During rehabilitation of the 94.6-MW Rapide-des-Quinze project in Québec, Canada, divers performed extensive underwater work. From pre-rehab inspections to replacement of the embedded parts of the draft tube to rehabilitation of the faces of the powerhouse and spillway, divers overcame many challenges to complete a successful project.
By André Frigon, Yves Richer, and Jérà´ me Dion
Work was recently completed on a major rehabilitation project at the 94.6-MW Rapide-des-Quinze hydroelectric facility on the Outaouais River in Québec, Canada. The main objectives of the rehab were to extend the life of the installation, ensure its proper operation, and meet modern safety standards.
This rehabilitation, which began in 2001, involved extensive underwater work on the spillway and powerhouse. Hydro-Québec considered dewatering the draft tube portion of the powerhouse and upstream portion of the spillway to perform the work needed, but this option was technically difficult and more costly than performing the work underwater.
Hydro-Québec had previous positive experiences with underwater work during a rehabilitation, which led the company to choose a similar route at Rapide-des-Quinze.
Background on the plant and its rehabilitation
Rapide-des-Quinze was built in two phases. The first four-unit powerhouse and main spillway were completed in 1923. In 1948, after raising the elevation of the reservoir by 7 meters, Hydro-Québec installed a second powerhouse with two more units. The units are fed by paired intakes and paired draft tubes.
Over time, sulfatation and the cycle of icing and de-icing caused significant degradation of all concrete structures. Hydro-Québec performed rehab work at the project in 1966 and again in 1972. But the degradation persisted, so the utility carried out analyses in 1996, 1999, and 2000 to determine the effects on plant safety and operations. These analyses showed that concrete degradation had caused significant operational problems, with gates either being very difficult to operate or no longer closing. These problems severely undermined the ability of the plant to operate safely for the public and the staff.
In February 2001, Hydro-Québec’s board of directors approved a C$80 million (US$65 million) rehabilitation project with three main objectives:
— Rehabilitate all concrete structures (spillway, water intakes, draft tubes, and piers) to minimize degradation from the ice cycle and alkali-aggregate reactivity;
— Guarantee spillway functionality by installing motorized heated gates, replacing the sluice gate and embedded parts (guides and sills), and installing two lifting carts; and
— Optimize operation by replacing or rehabilitating water intake guides and sills, trashracks, butterfly gates, emergency gates, stoplogs, and draft tube guides and sills; enlarging the water intake deck on the first powerhouse; and replacing or rehabilitating the penstock lining for all six units.
Actual cost of the rehab reached C$145 million (US$118 million) because the scope of work required was wider than Hydro-Québec originally anticipated. Construction Demathieu & Bard Inc. in Lafontaine, Québec, was the general contractor for the rehabilitation; Kamtech Services Inc. in Trois-Rivieres, Québec, was the mechanical contractor; and SPG Hydro International Inc. in Ste.-Julie, Québec, carried out the underwater portions of the rehab work. SPG had assisted Hydro-Québec with the analyses carried out in 1996, 1999, and 2000.
Performing the underwater work
The underwater work performed by SPG consisted primarily of rehabilitating the concrete around the piers of all six units, replacing all the embedded parts of the lateral guides, and installing new sills for the downstream draft tube gates. Associated underwater work consisted of consolidating the structure by installing rock anchors to the sills of two intake units and anchor bolts to one of the lateral piers of one unit, as well as drilling drainage holes in the intake sills of two units and in the draft tube sills of all six units to relieve hydrostatic pressure upon dewatering.
First, SPG carried out pre-rehab inspections of all the parts to be replaced or rehabilitated. These inspections were necessary to verify the scope of the work and to validate the specifications and drawings being used. This work involved detailed and precise measurements of each mechanical component of the guides and sills. SPG used a reverse pendulum vertical reference, which relies on the buoyancy of water, to determine the position of the three faces of each lateral guide and the existing embedded parts, as well as to measure the flatness of those parts.
