Fatigue analysis of corroded pipelines subjected to pressure and temperature loadings

In this paper a methodology for the fatigue analysis of pipelines containing corrosion defects is proposed. This methodology is based on the nominal stresses from a Global Analysis using a one-dimensional Finite Element (FE) model of the pipeline together with the application of stress concentration factors (SCFs). As the stresses may exceed the yielding limit in the corrosion defects, the methodology also adopts a strain-life approach (ε–N method) which is capable of producing less conservative fatigue lives than the stress-based methods. In addition the proposed methodology is applied in the assessment of the fatigue life of an onshore-hot pipeline containing corrosion pits and patches. Five corrosion pits and five corrosion patches with different sizes are considered. The corrosion defects are situated on the external surface of the pipeline base material. The SCFs are calculated using solid FE models and the fatigue analyses are performed for an out-of-phase/non-proportional (NP) biaxial stresses related to the combined loading (internal pressure and temperature) variations caused by an intermittent operation with hot heavy oil (start-up and shut-down). The results show that for buried pipelines subjected to cyclic combined loadings of internal pressure and temperature fatigue may become an important failure mode when corroded pipeline segments are left in operation without being replaced. -- Highlights: • An ε–N methodology for the fatigue life assessment of corroded pipelines is proposed. • The methodology includes: global analysis, stress amplification, and strain life calculation. • Different-size corrosion patches and pits on the external surface of the pipeline were analyzed. • It's shown that fatigue is a concern when corroded pipeline segments operate for many years