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Applications of RSTRENG Software for Corrosion and Metal Loss Assessment

ASME B31.G Original and ASME B31.G Modified are Level 1 calculations used to assess corroded pipelines for removal, repair,

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ASME B31.G Original and ASME B31.G Modified are Level 1 calculations used to assess corroded pipelines for removal, repair, or continued operation provided the metal loss was calculated to fall within safety limits. While the B31G criterion helped operators avoid many unnecessary pipe cutouts, there was a concern that the criterion was too conservative and led to cut outs that may not have compromised the safety of the pipeline. Thus, the modified B31.G method was adopted in the ANSI/ASME B31 code. A key difference between the two is the fact that ASME B31.G Original represents the entire effective area with a single parabola using a 2 term folias factor, while B31.G Modified represents the entire effective area with a modified rectangle using a 3 term folias factor. They each use two measurements: maximum depth and length. For hand calculations, a factor of 0.85 was recommended for the modified criterion, rather than the initial 0.67 in the original ASME B31G.  

Then, an effective area method was developed using the measured profile of the corroded area, providing more accurate predictions. This effective area method was called RSTRENG. RSTRENG is a level 2 calculation that requires more data and more intensive calculations. It is sanctioned by DOT CFR 49, 192, and 195 to calculate the remaining strength along extended length of metal loss of the pipe. It is the most accurate method to perform remaining strength calculations and has been approved by PHMSA. It calculates failure, safe operating pressure, and factor of safety based on the extent of metal loss.

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