Improvement of Pipeline Fatigue Life Estimation

Abstract

Fracture mechanics methodologies for calculating fatigue lives have been successfully applied by pipeline operators to estimate integrity reassessment intervals. Their application in the definition of pipeline system fatigue lives has been overly conservative in actual practice. The source and magnitude of the conservatism inherent in the calculated fatigue life estimates needs to be identified so operators have a better indicator of when reassessments should take place. The pipe life estimation is especially critical for Electric Resistance Weld (ERW) and Electric Flash Weld (EFW) pipeline systems with longitudinally oriented defects. Prior work on improving fatigue life was initiated through studies completed by Pipeline Research Council International, Inc. (PRCI) to evaluate the sources of differences between fatigue life estimates produced by industry fatigue analysis software and different metallurgists. Two significant sources of conservatism in the fatigue life estimation process were identified: the fatigue crack growth rate (da/dN) and the bulging correction factor applied to axial surface flaws. The experimental and numerical simulation techniques considering the impact of these factors on rate of fatigue crack growth of pipeline axially oriented defects are described in this paper. Finite element modeling was used to simulate pipe bulging in the presence of axial flaws. The effect of the pipe thickness, diameter and flaw geometry was compared with treatments included in existing defect assessment standards. The results illustrate that for longer and deeper flaws existing treatments over represent the local bending due to pipe wall bulging. This results in unnecessarily conservative (shorter) fatigue life estimates. The crack growth rate (da/dN) was measured in a compact tension specimen material fatigue testing program. The test results included a range of ERW and EFW pipe materials with varying vintages and grades. The measured fatigue crack growth rate for the materials tested was found to be lower than that recommended by existing industry standards. This adds to the over conservatism of current approaches. The numerical simulation and materials testing results and related recommendations presented in this paper are compared to existing codified treatments to quantify the level of conservatism inherent in the current state of practice. Recommendations are provided to enhance the precision and better manage conservatism in fatigue crack growth rate calculations. Increased accuracy serves to improve integrity management and would be of interest to pipeline operators, consultants and regulators.

Fatigue Life Models for Pipelines Containing Dents and Gouges

“…Likewise, as shown in Appendix A.3, Coelho et al [39] compared their FEA results of stress intensity factors for a wide range of surface crack sizes (0.25 < a/t < 0.8) with API 579 and BS 7910 estimates. They obtained the same conclusions made by Tiku et al [52], that is, 1) API 579 tabular data agree with the FEA results for all crack sizes, and 2) BS 7910 overestimates the FEA results for deep cracks with a/t > 0.5.…”

“…Figure A2 compares the FEA results of stress intensity factors for axial outside surface cracks in a pressurized cylinders obtained by Tiku et al [52] with API 579 and BS 7910 estimates of the K factors for cylinders. The stress intensity factor solutions for flat plates obtained Newman and Raju [47] are also included for comparison.…”

“…PRCI has sponsored a multiple-year project for measuring FCGR data for pipeline steels and welds [52,64,65]. First, Tiku et al [52] carried out 72 FCGR tests on 12 different pipeline steels with six grades of X42, X46, X48, X52, X60, and X70. For each grade, eleven samples were from pipe base materials, and one was from an ERW weld seam.…”

“…Figure 15 shows the FCGR test data obtained by Tiku et al [52] for base materials of six pipeline grades. It is evident that PRCI test data agree the BS 7910 mean curve for both stress ratios.…”

“…Based on their FCGR tests, Tiku et al [52,65] obtained two sets of mean FCGR curves for base materials of pipeline grades for the higher stress ratio of R > 0.5:…”