When a unit at Salt River Project’s 129-MW Horse Mesa project suffered a catastrophic failure, a creative option was needed to repair the unit, whose intake structure is 160 feet below the lake surface. The solution chosen, called “saturation diving,” involved using special equipment to replicate living at underwater pressures during the task.
By Mike Langen and Mark Estes
As temperatures pushed upwards of 100 degrees in June 2012, crews at Phoenix-based electrical utility Salt River Project (SRP) knew that demand for electricity would also rise. Operators began powering up Unit 4 at the 119-MW Horse Mesa project to help meet the mid-day demand cycle. But, during the unit start up, operators heard loud noises in the penstock and shut the unit down immediately to investigate.
What they were about to discover would send the nation’s third largest public power utility on a 15-month effort to repair and improve one of their most important assets.
Horse Mesa Dam, located about 65 miles northeast of Phoenix, is the largest of SRP’s four hydroelectric facilities, located along the Salt River in Arizona. The dam was constructed between 1924 and 1927. It has three conventional turbine-generator units. Unit 4, a pumped-storage hydroelectric unit, was installed in 1972. It is used in coordination with another pumped-storage unit downstream at 60-MW Mormon Flat Dam. The Horse Mesa pumped-storage unit pumps water into the upper reservoir, Apache Lake, when electricity demand and costs are low, then releases the water through Unit 4 into Canyon Lake, the lower reservoir, to produce power when demand for electricity is high.
|Located northeast of Phoenix, Ariz., Horse Mesa Dam is 305 feet high and 600 feet long and impounds Apache Lake.|
Water enters the unit from Apache Lake through a concrete intake structure that has two vertical concrete guide vanes running the length of the intake. These guide vanes were designed to provide improved hydrodynamic flow during the pumping cycle. Immediately following the emergency shutdown of Unit 4, SRP employees took the unit offline, drained water from it, completed the appropriate work safety clearances and began an inspection process. They discovered that one of the guide vanes had collapsed, sending huge chunks of reinforced concrete down the penstock and into the turbine. The dam itself was unharmed and other structures, including the penstock and generating unit, were structurally sound.
While crews set about removing the dislodged material from the turbine and penstock, SRP’s engineering and hydro generation departments began to explore repair alternatives.
“To evaluate our repair options, a multi-disciplinary team was assembled that included outside contractors and outside consultants, as well as a number of departments across Salt River Project,” said Roger Baker, principal engineer, hydro generation. “Seventeen options were considered, factoring in a variety of impacts. The gorilla in the room was the location of the intake structure, which is about 160 feet below the surface of Apache Lake.”
Draining the lake would have significantly impacted water operations; SRP delivers up to 1 million acre-feet annually to municipal, industrial, agricultural and residential users within metropolitan Phoenix. The move also would have affected the many recreational and commercial users at Apache Lake. So SRP and consultant Stantec Engineering came up with a solution that addressed the unique needs of the problem at hand.
Repair work would be done by diving crews working around the clock, using special equipment to replicate living at underwater pressures during the task, using “saturating diving.” In saturation diving, the divers live in a pressurized environment that can be maintained for extended periods (up to a month or longer) and they are decompressed to surface pressure only once, at the end of their tour of duty. By limiting the number of decompressions, the risk of decompression sickness is significantly reduced and the amount of time available for performing work is greatly increased (see sidebar on page 53). “The option of using saturation dive crews offered the earliest return-to-service date and also allowed repairs to the dam’s other three unit intake structures at the same time,” said Baker, who served as project manager. The intake structures for Units 1 through 3 are also located on the Apache Lake side of the dam, in deep water.
|Nine additional guide vanes were added to the original vane design to improve the flow of water through the intake.|
SRP selected Seattle-based Global Diving & Salvage Inc. for the project.
“This was a textbook case of all hands working well, successfully, to meet a demanding goal,” stated Baker, in discussing the efforts of SRP’s Mechanical C&M Group (which fabricated all of the components), Stantec and Global Diving & Salvage. “The best of the best worked on the project.”
