Offshore wind turbine from aerial view

On the high seas, the wind yield is particularly high.

© iStock.com/fokkebok

The next-generation offshore wind turbines are giants of the seas: From the surface of the water to the tip of the rotor, the giant towers measure 260 meters and thus tower over Cologne Cathedral by more than 100 meters. The rotor blades have a greater wingspan than an Airbus 380 jet. The new turbines generate up to twelve megawatts, enough to supply 19,000 households with electricity.

The new gigantic turbines pose completely different technical challenges. BAM has been researching the safety of offshore wind turbines for almost two decades and thus supports the switch to renewable energies. The focus is primarily on the parts subject to particular mechanical stress: Foundation piles, support structures and rotor blades. "Huge forces act on the structures," says BAM engineer Matthias Baeßler. "On the open sea, the towers have to withstand strong currents, swells and raging storms. This is especially true for the new giant towers."

Supported by a single pile of steel

All the more surprising, then, that the giant offshore wind turbines are supported by a single steel pile, or monopile, driven deep into the seabed.

Of course, with the latest generation of offshore turbines, these monopiles are also gigantic: 40 meters in length, the steel tubes measure nine meters in diameter. "If the tubes were lying on the ground from assembly, you could easily push a single-family house into them," says Matthias Baeßler.

The monopiles that support the turbines measure up to 40 meters

The monopiles that support the turbines measure up to 40 meters.

© Steelwind Nordenham GmbH, 2020

20 meter long welds

Still on land, the tubes are welded together from individual steel sheets. "These weld seams are several meters long, sometimes up to 20 meters," explains Thomas Kannengießer, who is an expert on safe welded joints at BAM.

Today, a high-tech steel is mostly used that is stronger than conventional steel. Thomas Kannengießer is researching how the complex alloys can be optimally joined. Above all, it is important to avoid cracks during welding, which can be caused by the high heat. BAM is testing how to preheat the steel to obtain an ideal weld.

Once the steel sheets have been welded together to form the 40-meter-long tubes, they are driven into the seabed at sea with a pile hammer so that the supporting structure of the wind turbine can later be securely fixed to them.

Before transportation, the tubes are welded together from steel sheets

How do you avoid "pile dents"?

Due to the large forces involved and the resistance of the ground, large deformations can occur at the base of the piles. "However, this must not happen under any circumstances, because it would compromise the structural safety of the entire structure," explains Matthias Baeßler.

That's why his colleague Pablo Cuéllar is developing models on the computer that describe the interaction of the piles with the seabed. They should help clarify how best to drive them into the ground without causing the deformations. And how thick the steel sheets need to be to anchor a giant tower 260 meters high in place.

Field tests with different soils

The BAM scientists will soon be testing their models in field trials - on land, of course. Together with the Technical University of Berlin, they will drive piles of different thicknesses into soils of different compositions and observe whether deformations form at the base.

Piles as small as about one meter in diameter and 10 meters long will suffice for them to do this. "This is completely sufficient to clarify whether our computer model provides reliable statements," says Matthias Baeßler.

After the tests, the BAM scientists will be able to say how precisely the piles must be dimensioned in order to securely fasten the new offshore wind turbines. Giant towers that reach higher into the sky than the largest Gothic cathedrals in the world.

For more information on our renewable energy research, please visit www.bam.de/renewable-energy.