Failure Pressure Prediction of Cracks in Corrosion Defects in Pipelines using eXtended Finite Element Method

• Author / Creator

  Zhang, Xinfang

• Pipelines are one of the safest means of oil and gas transportation. In Canada, 97% of all oil and gas production is transported by pipelines. However, their increasing age can constitute integrity concerns due to the presence of defects such as cracks, welds, dents, and corrosion. These defects can form due to coating or cathodic protection degradation or external mechanical damage. When flaws are detected in pipelines, it is extremely important to have an accurate assessment of the associated failure pressure, which would inform the appropriate remediation decision of repairing or replacing the defected pipelines in a timely manner.

  There are different codes for the assessment of single defects in oil and gas pipelines. The most common codes for crack-like defect assessment are API 579 and BS 7910, there are also numerical programs such as CorLAS. The most popular methods for corrosion defects are RSTRENG, Modified B31G, and LPC methods, there are also computer programs such as CPS. Other methods such as Finite Element Method (FEM) have also been used for assessing the crack and corrosion defects. However, cracks in corrosion (CIC) represent a class of defects, for which there are no agreed-upon method of assessment, with no existing analytical or numerical models to predict their failure pressures. In general, compared to the traditional FEM, which requires extremely fine meshes and is impractical in modelling a moving crack, the eXtended Finite Element Method (XFEM) is computationally efficient while providing accurate predictions. Hence, the aim of this study was to provide a guideline for the assessment of CIC defects in pipelines using XFEM.

  A parametric study was conducted in 2D to investigate the effect of different CIC parameters (initial crack depth, corrosion width, and corrosion profile) on the failure pressure and it was found that initial crack depth had a significant influence on the failure pressure among these parameters. In addition, mesh size sensitivity was investigated successively in the elastic-only material model and elastic-plastic material model. The elastic-only material model was found to exhibit an unrealistically higher failure pressure than the elastic-plastic material model.

  Several existing defect assessment methods that evaluate the failure pressure in crack and corrosion defects were reviewed. The versatility of RSTRENG, LPC, and CorLAS in predicting the failure pressure was also discussed. The study revealed that for the corrosion-only defect, the LPC method predicted the closest failure pressure to that obtained using XFEM (3.5% difference), whereas the RSTRENG method provided a more conservative prediction with 19% difference. CorLAS method provided accurate result for the crack-only defect with a 7% difference.

  The finite element program ABAQUS was used to model a series of burst tests. In CIC modelling, the CIC defect was modeled as an artificial corrosion defect with an elliptical profile and a V-notch shaped crack placed in the centre of the corrosion. The failure pressure was predicted when the crack penetrated the inner surface of the pipe. Based on this criterion, it was found that for shorter cracks, the failure pressure decreased with increasing initial crack depth; when the initial crack depth exceeded 50% of the total defect depth, the CIC defect could be treated as crack-only defects, since the failure pressure for the CIC model approached that for the crack-only model for ratios of the initial crack depth to the total defect depth of 0.75 and 1. However, for longer cracks, the initial crack depth was found to have a negligible effect on the failure pressure, implying that the CIC defect could be treated as either a crack or a corrosion utilizing the available assessment methods.

• Subjects / Keywords

  • Failure pressure
  • XFEM
  • Cracks in corrosion
  • Maximum principal strain
  • XFEM calibration and validation
  • Fracture energy

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-wrha-qs50

• License

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