Stantec Engineering, with input from Global Diving & Salvage, developed a new, custom-designed structure to replace the vanes that not only would be durable but also would improve flow characteristics in the Unit 4 intake. Using computational fluid dynamics modeling, designers modified the original guide vane design by adding nine horizontal vanes to provide improved flow. Instead of concrete and rebar, the new vanes are made of structural steel filled with a cementitous grout. The new intake assembly has 15 total pieces, with segments varying in weight from 1.5 tons to about 5 tons. The 15 segments are connected to each other and held in place using mechanical connections requiring almost 10,000 individual parts.
SRP also planned to take advantage of the temporary outage to fabricate and install new trashracks for Unit 4 to prevent debris from entering the intake and going into the hydroelectric unit. Based on the design of the existing racks, the structural steel trashracks comprise three vertical rows of screens, each about 17 feet by 10 feet.
The utility also planned to use the saturation divers on-site to complete some long-needed repairs to Units 1 through 3, specifically installing new stainless steel bulkhead sealing surfaces on each of the three original unit intakes.
By March 2013, the design was complete, fabrication of components by the Mechanical C&M Group had begun and it was time to bring in the divers. During a 10-day period, Global Diving & Salvage brought in enough material and equipment to support a 23-man crew and the 6,400-square-foot barge that serves as the hub of operations and the floating community for the divers. As the trucks arrived and unloaded at Three-Mile Wash 15 miles upstream from the dam, crews assembled the barge and readied it for operation.
“Logistically, this was one of the tougher jobs I’ve worked on,” said Ryan Smith, Global Diving & Salvage diver and support crew member. “We had to bring 29 large trucks down narrow, dirt roads with switchbacks. Sometimes we had to change drivers and use guys who could handle the tougher stretches. All trucks had to be escorted in and out of the site by the highway patrol.”
Global Diving & Salvage also had to obtain a number of hazardous material permits to transport the pressurized tanks of oxygen and helium needed for the job. along interstate highways and local roads.
Once assembled, the barge was moved in April down the lake into place inside the dam’s protective logboom barrier near the dam’s upstream face. Crews then prepared for an intensive 24/7 routine that continued through early September. Global Diving & Salvage used two teams of two divers working around the clock.
Each member of the dive team is trained not only as a diver but also as a construction worker. Divers remained under pressure in their barge-located habitation unit, in the diving bell, or at the bottom of the lake for 30 days. Divers are transported to and from the work site under pressure using a diving bell. Bell runs, the tie the diving bell is separated from the system, are routinely 10 to 12 hours. Topside crews of 10 to 12 men alternated in shifts on the barge to support the underwater work. Each shift was 12 hours daily; the day crew worked noon to midnight and the night crew, midnight to noon. On a typical day, divers awoke in pressurized quarters and reviewed plans. They would communicate with the dive supervisor, who was located outside the quarters, outlining what they wanted to accomplish that day and discussing any pertinent diagrams or schematics. A set of two divers descend to the working depth, at which point the door on the bottom of the bell will open (equal presure inside and out). One diver will exit the bell and work for four to five hours while being tended by his bell partner. He then returns to the bell and the second diver enters the water for his “shift.” At the completion of the work shift, the bell is sealed and raised to the surface and locked back onto the system. One team exits the bell and another team replaces them to repeat the process.
|With the intake of Unit 4 located 160 feet under the surface of Apache Lake, the “base” for the divers was installed as a barge over the intake structure to facilitate efficient transport to the intake.|
At shift change, the incoming team compares notes with the crew coming out of the bell, talking about what they encountered and placement of tools and materials. “You need to plan every move about 10 steps ahead so you can make the most of your time down there. While the support crew topside is at your disposal and your partner is in the bell, essentially you’re out there by yourself,” Smith said.
The support crew anticipates that not everything will go as planned. Some decisions have to be made immediately to ensure the project moves forward and the diver’s time is used to the fullest. “Sometimes the design doesn’t quite mesh with what the divers find, so we have to decide on the fly and make modifications,” said Curt Freeman, SRP site coordinator for the Horse Mesa project. “We’ve had to resolve issues at midnight, 3 a.m., on Sundays and during holidays.” To ensure continuity, Freeman lived on site at the Horse Mesa camp for the duration of the job.
