By Mo Ehsani, Ph.D., P.E., S.E. and Carlos Peña, M.S., P.E.
The use of FRP structural linings to strengthen and/or rehabilitate existing pipelines is increasingly gaining widespread acceptance among power plant and utility facility managers. The versatility of the linings to conform to a wide range of diameters and lengths, their high strength properties, light weight, impermeability, thinness and fast rate of application/installation are some of the reasons why many managers prefer FRP linings to other retrofit alternatives (Concrete Repair Bulletin, 2008). However, some managers may still have the misconception that FRP lining installation requires significant operational downtime. Recent advances in FRP technology, as well as improved installation methodologies have significant decreased the installation time. This article illustrates the impact of these methodologies on the largest FRP pipeline structural rehabilitation job done in a single phase in the world.
FRP linings typically consist of fabrics made with high strength fibers that are soaked in an adhesive resin, and are applied like wall paper to the interior or exterior of the pipe surface. The high strength fibers are typically composed of bundles of very thin strands of glass, carbon, aramid or Kevlar. Once the resin cures, the fabric turns into a very thin (typically about 0.05 inch thick) composite laminate. The density and orientation of the high-strength fibers, as well as the fiber type are parameters that the engineer can vary in order to create customized FRP linings that meet specific project criteria.
When applied to the inner surface of a pipeline, the FRP lining becomes a trenchless structural rehabilitation alternative, where all labor, equipment and materials are introduced into the pipeline through service access points, thus avoiding the need for excavation. Since many major pipelines lie under freeways and urban or industrial developments, excavation is not possible without major disruptions to traffic, production, or other normal operations. The economic impact of the disruptions, coupled with the significant investment required to replace deteriorated pipelines, increase the appeal of this trenchless retrofit option.
The low-pressure pipeline at the "El Encanto" power plant (Fig. 1) conveys river water from an upstream dam to the turbine complex downstream. The pipeline is built of cast in place reinforced concrete, has an inner diameter of 7 ft, and a total length of 5,742 ft. The water flows by gravity, but because of the elevation difference between the dam and turbine complex, as well as the continuous changes in the vertical and horizontal alignment of the pipeline required to conform to the mountainous topography, the water flow is pressurized.
Although the structural design had properly addressed the strength requirements of the pipeline and accounted for the design pressure and hydrodynamic loads, the pipeline's serviceability requirements were overlooked. The pipeline exhibited significant longitudinal and transverse cracking during a pressurized test and as a result, as much as 20% of the flow was lost due to leaks.
It was at this point that the authors were contacted for engineering consultation to repair the cracks and a site visit was quickly arranged. One of the authors inspected the cracks, reviewed structural plans and available local engineering reports pertinent to the leak issues, and indentified the main causes of the problem.
In order to arrive at an optimal engineering solution, all the problems mentioned above needed to be properly and simultaneously addressed. An additional consideration was the urgency of minimizing the time required to implement the repair, since -- obviously -- the power plant could not produce electricity while the pipeline was shut down. The solution implemented is discussed in some depth, as follows.
An FRP lining consisting of one layer of bidirectional glass fabric was designed to provide a humidity barrier, to offer an effective crack control mechanism, and to supply additional hoop strength to account for future losses of hoop steel due to corrosion. Since in all likelihood the corrosion process at the reinforcing steel had already started due to the two-way humidity paths generated by the existing cracks, the additional hoop strength provided by the FRP effectively increased the useful life of the pipeline. It should be noted that the humidity barrier is effective against water leaking into and out of the pipe, due to seepage or internal pressure effects, respectively; however, the corrosion of the steel reinforcement will not be slowed significantly as a result of the humidity barrier, since seepage water will continue to provide the means for this process to continue. While nonstructural linings can also provide two way humidity barriers, nonstructural linings cannot account for the loss of structural integrity caused by the ongoing corrosion due to the presence of seepage water.
Moreover, the adhered FRP laminate was designed to achieve full deformation compatibility with the pipe as the pipe expands due to pressurization, and the bidirectional orientation of the high strength glass fibers in the fabric guarantees that existing and/or future cracks are intercepted in orthogonal directions providing superior crack control. Nonstructural linings, on the other hand, cannot serve as an effective crack control mechanism.
Finally, an epoxy top coat was applied as a cover for all the installed FRP. This coat provides resistance to the abrasion caused by sediment carried by the river water, and additional leak proofing by covering any pin holes remaining in the FRP lining. The coating has a concrete gray color, which facilitates quality control by providing visual means of verifying that the entire light green-colored FRP lining is fully covered, and that any uncoated spots can be easily detected.
