Wind farm in focus: De Veenwieken Wind Farm

In the eastern part of the Netherlands, between Ommen, Hardenberg en Dedemsvaart, where till recently green pastures dominated the view, ten Enercon wind turbines are slowly rising on the horizon. All in close cooperation with the local citizens. On 7 September, this onshore wind farm De Veenwieken, opened the construction site to the public.

De Veenwieken Wind Farm is an initiative by renewable energy company Pure Energie and De Wieken BV, a partnership between Windunie, energy company Greenchoice and ABN AMRO Energy Transition Fund. Six turbines are owned by De Wieken BV and four by Pure Energie. At the time of the open day, one wind turbine was completed and a second almost. In the meantime most of the turbines are installed.

In total 154 persons could sign up for the open day where they, divided in 7 groups over the day, would be guided to two turbine construction sites and be informed on the wind farm activities. Two days after the announcement, the open day was fully booked, confirming the interest from the surrounding neighbourhood to be informed. The people attending the open day included local citizens, even children, but also people with knowledge or general interest in wind turbines. At the two sites, that of turbine number 9 and 10, they were welcomed by representatives of the project partners who explained more on the wind farm in detail from their field of expertise and would answer questions from the visitors.

Wind turbine number 10
At the first location, the construction site of wind turbine number 10, the visitors were welcomed by Kay Woesthuis, project manager for Ventolines, the company contracted by the wind farm owners for the management of the wind farm construction, and Alex Wielinga, Site manager of Enercon, the German wind turbine manufacturer that is supplying the ten wind turbines.

Several companies, including contractors and suppliers, are working at the wind farm during its construction. Woesthuis: “When we started in January of this year, all you could see were green pastures and a small road. We had to enforce the road from three to five metres for it to be suitable for the many heavy load transportations and build additional roads.” These activities were performed by KWS, part of construction company Koninklijke VolkerWessels, in the period from January up to May. KWS is responsible for all the civil work, including also the construction and, once the turbine is installed, the reconstruction of the crane hardstand to its normal required format. At around the same time, grid operator Enexis completed the grid connection, comprising two connections from a newly built site substation to the transformer stations in Ommen and Dedemsvaart. Between May and June, infrastructure company Van den Heuvel installed the cables between the ten wind turbines and the site substation. All of these connections had to be built new.

Woesthuis: “Each crew member receives a safety helmet with a unique number. We just handed out helmet number 307! On the busiest day there could be a maximum of 50 persons working at the same time but on average there are around 30 persons.” This does not include the transportation people, he adds. All in all there are some 500 persons active on the project in total. From Ventolines Woesthuis is working in a team consisting of two contract managers, a site manager, package manager, electrical engineer, safety manager, someone who ensures all permits are in place and safeguarded, a document controller and of course communication officer Oppenhuizen. On the document controller Woesthuis elaborates: “Per turbine we receive between 400 and 500 documents to review. Every step we take in the project is documented.”

Turbines without lighting
Ten Enercon type E103 turbines with a rated power of 2.35 MW are being installed. Each with an axe height of 98.5 metres and a tip height of 149.5 metres. As was also observed by one of the visitors, the selected wind turbines are relatively small compared to the large onshore turbines currently available on the market or being used in several other onshore wind farms being built somewhere else in the Netherlands. Matthijs Oppenhuizen, the communication officer for Pure Energie and this project, elaborates: “At other locations we build turbines with tip heights of 210 metres. To compare, when the blades are twice as long you will generate around four times more electricity.” The choice for this turbine is the result of governmental ruling indicating that only wind turbines without lighting could be used in this particular wind farm. Turbine lighting is at the moment still required when installing turbines with a tip height bigger than 150 metres, he explains. However, there are developments in sensor-based lighting which are only activated when there is air traffic within a certain proximity range of the turbine.

The Enercon E103 starts operating at 3 m/s. The chosen turbine type provides an advantage according to Wielinga: “The E103 is the only wind turbine that can still operate at wind speed 10. This is possible due to the blades being able to turn.” The turbine models are continuously under development, he adds. “Nowadays, with computers we can pre-calculate almost everything, including where the forces are concentrating. You can already see that the turbine designs have changed based on this. The early designs where more square. This particular turbine is egg-shaped but it might very well be one of the last wind farms where you will see this shape.”

