Impact of Local Metal Loss on the Creep Life of an Elbow Pipe

Abstract This paper is the third of a series of research studies that investigated the role of metal loss on the creep residual life of pressure components evaluated via inelastic FEA according to API 579-1 [1][2][3]. In the first two works, general and local metal loss was simulated for a straight section of a low-alloy pipe operated in the creep range. The results, in terms of creep damage, were obtained with and without on-going corrosion, i.e., with and without element deactivation in the FE model, and incorporating the two creep-rupture models included in API 579-1, namely the Larson-Miller theory and the Omega method. Aim of the works was the determination of the degree of conservativism of the common, and practical approach of performing creep inelastic FEAs in fully-corroded conditions. In the present paper the investigation was moved from the idealized case of the axis-symmetric LTA on a straight pipe to a more realistic scenario: the localized corrosion on the extrados of an elbow pipe under internal pressure and operated in the creep regime. The complex geometry required the setup of a 3D FE model and was challenging in terms of optimization and calculation times. Moreover, the characteristic stress pattern that develops in an elbow pipe under pressure had a significant effect on the mutually reinforcing interaction between metal loss and stress relaxation, with an important impact on the calculated creep life.