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Coastal caissons
8/10/03
Danish contractor Per Aarsleff recently finished one of its largest-ever foundations projects - the Nysted offshore windfarm in the Baltic Sea

At the end of May, the last consignment of massive gravity foundations for Denmark's DKK1.6bn (Euro 215M) Nysted offshore windfarm started its 200km journey from a casting yard on Poland's Baltic coast.

The event marked the final phase of Danish contractor Per Aarsleff's Euro 40M contract for client consortium Energi E2, DONG and Sydkraft to build and install 73, 1,300t concrete caisson foundations for the energy facility off the coast of eastern Denmark.

Once at Rødsand in the Femer Bælt, about 9km off the coast of Lolland, Per Aarsleff's team placed and ballasted the caissons on prepared stone pads on the seabed. All were in place by the end of June, three weeks ahead of schedule.

The caissons, stand in 7m to 13m of water and each is ballasted with 500t of rock and sand, arranged in a parallelogram grid of eight rows of nine. The rows are 850m apart and the caissons are at 480m centres in each row.

The main 72 foundations will support 2.2MW Bonus Energy three-bladed turbines on top of tapered tubular steel towers which rise 69m above sea level.

The remaining caisson will support the transformer substation for the 159MW windfarm, which covers an area of approximately 24 square km.

"The project went very well and fortunately during February and March we have had the least windy weather here for 25 years," says client foundation supervising engineer John Nielsen.

Per Aarsleff built the caissons in casting yard in a free bonded area of the Odraport at Swinoujscie on Poland's Baltic coast. The sheltered basin was ideally suited for caisson construction and, coupled with the local labour that is significantly cheaper than in Denmark, was a contributory factor in Per Aarsleff winning the contract.

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