The first order of business for the divers was to remove all leftover pieces of the failed vane. They also dismantled and removed the remaining guide vane. Divers used diamond wire saws to cut the remaining vane into pieces that were hauled out by pulley and crane.
Divers then prepared the surface inside the intake for installation of the new vanes. The vertical vanes, which were fabricated in three sections, were assembled on the deck of a barge.
The barge crane then lowered them, one at a time, into the water and landed them on a track that had been installed by the divers. Wheels mounted on the base of the vanes allowed them to be rolled on the floor of the intake structure along the track. Flotation devices were mounted along the top of the vanes to keep them upright while the divers guided them into position.
“The intake was at about a 15-degree slope, so you had a haul-in line to keep it moving forward at a constant speed, and a holdback line attached the vane to keep it from running away from you. The diver directed the topside operators as we guided these things into place,” said Mike Langen, Global Diving & Salvage’s vice president of marine construction.
Once positioned in the correct location, the gap between the vertical vanes and intake structure was filled with a high-strength grout. Divers anchored and secured steel forms to contain the grout. With the two vertical vanes secured in place, divers then used a highline-type rigging system to install the nine horizontal vanes, creating a structure that resembled a giant tic-tac-toe board.
“It was a little hairy installing the guide vanes, because they are very heavy and bulky,” Smith said. “We went through the process slowly and methodically making sure everything came together and in place.”
With the structure erected and anchored in place, the vanes were filled with a non-aggregate concrete mixture especially developed for underwater placement. This mixture added weight and stability to the new structure. Divers finished working on Unit 4 on Aug. 21, installed the new trashracks and then focused their attention on Units 1 through 3.
While the work was not quite as involved, there was some adjustment as the intakes for the original three units are about 100 feet deeper than the Unit 4 intake. Divers installed a 12-foot by 14-foot, 1-inch-thick stainless steel bulkhead seal frame around the bell mouth of each intake.
The project was complete on Sept. 3, 2013, and the barge moved upstream past the log boom and back to Three-Mile Wash on Sept. 6.
“I’ve done larger, longer projects, such as transmission line construction, 500-kV switchyards and substations, but never anything quite this intense – where there was no downtime,” Freeman said. “I was impressed with the way Global Diving & Salvage handled safety issues. They had their version of a tailboard when diving crews were switched out and topside daily on the barge. They also did safety audits weekly.”
Saturation Diving 101
Divers working at depth are exposed to pressures greater than standard atmospheric conditions.
When diving, a diver’s body absorbs gases in proportion to the surrounding pressure. As a diver surfaces, pressure decreases and the excess gas is released. By ascending slowly, gas is absorbed into the bloodstream, travels to the lungs and is exhaled safely. Ascending too fast from deep water causes gas to form dangerous bubbles in the bloodstream – bubbles that can cause decompression sickness, also known as “the bends.”
Saturation diving reduces the risk of decompression sickness and greatly increases the efficiency of working in deeper water. In saturation diving, divers breathe a blend of oxygen and helium in a pressurized environment that is maintained for extended periods; they are transported to and from the underwater work site inside a pressurized diving bell. Divers are decompressed to surface pressure only once, at the end of their “tour of duty.” Limiting decompressions reduces the risk of decompression sickness and increases the time available for working.
Bell “runs” to the work site are usually 10 to 12 hours. The bell descends to the working depth, at which point the door on the bottom of the bell opens – pressure inside the bell equals that outside to keep the chamber from flooding.
A diver exits the bell to work about five hours, while the other diver remains inside and helps monitor conditions. When the five-hour shift ends, the diver outside returns to the bell and the second diver enters the water. After both shifts, the bell is sealed, raised to the surface and secured back onto the pressurized environment. One two-diver team exits the bell and another team replaces them. Operations are continuous; 24 hours a day, seven days a week.
Mike Langen is vice president of marine construction with Global Diving & Salvage and Mark Estes is a senior corporate communications strategist with the Salt River Project.