SPG then used these measurements to create a three-dimensional model of the entire gate shaft. Divers measured the sill using a 3-meter straight edge, then added that element to the model.
In order to work on the draft tube gate components and allow work at the intake in dewatered conditions, SPG had to install upstream watertight cofferdams and downstream in-water barriers. To ensure a proper fit, divers inspected the concrete the cofferdams would rest against. Divers used underwater video cameras to document the inspection of the concrete faces of all piers to be rehabilitated. Based on this inspection, divers performed some concrete repair work on a submerged ancient tidal zone of the upstream face of the water intakes before installing the two upstream cofferdams for each unit. The same inspection and rehab were done for the concrete at the vertical faces of the draft tube piers before SPG installed the confinement barriers designed to provide diver safety and protect the work area from turbulence induced by the adjacent operating units.
At the spillways, underwater inspection and rehab work to provide a sealing surface for the watertight cofferdam on the upstream concrete face included sawing and demolition of concrete and even bedrock in the shallow areas.
After the preliminary inspection of the draft tubes was complete, SPG began the demolition phase to remove the embedded parts that would be replaced and provide an adequate void for the new embedded guides and sill parts. Demolition was performed using 400- to 1,660-millimeter hydraulic saws, 15-pound pneumatic chipper breakers, and 30-pound rivet busters. Drilling was performed using 12- to 90-pound pneumatic rock drills and hydraulic drills.
Installation of the upstream cofferdam and dewatering of the intake passages would result in great hydraulic pressure on the sills and walls of the passage. Those elements were not designed to resist such pressure. Therefore, SPG also had to perform work to prevent possible failure upstream that would cause major structural damage and hazards to human safety as a result of flooding of the dewatered working area. Divers drilled 2- to 3-meter-deep drainage holes 3 inches in diameter in the sills of Units 5 and 6 in order to reach the concrete-rock interface and allow the release of possible hydrostatic pressure. Finally, they installed pre-constraint rock anchors at the sill level of Units 5 and 6 to secure the sills as well as installed anchor bolts on one of the lateral piers of Unit 6.
To replace the embedded parts for the downstream draft tube gates and rehabilitate the concrete around the piers of all six units at 94.6-MW Rapide-des-Quinze, divers performed about 4,500 dives.
Safety measures required that, in addition to the unit on which the divers were working, one unit on each side had to be shut down until the cofferdams were installed. The planned downtime objective SPG had to meet for these units was only one week to install the upstream and downstream cofferdams needed to perform its rehab work to the draft tube gate components and the other contractors’ dewatered work. Because of the major rehab work required on some sealing areas for the cofferdam, up to one extra week could be necessary.
Divers anchored the downstream cofferdams to the vertical downstream face of the draft tube and to the river bed. To minimize vortex effects inside the work zone, divers used steel plates to close the gaps between the cofferdam and vertical face of the draft tube and the river bottom. Water in the work zone needed to be as still as possible because the precision of installation required SPG to use a reverse pendulum to measure vertical position and to correctly adjust the various components to be embedded.
Once a turbine-generating unit was isolated on the downstream side and dewatered on the upstream side, the planned objective was six weeks to rehabilitate the pair of draft tube guides and sills. This included removing embedded parts, installing new ones, drilling the drainage holes, and coping with any surprises. With the wider scope of work and some design changes, up to 12 weeks was required to complete the work.
Tight tolerances made the project even more challenging. Hydro-Québec required that the new parts have a tolerance of 0.5 millimeter over 2 meters for sill flatness and 1.5 millimeters over the total width of 4.5 to 6 meters for horizontality. Required tolerances for verticality were 1 millimeter over 3 millimeters for flatness and 1.5 millimeters over the height of 10 meters for Units 1 through 4 and 15 meters for Units 5 and 6. The tolerances were tight in order to meet criteria for leakage flow. The quality control program required measurements and video documentation before and after every step of the work.
