Building a wind turbine in the Groningen potter’s clay is like placing a lamppost in pudding. That comparison clarifies a lot; the strictest requirements are imposed on the foundations of the turbines. It was just one of the many challenges facing ABT subsidiary Windbase at the Geefsweer-Oosterhorn Wind Park to the southeast of Delfzijl.
It is all about the project interest
The metaphor of potter’s clay and pudding originates with Maarten de Keijzer. He is project leader and delegated client on behalf of initiators Yard Energy, Eneco and several private investors. As such, Maarten is responsible for the realisation of these 32 wind turbines with a capacity of 4.3 MW each. The foundations of the last turbine were poured at the start of December. The diameter of the rotors of the wind turbines is 136 metres and the height at the tip is 213 metres. Together they will provide 128,000 households with sustainable power in the future. The Geefsweer-Oosterhorn Wind Park will be completed in February 2021.
Planning is the biggest challenge
Maarten de Keijzer: ‘The biggest challenge for this project was not the potter’s clay, but the planning. The Council of State still had to render a decision and our turbine selection occurred much later than expected. This jeopardised the deadline for the Renewable Energy Production Incentive Scheme (SDE) subsidies. It meant that we had a lot less time for the definitive design. This procedural delay had major consequences for the engineering of the foundations, but also for the required infrastructure. In such situations it is nice if you can bring all required knowledge together immediately. We had to be able to switch quickly with consultant Windbase and contractor Volker Wind.’
This resulted in a very open way of cooperating that is fairly unusual in the world of wind energy. The partners were given room to actually solve the problems that had arisen together. The project interest was always of primary importance as there were no tight contractual boundaries. This meant freedom but also shared responsibility.
Transparency and trust
Cooperation was based on transparency, great mutual trust and the absolute will to find a solution together. Consultant Niek Pouwels of Windbase/ABT and senior project leader Ronny Hartemink of Volker Wind: ‘We were confronted with unexpected situations we had to deal with together. The knowledge and experience of our various specialisations such as concrete structures and geotechnology were applied fully. We were able to provide integral advice. How to handle the unpredictable subsoil as well as a proposed sand elevation? The soil condition was a truly complicating factor. We conducted pile-driving tests to see how the foundation pile would act. Maarten de Keijzer adds: ‘You really need each other. We are of course talking about technology, but is remains work carried out by people. The tight schedule sometimes demanded solutions really at the last minute. If that is the case you cannot sit there and look at each other or pass on your responsibility. Or worse try to score points off each other contracts in hand.’
Unusual solutions
Niek Pouwels: ‘In order to explore the project in depth from a technical perspective, we continuously had to look beyond the obvious. What to do about the layout of the civil infrastructure, transport, the assembly locations for heavy cranes, the manner in which the foundations were to be created, the construction? Will pouring take place in one go or will it take two times? The size of the turbines creates a significant dynamic load. This has an impact on the construction of the foundations. There was a very low blow count during the driving of the concrete piles. Test probes were carried out to demonstrate bearing strength and a stat rapid test load was performed. We managed to stabilise the anchor cage while pouring by using wooden piles directly below it. These are unusual solutions.’
Ronny Hartemink: ‘A useful and practical solution that was also new to us. Our unique formwork design and lean way of working allowed us to construct the foundations quickly and safely. You strengthen each other during the process by consulting with each other very frequently and very intensively. We would meet at least once per week, but frequently more often. And that allows you to accelerate as well.’ It was not just the turbines that required assembly places, the cranes did as well. And there was also limited time to convert the Definitive Design (DD) into an Implementation Design (ID). This meant we had to switch quickly and sometimes improvise. Volker Wind’s senior project leader: ‘The form of cooperation was special, but also exciting. It was a story with many uncertainties. As a contractor, we had to scale up gradually to a team that was more than twice the size it was at the start. And that means you do have to have those people in time.’
Many ‘lessons learned’
Project leader Maarten de Keijzer: ‘It was a matter of pioneering and being alert. We considered together what was feasible in a smart way. A work that is under so much time pressure means that you have to include each other in the process and continue to ask questions. This will let you know where the vulnerabilities are and you learn to understand each other’s problems. The construction of this wind park involved many ‘lessons learned’. It was a project that was managed exclusively on the basis of the planning and our partners understood very well what we as client wanted.’
Header: Niek Pouwels (left) and Maarten de Keijzer © Jacques Kok