Floating wind turbines: a promising start

Floating wind turbines have a very short history and the market is still in its infancy, but today it seems that many parties believe in the technique’s future. So what is the state of the market today, and what are the pros and cons of these large floating power plants?

In 2009, Norway had a world premiere when the state-owned energy company Statoil installed a full-sized floating wind turbine off the coast of the southern Åmøyfjord, where it had been assembled. The turbine is called HyWind and was built in collaboration with Siemens and offshore technology company Technip. The turbine, with a power capacity of 2.3 MW, has been producing energy ever since. In 2011 it produced 10.1 GWh; much more than Statoil had expected, confirms Statoil’s spokesman Morten Eek. So far, 2012 has been somewhat less successful, but that was due to technical issues in the power grid, not problems with the turbine itself.

For three years, the HyWind turbine was a lone pioneer. Until last spring, that is, when the WindFloat and its 2 MW Vestas turbine was moored to the seabed with four cables off the coast of the northern Portuguese town of Porto. Interestingly, this turbine uses the same export cable that was laid there in 2008 for the Pelamis wave power prototype. If all goes well, the turbine will be accompanied by five more floating turbines within the next few years.

The utility company Energias de Portugal (EDP), one of the developers of the WindFloat, estimated that at water depths of between 40 meters and 200 meters, the Portuguese waters can potentially deliver more than 10 GW of wind energy. Unfortunately, these depths are too deep for conventional offshore turbines, so it seems that investigating floating concepts could well pay off in the long run.HyWind Source Statoil

The bulk of the current plans for European offshore wind power are slated for construction in the shallow North Sea. However, many European countries, including Portugal, do not border shallow seas. Instead, their seabeds have steep descending slopes and much greater water depths that are not suited for conventional techniques. If a viable technique can be developed to deal with this problem, then many more countries could profit from offshore wind energy. The big winners of such a scenario would be remote places and islands like Hawaii, where energy is scarce and expensive.

Three main floating turbine techniques can be distinguished. The first technique, which was used on the HyWind in Norway, is called the ‘spar’. A spar is a long tube that is erected by adding ballast – water and stones – to one end. This results in a structure of which the largest part is actually beneath sea level. In the case of HyWind, the underwater structure is 100 meters deep, whereas the part above water reaches a height of 65 meters (the turbine blades add another 40 meters). This ratio is important to ensure the stability of the turbine. In no circumstances can it be allowed to fall over, and the deeper it protrudes into the water, the less it moves. Depth increases stability, just as it does in a fishing bobber. The spar is moored to the seabed with several cables.

The second technique is called the semi-submersible installation. This is a large floating platform with three or four interconnected columns that are placed as far as possible from each other. One or more turbines can then be mounted on top of this platform. The columns contain ballast and are partly submerged – hence the name. This technique obtains its stability from the large surface area and weight. The WindFloat in Portugal was built using this technique, which originated in the offshore oil and gas industry, as do many other floating technologies.

The last technique is called TLP (tension leg platform). This technique is a hybrid of the two techniques described above. Like the semi-submersible, it has several legs, but these legs come together in the middle to support the turbine like a spar structure. However, instead of a large ballasted submerged structure providing the stability, the TLP is prevented from capsizing by stiff cables, or usually pipes, dragging it downwards. TLPs are lightweight structures and inexpensive to build, but are complex to connect to the seabed and to stabilize. The Dutch firm Blue H technologies tested an 80kW TLP-based floating wind turbine off the coast of the Italian region Puglia as early as 2008.

Scale models

The Dutch research center MARIN (Maritime Research Institute Netherlands) plays an important role in the worldwide development of floating turbines. The institute has its background in testing floating structures for the offshore oil and gas industry, but in recent years Marin has tested several floating turbine scale models. It has also recently built a new high-quality wind generator specifically to test the behavior of floating turbines in bad weather. “Waves and currents are the most dominating factors for floating equipment in the oil and gas industry”, says researcher Erik-Jan de Ridder from Marin, “But for floating wind turbines, wind appears to be the biggest factor. That is why we had to generate higher quality wind in the model basin than was previously necessary.”

To simulate real wind conditions, MARIN set up multiple large fans and put them in a box with holes to streamline the wind. “Without that box, the fans produce too much turbulence, and you won’t see this turbulence in the real world. We do not have the beautiful wind that can be produced in wind tunnels, but we are getting pretty close”, says De Ridder. This gives MARIN’s model testing basin a unique position in the world. “There are several other water tanks in the world, for example MarinTek in Norway and tanks in Brazil and China, but they are not as popular as ours.”

