Influence of sour environment on crack initiation at corrosion pits under cyclic loading

  • Farnoosh Farhad

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

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, in which case the damage process
is more commonly known as corrosion fatigue. 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. The transition from corrosion pit to corrosion fatigue crack, the so-called pit-to-crack transition, is a significant portion of the total fatigue life and yet the underlying mechanism is still not well understood. This constitutes an
important knowledge gap, which has to be addressed in order to advance the life prediction methods for the assets. A sensible approach to progressing the understanding of the pit-to-crack transition, thus providing further clarity regarding the overall process of corrosion fatigue, is to conduct testing and condition monitoring under representative conditions of the field environment and service loading conditions. Most approaches
that attempt to model corrosion fatigue focus on the crack propagation stage rather than crack initiation. Available models used to predict the fatigue crack initiation from corrosion pits are based on the transition criterion, which is defined using long crack growth rate data and linear elastic fracture mechanics (LEFM).These prediction models assume pits as small cracks and use LEFM criteria to predict long crack. This approach neglects the short crack initiation life, which should not be neglected especially in the
high cycle fatigue regime. Limitations in applying this criterion to short crack initiation from pits mean that determining a criterion for short crack initiation from a pit remains a major challenge. In this project, it is hypothesised that corrosion pits do not behave as cracks but they may be the primary location for crack initiation. It is also hypothesised that the geometry of the pits and the level of stress value play important role in the crack initiation time.

Oil and gas production pipelines often operate under sour corrosive environments in combination with cyclic loading. It is well known that an aggressive environment can reduce the fatigue life of materials. Consequently, it is important to replicate in-service
conditions during testing to obtain reliable data regarding corrosion fatigue behaviour and in particular pit-to-crack transitions in API-5L X65 grade steel pipelines that are commonly used for oil and gas products risers due to their good weldability, mechanical properties and low cost. The material of interest in this study is seamless API-5L X65 grade pipeline steel that was provided by the industrial sponsor. This work investigates
the effect of sour corrosive environment on fatigue behaviour of corrosion pits in X65 grade steel risers utilising experimental, numerical and analytical methods.

This PhD project was set up to investigate the influence of sour corrosive environments on the pit-to-crack transition in X65 pipeline steel. The main aspects of the investigation include: (a) whether the sour environment could change the nature of the crack initiation process (e.g. fatigue damage development time and crack location) relative to fatigue damage process in the air environment; (b) whether it was possible to develop a predictive model for the pit-to-crack evolution based only on the pit geometry and size,
and the material mechanical properties in the sour environment.

An electrochemical method was used to create corrosion pits on the specimens and good control over the desired size of corrosion pits was achieved. An environmental cell was designed and constructed to undertake in-situ corrosion fatigue tests in a sour corrosive environment containing toxic gas of H2S under uniaxial fatigue loading. The bespoke test cell allows periodic non-destructive X-ray micro-computed tomography of the specimen in-situ during fatigue testing and thus enables monitoring of material
degradation 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 (SEM) of post-test metallographic
specimens. Load-controlled fatigue tests were carried out on smooth and pre-pitted specimens in both air and sour corrosive environment. Results presented herein demonstrate the performance and reliability of this approach. In addition, fractography of the specimens was carried out to more investigate the location of crack initiation from the pits. S-N curve was established in both air and sour corrosive environments to
be used in a predictive model. Local stress-strain behaviour at corrosion pits and its effect on fatigue crack initiation were investigated using elastic-plastic finite element analysis of specimens containing a single corrosion pit under static and cyclic loading. Analytical methods were also used to predict the local maximum stress and strain at the pit, which showed similar results to the finite element analysis result. Finally, a model was proposed to predict the pit-to-crack transition life that showed good agreements with experimental results. The local stress and strain ranges at corrosion pits were calculated‎ by‎ simultaneously ‎solving‎ the‎ material’s‎ cyclic‎ stress-strain relationship with either the Neuber’s or‎Glinka’s‎rule.‎Thereafter, the obtained local stress amplitude and mean stress were used to predict fatigue crack initiation life (Ni) using Smith-WatsonTopper (SWT) equation for ambient condition and the measured material’s‎S-N data test for the sour environment.
Date of Award2019
Original languageEnglish
Awarding Institution
  • Coventry University
SponsorsTWI, Ltd.
SupervisorDavid Smyth-Boyle (Supervisor), Xiang Zhang (Supervisor) & Kashif Khan (Supervisor)

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