Double steel savings in updated foundation design
There are many forces coming from an operating turbine. This has to be taken into consideration when designing the foundation for a wind turbine. At site number 10 the foundation design was clearly visible; a large round concrete surface with, at that moment, a long circle of bolds and the cases for the power cables sticking out. Wielinga explains that underneath the foundation there are 26 piles installed at a depth of around 17.5 metres. They used vibro piles. A reinforcement cage is added inside all of them. Of these, 13 are placed in a slight angle. The latter is to support the dynamic forces coming from an operating turbine. The piling activities for one foundation take around two days. “We are building in a relatively wet area, therefore we also had to organise draining solutions, all in agreement with the local water authorities.”

After that, 2.5 metre of soil, approximately 900 cubic metre, is dug out around the piles. Part of it is given to neighbouring farmers. This is followed by cutting of the 26 piles and installing the cases for the power cables. What follows next is the installation of an anchor cage which connects all the pile ends and pulls them together. This is done with a tension force of around 50 tonnes. This is followed by steel braiding. Around 23 tonnes of steel is processed in the foundation. Wielinga: “The old design included 60 tonnes steel, however, by reviewing the design and performing new calculations we managed to come up with an amended design which needed that much less steel!” This also has to do with the fact that wind has developed into a more mature industry, he adds, ‘In the early days, we didn’t take risks, now we have gained much more experience to do so.” In a next step, a formwork is added and concrete is then poured after which the foundation is left for 5 to 6 days to harden. Around 900 tonnes of concrete is used for each foundation. Wielinga: “With every step in building a foundation we are careful not to make too many damages as each damage weakens the construction.” All in all, the whole process of building a turbine can take around 32 days, he explains.

Installation in up to 12 m/s wind speeds
The construction of the wind farm is going according to plan. Although they had actually hoped to have the second turbine installed before the open day, says Oppenhuizen. There were however some delays on the side of one of the suppliers. This meant that at the next stop, the site of turbine number 9, the visitors could still see the tower sections and blades lying on the ground and the cranes waiting for action. Two cranes at a time are used for the installation of one turbine, a 120 metre large crane with a maximum lifting capacity of 600 tonnes and a smaller 250 tonnes auxiliary crane. The latter is mainly used for assembling the large crane and, when needed, placing components in an upright position. There are 2 large cranes working on site simultaneous.

The tower is built up from 5 sections. A flange connection with bolds is used to install them. The hub is first installed, followed by the 3 blades in one go. This is the most tricky part of the entire installation process explains Daniel Wolken, construction manager of Enercon. The blades are designed and manufactured by Enercon in-house. They are produced from thin layers of carbon fibre but also wood. The latter is to reduce the weight but wood also has the quality of being bendable. The light-weighed quality of the blades makes it more difficult to install.

Woesthuis explained earlier on that they are able to perform the installation in wind speeds up to twelve metre per second, after that it gets too risky. Weather permitted, they can assemble the crane, install the turbine and dismantle the crane again within a period of two and a half weeks. With Autumn around the corner, bringing more chance of unfavourable weather, he estimated that the installation works could be completed sometime in November.

This doesn’t mean that the wind farm is then also fully operational. All turbines will still need to be commissioned and after that, tested. Oppenhuizen: “Each turbine will have to operate for at least 300 hours in a row without technical failures before being handed over to our project team and then the client. We expect the latter to take place in the first quarter of next year.”

Cooperation with local citizens
Building a wind farm will never go unnoticed explains Oppenhuizen. Especially in a normally relatively tranquil area. Hundreds of transportations will take place, bringing building material, sand and components, used for site preparation and construction of the turbines themselves and the installation of the cables and substation. “For the large components we have created a separate exit from the N36 and try to do most of the transportations at night to minimise the annoyance,” he says. They also have someone as a dedicated point of contact in case there is a problem. He is especially proud of the cooperation with the local citizens. Representatives from the citizens of the municipalities of Ommen and Hardenberg, are united with representatives of the province, municipalities and the wind farm owners in a so-called guarantee committee (Waarborg Commissie). “We have regular meetings and this has worked out very well’, he concludes.

Wind Farm Figures
De Veenwieken Wind Farm
Owners: De Wieken BV en Pure Energie
Turbines: 10 Enercon E103
Rated output: 2.35 MW
Total capacity: 23.5 MW
Substation: 2
Grid connection: At Dedemsvaart and Ommen
Contractors: Ventolines, Enercon, Van den Heuvel, Enexis, KWS
Operational: Q1 2020

This article appeared in Wind Energy Magazine No 3 2019. Text & images: Sabine Lankhorst