Nova Scotia Power’s hydro projects feature more than 10 miles of pipelines, commissioned between 1922 and 1967. To address problems with aging facilities, the utility is in the midst of a pipeline replacement program. This work, scheduled to be completed in 2014, has brought many challenges and resulted in several innovations in the utility’s water conveyance system.
By A. Elaine Locke, James B. Yates, Donald F. Horne, Terrence F. MacIvor, and C.T. Chen
Nova Scotia Power Inc. owns and operates 33 hydro facilities throughout the Canadian province of Nova Scotia. These plants contain 53 turbine-generating units with a total capacity of 360 MW. The facilities range in size from 0.5-MW Fall River to the 200-MW Wreck Cove plant. Many of these facilities, including their water conveyance systems, require refurbishment to ensure safe operation.
For 13 of these units, water is transported from the head pond to the unit via a pipeline. In total, there are more than 10 miles of pipelines at Nova Scotia Power hydro facilities. These pipelines vary in diameter from 4 feet to 11 feet. All original pipelines, except one, were of wood stave construction and were commissioned between 1922 and 1967. (See Table 1 on page 20.) However, wood stave pipes have a service life of about 50 years. This means most of Nova Scotia Power’s pipelines have been or are scheduled to be replaced. Replacement of each pipeline brings unique challenges and rewards.
Discovering FRP pipe
Nova Scotia Power first installed wood stave pipe at its 4.2-MW Sandy Lake facility, in 1928. In 1973, the utility replaced almost 600 feet of the 8-foot-diameter pipe, which had reached the end of its useful life, with more wood stave pipe. In 1978, the remaining mile of pipe at Sandy Lake was scheduled for replacement. This time, Nova Scotia Power used a new kind of pipe, fiberglass-reinforced plastic (FRP). FRP pipe is manufactured in the province, making it a potentially economic alternate to wood stave.
FRP is flexible pipe that depends on the soil for support. The bedding for this pipe must be at least 6 inches of well-graded granular fill to allow for drainage. If the bedding does not drain properly and heavy rainfall occurs while the pipe is dewatered, the pipe could float out of place. The bedding used at Sandy Lake may not have been as free draining as necessary for FRP pipe, as there have been several occasions when the pipeline has heaved during a heavy rainfall. Overall, however, the installation was a success.
In 1980, a wood stave pipeline of the same diameter as Sandy Lake was to be replaced at 5.4-MW Hollow Bridge. FRP was again the pipe of choice, but Nova Scotia Power paid more attention to the gradation required in the bedding. The replacement pipe has performed without any problems.
Life extensions of wood stave pipelines
During the 1980s, Nova Scotia Power performed maintenance to prolong the life of its wood stave pipes. At the utility’s 7-MW Paradise project, the 4.83-foot-diameter wood stave pipe was originally installed in 1950. In 1987, about one third (3,200 feet) of this pipeline was replaced with more wood stave. Because much of the Paradise pipeline was not accessible by road, the replacement staves were flown to the site by helicopter. The material chosen for the new staves, on the basis of cost and availability, was local pine. Pine needs a significant treatment of pesticide and anti-fungal preservative compound to resist insect attack or rot. In addition, many of the replacement staves had knots, and many of the existing bands were reused. As a result, the life of this section of pipeline was considerably shorter than the 50 years of life that could be expected with better wood and all new hardware.
Nova Scotia Power carried out a major life extension project on the entire Paradise pipeline from 2001 to 2004. By replacing staves, adding patches, and tightening and replacing bands, the utility hopes the pipeline can function for at least another five years before being replaced. The future plan for the pipeline is to replace the section nearest the head pond in 2009 with a canal that would carry water to a new FRP pipeline.
Replacing pipelines with canals
In 1994, Nova Scotia Power decided to replace 4,600 feet of pipe at its 6.8-MW Hell’s Gate project. The pipeline for this facility hugged the top of a rocky valley along the Black River. Because of the length of this pipeline, the utility decided to replace it over two construction seasons. The first season involved replacing the 2,000-foot section of pipe upstream of the surge tank. Tenders were issued for wood stave, steel, and FRP pipe as possible replacement materials.
