Analysis of the Stress Basis to Use for Creep Evaluation of Pipelines in Fitness-for-Service Assessments

High-energy pipelines are operated at very high pressures and temperatures, with a typical lifetime in the order of decades. Therefore, FFS assessments are periodically performed to evaluate piping safety against creep. Major design codes provide allowable stresses in the creep range; nevertheless, an actual stress basis is needed to calculate creep damage in a FFS procedure. In this paper, three stress bases were considered: the first one includes the primary circumferential and axial stresses, taking also into account the pressure inelastic redistribution. The second basis includes primary hoop and sustained code stresses. Finally, the third one considers EN13480 axial creep stress, including one third of the secondary expansion range. The stress bases were calculated for an example pipeline through a standard stress analysis and the API 579-1 standard was used to evaluate piping safe life. An inelastic, time-dependent analysis of the pipeline, including plasticity and creep, was then carried out via a shell FE model and the lifetime computed using a Level 3 procedure. The comparison between the investigations showed that the primary stress bases were non-conservative for creep evaluation. The combined primary and secondary stress basis led instead to a lifetime consistent with that calculated through the Level 3 procedure.