C-Power N.V. Cast In Belgium, 2,700-Tonne Concrete Wind-Turbine Bases Are Headed Offshore.

As offshore wind turbines grow in power, they need increasingly robust supports. Six of the world’s biggest new-generation concrete gravity bases have recently been placed in depths up to 27 m in the North Sea to carry 5-MW turbines.

Installed 30 km off Belgium since late April, the hollow, prestressed 2,700-tonne concrete bases are the first of the $240-million, 60-unit Thornton Bank farm. The 300-MW first phase is due for completion this September.

With 126-m-dia. blades, the turbines will sit on 6.5-m-dia. steel pylons rising 77 m from the gravity bases. The 44-m-tall bases comprise cylindrical shafts cast atop 17-m-tall cones, which spread to 23-m-dia. Their walls are generally 50-cm-thick.

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A traditional steel “monopile” driven into the seabed was studied but rejected, says Geert Dewaele, construction manager for the owner, C-Power N.V., Zwijndrecht, Belgium. Needing to be 6-m-dia. with 6-cm-thick walls, “no company was able to put these piles in place,” he adds.

Dredging International N.V., Zwijndrecht, leads construction and installation of the gravity bases, under a $65-million contract started last August.

“As every single casting section [of the cone] has a different diameter, there [were] five separate sets of formwork in service,” says a spokesman for Austria’s Doka Industrie GmbH., Amstetten. Units of the firm supplied the climbing formwork to the casting site in Ostend, Belgium.

A 440-tonne-capacity crane barge transported and placed the last base into a dredged trench early in June. After backfilling the trenches and placing rock erosion protection, the contractor will ballast the bases with sand.

Then, under a separate $75-million contract, Germany’s REpower A.G., Hamburg, will be responsible for placing completed assemblies of turbines, blades and pylons. The first steel pylon was due in Ostend about June 19.