The Southeast Alaska Power Agency governing board last week approved moving forward with a remote inspection of the tunnels underlying the Tyee Lake hydroelectric plant.
In his report to the board on February 8, SEAPA power systems specialist Ed Schofield explained a remotely operated vehicle would be needed to perform an inspection of the facility’s water conveyance structures. Unlike the dam at Swan Lake, Tyee is a natural lake which is tapped for the facility. Water is conducted to the powerhouse through an intake in the lake into a drop shaft, then through unlined rock tunnels about 8,200 feet long and 10 feet across. These convey water from the lake to a 1,500-foot steel penstock, about five feet in diameter, which houses the facility’s two generating units.
Within the industry it is typical to drain and inspect hydro water conveyance structures at five-year intervals. But because of a collapse in the lower power tunnel during original construction, draining of the Tyee water conveyance structures has been avoided for 33 years.
“We don’t dewater the Tyee tunnel due to the possible risk of collapse,” explained SEAPA CEO Trey Acteson.
Schofield reported that technology has finally caught up to the problem, and a remote operated vehicle (ROV) could be used to inspect the tunnels and pressure shaft. This device would record video and compile a three-dimensional sonar map of the tunnel walls for the full length of the tunnel. This information would then be used to assess the general condition of the tunnel and integrity of the rock primarily in four known fault zones that were encountered during the initial construction of the tunnels.
“One of those fault zones have already failed and has had to be repaired,” he said.
The costs of the project would run at around $250,000, which was not included in the current year’s budget. However, a $535,000 submarine cable assessment which had been previously budgeted for is unlikely to occur this year, so the funds would be drawn from there.
Part of the cost is explained by the price of the equipment, coupled with the uncertainty involved. Plumbing the Tyee tunnel’s depths would be pushing the limit of what has been done previously with ROVs, each unit of which can cost in the neighborhood of $50,000, said Schofield.
“We’re right on the cusp of breaking into new territory,” he said.
If undertaken, a second unit would be kept on hand in case a retrieval of the first unit becomes necessary. In that event a delay to power generation could be extended.
“I’m talking days, not forever,” Schofield clarified.
Acteson conveyed to the board his staff’s opinion that the operation would be best undertaken during SEAPA’s regularly scheduled maintenance shutdown. Depending on what the ROV finds down there, the agency could then identify issues and develop a response.
“I’d rather know if there’s a looming problem or if there’s a lot of debris build up now that could lead to a longer-term plant outage,” he said.
Board members agreed, approving the proposal in a unanimous 5-0 vote.
Board members also approved a sizable project at Swan Lake, repairing the blades on its turbines for units 1 and 2. Schofield explained the blades are taking cavitation damage, with vapor bubbles forming on their low pressure sides. As the bubbles collapse on the runner during operation they erode the blade surface, which in turn disturbs the flow of water and speeds up the wearing away process.
An inspection of the facility last year indicated this, and to correct the problem staff recommend a repair of the blades. Cavitation is reversed by grinding out the compromised metal to a smooth, void-free surface. An electro-welding process is then used to build up the ground away surface with new material, followed by hand blending to bring the surface back to its original contours.
Requests for proposal on the project will be issued on March 7, with a deadline of April 9 set for bids. Like the ROV project at Tyee, the Swan repairs would be scheduled during SEAPA’s planned maintenance outage, tentatively in late May or early June. Working two shifts per day, the job would likely take 10 to 12 days to repair both units.
“I think it’s important that we go ahead and fix it during this first window of opportunity,” said Acteson.
Total costs would run to $395,000, which he explained would be a worst-case scenario. Staff has hopes that the actual extent of the damage needing repair would be less significant. Before work would begin, a third-party magnetic particle inspection would identify this, as damage cannot be evaluated visually. A second MPI would be undertaken after repairs were completed to confirm their quality.
In other board business, during his CEO report Acteson said he is working on a plan to restructure the northern-end brushing crew. Currently the crew has two full-time positions, and once one eventually becomes vacant he would like to transition the remaining position toward a trainee or relief post. Acteson noted contractors lately have been maintaining large portions of the 175 miles of transmission line, and that fully moving away from in-house work in that area would decrease the agency’s liabilities.
“This is one area of personnel safety that is our highest risk, bar none,” he explained to the board. “We’ve got guys out there in really nasty, wet conditions, snowy conditions, running chainsaws on slippery stuff, lots of hazards. What could possibly go wrong?”
Acteson noted there have been no lost time incidents so far under his tenure, a run he would like to maintain. Having a more flexible position would help reduce personnel costs and improve SEAPA’s training foothold for staffing the hydro facilities, he added.
“It could work as a clear conduit for folks who have high potential but perhaps haven’t developed all of the skill sets needed to move into the power plant environment,” he said.
Acteson said the item would be brought up when renegotiating contracts, later this spring.
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