Fatigue behaviour of corrosion pits in X65 steel pipelines

Corrosion pits are a form of geometrical discontinuity that lead to stress and strain concentration in engineering components, resulting in crack initiation under service loading conditions and ultimately fracture and failure. Initiation and propagation of cracks in offshore pipelines can lead to loss of containment and environmental and commercial impacts. In order to prevent such failures, tools to predict the structural integrity of pipelines need to be improved. This work investigates the fatigue behaviour of corrosion pits in API-5L X65 grade steel pipeline utilising numerical and analytical methods. Firstly, load-controlled fatigue tests were carried out on smooth X65 steel samples to establish S–N data. Secondly, local stress–strain behaviour at corrosion pits and its effect on fatigue crack initiation were investigated using elastic-plastic finite element analysis of samples containing a single corrosion pit under cyclic loading. Analysis of stabilised stress–strain hysteresis loops at corrosion pits showed that the local stress ratio at the pit changes from 0.1 to −0.4 while the applied stress amplitude increases with the same stress ratio of 0.1. Analytical methods were also used to predict the local maximum stress and strain at the pit, which showed a similar local stress ratio to the finite element analysis result but lower stress and strain ranges. Finally, fatigue crack initiation life was predicted using the combination of finite element stress and strain analysis and the Smith–Watson–Topper strain–life approach. An advantage of this method for life estimation is that this approach considers the local stress and strains at corrosion pits rather than applied stress.