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Hywind floating offshore wind turbine foundation


Kari Larsen

“We didn’t understand why it shouldn’t be possible to mount a wind turbine on a floating construction,” Sjur Bratland, Asset Manager for Hywind at Statoil.

He says Statoil believes floating offshore wind has great potential as “it is not limited by the same conflict of interests as onshore wind and offshore wind near shore.” Furthermore, wind farm developers have greater flexibility when it comes to wind farm design in terms of considering bird migration, shipping lanes, etc. Wind farms can then also be taken further offshore, reducing visibility from shore.

Bratland calls the floating offshore widn turbine foundation prototype outside Karmøy an “eye opener”: “Many people did not believe in this concept a few years ago. It is also a verification of our model produced in 2001, which was verified in a pool test in 2005 [at NTNU in Trondheim, Norway]; and now we want to verify that in real life.”

Once the substructure had been produced by Tecnip in Finland and towed to Stavanger, Norway, it was up-bended – i.e. it was filled with water and then raised from horizontal to vertical position. This and the commissioning were done near shore, protected from large waves. Three floating-to-floating lifts were used for this. Once the tower, nacelle and blades had been attached, the whole construction was towed into the field where the anchor had been pre-installed.

The offshore wind turbine foundation itself is based on a slender cylinder. “That means it has as little water plane area as possible,” Bratland says. “We are tying to reduce the impact of the base on the structure. We also have a low gravity point, which makes it quite stable. It has very little heave movement; it has a bit of pitch movement, but not much.” The blades are pitched in and out of the wind to reduce movements of the structure.

The reason for choosing steel for the foundation was that Tecnip had a good proposal using steel, but Bratland says the possibility of using concrete is still there. He underlines that the prototype is there to learn from. This is why there are over 200 measurement points around the construction to monitor the behaviour of the offshore wind turbine and mooring line, and the fatigue and stress on all elements.

No show-stoppers

The Hywind project has not run into any show-stopping problems so far- “it’s more about all the small details.

Siemens was a bit anxious about the floating-to-floating lift we were going to use, because they had not done it before. We had experience with it, so we felt that was something we had control over,” Bratland explains. Some were also anxious about the towing, whereas this again was not an area of concern for Statoil. “It was more that the wind industry did not have the offshore experience we had – we were more anxious about other issues.”

Changes for the future

Bratland says Statoil has made several experiences with the Hywind floating offshore wind turbine foundation project and that there are things that will be done differently next time.

“We have used a drag anchor, but I think we will be looking into some different issues there. We will also reduce the number of tug boats for towing the turbine – we were too careful. We would probably also like to do even more commissioning onshore or inshore, because it is much easier to commission onshore and inshore, than offshore.”

Bratland also believes Statoil will look into different access systems. “We are changing our supplier of access – not because we are not satisfied with the access supplier, but we want to try different contract formats and different access systems on Hywind.” Currently, access is obtained by pushing a boat against two bumpers on the foundation, which requires relatively calm seas of 1-1.5 m significant wave height. One objective is therefore to increase the acceptable wave height for access.

Bratland thinks it possible that his team over-dimensioned the construction to avoid failure, and that “we will look into how we can reduce some of the steel that we use because now we know more about the real fatigue.” Installation methods will also need looking into, as not all countries will have deep water close to shore, as is the case in the Stavanger area. The offshore wind industry also needs more installation vessels and a reliable supply chain offering competitive prices.

When looking at offshore wind turbines, Bratland says that as of today, Hywind is not able to support large turbines – not in terms of MW, but in weight. “We need a larger turbine on top to produce more power with the same infrastructure in place – that is the only way we can reduce the cost considerably. [So] we need lighter and larger turbines.”

When asked whether REpower’s 5 MW offshore wind turbine could be suitable, Bratland says that it is too heavy. “That turbine has a weight of 450-500 tonnes, and our substructure would then be extremely large and expensive with that top weight. If you could reduce a 5 MW turbine down to less than 200 tonnes, then we would be able to use the same substructure as we have today, and then we can get into a competitive position.”

Bratland cannot say when Hywind will be in commercial operation as it realised on future prices, new turbines, access systems etc. “That’s why we’re working so hard to convince the supply chain that they need to play a role in this.”

The Hywind project has cost Statoil NOK400 million – for building and operating it for two years. The operation aspect alone has cost NOK60m.

Bratland is also keen to stress that Statoil did not decide to become involved in the offshore wind industry to up its green credentials, but because it sees floating wind as a viable business opportunity.

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