By Elizabeth Ingram
|A diver with Underwater Construction Corporation completes a final inspection of the repaired area of a dam spillway apron.|
This article features three case studies of work performed under water at dams and hydro facilities in North America. The case studies illustrate challenges faced and lessons learned that other project owners can apply.
Work that needs to be performed under water at dams and hydropower facilities can run the gamut from assisting with unit repairs to modifying infrastructure. Challenges abound with this type of work, and the below three case studies clearly illustrate what those challenges can be, as well as how they can be overcome.
Upgrading a generator at Nine Mile
Nine Mile Falls Dam is on the Spokane River about 16 miles from downtown Spokane, Wash. The dam and associated 26.34-MW generating facility are owned and operated by Avista Corp. Associated Underwater Services is performing work as part of a generator upgrade for Units 1 and 2. Because of mechanical failures and the high cost to repair the early 1900s vintage generators, these units have not operated for several years.
Preliminary underwater work on the upgrade started in 2012 with AUS performing an underwater inspection, using a remotely operated vehicle, of the tailrace and inside of each of the four draft tubes. One major finding from this inspection was that the draft tubes supplying water to Units 1 and 2 were nearly full of sand and debris from not operating for several years.
Max J. Kuney, based in Spokane, was awarded the contract for the generator upgrade work. The construction project started in the summer of 2014. AUS, also from Spokane, was chosen to perform the underwater work. The critical path underwater work was to create a dry work area inside Units 1 and 2. This work started with the installation of two dewatering bulkheads upstream. This allowed work to begin on the removal of the old generators and the modification of the penstocks to accept the new generators. Sand removal started during the summer of 2014 and continued throughout the diving project, with new sand constantly migrating back into the work area.
The most challenging part of the project was dewatering of the downstream side of the dam. Working adjacent to the constant flow of water over the spillway created challenges in securing the work barge into position, as well as rough working conditions. To keep the work on schedule, AUS provided three dive crews with two day shifts and one night shift.
New draft tube stoplog guides would need to be installed underwater on the sides of the piers. AUS performed detailed surveys of the existing slots and found that the original guide slots were not in line with each other and were not plumb. A new design was developed by URS Engineering. The new design required the existing guide slots to be enlarged and new steel guides to be installed inside the larger guide slots.
To accomplish this task, a wall saw was set up on a vertical track to make vertical cuts in the concrete on both sides of the guide. The concrete was then chipped out by the AUS divers. A bottom sill plate needed to be installed, and this required several cuts in the floor to create a keyway for the new bottom sill plate. An underwater core drill was used to drill the holes for the rebar dowels, and these were installed with epoxy. The new guide beams were installed in one piece with a full-sized alignment frame. Once the new guide was in the slot, the entire assembly was jacked into position and rock bolted. The grout forms were wet welded into position by the divers. The forms were sealed with Splash Zone Epoxy and then the guides were grouted into position. After the grout cured, the alignment frame was removed with a crane. New guides were installed in all four draft tubes. New draft tube stoplogs were installed in Units 1 and 2 so that those entire units could be dewatered to continue the generator replacement work in the dry.
This phase of the underwater work was completed in April 2015. Work on the generator upgrade is still ongoing.
Dam spillway repair
Underwater Construction Corporation (UCC) inspection crews reported a bulged area of sheet pile on a dam spillway apron during the summer of 2014, uncovered as part of a scheduled inspection at a facility in the northern Midwest. Once a report had been submitted to the dam owner on the results of this inspection, UCC began working with the dam owner to develop a repair plan. Repair methodologies, drawings and material specifications were created. UCC then developed a project budget and schedule. Mobilization commenced in November 2014.
