COLOGNE, GERMANY — A 100 m wind turbine tower is made up of 30 or more massive ring sections held together by more than 25 km of welding. Inspecting welds on a big structure like this and correcting any flaws is a slow and laborious process.
"Our only option is to grind flaws out and do big repair welds, not just unsightly but costly and time consuming," said Alan Thompson, Manager of Welding Technology for Long Products at Tata Steel Europe.
Tata, TÜV Rheinland Sonovation and the Nuclear AMRC (Advanced Manufacturing Research Centre) came together to find a solution for this problem — in response to an Innovate UK competition to find improved manufacturing solutions for offshore wind energy structures.
TeamInnovate UK, the new name for the Technology Strategy Board, funded a feasibility study to explore whether NDT could be carried out during high-temperature welding. They looked at two areas: offshore energy and nuclear applications, where real-time testing to assure the quality and integrity of welding could bring real benefits.
Offshore wind turbines are largely constructed from massive steel pipes welded together from multiple ring sections — not just the visible tower, but also the below-water structure that supports it and anchors it to the seabed. Real-time inspection to ensure the welds can withstand the harsh conditions offshore not only boosts safety but considerably cuts the time and costs of reworking.
Similarly, in nuclear applications, being able to test welds during the high-temperature process would confirm their integrity and enable flaws to be corrected immediately — important in an industry where safety is absolutely paramount.
The result of the study is an innovative, proven NDT process which tests for flaws during the welding process, reducing time and money spent on repairs and improving not just aesthetics but build integrity and safety.
The consortium's solution was to integrate ultrasonic test equipment into an existing welding rig. The system was designed with heat-resistant probes and a cooling system to enable it to work at high temperatures (up to 450°C) and adaptive controls to adjust the speed and feed of welding.
The prototype was tested on a submerged arc weld around a large cylinder.
"We were concerned that the intensity of high-temperature welding would prevent us from detecting the signal or damage the equipment, but in fact the trials went very smoothly," said John Crossley, NDT lead at the Nuclear AMRC.
"With conventional ultrasound equipment, we can only inspect welds at temperatures of up to 50°C," commented Bjorn Snijders, Sonovation's lead engineer on the project. "The beauty of this technique is that flaws show up immediately in the image. You can see the length and height very accurately, enabling depth to be easily calculated."
As well as detecting imperfections in welds in real time at high temperatures, the system's adaptive controls enable welds to be filled uniformly and at greater speed, despite variations in size.
"We get a high completion rate so we don't need to add partial weld runs. Non-destructive, real-time testing is a true breakthrough. It will save companies considerable amounts of time and money correcting flaws," said Thompson.
A proven NDT procedure for high-temperature welding not only reduces costs and saves time, it also plays an important role in assuring build safety and integrity, particularly important for a civil engineering industry that builds ever bigger and more complex structures.
"We need to be doing more to reassure people that the structures we build are subject to stringent measures to ensure their quality and safety," noted Snijders. "Engineering is a long-term game. You need to have a solid body of proven results from studies to convince people to invest large amounts money in your technology."
The results of the feasibility study are a significant move forward towards NDT testing of welds at high temperatures. The next step is to move to a production-level test bed that offers more detailed analysis, such as how soon defects can be detected.
One particularly exciting finding from the study was that the adaptive controls on the new equipment could increase the speed of the welding process five-fold.
However, consortium members know that achieving these potential gains will require more advanced equipment, further development and a much higher level of familiarity with the process.
Sonovation is considering its next move, either providing machines with an NDT rig under license, or building and marketing an NDT welding machine itself — possibly in partnership with a welding equipment manufacturer.
"We wouldn't have been in a position to do this research without Innovate UK support and the facilities of the Nuclear ARMC. They make it possible for companies like us to work on projects that deliver real benefits but that aren't central to our manufacturing base," concluded Thompson.