|The horizontal steel plates lining the transition section of this penstock buckled and deformed, resulting in decreased output at the 4.6-MW Brian J. Gallagher Generating Station.|
Buckling of the steel liner of the penstock supplying the 4.6-MW Brian J. Gallagher Generating Station in Canada resulted in a 50% decrease in power output. The problem was repaired using carbon fiber-reinforced polymer.
By Jonathan Wise, Scott J. Newton, Rasko P. Ojdrovic and Dave Caughlin
Jonathan Wise is pipeline and industrial division project engineer with Fyfe Company LLC. Scott Newton is general manager of Mississippi River Power Corp. Rasko Ojdrovic is senior principal engineer with Simpson Gumpertz and Heger. Dave Caughlin is business development manager with Fibrwrap Construction.
Most of the hydroelectric generating stations across North America are 50 to 100 years old. As a consequence, they are approaching or have exceeded their design life. Owners are thus faced with a financial dilemma: refurbish, replace or run to failure.
One of the most significant assets in these generating stations are the penstocks. Unlike most other generating station assets, these pipelines are comparatively inaccessible and difficult to replace. Carbon fiber-reinforced polymer (CFRP) has been used to repair and strengthen pipelines, buildings and bridges for the past two decades. Timber, steel and concrete elements can be wrapped with FRP composite systems, in most cases as a strengthening element to a deteriorated structure. More recently, CFRP has been introduced as a repair alternative for failing pipe sections and joints, as well as for other civil structure applications requiring quick turnaround, low profile, and lightweight unidirectional tension members that can be installed with minimal impact to plant operations. As the global structural engineering industries have adopted this technology, so has the hydroelectric power industry.
In 2012, power output dropped by 50% in an instant at Mississippi River Power Corp.’s 4.6-MW Brian J. Gallagher Generating Station, which is located on the Mississippi River in Ontario, Canada. This plant was completed in April 2010. The turbine was taken offline, inspected and found to be in good condition.
Puzzled, Mississippi River Power examined the penstock and was shocked to discover that it was partially blocked. The steel plate lining the penstock had delaminated from the concrete encasement in the rectangular transition section that runs from the draft tube to the penstock. This allowed for cavity formation and penetration of water under the 3/8-inch-thick, 106-inch-diameter concrete-encased steel penstock. This caused the steel liner to buckle and deform through the section of the concrete-encased pipe and ultimately resulted in a partial blockage.
Mississippi River Power needed a repair solution that would: be designed as a stand-alone system (not using any strength from the host pipe); be applied on the inside of the pipe; minimize the loss of the internal diameter, be cost-effective, and have a design life of more than 50 years.
|The concrete surface was hydro blasted to a surface profile of level 3 prior to FRP application to ensure a proper bond between the CFRP system and the concrete.|
Traditional repair options
Numerous repair methods have been developed to deal with pipeline defects that in the end may cause a pipeline to rupture. These repair methods fall into two categories: external and internal. For this project, only internal repair options were viable due to the hardship of excavating the pipe under the river and removal of the concrete encasement above the steel pipeline.
Mississippi River Power considered three repair alternatives:
— Replacement of the buckled steel section;
— Internal steel patch plate liner with grouted annulus; and
— CFRP repair.
|The concrete surface was hydro blasted to a surface profile of level 3 prior to FRP application to ensure a proper bond between the CFRP system and the concrete.|
Replacement was quickly dropped from consideration because it would have necessitated diversion of the river and taken a period of years.
Welding of a series of internal steel patch plates onto the concrete encasement with a grouted annulus was considered. Steel patch plates can be designed to be slightly adjustable to minimize hydraulic section loss. They can be welded together to form a new full-circumference pipe within the existing concrete encasement. The annulus between the patch plates and concrete encasement would be filled with cementitious grout.
This option was eliminated for a number of reasons:
- Safety: There was limited access through an access manhole, the patch plates would need to be transported through the manhole into the penstock (requiring transport and handling of about 500-pound steel plates into and through a confined space on a slope), welding fumes can be a significant hazard, and the project would have a significant footprint due to the amount of material carried on and off site.
- Construction schedule, as work was projected to take just over a year to complete.
- Hydraulic loss resulting from the cross section being 4 inches or more smaller than the existing penstock.
- Total project cost would be about $2 million more than the cost of the CFRP repair option.
