Using an underwater remotely operated vehicle (ROV), the ASI Mantaro, ASI surveyed 7km of the tunnel in 'flooded conditions' at a depth of 160m.
Pacific Gas and Electric Company (PG&E), of San Francisco, California, operates the Middle Fork tunnel and power house, located in the Eldorado National Forest near Lake Tahoe. The hydroelectric facility draws water from the Hell Hole reservoir through the Middle Fork tunnel which empties into a penstock at the power house.
The inspection was determined necessary to identify the source of sedimentation deposits occurring in the turbines. Water supply is critical in this remote region for power, fire protection and consumption. Inspecting the tunnel in watered conditions eliminated potential structural risk and unnecessary waste of valued water resources.
The remote inspection protocol included the ASI Mantaro, a long tunnel inspection ROV, tethered by a sophisticated umbilical that transmits sonar and video data real-time to the surface via fibre optic telemetry. ASI has completed continuous surveys of 10 km from a single access point using this robotic system, including inspections of a 120 km water supply tunnel in Finland, 9.8 km tailrace tunnel in New Zealand and an 8km headrace tunnel in Chile.
Access to the tunnel required placement of 3 horizontal sheaves, by the ROV prior to the inspection.
The sheaves were placed at tight bends in the tunnel and kept the umbilical clear of the tunnel walls; reducing drag and wear on the umbilical. The ASI Mantaro was custom fitted with a buoyancy system to allow handling and placement of the sheaves in the tunnel. The buoyancy system was removed during the actual inspection to reduce overall vehicle dimensions for enhanced flexibility.
Upon completion of the inspection, the ASI Mantaro was re-fitted with the buoyancy system and the three sheaves were removed from the tunnel. The recovery and placement of the sheaves was accomplished without diver intervention.