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

Fatigue of X65 steel in the sour corrosive environment – a novel experimentation and analysis method for predicting fatigue crack initiation life from corrosion pits

Abstract Oil and gas pipelines manufactured from API-5L Grade X65 steel are generally subjected to cyclic loading and their internal surfaces are frequently exposed to corrosive sour fluids. Exposure of pipelines to these environments often leads to localized corrosion (pitting) and decreased fatigue life. Corrosion pits are geometrical discontinuities that may promote fatigue cracking by acting as stress raisers. In order to optimize asset inspection and repair scheduling, it is important to understand the fatigue behavior of X65 steel and in particular, the ability to predict the crack initiation from corrosion pit. To achieve this level of understanding, conducting fatigue tests in an environmental condition replicating the field environment is important. This paper presents the test protocol and results of environmental fatigue testing using bespoke laboratory apparatus to undertake in situ corrosion fatigue tests in a sour corrosive environment under uniaxial loading. The environment selected represent processes that are likely to occur at internal surfaces of oil and gas pipelines exposed to production fluids. The tests were carried out on smooth samples to obtain S-N curve in this specific environment as well as on pre-pitted samples. An electrochemical method is used to create corrosion pits on the samples. Also, a model is proposed to predict the crack initiation life from corrosion pit, using a local stress-based technique, which has been validated by experimental test results. Post-test fractography was carried out by scanning electron microscopy (SEM). The performance of our approach is demonstrated. The innovation is anticipated to encourage other workers to employ similar small-scale tests requiring toxic and challenging test environments

Laboratory apparatus for in-situ corrosion fatigue testing and characterisation of fatigue cracks using X-ray micro-computed tomography

This paper presents the design, construction, and assembly of laboratory apparatus to undertake in‐situ corrosion fatigue tests in a sour corrosive environment under uniaxial fatigue loading. The bespoke test cell allows periodic nondestructive X‐ray micro‐computed tomography of the specimen in‐situ during fatigue testing and thus enables monitoring of material degradation in‐situ as it progresses and in particular the pit‐to‐crack transition. This approach provides more direct information on crack initiation than complementary ex‐situ techniques such as scanning electron microscopy of post‐test metallographic specimens. Moreover, the apparatus was designed to allow a fatigue cycle to be interrupted and maintain the sample under static tensile load, during X‐ray tomography scans. This process reduced the risk of premature crack closure during interrupted tests. Results presented herein demonstrate the performance and reliability of our approach and will hopefully stimulate other groups to use similar “lab‐scale” initiatives