The owner of the facility decided CFRP repair was an attractive, viable option and contacted Fibrwrap Construction for a potential solution. Fyfe Company, the CFRP manufacturer, completed a CFRP feasibility study and then Fibrwrap Construction, Fyfe Company’s certified applicator, was contracted to perform the application of the Tyfo® Fibrwrap System, which is comprised of advanced FRP systems and supplied by Fyfe Company. Fibrwrap contracted Simpson, Gumpertz and Heger (SGH), a third party structural engineering company, to both complete a CFRP design for this hydro facility that would account for all internal and external loads acting on the host pipe and conduct onsite field engineering for the duration of the project.
Design development of the CFRP system
Before any design was created, it was essential to discuss site specifics with Mississippi River Power to determine the design parameters for this project. Table 1 summarizes the parameters SGH used to determine the CFRP repair solution.
After SGH completed an investigation of the penstock, it was determined that, due to the variable geometry inside the penstock, different design approaches were required for each area. Ultimately, the design considered three separate components: the circular cross-section, flat top and bottom portions of the transition piece, and circular sides of the transition piece.
First, for the circular section of the pipe, the CFRP liner was designed according to requirements in a draft standard from the American Waste Water Association.1 The CFRP liner was designed to resist the effects of the internal positive pressure, internal negative pressure, external groundwater pressure and temperature changes without relying on any strength contribution from the concrete encasement. For the design to meet the AWWA standard, there are several limit state requirements. The results showed that the design was controlled by buckling and that one longitudinal layer followed by six hoop layers of the Tyfo SCH 41-2X System, the CFRP component in the Tyfo Fibrwrap System, was required.
When designing for the top and bottom portions of the transition piece, SGH designed the Tyfo Fibrwrap System to act compositely with the reinforced concrete section as a one-way slab to resist the combined effects of groundwater pressure and internal negative pressure in flexure, and checked the design of a debonded CFRP. The general requirements of the American Concrete Institute’s 440.2R-08 guide were followed for flexural strengthening,2 with additional limit states investigated, such as CFRP debonding, steel reinforcement yielding and concrete crushing.
Results showed that the design of the Tyfo Fibrwrap system needed for the circular cross-section was also adequate for the top and bottom slabs of the transition piece, except for about 2 feet at the beginning of the transition piece near the rectangular cross-section. Within this 2-foot-long segment, four additional layers of the Tyfo SCH 41-2X system (CFRP) were needed.
Mechanical anchorage was needed in some locations of the repair area to resist vacuum forces and provide more force transfer to the concrete wall. For this purpose, Fyfe Company’s Tyfo SCH anchors were installed at a grid spacing of 14 inches throughout the top and bottom slab. Tyfo SCH anchors are designed using the pull-out resistances reported by the manufacturer and reduction factors selected from ACI 318-08, Appendix D.3 CFRP within the calculated spacing is checked to determine its adequacy to resist the loads in flexure, as a standalone plate between anchors.
At the downstream end of the repair region, where CFRP liner transitions to the existing steel penstock, CFRP is to be terminated with a WEKO seal that was manufactured by Miller Pipeline. At the upstream end of the repair, where the CFRP liner transitions to the existing steel draft tube, the CFRP repair was terminated with a steel termination detail consisting of a 12-inch-wide bent plate welded to a steel draft tube that was overlapped with the CFRP repair. The plate was anchored into concrete with Kwik Bolt expansion anchors, which were manufactured by Hilti.
Completing the work
Once Mississippi River Power approved the design of the repair from SGH, installation of the FRP was completed by Fibrwrap Construction. Prior to the FRP application, Fibrwrap Construction hydroblasted all the concrete surfaces receiving FRP to achieve the minimum International Concrete Repair Institute concrete surface profile (CSP) level 3, which is defined as a uniform exposure of the core’s coarse aggregate such that the surface will be void of latent materials.
The Tyfo Fibrwrap System was installed by bonding to the existing inner surface of the reinforced concrete encasement. The installation procedure consisted of the following steps:
— Apply a coat of Tyfo epoxy primer on all surfaces.
— Apply a coat of thickened Tyfo epoxy or tack coat on all surfaces.
— Install a layer of Tyfo SHE 51A (glass fiber-reinforced polymer or GFRP) over all exposed steel surfaces to provide a dielectric barrier and prevent any potential galvanic corrosion.
— Install a longitudinal layer of Tyfo SCH 41-2X (CFRP) followed by a coat of thickened epoxy or tack coat.
— Install initial hoops of CFRP, including a coat of thickened epoxy or tack coat after each layer.
— After the laminate cures, and where applicable in the transition structure, drill anchor holes and install ¾-inch Tyfo SCH composite anchors on a 14 inch by 14 inch grid, with 8 inch embedment and 6 inch splay on the penstock top and bottom surfaces and on the side walls where required.