Table 1: Schedule for Pipeline Work at Nova Scotia Power Hydro Facilities1
Nova Scotia Power determined that FRP pipe would be the most cost-effective option. In 1995, the utility replaced wood stave pipe at Hell’s Gate with FRP pipe in a fairly straightforward operation. Because the pipeline had to carry water between the first and second phases of the replacement project, a steel contractor fabricated a thimble to slide inside both the new FRP pipe and the existing wood stave pipe. The bands on the wood stave section were tightened to hold the thimble in place until the remaining wood stave pipe could be replaced the following year.
The second phase of construction was carried out in 1996. Nova Scotia Power issued a tender for the supply of FRP pipe, along with a separate tender for installation of the pipe.
Installation of this pipe required overcoming a significant challenge. The shelf on which the upper section of pipe sat was much narrower than the shelf for the lower section. To provide the base width required for the supporting backfill, bedrock had to be removed.
When tenders were received, the price for rock excavation was much lower than expected. Based on this, Nova Scotia Power decided to explore the possibility of excavating a canal instead of installing a new pipeline. Topographical maps indicated that the contours for 3,000 feet downstream of the head pond were at an elevation that would permit the excavation of a canal. Nova Scotia Power carried out a survey to confirm that actual infield elevations matched those on the topographical map. They did.
The next step was a geotechnical program, performed by geotechnical consulting firm Jacques Whitford & Associates, to determine the bedding, type, and permeability of the bedrock. Four boreholes were drilled at fairly even intervals along the proposed canal route. The bedrock was determined to be fine-grained grey slate, moderately jointed and slightly weathered. The bedrock had low acid-producing potential and permeability ranging from 1 x 10-5 to 3 x 10-4 centimeters per second. Although these findings were promising, time was a limiting factor. It was the middle of June, and manufacturing had to begin immediately if pipe was still to be installed.
Nova Scotia Power estimated that the capital cost of constructing a canal at Hell’s Gate was essentially the same as the estimated price for replacing the pipeline. The canal option cost estimate also included construction of a new intake structure and installation of FRP pipe where the elevation began to drop away and was no longer suitable for a canal. Based on a typical cross section of the canal, Nova Scotia Power personnel calculated that the head losses in the canal would be less than the losses in a pipe. Thus, constructing a canal could increase generation by 3 percent. Because the new canal was being built to the south of the existing pipeline, water could still travel through the pipeline, allowing generation to continue for most of the construction period. Based on these factors, the utility decided to proceed with construction of a canal for this facility.
The land adjacent to the proposed canal was property of Nova Scotia Power. However, this land belonged to the utility because it was a transmission right of way. Constructing a canal very close to existing transmission lines presents a challenge. About 1,000 feet of canal had to be built in the narrow space between the two transmission lines, which are only about 100 feet apart. The rest of the canal was constructed next to the 69-kilovolt (kV) line. The contractor carefully performed blasting and ensured that workers kept the safe working limits regarding approach to the transmission lines. The 69-kV line was taken out of service, and a hold off was placed on the 138-kV line during each blast. During blasting of the entire canal, only one pole was broken, when some material from a rock face flew and hit the pole.
There were two locations along the proposed canal route for Hell’s Gate where the elevation was too low to provide sufficient freeboard. In those locations, the contractor built a clay berm. In total, 150,000 cubic feet of rock was excavated to construct the canal. Most of this material was disposed of in an area on the downstream side of the canal.
The final design for the water conveyance consisted of 3,950 feet of canal and 400 feet of new FRP pipe (phase 2). In addition, 250 feet of FRP pipe installed during phase 1 in 1994 was reinstalled in order to align with the new pipe. An increase in generation of 5 percent was realized from this work.
Negotiating with affected people
Nova Scotia Power’s Bear River Development is comprised of two plants – 4-MW Ridge and 6-MW Gulch. The lower plant, Gulch, was commissioned in 1951, while Ridge was added to the system in 1956. Pipelines for both plants were 7.5 feet in diameter, but there the similarity ended. While the Ridge pipeline was a linear, downhill 3,100-foot run from head pond to plant, the Gulch pipeline twisted and turned and went uphill and downhill over its 9,800-foot path.
