Consolidation of Monopile Foundations at an Offshore Wind Energy Plants

Context

The Amrumbank West wind park 35km north of Helgoland has a surface area of 32km² and consists of 80 wind towers. The park was built in water that is approx. 20-25m deep. Each tower has a hub height of 90m above the water surface, each turbine has an output of 3.6MW, and the rotors are 120 meters in diameter. With a total output of 288MW, the park can generate energy for up to 300,000 households.

During the ramming of two monopile foundations, two construction joints were damaged between the lower section at a height of approx. 6m above the water surface and the upper section that accommodates the working platform.

Solution

In order to be able to establish the full load capacity of the two construction joints, 52 13.3m long Wire EX Tendons were used for each of the two monopile foundations. The tendons were installed in an area from 6.5m below the construction joint up to 6.5m above the construction joint, and that area was later concreted. By applying a post-tensioning load of 4,000kN per tendon, the full load capacity of the construction joints was reestablished.

The Wire EX Tendons that were used for this purpose had a total tonnage of 35t and were prefabricated and adapted to this special use by DYWIDAG in Langenfeld, Germany. The tendons were equipped with lost formwork in the form of a second sheathing that had to permit deformation by compression during the tensioning operation.

In order to simplify the assembly process on site, the required additional reinforcement was immediately added to the anchorage area. This way, the time required for assembling the tendons at the respective sites was minimized, which was extremely important for work in the North Sea.

The cavity between the tendons’ PE sheathing and the second sheathing was filled using corrosion protection compound, preventing any sea water that might enter into the monopile from reaching the tendon. Furthermore, full corrosion protection was also ensured during transportation of the tendons across the North Sea.

Due to the fact that the offshore jobsite was difficult to access, twice the number of jacks and hydraulic pumps needed for tensioning was provided in order to minimize the possibility of delays due to equipment failure. The jacks that were used were Type HOZ 4000/250 jacks as well as 495t jacks.

Work was successfully completed in limited space on site. In order to ensure a reliable monitoring of the forces acting on the tendons, some of the tendons were equipped with DYNA Force® Sensors.