High pressure, low on time, no light, no electricity and full of water:
How the inside of a French hydroelectric dam was quickly and precisely scanned under challenging circumstances.
The physical and figurative pressure is high when millions of litres of water are involved. Especially when they’re kept back by 87-year-old valves. Built in 1934 by German engineers as reparations post World War I, this hydroelectric dam in the Jura mountain range at the edge of France, has been working for 87 years. At full capacity, it generates enough electricity to power five thousand homes: more than 10 megawatts. It is now finally due to be retrofitted with two new turbines.
The original blueprints for the structure still exist, so it should be fairly simple, right? Just take the original measurements and build new turbines to fit the existing draft tubes. Unfortunately, it’s not as simple as that. Nearly nine decades of water rushing through the concrete structure that acts much like an exhaust pipe for the extreme forces that are channelled from the reservoir, through the turbines, are guaranteed to have changed the shape of the structure. Even then, one can only hope that the original blueprints are accurate; that there were no on-site deviations in the construction.
Even a small inaccuracy in the measurements could have devastating consequences. At best, it would mean that the power plant does not function at the specified performance standards. Things could be a lot worse, though: a small deviation in turbine 2 of the Sayano-Shushenskaya hydroelectric power plant caused a consistent, sustained vibration. Eventually, in 2009, this caused enough damage that the turbine was destroyed, and large parts of the power plant were rapidly flooded, resulting in enormous amounts of damage and costing 75 lives.
The draft tubes needed to be measured quickly, to not halt the power plant’s output for longer than necessary, and accurately to avoid any discrepancies and assure a long lifespan of the new turbines. Further complicating the task was the weather: at the edge of the dam, the water level was only about ten centimetres from spilling over the top. This required constant supervision by safety engineers to monitor the water level, as a change in the weather even kilometres upstream could rapidly affect the plant. If the situation changed, any team taking measurements in the draft tubes would have less than a minute to evacuate.
The solution was provided by Damien Delmont of 3DLM. Delmont would use the Artec Eva 3D scanner to scan the draft tubes and turn the scans into CAD models that could then be used to adjust the specifications for the new turbines accurately. The complex shape and size of the draft tubes meant that the 3DLM team had to work on scaffolding to reach the upper parts. An additional challenge came from the old valves, which were designed to not be water-tight, but to let through around ten litres of water per minute, meaning in places, the bottom surface was covered by as much as 2cm of water. The team erected a rudimentary wooden table to protect their electronic equipment and got to work with millions of litres of water above them.
The first step in the process was to add spray paint to the walls of the draft tube. The tube was divided into ten sections that would be scanned separately, which would allow the team to evacuate without losing too much data. They added squiggly patterns with the spray paint which would then assist them in saving time during the processing stage, when the ten separate scans would have to be lined up in Artec Studio software.
To double check that the Artec Eva scanned the floor of the draft tube accurately, even when submerged, Delmont used laser telemetry and thankfully, the measurements were correct. Delmont mentions that “The Eva proved to be an ideal scanner for this kind of work, because it is sufficiently precise and allows us to scan without placing any targets in such a humid, even wet environment that is relatively hostile to technology, in complete darkness, with no electrical outlets present.” Despite the challenges, the whole of the first draft tube was scanned in under five hours, taking around half an hour per section.
After the scan was finished, it was time to align the separate scans into one CAD model. Guillaume Demarche, Delmont’s colleague from Boreal 3D, said that “There wasn’t much work for us on the scan processing side in Artec Studio, which is how we planned it. Basically, after some minor clean up, I aligned the ten scanned sections and created a single 3D model encompassing all the surfaces of the entire draft tube.”
From Artec Studio, the 3D model was fitted with CAD primitive shapes. This could then be exported as STP files to SOLIDWORKS, which in turn was the final reference for drawing the model of the draft tube. Delmont mentioned that this process vastly decreased the overall time that the project would have taken with other methods.
Capturing the complexity of the large scale scan down to sub-millimetre accuracy proved fruitful: a layer of mineral build-up was discovered on the walls of the draft tube from nearly nine decades of water passing through. The build-up was up to 10mm thick in places, which is enough to alter the flow of water, even at such large volumes.
The team returned a few weeks later to scan the second draft tube and the well-structured plan meant that the highly detailed CAD model was delivered ahead of the deadline, which allowed the turbine manufacturer to use it for extensive CFD (computational fluid dynamics) simulations. This, in turn, would allow them to design the new turbines according to the optimal dimensions in relation to the draft tubes.
“In every project I’ve undertaken with my Artec Eva,” said Delmont, “it’s proven to be a critical solution for helping bridge the gap where no original drawings or documentation can be found, which is the case in about 70% of the retrofitting projects we’ve encountered.”
He continued, “With Eva, I am creating precise measurements and CAD models of objects only a few centimetres in size to others that are many meters long. I believe this is one of Eva’s greatest strengths, its versatility. The resulting CAD models are essential for everything from retrofitting to inspection, maintenance, and much more.”