Tri-State Generation and Transmission Re-Circ Pipe RepairsSteed Lopez2019-05-13T12:12:05+00:00
External post tensioning repair strengthened one 18-foot length, 108-inch diameter PCCP. Fifty three separate strands stressed to 47 kips each where used. Shotcrete replacement of original concrete shell was completed after installation of strand. Internal strengthening of three additional lengths of pipe using carbon fiber polymer materials was included.
1. High risk safety requirements in industrial power plant. Zero reported incidents.
2. Work completed on schedule and on-time during 10 calendar day shutdown.
3. Definition of work scope completed during construction phase of project.
Field Problems Encountered:
1. High ground water table at soil excavation for external pipe strengthening.
2. Determination of actual project scope delayed until completion of investigation completed at the outset of 10 calendar shutdown.
3. Discovery of cracking arrested by steel liner hindered repair process.
The Tri-State power station located in Craig, Colorado had its annual two week shutdown. During this shut down one of the three cooling towers was deactivated in order to perform maintenance. One of the major items to be completed during this shut down was the repair of the water transfer pipes from the cooling towers to the power station. This was a difficult task due to the compressed two week shutdown schedule. Electromagnetic testing took place in the first two days to determine which pipes needed repairs. There were two different types of repair possibilities to perform on the pipe: 1. Interior Carbon Fiber Reinforcing and 2. External Post Tensioning. The quantity of each repair was indeterminate until the testing was completed. Restruction Corporation was prepared to complete up to 3 interior repairs and up to 2 external post tensioning. Restruction Corporation mobilized to the site and was waiting standby for results of the testing. Strengthening construction started immediately once testing results were obtained. After testing, the engineer of record determined that 3 sections of pipe would need internal carbon fiber reinforcing and 1 section would require external post tensioning.
Post Tensioning Summary
The first step of the exterior post tensioning was gaining access to the exterior of the pipe via an excavation. This was done by a subcontractor to the power station and it complex excavation for several reasons:
1. The size of the excavation, (approximately 60ft diameter hole with a depth of 25ft)
2. The location of the excavation, being able to get the proper sloping of the walls. The repair pipe was directly adjacent to another pipe. This problem was solved by installing a trench box via crane between the pipes.
3. Constant presence of water due to the invert of the pipe being located below the water table.
4. Support of the pipe during post tensioning. Access to the whole circumference of the pipe was necessary to install the post tensioning. Two full circumference openings were dug at each end of the pipe and tendons were installed on both ends to help support the pipe (once the post tensioning had progressed enough to support the pipe the rest of the soil was removed.)
Restruction Corporations first step was to examine the exterior concrete shell of the pipe with the engineer. The original scope of work called for patching of any spalls in the outer shell and tensioning around the exterior of this layer. Upon investigation a large crack was found in the outer shell and upon further investigation it was found that the outer shell was debonded from the inner concrete shell along the majority of the surface area. Also the wires around the inner shell which are installed to help hold the inner level of concrete in place when the pipe is pressurized were almost all completely broken. A decision was then made to remove the entire exterior shell. This was done via chipping hammers, sledgehammers, and concrete hammers. Once the shell was removed it was found that the crack through the outer shell continued through the inner concrete shell. A core was then taken to check the depth of the crack. If the crack was full depth to the steel liner it was likely that the liner would show signs of corrosion and the section of pipe would be unsalvageable and need to be completely replaced. The core showed that the crack did go the full depth of the inner concrete shell but upon investigation the steel liner showed no sign of corrosion verifying that post tensioning was a viable repair.
The next step was to begin the tensioning of the strands around the pipe to provide the necessary support. The pipe section was 18 feet in length and the tendons were spaced 4 inches on center and stressed to a force of approximately 47 kips. The tendons on both ends of the pipe were stressed first so the soil support beneath the pipe could be removed. Stressing then continued stressing every other tendon so as to try to stress the pipe as uniformly as possible. A total of 53 tendons were installed. Once all the tendons were installed, conductivity between the steel liner and the tendons needed to be created. This was done by chipping a small area through the inner concrete to the steel liner on both sides and welding a conductive rod to both ends. A wire was then connected from the rods to every tendon. Once all were connected and conductivity was verified, the steel liner openings were filled with a repair mortar along with the core location. After passing a final inspection by the engineer, the pipe and tendons was encased with a fiber reinforced shotcrete shell and cradle. Prior to shotcreteing aggregate was used to fill the bottom of the excavation try to prevent the water from interfering with the shotcrete process. Once the shotcreteing was complete the pipe was covered with plastic and wet burlap sacks to keep moisture in the concrete and to prevent it from freezing. When a full cure had taken place the excavation was filled and compacted until it was back level to the surrounding areas.