The Columbia River was a deep shade of ocean blue, and the sky was surprisingly clear and sunny for a November afternoon. A perfect day for a dive.
On top of McNary dam, on November 4, two men unloaded a large, green remote operated vehicle (ROV) from the back of their trailer. The task for the day was to send the ROV down on the upstream side of the dam to conduct an inspection.
On that November day, the U.S. Army Corps of Engineers (USACE) Walla Walla District had called on Todd Manny and Jared Butler, the District dive coordinator and alternate for the USACE Portland District, to bring their ROV down to inspect McNary’s trash racks. The ROV in question was a large observation class ROV, designed to go as deep as 1000 feet in order to carry out inspections, using a high definition camera and sonar.
“The benefit of using the ROV is that, number one, from a safety standpoint you’re not putting a human in the water,” Manny said.
Indeed, diving is dangerous and it is far better to send an unmanned drone down to the bottom of a river than a human diver.
One of the biggest concerns is differential pressure, namely the suction caused by the turbines as water flows through the dam. This pressure can pull in an ROV or diver, trapping them or leading to serious or fatal injuries.
Before a dive, the nearby intake gates are closed, as well as the wicket gates around the turbines themselves. This stops water from flowing through the nearby turbines and becoming a hazard to a diver or ROV.
“We try not to send divers unless it’s absolutely necessary,” John Oberhelman, the Walla Walla District dive coordinator said. “We have a lot of criteria we have to follow. There’s a lot of paperwork that goes on behind the scenes.”
Cables present another potential hazard. While the ROV is operated remotely, it is not wireless and there is always a risk that the cable becomes entangled in debris or pipes below the surface. There are stories of ROVs getting so entangled that they ended up needing rescue by another ROV or human diver.
“This one has rescued three or four smaller ROVs,” Manny said, referring to the ROV they had brought to McNary.
Divers also have to be careful of entanglement. When suiting up, a diver is connected to a thick multi-strand cable called an “umbilical.” These cables, which look like yards and yards worth of yellow, orange and black licorice, supply divers with air, communication, a camera, and a light. In an emergency, an umbilical can also be used to pull a diver out of the water.
Manny and Butler hooked up the ROV to its 300 foot-long cable. They then ran through a series of checks, making sure that the light, camera and motors worked to their satisfaction.
Before the dive, the team gathered around the ROV to go over the plan and potential hazards.
“We always treat the ROV just like it’s a diver due to the cost of the equipment,” Manny said.
At last it was time to dive. The ROV was loaded into a metal basket and lowered into the water by McNary’s new Grove Carry Deck Crane, operated by Michael Gibbs and Joel Raplee of McNary’s rigging crew.
Once the ROV was low enough in the water to begin floating, Manny, now in the trailer with joystick controls in hand, backed it out of the basket and sent it down to begin the inspection.
Because of its multiple electric motors, the ROV is able to maneuver in many directions at once and its movement is often referred to as “crabbing.” This flexibility allowed Manny to send it down while zig-zagging back and forth across the width of the first stack of trash racks.
In a hydropower dam, trash racks are wide metal grills that are stacked on top of each other in front of the intake gates. These racks stop debris coming downstream from being sucked into the turbines and damaging them. As debris piles up, the racks need to be cleaned to prevent damage and to prevent obstruction of the water flow that would affect the performance of the turbines. Maintenance needs to happen regularly, but the best time to go down and inspect them is during the winter, when there are no salmon swimming through the area.
And so the ROV descended, documenting vertical metal bars dusted with green algae. While Manny piloted from the trailer, Butler remained by the railing, feeding cable over the edge to maintain the right amount of slack. Too much slack and there would be a risk of entanglement. Too little and the ROV would get stuck, fighting against its own tether.
As the ROV neared the bottom of the river, the debris could finally be seen. The first visible piece was a large log leaning up against the trash rack. Further down, a jumble of branches and logs littered the riverbed, along with what looked like a couple of large yellow IKEA bags. If the trash racks are to be serviced, a dive team will need to come clear the mess away before the racks can be pulled up for maintenance.
“A lot of the work we do from an ROV and dive aspect is data collection,” Manny said.
The information collected by the ROV will be used by engineers to write up more accurate cost estimates for the work it will take to complete the job. While the ROV used was called over from Portland District, most of the dives done at Corps projects are contracted out. These dives will continue on through the winter, in a race to finish their work before the juvenile salmon start their journey downriver in the spring.