PRCI Report 211


Pipe Axial Flaw Failure Criteria -- PAFFC, Version 1.0 User's Manual and SoftwareLeis, Brian N. and N. D. GhadialiPipeline Research Council International / 01-May-1994 / 47 pages

Battelle Memorial Institute
Need: In the early 1970's the Pipeline Research Council International, Inc.(PRCI) developed a failure criterion for pipes that had a predominately empirical basis. This criterion was based on flaw sixes that existed prior to pressurization and did not address possible growth due to the pressure in service or in a hydrostatic test or during the hold time at pressure in a hydrotest. So long as that criterion was used within the scope of the underlying database and empirical calibration, the results of its predictions were reasonably accurate. However, with the advent of newer steels and the related increased toughness that supported significant stable flaw growth, it became evident that this criterion should be updated. This updating led to the PRCI ductile flaw growth model (DFGM) that specifically accounted for the stable growth observed at flaws controlled by the steel's toughness and a limit-states analysis that addressed plastic-collapse at the flaw. This capability provided an accurate basis to assess flaw criticality in pipelines and also the means to develop hydrotest plans on a pipeline specific basis. Unfortunately, this enhanced capability came at the expense of increased complexity that made this new capability difficult to use on a day-today basis. To counter this complexity, this capability has been recast in the form of a PC computer program.
Benefit: This topical report contains the computer program and technical manual for a failure criterion that will predict the behavior of an axially oriented, partially through the wall flaw in a pipeline. The model has been given the acronym PAFFC which stands for Pipe Axial Flaw Failure Criteria. PAFFC is an extension of a previously developed ductile flaw growth model, L51543, and can account for both a flaw's time dependent growth under pressure as well as its unstable growth leading to failure. As part of the output, the user is presented with a graphical depiction of the flaw sizes in terms of combinations of flaw length and depth, that will fail (or survive) a given operating or test pressure. As compared to existing criteria, this model provides a more accurate prediction of flaw behavior for a broad range of pipeline conditions.
Result: The suitability of this software for its above stated purpose has been validated by its application to the wide database for full-scale pressure to failure tests of capped pipe sections with machined flaws. This database is presented in Reference 5 and discussed at length in Reference 6. As detailed in Reference 1, a biaxial stress field develops in the full-scale burst tests reported in Reference 5 as a result of the pressure acting on the pipe and the end-caps in these capped pipe sections. The influence of the biaxial stress field created by the pressure acting on the pipe and the end-caps in these capped pipe sections on the NLFM formulation embedded in PAFFC has been detailed in Reference 1. It has been shown there that any effects of this biaxial field on the predicted failure pressure are small and well within the effect of the uncertainty in typical materials properties within a length of line pipe. For this reason, the same formulation validated herein by its application to end-capped thin-walled axial flawed pipe sections will be equally accurate and valid for pipelines.

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