A wind turbine has thousands of bolt connections to clamp all flanges together. If they are not properly tightened, the reliability of the entire wind turbine is at stake. With its flange closing method, Boltlife can quickly determine whether each individual bolt has been brought to the correct load. In addition, they contribute to ensuring that wind turbines achieve their promised lifespan of 25-30 years or more, without costly bolting campaigns.

An interview with Chiel van Rooijen and Joost Prieshof.

For what problem have you found a solution?

"Bolt connections are typically tightened with torque tools. When the bolt or nut offers a certain resistance, the hydraulic wrench stops. In the laboratory, the resistance of each bolt seems the same, but there may be up to 40% deviation, which in offshore practice can lead to a spread in load up to 70%. A deviation in the load affects the entire flange. If a too low or high load to the bolt is applied during new construction or maintenance, it will lead to a direct influence on the fatigue life of the bolts in the operational phase, resulting in a loss of load or, worse, bolt failure.

Offshore wind parks are still relatively new, and the first wind turbines had larger safety margins built into the design. Since wind turbines are getting bigger and heavier, the safety margins become smaller. The limitation of the bolted connections is in sight. With a 70% spread in load, it is assumed that a wind turbine will not achieve its expected lifespan of 25-30 years without costly repairs or bolt replacements. Most companies in the offshore wind industry are aware of the fact that the quality of the bolted connections is a serious concern.”

What is the core of your solution?

"The quality of a bolted connection depends very much on the load in the bolts. Torque wrenches are not a good way to achieve or determine the correct load. In fact, torque (in Nm) and load (in kN) have no direct predictive relation to each other. In our process, we send a sound signal through the bolt and measure the time the signal takes before it comes back. Due to the bolt specific calibration curves, we generated in preparation, we can determine how much load we put into the bolt. At a later time, we can check this load at various intervals. Such a re-measurement takes only 20-30 seconds per bolt. Our solution consists of a complete procedure for applying the load using various aids as well as severe quality control of the measurements and measuring technicians.”

What is so pioneering about your solution?

"Measuring bolt tension is not new in itself. This was already used at the Golden Gate Bridge. Its application in the offshore wind sector is innovative. We take a different approach at bolted connections as well as the procedures of tightening and load spreading are unique. All data is processed, checked, analysed and stored, including various additional data points like time stamps and temperature. We have gained a lot of know-how during our various projects and use well-trained measuring technicians. In addition to that, we can also determine the quality of the connections on existing projects, preventing potential problems, even if they were not measured before."

What are the benefits?

"Less maintenance, more efficiency and a much better connection. Because the bolts are at the correct load, less maintenance is needed and they do not need to be replaced at a later stage.

When the old school torque procedures are used like in for example the DASt norm, the bolts in the connections should be replaced once every 2-7 years. This is caused by the enormous fluctuations in loads over the bolts in the flange connection. This puts a big strain on the fatigue life of the bolts. Maintenance and replacement are costly and time-consuming.

Our method is more efficient because we can measure the bolt connections faster with fewer people and less equipment. We measure all connections and therefore do not sample as a common practice and our method is therefore less time consuming as well. Because everything is traceable, it is also very interesting from an insurance point of view, with better connections the risks greatly reduce. By determining the load, we contribute to ensuring that wind turbines will reach their intended lifespan of 25 years or more, without any major unexpected maintenance costs.”

How far are you now?

"We employ 30 people, whom we have trained in our measurement procedures. We carried out our first projects now. For E.ON we did a full bolting and measuring campaign working on monopile to transition piece connections as well as taking all the measurements on the tower assembly of the new Arkona Becken wind farm. For another client, we were involved in a large re-bolting campaign of the foundations. We consider these projects as pilot projects to gain experience and to develop a blueprint for future projects."

What are your challenges?

"We are used to be positioned as a kind of warrant between the end user and the contractor. The old methods are sadly still very much part of the current installation methods and we always try come up with new solutions that work best for all parties providing we achieve the best possible bolted connections. In general, it requires diplomacy and a flexible innovative approach to keep all parties satisfied."

What are your next steps?

"The question is how quickly we should and can grow. To simplify this, we are working on an organisational blueprint for future projects and developing a training program to increase our scalability. In addition to that, we will be working on certification. Ultimately, we want to be able to offer a complete campaign as a (sub)contractor. "

What did Offshore Wind Innovators bring you?

"Martin Weismann helped us to approach the market. We are a small company with large customers. It is helpful when someone offers guidance and can introduce you to the right people. Martin Weissmann knows his way around in this industry."

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