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High-Resolution Tunnel Surveys by ROV [Sea Technology](Sea Technology Via Acquire Media NewsEdge) ASI ROV Multibeam Sonar System Inspects Hydroelectric Infrastructure The La Higuera hydroelectric project is a 155-megawatt, run-of-river development in the Andes Mountains on the Tinguiririca River, located approximately 150 kilometers south of Santiago, Chile. The project is managed by Tinguiririca Energía, a company that is co-owned by Pacific Hydro and SN Power. Construction of the project was completed in 2009 and includes an 18-kilometer-long headrace tunnel that delivers water to the power plant. In August 2010, a short section of that tunnel collapsed, forcing a 20-month outage while repairs were undertaken. As part of the follow-up monitoring program, the owner installed a variety of sensors for realtime monitoring of geotechnical parameters in the tunnel while it is operational. To gain a more comprehensive overview of the tunnel condition, the owner wished to inspect the tunnel during short outages without having to de-water it. The owner was familiar with long tunnel inspections using ROVs and issued a tender calling for proposals that would provide high level point-cloud data and sonar imaging that could be used to verify the general condition of the tunnel, look for deformation of the tunnel cross-section and detect cracking, displacements or separations of the tunnel lining. ASI Group Ltd. of St. Catharines, Canada, was awarded die contract to provide such a vehicle. ASI has been providing long tunnel inspection services for more than 20 years and has the longest fully tethered systems for each size class of ROVs in the world, with the longest tether being 10 kilometers in length. Tunnel Inspection System Due to the known condition of extremely poor visibility caused by the high sediment loads in the water, the entire inspection was planned and conducted using sonar technology since no useful visual feedback could be obtained with the system cameras. ASI proposed a multibeam sonar system to collect the tunnel profile sections. Since 360° coverage in a confined environment was required, multiple heads would need to be integrated since no single 360° sonar head was available at the time. AS! selected the Teledyne RESON (Slangerup, Denmark) SeaBat 7125 multibeam sonar system since it had a good track record and had previously been operated as a dual-head system, although never configured to operate with three heads, which ASI's inspection plan called for. The manufacturer also provided data acquisition and processing software, making them a "one-stop-shop" for development of this sensor package into one integrated system that could be applied in the tunnel. To provide full imaging of the tunnel surfaces, ASI selected the Teledyne BlueView (Bothell, Washington) M900 multibeam imaging sonar, using three units again to provide coverage of the full interior. As with the SeaBat 7125 units, three sonar heads had never been integrated onto a single platform for simultaneous operation. Accurate positioning in the tunnel was critical for development of the georeferenced data that the client required. Since the tunnel is a closed environment with no updates from satellite for DGPS, a high-grade inertial navigation system was required. ASI selected the Teledyne CDL (Aberdeen, Scotland) MiniPOSNAV3 as the system to provide the positioning input to the data acquisition software for the multibeam data. ASI used a Forum Energy Technologies (Houston, Texas) Sub-Atlantic Mohican ROV as the base platform to replace their original long tunnel system, the ASI Mantaro, which could not support integration of the multiple sensors being proposed. ASI modified the power supply system of the standard Mohican ROV so it would operate over the 10-kilometer umbilical and added modular frame and buoyancy components to support the multibeam array and INS packages. The completed system was the integration of the following sensors to the ROV: three SeaBat 7125 multibeam sonar, three BlueView M900-130 sonar, CDL MiniPOSNAV3 (T24) INS, Tritech (Aberdeen, Scotland) Super SeaKing DST dual-frequency digital chirp sonar, and modified power supply to operate over a 10-kilometer tether. Results The system was first deployed into the Tinguiririca Tunnel that is located immediately upstream and is part of the adjoining La Confluencia project (both tunnels have the same name). Technical issues prevented all of the multibeam systems working consistently together, but the overall operation and collection of data proved the concept. The inspection results also provided additional details on an anomaly that was previously identified in an earlier inspection, conducted by ASI with their Falcon ROV that was fitted with an Imagenex (Port Coquitlam, Canada) scanning profile sonar and BlueView sonar systems. A small relief target was identified with the scanning profile sonar- a coincidence considering the scan came around at just the right time to catch the feature. The benefit of the multibeam system is that it can collect several thousand points within a small area that better defines surfaces. The follow-up inspection captured not just the one instance of the raised target but completed the full model of the area so that the feature could be put into context with its surroundings. It was now known that the feature of interest was not just a discrete item but had length, so it was unlikely to be a rock. A few months after the inspection, the tunnel was de-watered during a planned outage for routine maintenance. Arrangements were made to conduct a drive through the tunnel to locate targets that had been seen in sonar images and compare them with actual features. The target of interest that was modeled by the sonar systems was identified as pipes that were installed through the lined section. They allow water to drain through them from the tunnel on the upstream side of the lined section. The BlueView imaging sonar also provided excellent coverage and correlation of features. The displays were arranged to represent the tunnel walls as if the observer were walking down the tunnel. Deliverable products from these surveys include 3D point cloud models of sections of interest that can be compared with previous surveys to determine areas of cross-sectional change and sonar images with annotations showing isolated targets and features of interest. The 3D models can also be used to provide animated fly-throughs, an excellent tool for presenting data to a wide variety of stakeholders in a clear and familiar manner. * Bob Clarke has 30 years of experience in the marine industry, including field and management positions in commercial diving, remotely operated vehicles, and atmospheric diving systems for inspection, construction and salvage/recovery operations. He presently manages ASI's fleet of ROVs and directs company development of new equipment and technologies as they relate to underwater inspection and intervention. Rick Fletcher is a civil engineer with more than 40 years of experience in the management and control of a variety of civil projects, including nuclear power plants, hydroelectric projects, metal processing plants, airports, marine infrastructure, transit and public utilities. For the past eight years, he has been involved in managing the development process for six run-of-river hydroelectric schemes in the upper Cachapoal River basin. (c) 2014 Compass Publications, Inc. |