Offshore Wind Turbine Foundations – In Denmark and Belgium-PDF

PDF

Pdf Summary

The paper reviews lessons from offshore wind turbine foundation projects in Denmark and Belgium, focusing on two gravity-base designs that reflect the rapid growth in turbine size and the move into deeper, harsher waters.<br /><br />At Nysted, Denmark, 72 turbines of 2.3 MW were installed in relatively sheltered waters, 6–9.5 m deep, over stiff moraine clay. Boulders made monopiles unsuitable, while the soil supported gravity foundations. To enable transport and lifting, the concrete foundations were kept relatively light and built with open cells; heavy olivine ballast was added on site for stability. Close collaboration between designer and contractor helped tailor the design and installation process. The foundations were installed on schedule.<br /><br />Thornton Bank, Belgium, was planned for 60 turbines of 5 MW in the rough North Sea, at depths exceeding 25 m. Its foundation concept uses a post-tensioned concrete conical shell and cylindrical shaft. The design aims to provide stiffness, fatigue and crack resistance, efficient load transfer, and ballast capacity while limiting offshore work. Moving sand dunes and seabed scour also require careful consideration.<br /><br />The projects show that foundation design must integrate turbine dynamics, nonlinear soil stiffness, environmental loads, geotechnical conditions, construction, transport, and installation constraints. Design iterations between turbine and foundation engineers are essential, as are clear interfaces among project parties. Foundation weight must balance permanent stability against the limits of lifting and transport equipment.<br /><br />The authors conclude that no standard solution suits large turbines in deep water. Each project—and potentially each turbine location—requires a tailored design. Contracting approaches also affect design and cost: split contracts can allow foundations to be optimized for a contractor’s methods but demand strong client-led interface management; turnkey contracts can improve coordination, while client-led detailed design may reduce tender costs and risks but requires more generic foundation designs.

Keywords

offshore wind turbines

gravity-base foundations

Nysted Denmark

Thornton Bank Belgium

foundation design

deep-water installation

geotechnical conditions

post-tensioned concrete

ballast stability

contracting strategy