Buried piping is subject to unique environmental exposure, loading, and restricted access once put into service. Buried piping is susceptible to various corrosion mechanisms on the outside of the pipe as well as internal corrosion mechanisms similar to other aboveground piping. Inspection, repair and replacement of buried piping to address such issues are inherently difficult and costly due to the access issues. To address such difficulties and avoid excavation, a carbon fiber reinforced polymer (CFRP) repair can be applied to the internal diameter of a buried pipe to provide a structural pressure boundary to strengthen or replace the existing piping over a specified length.
Pipe repairs using CFRP have traditionally been designed using a Load and Resistance Factor Design (LRFD) approach for determining the demand and the strength of the repair. However, inclusion of CFRP design rules into allowable stress design (ASD) based Section XI of the ASME Boiler and Pressure Vessel Code, which has not adopted LRFD, requires use of safety factors applied to the strength for the appropriate design rules. Both ASD and LRFD have the same philosophy that the stress from the applied loads must be less than the material strength by a certain factor, and the basic difference between the two approaches is how to determine the appropriate factor of safety to cover all unknown variations in load, material strength, installation, and other.
This paper provides a basis for development of safety factors for design of CFRP repairs of nuclear safety related buried metallic piping to meet the required maximum acceptable probability of failure and reliability in accordance with NEI 96-07.
