Credit: Pareto

By Susan Kraemer

Monopile, and some jacket foundations, still dominate the offshore wind market in commercial operation. Prototype floating foundations are performing well in demo or pilot projects, but scaling up is only just beginning.

Even the most recently announced offshore wind projects, with state-of-the-art turbines - up to 6MW and more - still employ conventional foundation technology. Geography and fear explain why that is.

Geography’s favourite

Vast unexploited shallow waters surround existing European projects, mostly less than 50 sea miles from shore, and under 50 meters deep; shallow and close enough to maximise the existing supply chain; conventional monopile and jacket foundations.

Instead of going further out into deeper waters, adding to farms within this shallow acreage makes economic sense, says Tim Fischer, Vice Director at Ramboll Wind. He questions the need for radical new design, since traditional monopile and jacket foundations still have potential for incremental cost reduction.

“Sometimes it's good to use technology you have - and optimise it further,” he says.

“It's like the hundred-year-old car business. Every year they‘re still able to reduce costs 2% by optimising small things. But they do it by keeping modest: they are not developing cars with 5 wheels.”

Failure’s cost

Last year DONG Energy installed the first jacket with suction buckets as a full-scale prototype at Borkum Riffgrund 1.

Ramboll designed the steel jacket foundation and buckets, establishing foundation loads in relation to soil stiffness to ensure safe levels of suction in installation. The interaction between jacket and soil will be monitored over time, through more than 100 instruments designed and installed by Norwegian Geotechnical Institute (NGI) who delivered the geotechnical design.

But because in 2005 a previous bucket design in Germany had failed during installation - even though the cause was accidental - no one attempted bucket for eight years.

Offshore wind is high risk. It can take years to re-try a new technology after failure.

Credit: korhil65

Paradigm shift

Meanwhile, most of the global potential - apart from the North Sea - is in deeper water, where fixed foundations are more challenging. Norway’s state-owned oil company Statoil has been Europe’s early mover, refining a floating design since the early days of the offshore wind industry.

“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,” says Trine Ingebjørg Ulla, Head of Wind Asset Management at Statoil. “That’s easier to fabricate than the semi-submersibles where you have all these parts you have to put together.”

Five years after deploying the single-turbine 2 MW Hywind 1 prototype, Statoil’s next step is now awaiting final permits. Its 30 MW Hywind Scotland project off Scotland’s northern coast will be at a 100 metre depth, supporting 6 MW turbines atop five 80 metre floating foundations.

Principle Power, a start-up which just merged with long-time partner Marine Innovation & Technology (MI&T) deployed its first prototype four years ago. The WindFloat, a three-legged triangular pontoon stabilised by semi-submersion has been stationed off Portugal, supporting a 2 MW turbine which has generated power since 2011.

Four years later, Principle Power is now also developing their first 30 MW project. Their WindFloat Pacific, partly funded by the US Department of Energy, is to be stationed off the coast of Oregon and comprises 5 floating foundations supporting 6 MW turbines.

Prototypes proved the technology is cost-effective and stable

Installation of these floating technologies is relatively minimal.

WindFloat is loaded with its turbine at the shipyard and simply floated out upright. Hywind is slowly ballasted with water and rocks to upright itself, and towed out using just small barges, embedding three drag anchors once on site.

Both floating prototypes have successfully survived storm conditions.

“The criteria in Portugal for a “one-year storm” is seven metre significant wave height and 11 metre maximum. At seven metres you should operate and 11 metres is survival conditions. We have survived these conditions three times so far,” CEO Alla Weinstein told Wind Energy Update last year.

Statoil’s Hywind has survived 90-mile-an-hour winds. “It’s been exposed to waves up to 19 metres, and winds up to 40 metres per second; a strong hurricane for Norway. It stood there very stable and moved up and down with the wave,” Ulla relates.

Several innovations contribute to Hywind's stability. At just 12 to 14 metres diameter, its slim profile reduces reaction to the waves, and heavy ballasting lowers its centre of gravity.

“We’ve developed a patented motion controller that is talking to the pitch control system of the turbine, so whenever you have the wind hitting the turbine; moving it backwards in a pitch movement, it will just release some of the forces from the wind. That lets some of the forces go, by pitching the blades, and that moves it back up again.”

A slow-moving affair

Statoil pioneered offshore foundation technology a half century ago when offshore oil and gas was a nascent industry, and began investigating floating offshore wind foundations in 2002. Yet the company doesn’t expect to finally be delivering utility-scale offshore wind foundations until 2020.

Japan’s heavy hitters like Mitsui are only just now entering their first rounds of testing of prototypes supporting one turbine at a time, like Principle Power and Statoil four and five years ago. They favour advanced spar designs, or various three-pronged semi-submersibles like the Windfloat.

Despite much-touted alternatives, clearly, actual innovation in offshore foundations is a very slow-moving affair, and perhaps that’s not a bad thing. When I asked Ramboll’s Fischer why even well-funded Statoil is moving so slowly on deploying floating technology at utility scale for offshore, he was unperturbed.

“Slow and steady wins the race,” he said.