As hydroelectric power water conveyance conduits across the country age, Owners are being challenged to complete difficult inspections and are required to make decisions around rehabilitation versus replacement of these significant assets. MP had a similar situation develop when they had to deactivate the FERC-regulated water conveyance system at the Thomson Hydroelectric Development due to a flood-related 2012 breach along the forebay embankment. The penstocks were originally constructed in 1905 and were upgraded at various times throughout the following years. To evaluate the reliability of the riveted and welded steel pipe conduit system, MP elected to conduct a detailed inspection and structural assessment of the conduit system while the project was idle. This project was different than typical penstock evaluations, as MP elected to clean the interior of the steel pipes to allow for the detailed inspection and prepare for lining. This allowed the identification, documentation, and quantification of specific defects. The field inspection identified areas of severe pitting, holes, bulges, buckles, and deformations in the pipes. The inspection also included ultra-sonic thickness measurements, out-of-round measurements, and obtaining steel coupon samples from the various pipe construction eras for use in development of the allowable stresses, which are based on the tensile strength data obtained from the coupon samples. Using the field and laboratory results, hoop stress calculations were performed for each pipe section. As the analyses identified general structurally deficient areas, additional field data was collected to narrow in on the specific extent of problem areas. This iterative approach of progressively more detailed pipe inspection and assessment identified only those localized conduit areas that were found to be structurally deficient. This approach ultimately allowed MP to retain a majority of their aging asset with minimal rehabilitation work at substantial cost savings. GEI worked closely with MP and their preferred contractor who was engaged early in the design process to develop cost-effective and efficient penstock remedial repair designs. The rehabilitation work included localized pipe repairs, pipe replacement, pipe slip-lining, polyurethane lining and coating, and installation of an active cathodic protection system. This project can serve as a guide for other Owners looking to evaluate and repair their aging conduit infrastructure.
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