Emergency Repair of a Mile Long Cast in Place Reinforced Concrete Penstock Using Fiber Reinforced Polymer (FRP) Technology

This paper describes the emergency repair of a newly constructed cast in place reinforced concrete penstock located at "El Encanto" Hydroelectric Power Plant near San Jose, Costa Rica. The 7 foot (2.1 meter) diameter and 5742 feet (1750 meter) long penstock conveys river water from an upstream reservoir, along mountainous terrain, to a turbine complex building. While the water flows by gravity, the flow is also pressurized due to the difference in elevation between the reservoir and the building. In the spring of 2009, when plant construction was completed, the penstock was subjected to a pressurized flow test and extensive leaking was observed from surface cracks though out its length. As a first repair measure, the penstock was dewatered and all visible cracks were sealed with typical leak repair products; however, when the penstock was re-pressurized, the cracks reopened and leaking continued. As much as 20% of the flow was lost and as a result, the volume of water reaching the turbine complex was insufficient to efficiently generate electricity. The repair alternative that was successfully implemented considered the installation of a Glass Fiber Reinforced Polymer (GFRP) fabrics on the entire inner surface of the penstock; nearly 150,000 ft² (14,000 m²) of GFRP lining was installed, making this the world's largest FRP pipeline repair project on record performed in a single phase. Among the challenges of the repair project were the significant variations in the vertical and horizontal layout of the penstock due to the mountainous terrain, the limited access to the inside of the penstock through four service access points and the use of a local labor force with no prior experience in FRP installation projects. To minimize the plant shutdown time, the fabric and epoxy manufacturing plants were placed in accelerated production runs and all materials were shipped while the engineering design phase was finalized. Two engineers and four field supervisors were sent to run the installation phase and train the installation crews integrated by 40 local laborers. The installation of the GFRP liner and a protective epoxy top coat was completed in a record 15 day period and the penstock was successfully re-pressurized in July of 2009.