Talking To Statoil: The Engineering Of The 30 MW Hywind Scotland

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Statoil’s long-awaited Hywind Scotland, the 30 MW floating wind farm at Buchan Deep, 25 km offshore Peterhead in Aberdeenshire, Scotland, is ready to go.

With final consents in hand last year, finance was the last hurdle, along with the addition of a small 1 MWh battery. Now, with the final investment decision (FID) for the project arranged at $235.8 million, construction has begun on the floating Hywind project.

Statoil Hywind
Years of R&D have preceded this moment

The Norwegian state-owned energy giant Statoil had been moving at a measured and deliberate pace into floating offshore wind with its first single Hywind floating platform.

The original Hywind floating foundation supported a single 2 MW Siemens turbine, and has been operating successfully since 2009, proving the concept for today’s 30 MW Hywind Scotland composed of five turbines of 6 MW each.

“Back in 2002 when we started developing the Hywind floating foundation design, full scale meant a few megawatts. That was state of the art,” Statoil Head of Renewable Asset Management Trine Ingebjørg Ulla told Cleantechnica

“In 2009, basically seven years after we started thinking about it, we installed that 2 MW prototype in Norway. And it is still out there, producing. The basic idea was to see if we could use it in the North Sea outside Norway because we have deep water.”

The deep water was crucial. Ulla said that when offshore wind starts to leave its birthplace in the North Sea, then floating foundations become viable, because most of the world’s offshore wind potential is in water too deep for fixed foundations.

Offshore wind was initially developed in a unique geography. The North Sea is unusually shallow and sandy for more than 12 miles out from shore

“You have water that ranges from 15 to 25 meters in a very, very large area, so you could actually make these fixed foundations very efficient,” she said.

While these shallows are where the world’s offshore wind industry was born; now China, Japan, and the US are just starting to develop their own offshore wind.

These potential new markets for offshore wind have water too deep for fixed foundations.

Globally, the deeper water offshore wind potential is huge. The technical potential to supply the US, for example, is nearly 17,000 TW, more than all the electricity it currently uses

With cooler air, fewer space constraints, and stronger winds, and even with additional space for more wind farms, all offshore wind makes it possible to more fully exploit the world’s wind potential. NIMBYism also makes further-out wind farms a safer bet by reducing the financial risk of delay, but the further out offshore wind goes, the deeper the water tends to be.

For all these reasons, floating foundations represent the future of offshore wind.

The 30 MW-turbine Hywind Scotland wind farm will be arrayed at water depths ranging from 95 meters to 120 meters, like the deeper water typically found outside the North Sea.

The design concept for each of Statoil’s five Hywind floating turbine holders has the elegant simplicity of a needle: floating improbably upright.

Needle design

“We save a lot on the consistency of fabrication, because it’s only one thing; it’s a pipe basically with a lid in the bottom,” Ulla explained.

“That’s much easier to fabricate than the semi-submersibles where you have all these parts you have to put together.”

This utterly simple floating foundation makes installation absolutely minimal. Once in the water, the cylinder is slowly ballasted with water and rocks from the bottom to upright itself, and then can be easily towed out with the use of just small barges. This is revolutionary.

Normally foundations for offshore wind farms need enormous and specialized ships to install foundations into the seabed. There are only a handful of these gigantic purpose-built offshore wind installation ships in the world, which are able to install the massive foundations needed. Installation cost holds the biggest cost-reduction potential for offshore wind.

At just 12 to 14 meters diameter, each “needle” has a slim profile that reduces reaction to the waves, and heavy ballasting that lowers its center of gravity. The five floating foundations making up the 30 MW wind farm, are simply connected by cables. Three drag anchors for each one embed themselves in the sea floor as the movement of the waves shifts back and forth.

Because of the low center of gravity, the Hywind concept has proven itself to be remarkably stable in its seven years at sea

“A floating wind turbine is exposed to three separate forces, from the wind, from the waves and from the current. And all of these are affecting its movements, the way it twists and turns, the way it heaves up and down, and the way it pitches sideways”

“The most important thing is to be able to control that movement,” she said. “In that respect this Statoil needle design has some very beneficial features.”

Statoil’s Hywind prototype off the coast of Norway had survived 90-mile-an-hour winds in its years at sea. “It’s been exposed to waves up to 19 meters, and winds up to 40 meters per second; a strong hurricane for Norway,” Ulla explained. “It lies there very stable and just moves up and down with the wave.”

The new Scottish location has strong and steady winds, in the top class for wind speed — averaging 10 meters per second, which is much less than that hurricane extreme. Average wave heights there are up to 2 meters.

The higher and steadier wind speeds found offshore are one reason to develop offshore wind, because output rises as a cube of wind speed. Output increases 8 times in each doubling of wind speed, and is rated from one to seven with seven being the maximum. At 10 meters per second the Scottish location is a top-ranked seven.

Air density and altitude also affect power output; and air density is affected by temperature. The cold winds found off the coast of Scotland – at the greater height of these turbines – will be much more productive than even the same wind speeds harnessed by the same size turbine over warm land in the summer.

Hywind Scotland turbines by size

Offshore turbines are much taller and have much greater power than onshore wind turbines, where 2 MW and 3 MW turbines are the norm

Siemens has just announced that it will be supplying the five 6 MW turbines for the Hywind Scotland project. Siemens and DONG Energy have led the way in pioneering gigantic offshore turbines, and there are even testing facilities now that have planned to be able to test turbines up to 15 MW over the next years, based on their talks with leading turbine makers.

As with any technology, costs normally come down a bit with each subsequent deployment, but here Statoil has broken a record.

The 30 MW Hywind Scotland will cost $235.8 million. This represents an amazing 60% to 70% cost reduction – on a per megawatt basis – compared to the single turbine 2 MW Hywind pilot from 2009 that made this possible. Onshore wind is much cheaper than offshore of course, but as we start to refine the engineering and deploy more offshore wind, offshore wind costs are coming down too.

Statoil has plenty of experience it can apply to developing offshore foundations for wind, having pioneered offshore foundations for oil and gas a half century ago

“We started the same way in the oil and gas business. We started offshore by attaching things to the bottom,” Ulla said.

“But there too, you’ve seen the movement away from the fixed and over to the floating. So there’s been a lot of floating developments in very, very deep water over the last few years.”

Image Credits: Statoil

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