January 15, 2007

Focus on O&M (January 2007)

Sonar selected for soundness

To deal with the visibility issue, the specification called for the use of sonar. Although some at the plant originally were concerned that sonar might not be able to detect small voids in structural surfaces, at the end of the day the technique was unanimously accepted for its unquestioned ability to generate profiles of surfaces and repairs and precisely locate the Jersey barriers.

Even if a crystal-clear picture could have been guaranteed by firms bidding for the inspection dive, the one must-have item in the specification was the ability to know where the ROV was in the upper reservoir on a real-time basis. An inability to meet this requirement was cause for one bid to be eliminated.

The winning bidder offered an interesting combination of technologies that allowed the onshore team to watch the underwater video and sonar and simultaneously track the position and orientation of the ROV. Prior to starting the actual inspection, the survey points were downloaded into the inspection firm's computer along with a digitized map of the repair area. Being able to see not only where the ROV was but also in what direction it was pointed allowed for immediate position corrections and/or hover and hold commands.
 

Winter complicates the work

The inspection took three days—one day to set up and two days in the water. Although the access road had been cleared of snow earlier in the morning, it took more than an hour to haul the dive boat up to the reservoir, a trip that normally takes 20 minutes (Figure 5). Ice, and stopping to allow logging vehicles to pass, caused considerable delays. Once the crew arrived at the reservoir, they had to shovel snow from the steep access into the water.

 

5. Icebreaker. The dive boat with the sonar-equipped ROV had to pass over icy roads to reach the reservoir. Courtesy: FirstEnergy Corp.

It took more than four hours to set up the equipment. The first day ended with the dive boat being in the water for a 90-minute shakedown cruise. With below-freezing temperatures expected that night, the decision was made not to put the ROV in the water to check out its systems. There were just too many seals that could have frozen later if the vehicle had been "put up wet."

On the second day, the access ramp was cleared of the previous night's 6-inch snowfall and salted before the ROV team arrived. Pre-inspection procedures included a guidance system check that compared the GPS data from the vehicle with survey points on the digitized drawing of the reservoir. Then the ROV was placed in the water and the inspection began. For the first 30 minutes, the vehicle produced a steady stream of video and sonar data. But then the video signal was lost along with control of the port thruster. Some quick troubleshooting diagnosed the problem as an electrical connection that had opened.

After the repair was made, the ROV was put back in the water for more than three hours, with the shore-based team viewing the displays and guiding the boat-based ROV team. Even without the turbulence from the thrusters the viewing clarity for most of the inspection was poor. If not for the sonar data, the inspection would have been unsuccessful. With more subfreezing weather in the forecast, the second day ended with the ROV safely stored inside the Seneca Plant overnight to prevent the seals from freezing.

The third day began—much like the second—with nearly 6 inches of snow on the ground. During an inspection of the equipment, it was clear that the dive boat's engines were starting to feel the effects of three days of being exposed to near-zero temperatures and heavy snows. For example, the diesel-generator that powered the ROV had trouble starting. However, within two hours the talented ROV team put all of the equipment in working order and completed the inspection, including the six acres of the Fluent Run Depression and the Jersey barriers. Based on past experience, the inspection could not have been completed in two days if a diver had been used. Time and again, the ability to know where the ROV was, even in periods of near-zero visibility, allowed the team to gather large amounts of data in a relatively short period of time.

Buoyed by the success of the ROV, the decision was made to use it for one additional inspection. During the plant's pumping and generating modes, there is a brief time when there is no water flow between the upper reservoir and the plant. It was decided to move the ROV to the 19-foot-high head conduit and inspect the intake structure. Navigating the ROV between the intake columns was easily accomplished. But the inspection had to be cut short when the plant returned to generation mode. Because the team proved it could be done, ROVs will be used for future high-head conduit inspections.