When it was time for pipeline replacements, the more straightforward Ridge replacement was carried out first, even though it was five years younger than the Gulch pipe. Nova Scotia Power placed FRP replacement pipe at Ridge in 1993. Five years later, the utility tackled the more complicated Gulch replacement.
When Nova Scotia Power Commission, a predecessor to Nova Scotia Power, was considering options for the initial Gulch pipeline in the early 1950s, part of the preferred route ran over land belonging to the Bear River Indian Reserve. To deal with this issue, Nova Scotia Power Commission acquired a 66-foot-wide parcel of land to construct the wood stave pipeline through the reserve. When the pipeline was scheduled to be replaced in 1998, Nova Scotia Power personnel met with the nation’s Chief and Band Council. The first option presented was to replace the pipeline with another pipeline on the existing 66-foot easement.
Nova Scotia Power used fiberglass-reinforced plastic pipe to replace the original wood stave pipeline at the 6-MW Gulch facility in 1998.
Because this option presented difficulties with restricted access, a second option was considered. If the First Nation Community would agree, Nova Scotia Power sought to replace the pipeline with a canal. To illustrate the proposed canal route, a scale model was constructed. To aid the explanation, Nova Scotia Power prepared a computer animation of a flight along the canal. However, negotiations could not be concluded after several months, and the start of the construction season was approaching. Nova Scotia Power decided to install an FRP pipeline that would follow the alignment of the wood stave pipe.
The 1953 Easement Agreement with the First Nations contained a provision that Nova Scotia Power would “construct and maintain at all times hereafter” two bridges and an underpass for use by the First Nation Community. To replace the bridges, the layout for the new FRP pipe included two locations where the pipe was buried to a depth that would permit access over it.
The route for the Gulch pipeline included a 75-foot-deep ravine about 2,000 feet downstream of the head pond. When the wood stave pipe was installed in the 1950s, a 60-foot-long and 30-foot-high concrete slab bridge had been constructed to smooth out the bottom of the sharp vertical curve to a profile acceptable for a wood stave pipe. This bridge also provided the underpass along the stream required by the First Nations agreement. During the pipeline replacement project, the contractor demolished and replaced the severely deteriorated concrete bridge with a corrugated metal arch culvert 16 feet high and 21 feet wide.
Nineteen-foot-high metal bin-type gravel-filled retaining walls at each end of the culvert also provided support for granular material required for the pipe foundation and backfill. Although these bins allowed the base of the pipe to be raised, a downhill slope still remained through this ravine. To allow the pipe to be completely drained during dewatering and to prevent an airlock during pipe filling, a valve was installed at this location.
This culvert was also the start of the pipe installation that ran through the First Nations’ lands. The Bear River Band Council granted six-month temporary access through the community to assist accessing the construction activity, but construction itself had to be completed along the 66-foot right of way. The access along the pipeline had to be confined to the area where the pipe was located. To accomplish this, the contractor removed existing wood stave pipe starting at the ravine and working toward the surge tank. The pipe bed was then available as access for installation of the new FRP pipe.
The first length of new pipe was installed next to the surge tank, then the new pipe was laid working toward the ravine. The cross section of the land next to the surge tank fell quickly away to the river below. To provide sufficient cross sectional area for the FRP backfill, the contractor installed a Geoweb wall, a reinforced soil retaining wall supplied by Presto Products, to raise the land to an acceptable elevation. This flexible composite structure consists of stacked sections of polyethylene cellular confinement facia units, infilled with gravel.
Because the lower pipe placement was restricted to the 66-foot strip of land, the existing wood stave alignment had to be closely followed. However, wood stave pipe is more capable of snaking through a crooked alignment than is FRP pipe. Installation of FRP pipe permits an offset of about 1 degree per length. This was not enough to mirror the wood stave alignment, so several mitered deflection sections had to be installed, complete with thrust blocks.
