This article exhibits how modern inspection methodologies combined with innovative computational analysis practices demonstrate the value of conducting fitness-for-service (FFS) assessments on sectional piping. In this instance, a fitness-for-service assessment was performed on two sections of a pipeline experiencing external corrosion at the pipe-to-elbow seam welds. A full external laser scan and spot ultrasonic thickness (UT) readings were used to measure the corroded geometry and verify accurate measurement of the remaining thicknesses in various corroded locations. This allowed for the actual corroded profiles to be accurately modeled using finite element analysis (FEA).
Complications were present when modeling the observed metal loss. Using a fresh approach to finite element mesh generation modeling, the actual measured corroded geometry was modeled without the need for overly-conservative geometric simplification. A Level 3 FFS assessment was performed in addition to a remaining life assessment based on observed corrosion rates. The result of this analysis was that the piping could remain in service for at least two additional years before needing repair.
Case Study overview
A recent inspection of a portion of piping identified two instances of localized metal loss in the vicinity of two 48-inch pipe-to-elbow welds. This inspection consisted of a full external laser scan of the regions of metal loss, as well as various ultrasonic thickness (UT) readings of the corroded areas. The full external scan provided a depth of metal loss (measured from nominal) at every 0.04” axial increment, and every 0.04” circumferential increment throughout the corroded region. Spot ultrasonic thickness (UT) readings were also taken in order to verify accurate measurement of the remaining thicknesses at various corroded locations. These UT readings were used along with the external scan data to create the actual corroded geometry and position this corroded geometry with respect to the various structural discontinuities along the pipe-to-elbow configuration. This allowed for the use of finite element analysis (FEA) to precisely model the corroded profiles.