IN DEPTH: Japan's fixed foundations

The Hibikinada project has a concrete base built for stability in an earthquake

The Hibikinada project has a concrete base built for stability in an earthquake

Pioneering floating wind projects may have stolen the limelight in Japan, but cheaper fixed-foundation offshore developments are likely to be built on a commercial-scale first.

Although the island nation’s narrow continental shelf means its potential for shallow-water projects is limited to a maximum of 23GW, compared to 519GW of floating wind, the cost of energy of the former is expected to be four to six times cheaper — ¥50 ($0.48) per kWh against ¥200-300.

So on top of investing billions of yen in floating pilot schemes, the government is ploughing ¥4bn into two shallow-water demonstration projects that are testing different types of fixed-foundation technologies.

Just off the eastern coast, near Choshi in Chiba prefecture, a concrete gravity base (CGB) topped with a 2.4MW Mitsubishi Heavy turbine was switched on in March 2013; while in the southwest, off Hibikinada, Fukuoka prefecture, a hybrid steel and concrete foundation fitted with a Japan Steel Works (JSW) 2MW machine was brought on line last summer.

“Floating wind turbines will be very important, but also very expensive, so we have also been experimenting with [seabed] fixed solutions,” states Michio Hashimoto, the director general of the New Energy and Industrial Technology Development Organisation (Nedo), the government agency that is overseeing both projects.

“Though it is more expensive than onshore, we see the fixed type [of offshore wind turbines] to be potentially economical if the technology is further developed and the installation is improved.”

The Choshi CGB, installed in 12 metres of water at Byobugaura, is a “self-installing” flask-shaped unit similar to designs used at European shallow-water projects such as the pioneering Nysted development off Denmark and Belgium’s Thornton Bank, but never tested in seismically active regions such as Japan.

foundations_2nd.jpgThe earthquake-proof, pre-stressed concrete foundation, measuring 21 metres in diameter and weighing 2,300 tonnes, was fabricated by Kashima Construction in Kashima port. Installation in early 2012 was a tricky affair, as the development site lies at the crossing point of the powerful Oyashio and Kuroshio ocean currents. Veteran divers brought in to help position the CGB on the seabed were often in danger of being swept away in the ripping flow.

After being floated out to site, the CGB — a concept based around an arrangement of internal chambers that can be gradually filled with water to lower the foundation to the seabed — was topped out with the wind turbine’s tower and 92-metre-diameter rotor and nacelle.

The pilot, being operated with utility Tokyo Electric (Tepco), will be run until spring 2015 in average winds of seven metres per second (m/s).

“The performance of the substructure, I have been informed, is very good and the turbine has been running at better than 35% capacity, which [given the wind resource] is quite good too,” says Hashimoto.

At Hibikinada, the hybrid foundation concept used for the 82-metre-diameter direct-drive JSW turbine is an industry first. Built and installed by Goyo Construction in 15 metres of water, the Kitakyushu demonstrator is based on a design that mates a four-legged trussed-steel jacket with a concrete caission base — seen as a better bet for stability in an earthquake than a conventional piled-in steel jacket.

“This was a design meant to provide something entirely new [for mid-range water depths]. With a monopile or gravity-base foundation there are very big wave loads, but with this hybrid jacket the waves pass through,” states Hashimoto.

“Also there is potentially less scour [where sediment-laden seafloor currents eat away at the soil around the base of the foundation, destabilising the turbine].”

Developed jointly with Japan’s Electric Power Development, the Kitakyushu demonstrator will be run in winds averaging 6m/s. Like the concept chosen for Choshi, this unit is seen as having “low environmental impact” with little installation noise pollution — which is important in a country with such an influential fisheries lobby.

The Choshi and Hibikinada sites were picked from a shortlist of six on the grounds of their “stable wind conditions and interesting technical challenges”.

“These projects are not only a test of the technology concepts, but are also helping us gain experience in building foundations in Japan and look beyond the monopile, which is currently the most economic offshore foundation,” says Hashimoto.

“The monopile is a known technology. The Choshi and Hibikinada demonstrators are allowing us to expand the technical options for the Japanese [offshore wind] situation, although they have been installed in water depths that could have used monopiles.

“But we have been somewhat conservative [in terms of water depths and technology concepts] because if these projects were to fail, it would effectively have killed off Japanese offshore wind.”

The idea of the two Nedo pilots is plainly commercial: to prove up the technologies for use off Japan, while showcasing them for export into international markets.

“It is true that Japan’s shallow-water wind resource is somewhat limited, but still the fixed foundations will come first before [widespread take-up of] floating wind turbines,” states Hashimoto. Choshi and Hibikinada, he notes, can point the way for a government looking for a step-wise “prioritising of near-shore wind before going into the more expensive option of floating wind”.

“Talking to Japanese wind farm developers, they see the benefit in having different types of foundation solutions, along with monopiles, to provide technical options in securing backing from investors in future projects — maybe first in the areas around Choshi and Hibikinada, where we are proving these technology can work economically.”

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