Illustration: Gert K. Nielsen

Wind turbines to venture out into the deep sea

Tuesday 23 Apr 19


Henrik Bredmose
DTU Wind Energy
+45 45 25 43 15

Six partners in FloatStep

  • DTU is leading the FloatStep project, developing calculation models and participating in model tests.

  • DHI is developing calculation models and performing model tests involving waves and wind.

  • Siemens Gamesa Renewable Energy is developing calculation models and participating in a full-scale demonstration project in Norway.

  • Stiesdal Offshore Technologies is developing the TetraSpar foundation and leading the full-scale demonstration project in Norway.

  • Stromning is developing computational fluid dynamics (CFD) methods for detailed computer calculations of flows around the floater.

  • The University of Western Australia is providing data analysis and offshore hydrodynamics expertise.
Floating offshore wind turbines are gaining ground. A new Danish project aims to make it cheaper and easier to produce floating foundations for deep water.

There are only 11 floating offshore wind turbines in the world. The first Danish-developed floating wind turbine foundation is coming soon. It aims to pave the way for the wind turbines of the future at sea.

DTU researchers are currently working with leading companies in the field to optimize the design of the floating wind turbine foundations that make it possible to place offshore wind turbines in very deep water. Innovation Fund Denmark has invested EUR 2.2 million (DKK 15.8 million) in the FloatStep project.

The aim is to make it cheaper and easier to produce floating platforms and service the market for deep sea floating wind turbines, which is making the transition from ‘proof of concept’ to an emerging commercial market.

Offshore wind turbines are gaining more and more ground. But in coastal areas of USA, Asia, France, and Portugal, the sea is too deep to affix wind turbines to the sea bed, as is practized in the North Sea, where depths are often less than 60 metres. At greater depths, floating wind turbines are therefore the solution.

“Demonstration projects have so far shown that floating offshore wind turbines are a viable technology. We are also seeing political interest from USA and Asia in acquiring offshore wind energy. Denmark has to get involved in this market,” says Henrik Bredmose, Professor at DTU Wind Energy and the project’s academic officer.

“Denmark has been a pioneer in offshore wind energy. We now need to take the lead in relation to floating wind turbines and take advantage of the experience we have accumulated with fixed-base turbines in the North Sea and as a wind turbine supplier for the floating demonstration projects.”

Photo: Gert K. Nielsen
The TetraSpar foundation, developed by Stiesdal Offshore Technologies, was tested in DHI’s wave tank in 2017. Photo: DTU 

Better calculation models

To meet the challenge, DTU and the research consortium will draw on extensive experience with floating wind turbines—from industrial projects, EU projects, and model experiments at DHI . Over the next four years, researchers will use their knowledge to improve the tools and methods used to design and launch the floaters that the offshore wind turbines are founded on. This means that the project parties will develop and test new and improved calculation models that can be used for all types of floaters.

The calculation models will be validated against analyses of full-scale measurements from Norway, advanced calculations, and model experiments in DHI’s wave tank, in which a floating base has been exposed to strong winds and extreme waves, similar to the conditions that occur in the Atlantic Ocean.

Innovation at sea

Floating wind turbines are already being tested and developed in several places in the world. Under the Norwegian Hywind model, wind turbines are placed on a floating buoy, with ballast to a depth of 80 metres, and moored to the seabed. In the French Ideol project, the floater is shaped like a large barge on the surface.

In the Danish FloatStep project, researchers are testing a floating TetraSpar foundation, which is anchored to the seabed using chains to prevent the offshore wind turbine from floating away.
This foundation was developed by Stiesdal Offshore Technologies, and will be tested in 2020 with a full-scale 3.6 MW wind turbine in the sea near Stavanger, Norway.

The TetraSpar model is based on a mindset that is radically different from that of other floating wind turbine foundations.

“Normally, you design the general structure of an offshore wind turbine, and then look at how to make it,” says Henrik Stiesdal, CEO of Stiesdal Offshore Technologies.

“We base our design on the use of factory-made components for the wind turbine and foundation, which is far cheaper. Using these components, we can build a simple robust structure. The result is a dramatic reduction in costs compared to what we have seen so far.”

Photo: Gert K. Nielsen
The nacelle and rotor of DTU’s 10 MW wind turbine being tested in DHI’s deep-water pool at a scale of 1:60. The polystyrene balls are for optically measuring the turbine’s movements. Photo: DTU.

The Tetra Spar floater can be placed in water more than 100 metres deep. But the further out to sea you go, the more factors that come into play,” says Henrik Bredmose:

“Everything becomes more extreme at sea. The further out you go, the stronger the wind, and the more extreme the waves. We are therefore developing models to calculate how strong waves affect the floater and the mooring that holds it in place.


Waves represent a challenge for floating wind turbines in deep water. The FloatStep project is optimizing the design and installation of wind turbines.

Illustration: Gert K. Nielsen

Installation on the open sea

Installing a floating offshore wind turbine is more susceptible to the elements than a fixed-base turbine. To minimize costs, the TetraSpar floater is factory-made and assembled at the harbour. It is then sailed out to sea and attached to a mooring system, and the keel is lowered. But how big can the waves be when the wind turbine is being installed on the open sea? This is one of the questions being investigated using model trials and calculations.

 Illustration: Gert K. Nielsen

2. Vibrations in the turbine tower

Strong winds and high waves expose floating offshore wind turbines to vibrations on the open sea. The wind turbine tower and floater must therefore be carefully balanced to ensure that powerful oscillations do not arise. When the wind turbine reaches its maximum power, it adjusts the blade angle, minimizing the pressure on the floater—just like letting the sails out on a boat. Special control is required to avoid excessive floater rolling. Faster methods are therefore being developed for pre-design, that take into account tower oscillations and blade adjustment.

Illustration: Gert K. Nielsen  

3. Extreme waves hit floaters

Floating offshore wind turbines can be hit by extreme waves. It can create great pressure when water crashes into the steel structure and creates oscillations in the foundation and turbine. Detailed computer calculations (computational fluid dynamics) of the flow around the floater are used to show how the extreme waves hit the floating foundation and predict the effects. The calculations are based on a combination of simulations and engineering models.

 Illustration: Gert K. Nielsen  

4. Fatigue in the mooring system

Long waves, which come in groups, create oscillations in the mooring system. This can lead to metal fatigue in the anchor cables. Detailed calculations are therefore performed on these low-frequency waves. The calculation models will be verified using scale model testing at DHI and full-scale measurements from Norway




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