Salt River Project recently undertook a challenging underwater repair operation to avoid draining Apache Lake.
The unusual underwater operation allowed the Valley utility to repair the damaged intake structure for Horse Mesa Dam’s hydroelectric units, with the help of Seattle-based Global Diving & Salvage Inc.
“We were faced with some tough choices, with a primary focus being the preservation of our valuable hydro generation pump-back unit,” said Roger Baker, a principal engineer in SRP’s Hydro Generation department and the project manager of the Horse Mesa Dam job.
“We really didn’t want to lower the reservoir level by 170 feet and essentially drain Apache Lake — which we had to lower about 50 feet in 2006-07 for maintenance work that is planned every 20 years — so we looked at several other options.”
The complicated project was prompted by a June 2012 collapse of a guide vane inside the penstock intake, the huge pipe that passes through the dam from Apache Lake into the generating unit below on the Canyon Lake side. The dam itself was unharmed.
The damage forced the loss of 119 megawatts of electricity produced by the pump-back unit added to the dam in 1972, which hampered SRP’s pumped-storage system.
Horse Mesa, which creates Apache Lake, is the next dam downstream on the Salt River from Theodore Roosevelt Dam.
SRP’s Mechanical C&M (MCM) department, Global Diving & Salvage and Stantec, fabricated the new parts for the intake structures, logging about 9,000 work-hours to see the project through completion.
Global Diving & Salvage early this year set up a huge, working barge on Apache Lake next to the dam. Global Diving hauled 29 tractor trailer loads that included the barge system, cranes, boats and other support features — its 20- to 24-member crew working around the clock. Global Diving’s crew used a specialized technique called “saturation diving,” where the divers breathe a blend of oxygen and helium, and stay under pressure for up to 30 days. This allows the divers to work at depth for much greater periods than conventional diving allows.
While one diver remained in the diving bell, the vessel that brought them from the work area 160 feet below the surface of Apache Lake to their pressured habitat vessel on the barge, the other worked a five-hour shift. Roles were then reversed for the other five hours. Ten hours later, the second two-man crew repeated the process.
The new vanes fabricated and assembled by SRP’s MCM department are carbon steel forms coated with corrosion protection that are filled by underwater concrete. Global Diving’s tasks on the project included anchoring and securing the steel forms and filling them with a special non-aggregate concrete mixture especially developed for underwater placement.
SRP is the largest provider of water and electricity to the greater Phoenix metropolitan area, delivering about 1 million acre-feet to agricultural, urban and municipal water users and providing electric service to about 970,000 customers.