Data collection for condition assessment28 September 2021
WireScan technology provides dam owners with detailed data for critical condition assessment and Asset management
As ageing infrastructure continues to be a concern to owners, operators and governing bodies that are responsible for public safety, the inspection of assets above and below water that identifies structural deterioration or deformation is key to help plan maintenance schedules, budgets and reduce the risk of unplanned events.
Water control at dams is generally achieved using large gates or sectional stop logs that are inserted in slots down either side of a water passage or sluiceway. If these slots, frequently called gains, are misaligned or deformed, the gate or stop log cannot be inserted to isolate the sluiceway or may become jammed and stuck in place. Since these are critical to the control or water elevation and water passage, there must be a high level of confidence that the control mechanism will work in the event of an emergency. Even planned maintenance shutdowns frequently require unit isolation enabled by gates and stop logs; if the unit cannot be isolated, the whole plan for the unit maintenance cannot be executed.
Hydro Québec in Canada operates over 60 hydroelectric power stations with a combined output in excess of 37,000MW.
“We needed to develop a safe way to collect higher quality data for gate inspections says Guillaume Boivin, an engineer with L'Institut de Recherche d'Hydro-Québec (IREQ).
Traditional means used manual measurements and visual identification of anomalies, however, survey capabilities were limited. Manned intervention in confined and hostile underwater environments also posed safety concerns for Hydro Québec.
Working closely with their engineering and operations teams, the team at IREQ developed and patented WireScan, a laser scanning tool that produces accurate engineering-level, three-dimensional data of vertical surfaces that are above and below water, including dam walls, gates and stoplog slots. Hydro Québec was able to put it to the test on their own assets by completing over 75 inspections in various environmental conditions. With a mature product in hand and knowing its potential for other owners and operators, Hydro Québec awarded the commercialisation and manufacturing of the WireScan to the ASI Group in 2021.
ASI is a leader in underwater inspection methodologies using laser, sonar, video and other sensors deployed using remotely operated vehicles (ROV). Hydro Québec saw the technical expertise within ASI as well as the extensive market reach the company has developed over the years to service clients internationally with offices located in most parts of the world, including Canada, the US, Europe and South America. These capabilities will support WireScan utilisation for a large number of potential clients.
WireScan uses the patented twin laser technology to scan a surface in combination with two isolated vertical reference lines. The vertical reference lines are independent of the surface being scanned, providing an external and true vertical reference that the measured surface can be related to. Even if the “vertical” surface is straight, it may be misaligned with true vertical, leading to gates or stop logs becoming jammed when the slot alignment gets out of tolerance. The entire WireScan system is calibrated in the factory to high precision tolerances.
The modular design allows for easy transport and operations. Only two operators are required; one operator monitors the data acquisition for quality, and the other deploys the system. Set-up can be completed within two hours in typical environments. When the system is in position, it performs a self-check against the factory calibrations. WireScan is then surveyed in to reference points, allowing the collected data to be translated into the client’s local coordinate system.
Scanning speed is dependant on the level of detail required – fast deployment will space the scans vertically further apart while slow scans will create a very high density of points. Under challenging conditions where the vertical reference lines cannot be totally isolated from flow resulting in slight movement, the system constraints can be adjusted to permit scanning. The result is that by loosening the constraints to the vertical reference lines, the resulting data will have a similar loosening of the ± 1 mm expectation. (The data will still be measured to that level, but since it is tied to the reference lines, if the reference lines move, the data will get “pushed” as well.)
Optimal conditions provide the best results. Calm, clear water supports WireScan’s ability to measure with a precision of +/1 mm above and below the waterline over a maximum height of 30m. Some modifications during the system development have led to a more flexible method of installation to provide lateral translation of the unit across a gain. This makes it easier to collect data of the concrete condition adjacent to the gain.
ASI took advantage of the technology to provide it as an alternate methodology on a project where sonar technology had already been proposed. The level of detail that can be collected with WireScan far exceeds what is possible with acoustic systems and lined up with the client’s original specifications. Deployment of the system had to be underneath a suspended gate, making for a challenge not only in putting a suspended scaffold in place, but also for surveying in the WireScan to match it up with the structure’s control points. With limited and often obstructed lines of sight between the surveying laser tracker and the WireScan slight adjustments to the positioning of the system were frequently required.
Excellent data was collected over the course of a few days for the four gains of interest. Multiple passes were collected along each gain as a means of quality control and to cover areas adjacent to the gains. Processing the data into relative distances between the upstream and downstream faces of the gains helped engineers determine if the clearances were still within tolerance or required maintenance action.
Report by Robert Clarke, Vice President, ASI Remote Inspection and Surveying; and Merry Dang, Vice President Business Development, ASI. www.asi-group.com