The time urgency associated with the power plant's imminent start of operations cannot be overstated, which required placing the entire design and manufacturing process on a very short schedule. Epoxy and fabric manufacturing plants were placed on accelerated production runs and part of the production was prepared for air cargo transport.
A technical team comprising two structural engineers and three field supervisors traveled to Costa Rica to oversee the project and train the local installation crews. A technical team fluent in Spanish was required in order for the job to run smoothly.
The 5,742 ft long pipeline had four lateral access points at the locations of relief valves, with spacing ranging from 1,000 ft to 1,500 ft. These 24" x 24" access points were used by the crew to supply FRP materials, tools and equipment to four installation stations inside the pipeline.
The installation direction was opposite to the flow direction to prevent the tendency of the joints in the FRP lining from being lifted by the water flow. Each installation station consisted of a 5-man crew inside the pipe applying the FRP lining to the pipeline's interior walls, and another 5-man crew performing support activities such as transporting the rolls of lining material from the access point to the installation point, cutting and preparing the FRP rolls, etc.
Specially designed construction joints were prepared at the starting point of each installation run, which also became the end points of the installation front that started at a downstream access point. The joint was later sealed with an epoxy paste. Nowhere in the 5,742 ft length of the pipeline were FRP lining edges left exposed to peeling from water flow, maximizing the water tightness of the installation.
The average rate of production of each of the four installation stations was 2,500 ft2 of FRP lining installed in an average 8 hour work day. The operation continued seven days a week, allowing the complete installation of approximately 150,000 ft2 of the FRP lining system in 15 calendar days. This also included the application of the epoxy top coat, which, as stated previously, was used to provide abrasion protection for the FRP, as well as to seal any remaining pores in the installed FRP laminate. The application took place before the lining was fully cured (surface was still tacky on contact) to ensure maximum bond.
More than one mile of a 7-ft diameter pipeline was successfully retrofitted to its original condition in three weeks (one week of prep work and two weeks of FRP lining installation). The FRP lining is expected to require no maintenance and to have a useful life that will at least match the pipeline's operational lifetime.
The FRP retrofit of the 5,742 ft long and 7 ft interior diameter, cast in place concrete pipeline at "El Encanto" Hydropower Plant in Costa Rica is the largest reported FRP pipeline retrofit job completed in a single phase to date in the world.
The unique features of this project such as the extent of the prep work required to smooth out the irregularities caused by the cast in place procedure, the significant complexities introduced to the engineering design caused by the length of the pipeline, the transitions to steep slopes and the changes in the vertical and horizontal alignment of the pipeline due to the mountainous topography, the training of local installation crews with no previous experience with FRP, the urgent need to minimize downtime of the power plant forcing quick design, materials supply and mobilization of technical team, and the fact that the project was completed ahead of schedule, made the successful completion of this project an outstanding achievement in civil engineering.
The fact that over one mile of a large diameter pipeline can be retrofitted to its original condition with minimum downtime and no excavation was required, especially under the unique challenges mentioned above, is a testament to the versatility and effectiveness of this FRP technology and the experience and technical capabilities of the project team.
Since retrofitting pipelines inside power plants, water/wastewater plants or other industrial facilities, usually is scheduled during programd maintenance shutdowns, the amount of retrofit that can be done obviously depends on the time allocated for the shutdown. However, considering the production rates that can now be achieved in the installation of the FRP lining, it may now be feasible to schedule the structural rehabilitation of complete pipelines during a typical shutdown period. Moreover, FRP may now be considered as a viable alternative for emergency repair of pipeline segments that cannot be excavated.
Concrete Repair Bulletin (2008). "Restoration of Large Diameter Prestressed Concrete Cylinder Pipelines, Water Flow, NM," International Concrete Repair Institute, 21(6), 45-47.
About the Authors:
Mo Ehsani, Ph.D., P.E., S.E.: President and CEO of QuakeWrap Inc. and Professor of Civil Engineering at the University of Arizona. In the 1980s he pioneered the research on the structural applications of FRP technology, and is internationally recognized as an expert on the subject.
Carlos Peña, M.S., P.E.: President and CEO of QuakeWrap México and Professor of Civil Engineering at the University of Sonora. Oversees all technical operations of QuakeWrap Inc. and has more than 25 years of experience as a structural consultant in México and the U.S.