Associated Underwater Services, Inc. (AUS) conducted a Remotely Operated Vehicle (ROV) survey of Twin Falls Power Plant tail race at the Snoqualmie River near North Bend, WA. The project was conducted for ENEL Green Power North America (EGP) who operates the hydroelectric facilities at the site.
The tail race tunnel is over 4000 ft long with a nearly 200ft wye connecting it to the North and South Turbines. It is accessed through a bypass tunnel that merges with the north leg of the wye approximately 70 ft downstream of the tunnel origin. The north leg of the wye upstream of the bypass tunnel was not inspected, nor was the south leg of the wye.
The purpose of the project was to visually inspect the tunnel for obstructions or debris. Dimensional information was obtained every 50ft along the tunnel using a profiling sonar. A dive crew prepared the tunnel for penetration by the ROV and inspected the downstream tunnel exit.
The inspection of the Twin Falls Tail Race Tunnel required the use of a specialized ROV outfitted with navigation and profiling sonars. The project required that the equipment be mobilized and set up at two locations; near the bypass tunnel in the powerhouse access tunnel and overlooking the Snoqualmie River near the tail race tunnel exit.
AUS provided their Seaeye Falcon ROV to conduct this inspection due to its capabilities and size. The vehicle was ballasted to be neutrally buoyant in fresh water and uses five electric thrusters for propulsion through the water. Four horizontal vectored thrusters are used for forward, reverse, and lateral travel, providing excellent maneuverability. The Falcon also has a vertical thruster to enable the operator to move the ROV vertically through the water column. The vehicle was equipped with three variable intensity 75 watt quartz-halogen lights to illuminate the area of inspection.
The size and weight of the Seaeye Falcon ROV allows the vehicle to be deployed in a variety of access ports and penetrate long distances.