Full circle: decommissioning the first ever offshore windfarm

21 November 2017 (Last Updated June 5th, 2020 13:29)

Vindeby was the first offshore wind farm ever built, forming the foundation for today’s wind industry. Now Ørsted has decided to decommission the project, but just how much have we learnt from it? Molly Lempriere talks to Ørsted’s Lars Bie Jensen about Vindeby.

Full circle: decommissioning the first ever offshore windfarm
“I think you cannot understate the importance of Vindeby, this is the pilot project that paved the way so that the offshore wind industry could be possible.” Credit: Courtesy of Ørsted

In 1991, to the south east of Denmark, near Lolland, DONG, now called Ørsted, set about building a pilot project. Vindeby was the first ever offshore wind farm, built to test whether or not it was possible for turbines to exist in the harsh environment of the sea, and whether their power yield would be commercially viable.

Eleven fully constructed turbines were lifted into the waters between 1.5 and 3km from shore. They were connected to the grid and powered 2,200 homes for 26 years, producing 243GWh throughout their lifetime. Standing at just 54m, Vindeby’s 0.45MW capacity turbines had a total export capacity of 5MW. This may seem small compared to today’s standards, but Vindeby set the foundations that led us to the colossal wind farms built today, including Ørsted’s latest project, Hornsea One, which is expected to have a total capacity 1,200MW.

The project was a success, and Denmark is now a world leader in wind energy. More than 40% of Denmark’s electricity comes from wind power, and it is projected to reach 50% by 2020. The sector has also grown a staggering amount since the instillation of Vindeby, employing 30,000 people in Denmark alone and creating annual revenue of DKK 90bn.

In March 2017 the decision was taken to decommission the wind farm, marking another first for the company. Lars Bie Jensen, project manager for the decommissioning of Vindeby, explains the significance of the project.

Molly Lempriere: Could you tell me a little about the installation of Vindeby?

Lars Bie Jensen: The installation was 26 years ago and the turbines were built on gravitational foundations, concrete foundations that were cast in an artificial dock very close by. The dock was flooded and the foundations then floated out to sea into their positions. They were then balanced onto the bottom of the sea, and there was a concrete slab cast on top ready for the turbines. The turbines then came in one piece and were erected with mobile cranes. That was basically it.

ML: When they were first installed how long were they supposed to last?

LBJ: I don’t know, today the consent given for turbines shows they are expected to last 20-25 years. I don’t know if back then they did any business case on how long they should last.

It was a test to see if we could build offshore simply, of course some considerations as to how long they should last were made but I’m not aware of how long. I think they outperformed what people thought they would.

ML: So why did you decide now is the time to decommission them?

LBJ: Partly because the consent was expiring and we are obliged to take the park down. We looked into whether we could then extend the consent, which was a possibility. But we also looked at how the turbines actually looked, and it turned out that all of the gears needed considerable refurbishment to be able to last any longer. That killed the business case basically; it was just not viable to change the gear boxes and make then run for some more time. It was only 11 turbines, it’s not a high earn site, so there were quite a few odds against prolonging the life of Vindeby.

ML: How did you go about decommissioning Vindeby?

LBJ: This is the first time we’ve done this, at least at DONG energy (Ørsted). We looked at what we had, the documentation on it and started to think how could we build-back so to speak.

We decided to give the industry the opportunity to come up with ideas on how to decommission. So we made a tender, where we invited people to come with their ideas on how to decommission it. We didn’t dictate how to take Vindeby down. We got the tenders, looked at them and rated them, and found a concept that we, technically and price-wise, liked and awarded them the tender. In the tender we were also very aware of the recycling of the wind farm. Everything that could be recycled was used, including melting the steel from the turbines, using of the concrete rubble as new building material etc.

ML: Did you face many challenges in decommissioning?

LBJ: Yes and no, because the windfarm was 26 years old we had a little bit of a lack of documentation, partly because some of the documentation burned in a fire some years ago. In one of our power plants Masnedø where we had a documentation room, there was a fire. This was of course, before we had everything electronically, so there wasn’t much electronic data on Vindeby.

We had some design documentation, and we also gathered some from suppliers from back then, that we were working off of. First of all we took off the turbines and that went very well, no issue with that at all. After practising on one turbine we could take a turbine down in four hours, so that was quite straight forward and the guys did a good job.

Then we had the foundations, and there were more issues there. We were taking them down using a hydraulic hammer and a hydraulic scissors, simply cutting them down in place and then scooping up the rubble and transporting it away. But when we reached the bottom plate we found some extra concrete that we couldn’t find on the drawings [and] we found from the project itself.

This concrete caused a little bit of a problem because it was quite big so we needed to alter our method a little bit, we took on another tool, a rock wheel, simply a wheel with spikes on that could then scrape off the concrete in big chunks. That took a little bit longer than anticipated, simply because we didn’t anticipate the extra concrete. It wasn’t an issue; it just took longer to do it.

With the cables we planned to simply pull them out of the seabed, and that worked very well too so there was no issues there. It went as planned, it took a little longer but that was just due to the extra concrete we found.

ML: Do you think you learnt anything that you could bring into other decommissioning projects?

LBJ: If you think that this windfarm is 25/26 years old, these turbines are not produced any more. They’re very small. For the turbines I think we learnt more about durability, people will be looking at the blades, looking at the gearboxes, how they stood up for 25 years and there people will learn something.

I don’t know if anything could be learnt from the design of the turbines themselves. The turbines have evolved so much that I don’t think there’s that much learning. From dismantling the foundations, yes we learnt a little bit but that’s concrete foundations and as you probably know, most of the turbines today are standing on monopile structures. So actually the overall learning from Vindeby is not that much.

Of course we can always learn but it’s very hard to transfer the knowledge from this little 11 turbine, very low water, small turbine onto the bigger sites today at least from a technical perspective.

ML: How important do you think Vindeby was in other ways for the adoption, and acceptation of offshore wind and for Denmark?

LBJ: Denmark is the cradle of offshore wind, Vindeby was the first offshore windfarm, and here we simply proved the concept that, yes you can build offshore, and yes the energy yield of that is higher and the turbines can last out there.

So I think you cannot understate the importance of this, this is the pilot project that paved the way so that the offshore wind industry could be possible. It gave birth, so to speak, to the offshore wind industry. It took some years but that’s where it all started.