Abstract
AbstractOil 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.
Highlights
Previous work by the authors has shown the initiation of fatigue crack from corrosion pits in pre-pitted samples, captured by X-ray tomography by interrupting the environmental fatigue tests.[18] It was observed that crack initiation life, Ni, was about 90% of the total number of cycles to final failure, Nf. on consideration of this observation and the small thickness of the fatigue test specimens, Ni in this paper is approximated as 90% Nf. To study the effects of applied stress, pit geometry and pit size on fatigue crack initiation life, Murakami's square-root-area parameter model was used in calculating the stress intensity factor range, that is, using Equation 224,25: qffiffiffipffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ΔK = 0:65 ΔS π area
In earlier publications[18,19] we have summarized the discrete parts of our initial efforts toward developing a harmonized approach to environmental fatigue testing of alloys
Thereafter, the obtained local stress amplitude was used to predict the fatigue crack initiation life (Ni) using the material's S-N data measured in the sour environment as presented in Figure 7 and Equation 1
Summary
Previous work by the authors has shown the initiation of fatigue crack from corrosion pits in pre-pitted samples, captured by X-ray tomography by interrupting the environmental fatigue tests.[18] It was observed that crack initiation life, Ni, was about 90% of the total number of cycles to final failure, Nf. on consideration of this observation and the small thickness of the fatigue test specimens, Ni in this paper is approximated as 90% Nf. To study the effects of applied stress, pit geometry and pit size on fatigue crack initiation life, Murakami's square-root-area parameter model was used in calculating the stress intensity factor range, that is, using Equation 224,25: qffiffiffipffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ΔK = 0:65 ΔS π area
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