As offshore wind technology develops ever more gigantic machines further from shore into deeper waters, it is common wisdom that new foundation technologies are needed. But are they?
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 - with Statoil’s 30 MW Hywind project in Scotland, for example.
Even the most recently announced offshore wind projects, with state-of-the-art turbines - up to 6MW and more - still employ conventional foundation technology.
Why is there so little innovation?
Vast unexploited shallow waters surround existing European projects, mostly less than 50 sea miles from shore and under 50 meters deep. They are 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.”
Last year DONG Energy installed the first jacket with suction buckets as a full-scale prototype at Borkum Riffgrund.
Ramboll designed the steel jacket foundation and buckets, establishing foundation loads in relation to soil stiffnesses 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 the Norwegian Geotechnical Institute (NGI), which 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.
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 the financial wherewithal to safely develop floating foundations - its Hywind design is elegantly simple.
“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.”
Installation is minimal too. The cylinder is slowly ballasted with water and rocks to upright itself and easily towed out with just small barges, embedding three drag anchors.
Five years after its single-turbine 2 MW Hywind 1 prototype, Statoil predicts supplying offshore wind by 2020. Its 30 MW Hywind Scotland project awaits final permits off Scotland’s northern coast, at a 100 metre depth, supporting 6 MW turbines atop five 80 metre floating foundations.
Statoil pioneered offshore foundation technology a half century ago when offshore oil & gas was a nascent industry.
“We’re actually the world’s largest oil & gas operator in deep water - so we have the confidence to start thinking about this,” she explains.
Startup Principle Power just merged with long-time partner Marine Innovation & Technology (MI&T). Its WindFloat prototype, a three-legged triangular pontoon stabilised by semi-submersion, has been stationed off Portugal, supporting a 2 MW turbine generating power since 2011.
Floating is stable
Both floating designs survive storm conditions well.
“One year storm criteria in Portugal is 7 metre significant wave height and 11 metre maximum. At 7 metres you should operate and 11 metres is survival conditions,” CEO Alla Weinstein told WindEnergyUpdate last year.
“We've survived those conditions three times so far.”
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 lies there very stable and moves 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.
In addition, she explains: “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; letting some of the forces go, by pitching the blades and that moves it back up again.”
Statoil began investigating floating offshore wind in 2002. Despite much-touted alternatives, perhaps options for offshore foundations will arrive slowly.