‘I think all techniques can co-exist’

De Ridder does not want to choose between different float techniques. “They all have their own features”, he says, “A spar reaches very deep. That is no problem for countries like Norway, where they can be assembled in a fjord, and fjords are usually very deep. From there, they can be towed to their destination. But in Portugal, the harbors are not so deep, so there a semi-submersible would be a more obvious technique. I think all techniques can co-exist. They do not necessarily compete with one another.”

One distinctive feature all floating turbines – well at least semi-submersibles and spars – share in comparison to fixed turbines is that maintenance can be done in the harbor. De Ridder: “Let’s say that a turbine is really broke, to the point where it will take several weeks to fix it. It does not happen very often that weather conditions at sea stay calm for weeks on end. So if you have to repair a turbine at sea, chances are that it will take a lot longer than necessary. Moreover, personnel and materials are much cheaper in harbors than at sea.”

Europe seems to take the lead when it comes to floating turbines, but Japan and the US appear to be catching up fast.

Japan is in a transition period after an earthquake and the subsequent tsunami destroyed the Fukushima nuclear power plant one and a half year ago. The government recently announced subsidized plans for three floating multiple MW turbines: one semi-submersible 2 MW installation, one semi-submersible with a turbine that reaches 7 MW and one ‘advanced’ spar that can generate 7 MW. Moreover, Japan should be building the world’s first floating substation in the very near future, and in 2020 an installed capacity of 1GW of turbines should be floating in the waters around the north of Japan. The technology company Mitsubishi plays a leading role in this development.

The United States has also showed an interest in floating turbines. In 2010, the US government subsidized a large research project called DeepCwind, run by the University of Maine. The money was intended to be used in the US, but some of it ended up on the bank account of MARIN in The Netherlands. “They had to spend the money in their own country if possible, but we had the equipment that they lacked. We had a unique position with our new wind setup”, says De Ridder from MARIN. “We got that contract, and we were very happy with that.” This resulted in a collaboration with the University of Maine. “Together we are now designing a scale model for a wind turbine based on the lessons learned in the previous model test campaign.” One result of the research done by the University of Maine is an extensive list of floating turbine projects, which Offshore Wind Magazine has published in this article.

The United States are earnest in their ambitions, and the US Interior Department has contacted Statoil to set up a project off the coast of Maine. The Norwegians are eager to cash on their initial R&D investment in Norway and would like to build four 3 MW turbines with ‘second generation’ HyWind technology, confirms spokesman Morten Eek. It is not yet clear whether these plans will go through, however. The Interior Department (Bureau of Ocean Energy Management) is now trying to arrange for the necessary permits and licenses.

In the lee of these developments, many researchers are working to find solutions for one of the biggest cons of floating turbines: wear and tear. All of the sprockets and gears inside the turbine that transfer the movement of the blades to the electricity generator are affected by the movement of the turbine itself. Erik-Jan de Ridder: “Small movements at the bottom of the turbine mean a lot of movement at the top.” There are solutions to this, explains the researcher: “You can try to use direct drive generators, which have fewer parts that can be damaged. Or you can build larger platforms with greater stability. The problem is that these measures all increase the costs.” A third solution is more revolutionary: turn the blades 90 degrees, effectively turning the turbine into a vertical axis wind turbine, or VAWT. De Ridder: “This has some advantages. First, it enables you to generate the electricity at the bottom, where the forces are less intense. Moreover, maintenance can be done from ground level, which is easier and less expensive.”

A European consortium of research institutions and technology companies is now working on a concept that utilizes this technology. The Deepwind consortium, as it is called, will deliver a scale model that will be tested in the MARIN water tank in March 2013. According to De Ridder, the consortium will investigate whether generation capacities of 5 MW to 20 MW are feasible using a VAWT.

Another interesting development can be seen in the Mediterranean waters near Marseille in the coming period. The French company Technip, that also was involved with HyWind, will lead a consortium of six European companies that tries to build the 2 MW INFLOW (Industrialization setup of a Floating Offshore Wind turbine) demo project, a follow up on the Vertiwind vertical axis turbine project of Nénuphar in Lille.

Should developers of ‘conventional’ offshore wind turbines worry? Can floating turbines really compete? De Ridder doesn’t want to answer that question: “I just don’t know.” But he does know this: floating turbines should be employable in water depths up to 50 meters, which means that they are also suited for the shallow waters of the North Sea. If you combine this fact with the performance of the HyWind in 2011, which reached an unexpected capacity of 50 percent, according to Statoil’s spokesman Morten Eek, then maybe these developers have something to worry about.

By Tijdo van der Zee, Offshore Winde Magazine, 2013

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