|A diver is welding grout forms into position during work to install new draft tube stoplog guides at Nine Mile Falls Dam.|
The project required divers to repair the spillway toe of the dam by first cutting down the sheet pile to specified elevations using underwater cutting/burning equipment and techniques. Sheet pile was cut down 1 inch below the water line about 11 linear feet across. Divers then drilled a series of holes through the steel sheet pile into the existing concrete of the spillway for installation of eight anchors. Threaded rod anchors were installed and epoxied into place to reinforce the sheet pile and prepare for the new grout to be placed underwater. Solid carbon steel bar stock was welded vertically into place at the sheet pile knuckles to reinforce the structure following American Welding Society D3.6 standards. A geotextile membrane manufactured by US Fabrics Inc. was installed at the base of the spillway toe and custom grout bags were filled and anchored into the riverbed for future scour protection. Grout was than pumped in the voided area between the existing sheet pile and spillway apron. A final inspection of the repair was conducted and a comprehensive construction report was created as a submittal for future reference before demobilization from the site. The work took a total of 13 days.
Many challenges were successfully overcome during the work at this dam, including:
— Minimal water visibility, which required diving personnel with the skill set to work in this environment;
— Water temperature of 32 degrees Fahrenheit;
— Remote site location, which posed challenges with regard to equipment logistics; and
— Air temperatures of -10 to 20 F.
Underwater tunnel repair using a robot
The Puerto Rico Electric Power Authority (PREPA), owner of the 25-MW Yauco I and 9-MW Yauco II plants, was looking to perform maintenance on the tunnel systems to ensure their ongoing successful operation. At Yauco I, PREPA observed that debris was entering the turbines. This led them to suspect there was an issue with the trashrack downstream of the rock catcher and surge shaft. At Yauco II, the turbines and wicket gates required rehabilitation, but first the turbine shutoff valves needed to be rehabilitated so they could close properly.
|This image, taken using a remotely operated vehicle, shows both the trashrack sections that were out of alignment and a failed support beam.|
To solve these issues and prepare the two facilities for ongoing generation needs, PREPA turned to ARG Precision to design low-risk remediation methods that could be carried out while water remained in the tunnel. This was important because: PREPA acknowledged that dewatering the tunnels could potentially lead to areas of collapse, the Yauco II tunnel had a tap for drinking water, and minimal flows needed to be maintained to allow for irrigation water for the surrounding communities. It was determined that dewatering was not a desirable option for the project. Further, it was desired that the remediation plans be designed to allow for the tunnels used to resume generation on short notice to meet demand needs.
ARG Precision hired Hibbard Inshore as the contractor for the underwater works, opting for a robotic repair solution rather than using divers because the work areas were confined inside the tunnels and penstocks, access space was limited, and depth was significant. Initially, Hibbard Inshore was tasked to perform underwater inspection within each tunnel and the penstocks to quantify sediment levels in the Yauco I rock catcher, assess suspected damage to the trashrack, and evaluate the Yauco II penstocks for plugging so that the turbine shutoff valves could be replaced.
At Yauco I, the rock trap and trashrack were located at about 355 feet of depth. One 4-by-8-foot section of the trashrack (weighing 2,500 pounds) had detached and moved down the penstock to about 850 feet of depth. The rock trap’s drain was no longer clearing sediment and the 100-foot-long trap was overflowing with sediment and rock. There were about 277 cubic yards of debris identified, and the rock trap was capable of holding 133 cubic yards of debris before its dividers were completely covered.
After the inspection, ARG Precision and Hibbard Inshore were tasked with providing robotic solutions for removing the debris and damaged trashrack components, dredging the sediment out of the rock trap and repairing the trashrack at Yauco I while also designing and placing a temporary bulkhead in the penstocks of Yauco II so that the turbine shutoff valves could be replaced without dewatering.
