can also force the blades to feather, and with these two mechanisms we can turn the rotor into the wind with virtually no torsion at all in the construction,’ said Forland. ‘This is pretty standard technology from the North Sea offshore oil industry.’

One disadvantage of downwind turbines is that the blades rotate in the wind shadow cast by the mast. Every time a blade passes the mast, the wind force on it drops. This sets up a vibration in the blade that can lead to fatigue and eventual cracking. To avoid this, Sway has designed an aerodynamic housing for the mast to reduce pressure drop and prevent vibration.

‘Onshore, you also have a noise problem with downwind turbines —you hear a fluttering sound every time a blade passes a mast,’ said Forland. ‘But that’s not an issue 20 miles out to sea.’

And the downwind design should not cause problems with the turbine itself, he added. ‘Our likely turbine supplier has done a feasibility study and found no problems. The blades have to face the opposite way and some small changes are needed to the major gear hub but the supplier classes it as a standard turbine. It’s a big relief to us that only small alterations to the turbine are needed.’

Sway’s £14.8m research and development effort made extensive use of computer simulation, said Forland. ‘We have an integrated simulation system which couples aerodynamics and hydrodynamics and this helped us take a lot of weight out of the construction.’

Sway is carrying out detail engineering to enable it to build a full-scale prototype that will be deployed off the Norwegian coast in 2010, in full operational depth of water.

This puts it about a year behind the schedule of its competitors (one of which, StatoilHydro, is one of the major investors in Sway) but Forland is hopeful the greater production capacity of the Sway design will make it an attractive option.