Current federal regulations in the U.S. require excavation of all dents associated with metal loss due to corrosion identified through in line inspection surveys. Once a dent has been found to be associated with metal loss through excavation, there is little guidance to determine the serviceability of the anomaly.
Past research has provided methodologies to assess the fatigue life of plain dents, considering the shape of the dent, but there are no widely accepted assessment methodologies to predict the effect of associated metal loss due to corrosion on the fatigue life of dents.
This paper focuses on the fitness for service of dents associated with metal loss, particularly corrosion in dents. Currently, fitness for service assessments of plain dents provide an estimated remaining life of a dent based on the geometry of the dent and current pressure cycling of the pipeline. Dynamic pressure cycling at each dent location is estimated using the upstream and downstream pressure cycle data, elevation, and distance along the pipe. The dynamic pressure cycle data at each dent is then converted into equivalent stress cycles based on the results of rainflow cycle counting.
Finite element analysis (FEA) of a dent without metal loss and with metal loss is performed to compare the maximum stress concentration areas. The FEA program Abaqus is used with solid elements to model the dents. The differences between maximum stress concentration areas is compared for a matrix of extent of metal loss, and orientation of metal loss to analyze the general effect of metal loss and the interaction of metal loss in a dent. The stress concentration areas of dents without metal loss and with metal loss are then applied to current fatigue assessment methodologies provided in API 579 to analyze the effect of metal loss on the fatigue life of dents.