The first phase at Yauco I was to attach tooling to the ROVs to perform the cutting and lifting operations necessary to remove the debris, conduit and damaged racks from deep underwater. The surge shaft was used as an access point into the tunnel because it was more practical to lift the ROVs, pump and tooling into the 5-foot-diameter shaft than it would have been to try to access the area from the intake end several miles away.
|This old turbine shutoff valve was removed and replaced without dewatering the tunnel or second penstock.|
To fit into the shaft, a customized tooling skid had to be built for the Hibbard Inshore Mohican ROV. This skid would hold two five-function manipulator arms that could be used to operate tooling and to grab the structures to allow cuts to be made and rigging to be placed. The skid was built to allow the arms to fold at their shoulder so they would hang underneath the vehicle as it was inside the surge shaft. Cameras were added to the manipulator arms so the pilot could independently monitor the view from each arm.
The first tool attached was a rotary saw, which was used to cut through the steel conduit that was lying across the surge shaft. This conduit was removed by the vehicle through the top of the surge shaft.
A custom, high-head trash pump was lifted into the shaft to pump solids up to 2 inches in diameter the lateral distance of 100 feet as well as 355 feet vertically to remove sediment from the rock trap. The Mohican’s arms were used to maneuver the pump suction hose into all of the areas between the divider walls in the 100-foot-long trap to remove as much sediment and debris as possible. The ROV and pump removed 254 cubic yards of sediment from the rock catcher in this manner.
The Mohican ROV was then refit with a centerless saw that would allow it to make the deep cuts necessary to remove the existing trashrack support beam. The beam had buckled during failure of the trashrack and needed to be replaced.
Once the ROV had cut the beam and removed it through the surge shaft, the ROV was used to remove the two sections of the existing trashrack that were damaged too badly to be put back into place, including the section that had slid to a depth of 850 feet down a penstock with a 45-degree angle. The Hibbard Inshore team used the Mohican ROV, along with a Navajo ROV, a pneumatic lift-bag, rigging, winches and hoists to retrieve the trashrack sections.
The ROVs attached lines to each section, used the lift bags for lateral transport through the tunnel back to the surge shaft, and then lifted them to the surface.
The rebuild process then started by installing a new support beam. The remaining trashrack sections were brought back into alignment and structurally supported by bolting into the new support beam. The Hibbard Inshore team redesigned the trashrack sections and support beam so they would have increased strength and could be installed by a ROV. Brackets were used to hold the sections together with bolts, and the sections were secured to the new beam.
The new beam had further bracing at each end to support the rack sections. Once the beam was in place, the new trashrack sections, brackets and bolts were put into place. The Mohican ROV was fit with a tool to torque nuts onto the bolts that pulled the trashrack sections into place properly.
Upon completion of the repairs at Yauco I, the crew moved to Yauco II. Data was collected to allow Hibbard Inshore to work with its plug manufacturing partners on the design of a temporary bulkhead that could be inserted into the tunnel and carried into place by the ROV. This bulkhead had redundant seals as well as pressure monitoring to allow the crew to determine if any of the seals were not seated properly or were not holding. The bulkhead was tested and brought to full working pressure before being shipped to the Yauco facility.
After the ROV inserted the plug into one penstock and actuated the seals, it was confirmed that the seals were holding, and water was drained only from the downstream side of the plug, leaving the water in the tunnel, second penstock, and portion of the first penstock upstream of the plug.
ARG Precision then worked with PREPA to have the valve removed and replaced. Once this was completed, the temporary bulkhead was removed from the penstock by the ROV and was then placed in the second penstock to allow that valve to be replaced. After the valves were replaced, the new valves could properly seal, allowing PREPA to complete the rehab work necessary on the wicket gates and turbines.
During this project, PREPA, ARG Precision and Hibbard Inshore worked together to devise an inspection plan, without dewatering, in areas of its intake tunnels that were difficult to reach. Further, once problem areas were identified, the three companies went to work to devise tooling and methods to remove debris, remove damaged materials, replace a trashrack and replace two turbine shutoff valves in those same, difficult-to-reach areas. This allowed PREPA to eliminate undue stress and the potential of tunnel collapse from dewatering, continue to provide water for drinking and irrigation, generate when necessary and avoid the safety risks of using confined entry crews.
Elizabeth Ingram is managing editor of Hydro Review.