— Install remaining hoop layers of CFRP, including a coat of thickened Tyfo epoxy or tack coat after each layer.
— Install the final longitudinal layer and apply a skim of thickened epoxy as a top coat.
— Install termination details.
During the removal of the balance of the buckled steel penstock and surface preparation, several leaks were discovered. These leaks were particularly prevalent along the cold joint in the concrete encasement. As water and epoxy do not mix well, eliminating these leaks was critical to the success of this project. Consequently, before implementation of the CFRP repair, Fibrwrap engaged the services of both Kinetrics and Dumolin to eliminate the leaks. The typical leakage repair procedure consisted of:
— Surface preparation (routing and/or grinding);
— Drill access holes for the urethane grouting;
— Acid flush all injection sites;
— Install surface seal (epoxy paste or hydraulic cement); and
— Perform polyurethane injection.
Once the leaks were eliminated, Fibrwrap began to install the CFRP. However, during the inspection of the CFRP installation, locations where water was infiltrating the laminate were discovered. Despite the efforts of Fibrwrap to maintain a dry surface, water infiltration occurred between the time of CFRP installation and full cure of the epoxy. This occurred in both a finished laminate and partially finished laminate.
The typical repair procedure consists of:
— Drilling and injecting CFRP laminate in the vicinity of the weeps to address leakage in concrete.
— Observing the area for 24 hours. If weeping reoccurs, repeat the previous step.
— If weeping has stopped, patch the area with a minimum number of hoop-direction layers equivalent to the number of layers installed at the time the leak was detected (i.e. if leak was detected after the fourth layer, replace all four layers). Minimum width of hoop-direction repair was 2 feet, the hoop layer extended a minimum of 1 foot beyond the edge of the affected area, and each subsequent hoop layer extended a minimum of 1 foot beyond the previous layer. A base layer of Tyfo SCH 41-2X in the longitudinal direction was applied locally under the hoop layers and extended a minimum of 1 foot beyond the affected area.
Four weep locations were identified during CFRP application and were repaired as indicated above.
Quality control/quality assurance
During CFRP installation, Fibrwrap Construction prepared 2 feet by 2 feet representative mockup areas (two layers of Tyfo SCH 41-2X) for pull-off test at various locations to determine the bond strength between the CFRP and concrete. The installation of these mockup areas was directly reflective of the installation process for the adjacent CFRP repair areas. Once the fiber cured for 24 hours, direct pull-off tests were performed and passed the manufacturer-recommended 200 psi tension value depicted in the specifications and drawings. The tension tests pulled off from the surface of the concrete, demonstrating that the tension of the fiber was greater than the tension strength within the concrete.
To verify that the design values of the CFRP installed in the field were above the design values depicted in Fyfe Company’s FRP data sheet, 2 feet by 2 feet CFRP witness panels were created per shift and tested at a third party testing facility. The tension test results demonstrated that the CFRP materials used for these repairs exceed the design values published by Fyfe Company and the values used in the third party SGH design calculations.
The use of the Tyfo Fibrwrap system to rehabilitate damaged penstocks had significant economic and environmental benefits.
For the most part, the cost of designing with the FRP composite system can be broken down into two factors: direct and indirect costs of installation. Primarily, the accessibility of the penstock (buried or above grade) and the pipe’s diameter will determine whether rehabilitating the pipeline using CFRP is more beneficial than using other alternative methods (i.e., slip lining, replacing the pipeline, etc.). Installation of the FRP composite systems is typically faster and easier than the traditional methods.
While the direct costs of the work can be quite hefty, it is also important to investigate the indirect costs to arrive at a total project cost. One example is the avoidance of indirect costs associated with interrupting service during a repair (such as lost revenue from power generation or installing a bypass).
The repair project commenced in late July 2013 and was completed in early September 2013. Since substantial completion, an inspection was performed in September 2014 by SGH. SGH determined that the CFRP was in good standing condition and was functioning as expected, withstanding all internal and external loads of the system.
1“Draft Standard for CFRP Renewal and Strengthening of PCCP,” American Water Works Association, Denver, Colo., 2013.
2“ACI 440.2R-08: Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures,” American Concrete Institute, Farmington Hills, Mich., 2008.
3“ACI 318-08: Building Code Requirements for Structural Concrete and Commentary,” American Concrete Institute, Farmington Hills, Mich., 2008.
A webcast on this subject, called Rehabilitation of Penstocks and Structures vs. Replacement, is available for free viewing at any time. Visit the HydroWorld site at http://bit.ly/1FDgu4L.
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