There were other land-related issues to be overcome. Just as construction was about to begin, Nova Scotia Power discovered it lacked an easement document for two adjoining parcels of land. The utility decided to purchase the properties outright. To clarify title, the utility undertook a title search on both properties. Lot “A” was a straightforward purchase. However, Lot “B” was owned/ claimed by two parties. One party had not recorded the deed until several years after its purchase, and not before a second party claimed ownership. A second party had purchased an entirely different property, but that deed included two land descriptions, one of them for Lot ”B.” With both deeds recorded – one by default and one by a long overdue registration – Nova Scotia Power decided to purchase Lot “B” from both owners rather than delay pipeline construction.
Gravel is an essential component of an FRP installation. Fortunately, one of the two lots the utility purchased turned out to be a “gravel mine,” which produced gravel for the entire project.
With the land issues resolved, installation of the pipe proceeded. Once the bedding was placed, a routine developed for the pipe installation and backfilling. By Thanksgiving, about three quarters of the 198 pipe lengths had been laid. A heavy rain on Thanksgiving weekend prevented water from draining along the pipe. As a result, the first 30 lengths floated. They had to be removed and reinstalled, making the connection to the rest of the pipeline with a wrapped field splice. Crews worked at both ends of the pipeline so that the project was successfully completed on schedule.
Future work planned
Nova Scotia Power has not replaced any pipelines since the work at Gulch in 1998, but major life extension work has been carried out at several plants.
Two of the oldest pipelines are at 1.25-MW Coon Pond (also called Mill Lake) and 4-MW Tidewater. However, since these pipes were commissioned in 1922 and 1938, respectively, a major upstream storage reservoir, Pockwock Lake, was taken over by the Halifax Regional Municipality to form part of its water supply. Thus, the economic value of the Coon Pond development is marginal. Nova Scotia Power will make a decision on the future of this development in the near future. The original Coon Pond wood stave pipeline was replaced with more wood staves made from local pine in the early 1990s.
Life extension work carried out at Tidewater in the late 1990s had its challenges. Fumes from creosote applied during hot weather resulted in complaints from the neighboring family and subsequent stoppage of the work. It is also becoming increasingly difficult to obtain environmental approval for creosote applications. Nova Scotia Power is trying other products as replacements for creosote, but to date there is no clear winner.
Life extension also was carried out in the early 2000s on the utility’s biggest wood stave penstocks, at 18-MW Weymouth Falls. Stave replacements, patching, and band tightening on these two 11-foot-diameter pipes was carried out by a specially trained Nova Scotia Power crew.
The next pipeline replacement project scheduled is at 3.8-MW Dickie Brook in 2008. Life extension work was carried out at this facility in 2000. But, in 2008, this pipe will be 60 years old and at the end of its useful life. Preliminary engineering studies indicate that the 3,700 feet of 5-foot-diameter pipe probably will be replaced with FRP pipe, although a partial canal-pipe option also has been considered.
Nova Scotia Power is only halfway through its pipeline replacement program, which means there are still 5 miles of wood stave pipe to be replaced at its facilities.
The authors may be reached at Nova Scotia Power Inc., P.O. Box 910, Halifax, Nova Scotia B3J 2W5 Canada; (1) 902-428-7546 (Locke), (1) 902-428-7529 (Yates), (1) 902-869-5363 (Horne), (1) 902-478-6174 (MacIvor), or (1) 902- 428-7557 (Chen); E-mail: ann.locke@ nspower.ca, email@example.com, firstname.lastname@example.org, tmac@eastlink. ca, or email@example.com.
The authors wish to thank: Jim Gordon for wise counsel received over the years; Nova Scotia Power’s construction superintendents; Daeco Contracting and Elliot Excavators, two contractors that worked hard and intelligently to perform the construction work safely and efficiently; and Barry Arsenault, Nova Scotia Power’s computer-aided drafting specialist extraordinaire who produced numerous hydro drawings. Last but definitely not least, we thank Eric Brown, “Mr. Hydro” for Nova Scotia Power from 1982 until his retirement in 2007. His dedication to hydro engineering inspired the authors to prepare this article.
Elaine Locke, P.Eng., is senior civil engineer, Jamie Yates, P.Eng., is hydropower consultant, Don Horne is property specialist, Terry MacIvor is senior construction superintendent, and C.T. Chen, P.Eng., is hydropower consultant for Nova Scotia Power Inc.