Floating offshore wind farms in deep waters are one of the potential solutions to expose the turbines to high and consistent winds and harness inexpensive green energy. However, so far, the overall production and installation costs of floating offshore wind farms are uncompetitive, driving the cost per kilowatt-hour (kWh) up compared to bottom-fixed offshore wind turbines that stand firmly on the seabed.

The potential for building more traditional bottom-fixed offshore wind is limited. Although Europe has plenty of sea and coastline, these types of wind turbines are economically feasible only in waters up to 50 meters deep, which means the turbines must be relatively close to the coast – often within 10-20 km from land. And in these areas, due to socioenvironmental reasons, it is often not permitted to set up offshore wind turbines.

Therefore, it is necessary to find solutions to place wind farms further out at sea, away from coastal activities and protected areas, and out of sight.

“Only a fraction of the area is available for offshore wind development and with public support dwindling due to the “not in my backyard" syndrome maybe the solution is to make them float further away. This enables the developers to harness more consistent and stronger winds, in areas of the ocean that have more potent wind resources” says PhD student David Stamenov. His PhD thesis which he is about to defend is part of the CYBERLAB project.  

Mathematical model helps design optimal testing

To make this technology attractive it is necessary to develop stable, gigantic floating elements that support the turbines. Steel mass is one of the factors driving up the costs of floating turbines (in addition to marine operations and required logistics). On the other hand, it is clear that larger floaters in general means cheaper kWh, which is why developers are now approaching 20 MW turbines.

In this development, experimental testing is a crucial component to ensure that the turbines are stable in storms and can withstand waves and rough conditions over many years.

But what should be tested and how should this be done? This challenge has been addressed by the CYBERLAB project: Not by testing the full scale 100x100 meters floater far out at sea, but with a smaller model that was tested in a wave basin at SINTEF Ocean, a research institute in Norway. The project is now completed, and the researchers succeeded in developing a mathematical model that can determine which design parameters are most sensitive and impact the behavior of the structure. Based on that, an efficient testing campaign can be executed which will provide the best answers on how a real floating offshore wind turbine will react under the exact conditions it will be exposed to far out at sea.

“By using the mathematical model we developed, it is possible to determine what are the best experiments that will excite the model in a particular way and reveal its behavior and by performing significantly fewer tests than is usually done. We achieved the same results with two tests as what would be achieved with five to seven tests, “says David Stamenov.

Less expensive mooring system

Only very few projects around the world are developing solutions for floating wind turbines. All of them are experimental research projects generally working on the assumption that each wind turbine should be anchored to the sea bottom, which is also more complex and expensive at larger depths than closer to the coast.

Even though deep waters offer greater spatial opportunities for offshore wind farms, and also potentially the possibility to harness more consistent and stronger winds further out at sea, the floating wind industry has yet to attract mass investment. Although costs are dropping fast, there is still more work to do.  

The mooring system is one component that is yet to be optimized and a potential way to reduce costs. One option is to connect the floaters with shared lines in a repeating pattern, which can substantially reduce the number of seabed anchors required. But this too requires extensive research and testing before it can gain investor confidence.

“If you imagine giant structures tied together, you want to make sure, that the loads that you predict in the tests and numerical models are what the wind farm will actually experience. Because if you miscalculate the loads and overload some of these lines, and if they break, you could have a progressive failure where the whole farm gets blown out”, says David Stamenov.

No time or money for trial and error

There is also still no consensus on how the giant floaters are constructed optimally. At the moment there are hundreds of floater concepts of different kinds. The experiment was conducted using a far smaller replica of one of the floater types, suitable for work in the test facility.

• “It is not yet clear which type is the best possible floater, because they are huge, they have to resist the sea loads, but they also must be cheap, they have to be assembled somewhere, and a block of 100 by 100 meters is not easy to assemble”, says Giuseppe Abbiati, Associate Professor and Head of the Structural Engineering Section at the Department of Civil and Architectural Engineering at Aarhus University.

That is why efficient and realistic tests are crucial for development. The tests need to give answers to what happens when a gigantic block of steel is floating in the sea for 30 years.

“Trial and error have taught engineers valuable lessons over the years. You learn from your mistakes, and you improve. We are good at designing buildings and bridges because we have done it for 3.000 years. Floating wind turbines? Not really. So, to gain understanding and be able to say one floater is better than another might take a lot of experiments. Experimentation in this phase where technology is not well known is crucial”, says Giuseppe Abbiati.

For LinkedIn

Simulation to make testing of floating offshore wind turbines more efficient

Researchers from the @Department of Civil and Architectural Engineering, Aarhus University have developed a mathematical method to optimize the testing of giant floating offshore wind turbines. The hope is that more efficient trials can help accelerate the development of offshore wind farms in deep waters.

Floating wind farms are a possible key to unlocking the strong and consistent winds found in deep waters. But the technology is still expensive and complex to test. As part of the CYBERLAB project, PhD student David Stamenov and his colleagues have created a model that identifies which design parameters matter most – and how to test them efficiently when it comes to developing offshore deep water turbine parks.

To make this technology attractive it is necessary to develop stable, gigantic floating elements that support the turbines. Steel mass is one of the factors driving up the costs of floating turbines (in addition to marine operations and required logistics). On the other hand, it is clear that larger floaters in general means cheaper kWh, which is why developers are now approaching 20 MW turbines.

In this development, experimental testing is a crucial component to ensure that the turbines are stable in storms and can withstand waves and rough conditions over many years.

But what should be tested and how should this be done? This challenge has been addressed by the CYBERLAB project: Not by testing the full scale 100x100 meters floater far out at sea, but with a smaller model that was tested in a wave basin at SINTEF Ocean, a research institute in Norway. The project is now completed, and the researchers succeeded in developing a mathematical model that can determine which design parameters are most sensitive and impact the behavior of the structure. Based on that, an efficient testing campaign can be executed which will provide the best answers on how a real floating offshore wind turbine will react under the exact conditions it will be exposed to far out at sea.

“By using the mathematical model we developed, it is possible to determine what are the best experiments that will excite the model in a particular way and reveal its behavior and by performing significantly fewer tests than is usually done. We achieved the same results with two tests as what would be achieved with five to seven tests, “says David Stamenov.

Read more: (Link to article om cae.au.dk)

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