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Precast foundations anchor offshore turbines
Near the end of July 2008, the first wind turbine was set at the Thornton Bank Wind Farm, located in the North Sea on a 20 km (12.4 mile) long sand bank near the border between Belgium and the Netherlands. This marked a major milestone for an offshore wind power project owned by the Belgian company C-Power. This project will eventually provide 1000 GW?h of energy, which represents 1/3 of Belgium's 2010 renewable energy target and enough to provide electrical power for 600,000 Belgians for an entire year. Once the towers, nacelles, and rotors are installed on the remaining turbine foundations, the first phase of this three-phase project will begin producing energy. The next two phases of the project will install 54 additional wind turbines, bringing the total power capacity of the project to 300 MW.
The basis of design was developed by the owner's engineer, Technum/IMDC of Belgium, while detailed design was performed by the contractor's engineer, COWI of Denmark. The wind farm is located in almost 30 m (100 ft) deep water and nearly 30 km (18.6 miles) out into the sea, making it invisible from the Belgian coastline. To cope with the demanding loads and environmental conditions that the structures will be subjected to, the wind turbines are supported by steel towers resting on gravity base foundations made of precast concrete with vertical post-tensioning. The construction of the foundations was a major challenge not only because of the harsh environment that they are expected to withstand, but also because of the innovative method used to install them.
SITE PREPARATION
Two sites had to be prepared for the foundations-one at the onshore precasting yard and one at the offshore final location. The precasting yard, shown in Fig. 1, was located in a harbor at the Halve Maan workyard in Oostende, Belgium, almost 40 km (25 miles) from the final location. As shown in Fig. 2, the construction zones included 460 mm (18 in.) diameter piles, a 560 mm (22 in.) thick base plate, and 1.3 m (4.3 ft) tall concrete blocks. The space between the concrete blocks was open to allow heavy transporters to travel beneath the completed turbine foundation, lift it off the concrete blocks, and...