January 15, 2007

Focus on O&M (January 2007)

ROV solves several problems

In December 2005, FirstEnergy realized that it needed to perform an annual FERC Part 12 safety inspection of a portion of Seneca Station's 100-acre upper reservoir—a bowl-shaped structure the size of a NASCAR track that holds about 2.1 billion gallons of water—early in 2006. What made this task challenging (in addition to the Pennsylvania winter) was that the area to be inspected was submerged daily at a depth of up to 50 feet, depending on plant operations. Further complicating matters, material and equipment delivery would be difficult: During the winter, the U.S. Forest Service normally does not plow the 6-mile-long gravel road that provides access to the reservoir because it is used by recreational snowmobiles.

Despite these challenges, and the fact that a "wet" inspection of this size and complexity had never been done before at a FirstEnergy facility, Bill Harker— director of FirstEnergy's hydro and combustion turbine plants—gave Seneca Station's inspection team the go-ahead to investigate using an ROV (Figure 2). "Any time you have an opportunity to safely and cost-effectively shorten an outage by four days and still meet all of your inspection objectives, you have to thoroughly investigate the option," said Harker. "Over the past several years, we've had success using new techniques during planned maintenance outages of our hydro stations, and we hoped that we could add ROV inspections to the list."

2. The Phantom knows. The remotely operated vehicle used to inspect the upper reservoir of Seneca Station. Courtesy: ASI

How did Harker determine that using an ROV would shorten the Seneca Station outage by four days? In 2004, some repair work had been done on the upper reservoir in a section known as the "Fluent Run Depression," a natural depression capable of holding 40 million gallons of water. As a result, even after the upper reservoir was drained, the Fluent Run Depression still contained a mixture of water and mud that took more than four days to pump out. After the water was removed, the six acres of the Fluent Run Depression were repaired by adding two layers of liner topped with 18 inches of sand and ground asphalt (Figure 3).

3. Multiple layers. Repairs to the Fluent Run Depression required two liners and 18 inches of cover. Courtesy: FirstEnergy Corp.
 

However, when the upper reservoir was refilled, the initial rush of water washed away some of the sand and ground asphalt. To eliminate further wave damage, "Jersey barriers"—shaped like the state—were installed around the Fluent Run Depression section and the refilling was completed successfully (Figure 4). All instrumentation readings and twice-weekly physical inspections showed that the repairs worked as planned. Even so, the section still needed to be included in the FERC Part 12 safety inspection for 2006.

4. Stay in place. Jersey barriers were added to prevent scouring during refilling of the Fluent Run Depression. Courtesy: FirstEnergy Corp.

No divers allowed

When FirstEnergy managers were deciding whether to inspect the reservoir in a watered or de-watered state, they identified three big benefits of doing the inspection underwater:

• The Fluent Run Depression would not have to be drained, enabling the plant to return to service four days sooner.
• More Jersey barriers would not need to be installed to prevent any damage from the initial fill-in rush of water.
• Using an ROV to perform the inspection would solve a big problem: the water's murkiness.

The last benefit loomed particularly large. For flexibility's sake, human divers have long been the first choice for underwater inspections and repair work. But in this case, cost and safety issues dictated against the approach.

For a diver to work without having to enter a decompression chamber upon returning to the surface, the deepest the reservoir could be is 40 feet. To meet this condition over an extended period of time, the plant would have to be kept in reserve shutdown. This option was considered too costly. So, too, was this alternative: conducting the inspection over several days, and only when the upper reservoir water level was low. Needless to say, transporting a decompression chamber to the remote site also was ruled out, for reasons of cost.

Safety was another reason for not using divers; they could be disoriented by the poor visibility caused by the reservoir's turbidity. Cloudy conditions also might have rendered useless any videotapes recorded underwater, which would compromise inspection performance.

Once it was decided that the ROV approach had merit, a specification was created for vendors to bid on. Clearly laying out the technical and logistical parameters and expectations of the work served as the precursor of a cost-effective inspection solution for FirstEnergy. In effect, the specification eliminated three concerns that typically plague underwater work—visibility, orientation, and equipment malfunctions.