When the 65-year-old wood stave penstock of the 10.4-MW Pinnacles project in Virginia began leaking, owner Danville Power & Light decided to remove the structure and replace it with steel pipe. The penstock was built on a narrow bench cut into the side of a steep gorge, making access difficult. The owner used a helicopter to retrieve sections of the old penstock and deliver the new epoxy-lined steel pipe.
By Timothy R. Wagner and Paul D. Kalv
The 10.4-MW Pinnacles project, on the Dan River in the Blue Ridge Mountains, was built to provide a reliable electricity source for the city of Danville, Va. The facility, which went on-line in August 1938, uses the run-off from 33 square miles of drainage area, under a maximum operating head of 701 feet. A storage reservoir impounded by Talbott Dam discharges water into the Dan River. That water, combined with water from the surrounding drainage area, is diverted by Townes Dam into a 5,400-foot-long wood stave penstock and 4,600 feet of concrete-lined tunnel that leads to a steel surge tank 700 feet above the river. The water then flows down the mountain through a 1,200-foot-long steel penstock to the powerhouse, which contains three turbine-generators.
The Pinnacles project operated for more than 65 years with few changes. However, concerns about the condition and reliability of the old wood stave penstock led project owner Danville Power & Light, a division of the city of Danville, to plan a replacement. In 2005, more than a mile of steel penstock supplied by American SpiralWeld Pipe Company of Columbia, S.C., was lowered into place by helicopter.
Why the penstock needed replacement
During initial construction of Pinnacles, wood stave pipelines with a nominal diameter of 70 inches were used to build the 5,400-foot-long penstock. This type of penstock was most attractive to the design engineer and contractor because it could be assembled on site, a bonus given the extremely remote and rugged terrain.
Over time, expansion and contraction issues and general weathering of the material compromised the integrity of the wood stave pipeline. Considerable leakage at the seams and pipeline deterioration in some areas led to concern that the supporting mechanism of the pipeline, also wood, could be undermined or that the pipeline itself might collapse. The leakage problem was so bad that ice blocks as big as Volkswagen cars were forming and falling off the pipeline in the winter. To address these issues, Danville Power & Light made the decision in 2002 to replace the wood stave penstock.
Wiley & Wilson Inc. in Lynchburg, Va., the engineering company that coordinated design implementation of the pipeline replacement, prepared design specifications for wood stave or steel material, as well as construction contractor bids. Danville Power & Light awarded contracts to the lowest bidders meeting the specifications.
This penstock replacement project was rendered particularly complex because of the location of the pipeline and the nature of the surrounding terrain. The pipeline was above ground on a narrow bench cut into the side of a river gorge. The original pipeline design incorporated saddle supports, as well as pipe bridges to support the saddles and span areas where the bench was nonexistent. All of the pipe bridges were designed so that the bents used the same structural shapes, except for the section that bolted to the footings. This arrangement allowed the contractor to use an assembly-line approach to manufacture and install the bents.
The rugged terrain of the site was especially challenging for the pipeline replacement project. Workers could access the site at four locations, but those points were approachable only by walking – or, as some would argue, difficult hiking. Getting equipment to the site was even more challenging. As the project developed, access roads were cut through the site, but these roads had sections with slopes of 1.5:1 and 2:1. Only tracked equipment and four-wheel-drive vehicles with good tread could traverse those portions, and then only in low gear.
Danville Power & Light considered using old logging roads, but the steepness of the upper face of the site and the limited access to a narrow ledge along the pipeline route quickly pointed to the need to consider helicopter placement.
In addition, the ledge portion of the wooden penstock route was very narrow and would not allow the new steel penstock to be placed adjacent to the existing wood stave pipeline. This meant that all of the wood stave pipe and a portion of the existing steel pipe had to be removed, using a helicopter, before the new pipeline could be installed.
Wooden timbers on the old bridges supporting the penstock for 10.4-MW Pinnacles were replaced with structural steel members to support the weight of the pipe and the new catwalk.
After a thorough analysis, Danville Power & Light determined that the only reasonable installation option was to use helicopters to lower the new pipeline pieces in place. Any other method, including cutting new roads, would have required approval to remove trees. Even then, it would not have been possible to move the pipe on such a narrow ledge.
