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PRCI PR-185-9733
- Limit States and Reliability-Based Girth Weld Assessment Framework for Long Pipeline Segments
- Report / Survey by Pipeline Research Council International, 07/28/2000
- Publisher: PRCI
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L51822e
Edison Welding Institute
This report describes the development of a graphical framework enabling probabilistic assessment of surface-breaking planar defects in pipeline girth welds, in terms of multivariate limit state functions incorporating weld strength mismatch effects. This is part of an ongoing effort by the Welding Research Supervisory Committee to align girth weld fitness-for-service assessment methods with recent advancements in limit states- and reliability-based design for pipelines. A previous three-year program (PR-185-9429) by the same investigators produced a database of partial safety factors (PSFs), calibrated against a broad range of reliability targets, for use with limit state functions based on existing stress intensity (crack driving force) and plastic collapse solutions. Several inherent limitations of these solutions, including their inability to account for strength mismatch in the behavior of weld metal and heat-affected zone (HAZ) flaws, motivated the development of new relationships in the present work. The PSF-based approach to probabilistic assessment also had its limitations, namely that it did not provide all of the necessary tools to address multi-weld pipeline segments, and that it was somewhat cumbersome to apply in practice. A more direct method was therefore desired.
Benefit: Development of new parametric relationships incorporating mismatch effects involved nonlinear finite element analyses to obtain J-integrals for a large matrix of cracked girth weld geometries, following a modified GE-EPRI procedure. From these were derived material- and geometry-specific failure assessment diagram (FAD) loci, parametric in pipe diameter, wall thickness, crack depth, yield strength, strain hardening rate, mismatch ratio, weld width and applied stress.
Result: Correction factors were developed to account for strength mismatch effects on limit loads of flawed pipeline girth welds. These were combined with J-integral expressions to develop failure loci. Using mismatch-corrected, multivariate limit state functions, algorithms were developed to plot iso-reliability contours onto two-dimensional failure assessment diagrams (R-FADs), thus enabling fully probabilistic assessments based only on deterministic calculations. This technology was implemented in the form of a computer program titled GirthRel, which includes the capability to assess the overall girth weld reliability for pipeline segments of any specified length.
Edison Welding Institute
This report describes the development of a graphical framework enabling probabilistic assessment of surface-breaking planar defects in pipeline girth welds, in terms of multivariate limit state functions incorporating weld strength mismatch effects. This is part of an ongoing effort by the Welding Research Supervisory Committee to align girth weld fitness-for-service assessment methods with recent advancements in limit states- and reliability-based design for pipelines. A previous three-year program (PR-185-9429) by the same investigators produced a database of partial safety factors (PSFs), calibrated against a broad range of reliability targets, for use with limit state functions based on existing stress intensity (crack driving force) and plastic collapse solutions. Several inherent limitations of these solutions, including their inability to account for strength mismatch in the behavior of weld metal and heat-affected zone (HAZ) flaws, motivated the development of new relationships in the present work. The PSF-based approach to probabilistic assessment also had its limitations, namely that it did not provide all of the necessary tools to address multi-weld pipeline segments, and that it was somewhat cumbersome to apply in practice. A more direct method was therefore desired.
Benefit: Development of new parametric relationships incorporating mismatch effects involved nonlinear finite element analyses to obtain J-integrals for a large matrix of cracked girth weld geometries, following a modified GE-EPRI procedure. From these were derived material- and geometry-specific failure assessment diagram (FAD) loci, parametric in pipe diameter, wall thickness, crack depth, yield strength, strain hardening rate, mismatch ratio, weld width and applied stress.
Result: Correction factors were developed to account for strength mismatch effects on limit loads of flawed pipeline girth welds. These were combined with J-integral expressions to develop failure loci. Using mismatch-corrected, multivariate limit state functions, algorithms were developed to plot iso-reliability contours onto two-dimensional failure assessment diagrams (R-FADs), thus enabling fully probabilistic assessments based only on deterministic calculations. This technology was implemented in the form of a computer program titled GirthRel, which includes the capability to assess the overall girth weld reliability for pipeline segments of any specified length.
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