IN DEPTH: Statoil's Hywind 2

Some 25km off the storm-scoured coast of northeast Scotland, where the seafloor drops away from the continental shelf into the sub-zero darkness of the North Sea deeps, Norwegian energy giant Statoil is planning a world first: an array of floating wind turbines.

The five-machine 30MW Buchan Deep project, set to be anchored off Peterhead by 2016 in more than 100 metres of water, could be a commercial launchpad for a sector that saw its maiden full-scale prototype — the developer’s own Hywind 1 — switched on less than five years ago off western Norway.

For if the trials of the grid-connected quintet — consisting of bottle-shaped spar-buoy foundations topped with 6MW turbines — are successful, it would clear the way for Statoil to begin serial production for wind developments of over 500MW by 2020.

Buchan Deep has a lot to recommend it as a demonstration site [for the second-generation Hywind],” states Statoil’s head of floating wind business development, Trine Ingebjørg Ulla. “It is quite close to shore, so we can get out to [the array] and back relatively easily, the metocean conditions and water depths suited us, it has a [suitable onshore] grid connection — and there is a well-developed supply chain from the offshore oil industry in the area.

“And having the very large turbine size that is now available, we see a tremendous reduction in cost — by two thirds per megawatt to begin with — and more cost-reduction possible,” she says, adding that floating wind turbines could soon be cost-competitive with those on fixed foundations.

“This is a big leap forward. Buchan Deep is to reduce risk, to demonstrate that the costs are sufficiently reduced before starting commercial production. This is where the Hywind proves that it is more than a clever technology, that it is one that can be commercially viable.”

The Hywind prototype spinning in the deepwater wilds off the island of Karmøy, north of Stavanger, since 2009 has been quietly building a conclusive commercial case for the spar-buoy concept.

Designed around a hull fabricated by French contractor Technip at its Pori yard in Finland and topped out with a 2.3MW Siemens turbine, the Hywind 1 has performed superbly, weathering winds of 40 metres per second (m/s) and winter waves 15 metres high — and churning out close to 35GWh at an average capacity of 50%.

The numbers speak for themselves. The prototype, which is operating in 220 metres of water and linked to shore by a 10km export line, is producing 10-20% more power than same-size monopile-mounted offshore machines — bedrock proof of the wider economic viability of floating wind.

“Our experience on this pilot project has confirmed that our control systems works, that the design is good and scaleable, output is high and the operational behavior robust, so we are very confident in taking this next step with Buchan Deep,” states Ulla.

The Hywind concept is based on a well-understood offshore technology — the spar, which has enjoyed a long and happy history as a floating foundation for deepwater oil production platforms since the mid-1990s.

The second-generation Hywind 2 is an evolutionary advancement, most of all because the hull’s length has been trimmed to a more compact, lighter, “site optimised” design that stretches 76 metres below the surface — compared to the 100-metre draft of the prototype — which should open up a wider offshore market for the deepwater concept.

“We have a catalogue of designs — the more benign the metocean conditions, the slimmer and shorter you can make the spar,” says Ulla.

“The basic design is largely the same [for Hywind 2] but as we are reducing the draft and increasing the size of the turbine, there is some adapting to be done — the diameter of the spar will be increased — likely to between 12 and 14 metres under the waterline — and the solid ballast will be improved and optimised.”

A 6MW nameplate turbine for the demonstrator units “in the 150-metre rotor-diameter range” will be chosen in the next five to six months, she says.

“[Topping out a Hywind] with a 6MW turbine will be very different from the 2.3MW [prototype]. When you enlarge the diameter of a rotor from 82 metres to 150 metres you are exposing the whole structure to a different scale of forces — with the bigger Hywind the forces of the winds [on the rotor] will be greater than the forces from the waves and currents on the ballasted hull, compared with the first Hywind.”

The Hywind prototype was ballasted with water and an aggregate of mainly olivine — a green mineral that makes up much of the Earth’s mantle. For the new model spar, Statoil is exploring a shortlist of “denser, heavier rock materials” to reduce the amount of ballast needed, but it has not yet arrived at a final choice.

Likewise, a decision on anchors and mooring lines, which hinges on further geotechnical studies under way, is still to be taken.

There is a bigger picture to think about too: how the five turbines — which will be arrayed three-and-two with intrafield cables designed for a single unit to be disconnected without short-circuiting power production — will be spaced apart at Buchan Deep to work optimally together.

“The influence of the wake effects in a wind farm will be an issue. But this is one of the reasons we are doing a pilot — to see how a commercial scale project would react to the wake effect and how we can control the movements of the turbines in an array configuration.”

Last November, Statoil — which had been working with the UK Crown Estate for two years on the Buchan Deep project — was at last granted a lease. “This has been a long time in discussion, so we are very pleased to now have the leasehold so that we can embark on the next development stage,” says Ulla.

With environmental impact surveys being carried out and wheels in motion to get government consent on the pilot, Statoil is taking a closer look at the currents at the deepwater site, bringing in Australian contractor Metocean Services International late last year to moor a buoy outfitted with a sub-surface acoustic device to take current measurements.

“The winds we know quite well — they blow, on average, over 10m/s. We haven’t got very good current data — but we will soon. From what we can see, Buchan Deep has at least as fast currents as across the North Sea in Norway.”

Slated for a final investment decision next year, the pilot would represent the home straight for commercialisation of the Hywind. After the first phase of its testing programme off Scotland, the plan is to move the technology onto the industrial-scale assembly line, currently in Ulla’s diary for 2020.

With Buchan Deep now well and truly on track, floating wind technology is building up a head of steam.

Principle Power’s WindFloat semi-submersible design is being worked up for multi-unit projects off the US West Coast and/or Portugal, and phase two of the Fukushima Forward project off Japan will see a pair of 7MW turbines installed this summer.

“We are positively surprised by how quickly the [floating wind] market has developed in a short space of time,” says Ulla.

“When we started planning the pilot years ago we didn’t think we’d be able to use the size of turbine that is now available — we were thinking 3MW [units]. These bigger turbines, the 6-8MW ones, are changing the economics entirely.

“As far as the official number for the global potential for floating wind, it is hard to estimate, but clear that the technical potential is very high — probably much higher than for bottom-fixed wind turbines.”