After divers finished rebuilding the gate guides, they drilled drainage holes in the draft tube sills for all six units to prevent buildup of hydrostatic pressure during dewatering of the draft tube. The draft tubes were dewatered to allow subsequent sandblasting and painting work.
SPG employed ten to 16 divers on site during the work. Upstream dives were performed in 20 to 60 feet of water, and downstream dives in 20 to 40 feet of water. Over the four-year contract, a total of nearly 80,000 working man-hours were required to complete SPG’s portion of the project. This included modifications to the initial design and additional work arising during planned rehab. One example is the complete reconstruction of a sill under which a major void was identified. Of the total time worked, about 16,000 man-hours were worked by divers performing about 4,500 dives.
For such a project, safety and environmental issues are very important. Safety plans were well established, including the specific dive procedures of Hydro-Québec. Despite the numerous dives performed, only a single incident occurred. This involved a decompression omission due to a deeper dive than expected. The diver was rapidly treated in a decompression chamber on site and suffered no health problems.
Regarding environmental issues, various measures were used to mitigate the side effects of the work. These included using biodegradable oils and leak pans for the hydraulic power tools and recuperation and cleaning of the concrete preparation area and demolition debris.
Challenges of the project
There were three main challenges for the divers involved in the Rapide-des-Quinze rehab. Because of significant turbidity, all the underwater work had to be performed in water with visibility of less than 0.6 meter. There was no practical way to overcome this issue, which occurs because of the dystrophic (dark colored) water characteristics and the variable turbidity levels resulting from the work activities and the rate of water replacement through the protective barriers. Such rehab activities as drilling, sawing, chipping with power tools, and concrete injection generated much turbidity in such a confined work area.
A second challenge was the fact that the temperature of the water during the work period (February to December) varied from 0 to 15 degrees Celsius (C). To optimize dive operation and duration, divers used “hot water” suits. These are wet suits outfitted with a manifold and tubing that circulates hot water inside the suit. Water flows from a water heating furnace on the surface — through the umbilical that also carries air, communication, lighting, and the video signal — into the suit. The water supplied is about 24 to 28 degrees C. It exits the suit at the wrists and ankles, and a special pipe allows divers to use the heated water to de-ice pneumatic tools, which tend to freeze up in cold weather.
The short amount of downtime of the generating units, as specified, was a third major challenge for all parties. Over the course of the project, SPG had the opportunity to refine the specific approach for the job due to the repeatability of the work. This learning process helped keep unit downtime low, even with the enlarged scope of work. In addition, having key personnel on site from start to completion of the project helped with the success of the work. To minimize downtime, SPG accelerated the schedule by working on nights and weekends and pairing divers to accomplish tasks more quickly.
Measuring final results
The final tolerance results obtained on the embedded parts installed by the divers are equivalent to the work that was done upstream in a dry environment. All the unit draft tubes were dewatered without any leaks, meaning subsequent jobs could be performed on schedule. This proves that even complex work can be correctly achieved underwater when dewatering cannot be easily done, is not cost effective, or is simply impossible.
Mr. Frigon may be reached at Hydro-Québec, 855 Ste-Catherine Street East, 15th Floor, Montreal, Québec H2L 4P5 Canada; (1) 514-840-3000; E-mail: email@example.com. Messrs. Richer and Dion may be reached at SPG Hydro International, 2151 Leonard-de Vinci, Suite 101, Ste-Julie, Québec J3E 1Z3 Canada; (1) 450-922-3515; E-mail: firstname.lastname@example.org or jdion@ spghydro.com.
André Frigon, Ing., is a project director for Hydro-Québec, responsible for managing rehabilitation projects. Yves Richer, B.Sc., is president of SPG Hydro International Inc., which performed the underwater work described in the article. Jérà´ me Dion is a project manager for SPG Hydro.