Once the placement method was chosen, Danville Power & Light needed to select the materials for the steel pipeline. The utility initially planned to install steel pipeline with a cement-mortar lining. However, this material weighs 110 pounds more per foot than epoxy-lined steel pipeline. Although pipeline with the two-coat epoxy lining is more expensive, it ultimately would cost less to install using the helicopters. In the final analysis, Danville Power & Light chose 69-inch-diameter steel pipe manufactured by American SpiralWeld Pipe as the best replacement material. This type of pipe was included in the contract awarded to Waco Inc. of Sandston, Va., to install the pipeline. The total value of the contract was nearly $6.35 million.
Performing the replacement work
Numerous issues required attention during the pipeline replacement project at Pinnacles. For example, where bridges or bents were not required, cradle footings had to be new because demolition of the existing pipeline would damage the old footings beyond reuse. Wiley & Wilson designed the new footings to carry the load of pipe lengths up to 25 feet. Furthermore, two kinds of footings were designed – one for a rock-bearing foundation and one for soil-bearing – because of different ground conditions along the pipeline path.
Waco replaced wooden timbers supporting concrete cradles on a steel arch bridge with structural steel members to support the new pipeline and catwalk. The bridge’s wooden catwalks, originally 24 inches wide, had effectively shrunk to 15 inches because of shifting trestles. As a result, personnel had to turn sideways to squeeze between the pipe and handrail, often resulting in torn clothing. Also, the creosoted lumber was extremely slippery during the winter months – a definite safety issue. To remedy these issues, Waco built new 30-inch catwalks with steel handrails and galvanized grating for flooring.
In addition to using 25-foot lengths of pipe where possible, 20-foot and shorter lengths were installed to meet existing trestle supports and essential points of tangency. In addition to the epoxy interior lining, the pipe’s exterior was coated with a three-coat Xymax system to resist weathering and ultraviolet degradation.
Because of the considerable curves in the existing pipeline route, 132 of the 254 pipe lengths installed at Pinnacles required a mitered end, which allows joint deflections of up to 5 degrees.
Waco first removed the existing wood stave pipe and several sections of existing riveted-steel pipe at the sharper curves. Waco began disassembling the existing wood stave pipeline and banding the material into bundles that could be airlifted to the staging area at the top of the mountain using a small commercial helicopter. However, Waco soon discovered that it was much more efficient to simply cut the wood stave pipeline into 10-foot sections and airlift these sections. Demolition of the existing pipeline and trestles took about two months.
Waco then replaced the pipeline in three sections, with the pipe set sequentially using a small mechanized dolly that mimicked a backhoe assembly. Waco designed and built the dolly. The dolly raised the back end of the inserting pipe to facilitate the jointing operation.
Installation of the new steel pipeline took about two months after all the supporting steel trestles were in place. At the most efficient point of the installation, a joint of steel pipe was being placed by helicopter every five minutes.
Results to date
The entire pipeline installation project at Pinnacles was completed in ten months with an average work crew of only 15. In 2005, the project was recognized by the Steel Pipe Fabricators Association’s Pipeline Division as the Steel Pipeline Project of the Year.
Installation of this pipeline taught several valuable lessons applicable to other hydro project owners planning a similar project:
– If multiple pipe materials are available and are satisfactory for the project, bid these materials as alternatives;
– Keep the design of the pipeline and structural components as similar as possible so that construction of the project becomes an assembly line; and
– Take time to develop a demolition plan, and be willing to make adjustments if your first plan does not work.
Mr. Wagner may be reached at Wiley & Wilson Inc., 2310 Langhorne Road, Lynchburg, VA 24501; (1) 434-455-3681; E-mail: twagner@wileywilson. com. Mr. Kalv may be reached at 2010 Griffin Road, Leesburg, FL 34748; (1) 352-728-9834; E-mail: email@example.com.
Tim Wagner, project manager for engineering company Wiley & Wilson Inc., coordinated the design implementation of the pipeline replacement. Paul Kalv was director of Danville Power & Light, owner of the 10.4-MW Pinnacles facility, during the pipeline replacement. He is now electric director for the city of Leesburg, Fla.