Development of a Low-Cost Autonomous Underwater Vehicle (AUV) for Inspection of Water-Filled Tunnels During Operation
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During the past century, over 3500 km of blasted mountain tunnels have been commissioned for water transportation in Norway. Today, these tunnels are not easily inspected for internal collapses or debris without needing to stop production, de-water and subsequently inspect them by foot. Inspection alternatives such as Remotely Operated Vehicles (ROV) are often deemed too expensive. As for the de-watering method, it has been concluded that repeated drainage increases the risk of further damage to the tunnel internals. Alcoa s aluminum plant in Mosjøen is one of many businesses reliant on a water tunnel in their daily operation. However, they have reported concerns about a potential collapse inside their 4-km-long tunnel, without any cost-efficient way of inspecting it. In their case, shutdown and de-watering is out of the question. With the objective of locating and evaluating potential collapse(s) in Alcoa s tunnel, this master s thesis proposes inspection by means of a new low-cost Autonomous Underwater Vehicle (AUV). The AUV varies its buoyancy while being dragged by the water stream through the tunnel, producing a two-dimensional height vs. time plot of the tunnel contour. This plot can afterwards be compared to original tunnel schematics to indicate both obstacle position and height. The AUV s proof of concept is verified through collection of empirical data in a scaled down version of Alcoa s tunnel, the results of which have been accepted for publication by the American Society of Mechanical Engineers (ASME). In addition to providing valuable data in the scaled down tunnel, two full-scale hydropower tunnels have also been inspected during operation. The two tunnels, located in Sokna and Svorkmo, have a length of 600 m and 1100 m, respectively. Their successful inspection is regarded as important stepping stones towards the final inspection in Mosjøen. In the case of Mosjøen however, no AUVs were able to traverse the entire tunnel length. As a countermeasure for the unsuccessful inspections, simulations based on the tunnel geometry and flow data provided by Alcoa have been performed in MATLAB. These results indicate a collapse too large for the AUVs to pass, suggesting the presence of a major obstacle (∼3.5 m high) located somewhere in between 2400 and 3400 meters into the tunnel.