Session: 03-03-07 Feature Assessment Case Studies Cracking I

Paper Number: 134168

134168 - The Role of Residual Stress in Circumferential Stress Corrosion Cracking – An Initial Study 

Abstract: 

In the past decade, several near-neutral pH external circumferential stress corrosion cracking (C-SCC) was found in various pipelines, and some features have led to in-service or hydrotest leaks. Previous studies have emphasized the significant role of stress in the integrity management of C-SCC, primarily in two aspects: i) the initiation and propagation of C-SCC, and ii) the burst and fitness for service (FFS) assessment of known C-SCC features.

Earlier research revealed that most C-SCC colonies were located near the top or bottom of the pipe, often associated with vertical field bends or local deformation at the bottom of the pipe, and often they are found on slopes. The mechanisms of stress corrosion cracking suggest that axial stress must surpass hoop stress for cracks to align in the circumferential direction. Historically, the sources of axial stress were identified as geohazard-induced axial stress, pressure-induced axial tension, or cross-sectional or local deformation. However, stress analysis on several recently discovered C-SCCs in operational natural gas pipelines revealed low global axial stress at these locations. This finding, confirmed by the observation of minimal or no movement during cut-out, necessitates a better understanding of additional axial stress sources, such as surficial residual stress at these locations.

In this study, through thickness residual stress measurement and mapping were performed using high energy Synchrotron x-ray diffraction and Neutron diffractions on both cracked and non-cracked pipe sections cut from in-service natural gas pipelines. The test results indicated high residual stress with substantial amplitude variation through the wall thickness, in part, explaining the formation of C-SCCs in areas where axial operational stresses are low. Advanced microscopy using focused ion beam (FIB) and electron backscattered diffraction (EBSD) observed thin layers of significant plastic deformation at crack initiation sites, aligning with the residual stress observations. The fundamental difference in nature of the loading and stress between axial stress corrosion cracking (A-SCC) and C-SCC and its implication for FFS assessment is also discussed. The details of the testing, microstructural characterization and analytical work are summarized in this paper.

Presenting Author: Fei Long Queen's University

Presenting Author Biography: Fei Long obtained his PhD degree in materials science and engineering from Queen’s University in 2015, afterwards he worked as research associate in reactor materials testing laboratory, Queen’s University. He has more than 10 years of experience on electron microscopy, neutron and synchrotron x-ray diffraction.

Authors:

Fei Long Queen's University
Suraj Persaud Queen's University
Mark Daymond Queen's University
Ken Zhang TC Energy
Ronald Chune TC Energy