Corroded Pipe Remaining Strength (ASME B31G)

The remaining strength of a corroded pipeline by the original ASME B31G criterion, and whether it still supports the line's operating pressure. The criterion says a corroded area behaves like a blunt flaw whose severity depends on how deep it is relative to the wall AND how long it is relative to the pipe's ability to bulge around it. Depth alone is not the answer: a deep short pit can be tolerable while a shallower but much longer groove is not, because the bulging factor grows with length and drives the failure pressure down. That is the single most useful thing to know when reading an inline inspection report, and it is why anomalies are ranked by predicted failure pressure rather than by depth. Two things here are easy to get wrong and are therefore computed rather than left to the reader. The first is the BRANCH: B31G is a two-part criterion, and the parabolic form with its Folias bulging factor applies only while A = 0.893 L / sqrt(D t) is at or below 4.0. Past that the defect is treated as a full rectangular loss with no bulging factor at all, and carrying the parabolic form beyond its range overstates the remaining strength on exactly the long defects that matter most. The second is WHICH PRESSURE the acceptance is read against: the predicted failure pressure is not the criterion, the safe pressure after the safety factor is, and a defect whose failure pressure sits above MAOP can still fail the screen once the factor is applied. The original B31G is deliberately conservative -- it assumes a parabolic profile and a 1.1 flow stress -- and Modified B31G and RSTRENG use a more realistic effective area and typically permit higher pressures on the same defect. That conservatism is a feature when screening hundreds of anomalies and a cost when it condemns a joint unnecessarily, which is why a defect failing this is normally re-evaluated by RSTRENG with the detailed river-bottom profile before anyone digs. This screens a single area of general metal loss on nominal wall: it does not evaluate cracks, gouges, dents, seam or girth weld anomalies, interacting defects, or corrosion under external loading, and it is not a fitness-for-service assessment. ASME B31G, the operator's integrity management program, and a qualified engineer govern.

flow stress = 1.1 x SMYS; A = 0.893 L / sqrt(D t) selects the branch; at A at or below 4.0 the parabolic form P = S_flow (2t/D)[1 - (2/3)(d/t)]/[1 - (2/3)(d/t)/M] with M = sqrt(1 + 0.8 L^2/(D t)), above it the rectangular form P = S_flow (2t/D)(1 - d/t); safe pressure = that divided by the safety factor; and depth past 80% of wall is unacceptable regardless of length.

The original ASME B31G criterion by name, including the A parameter that selects between its two branches and the 80% screening limit. Acceptance is read against the SAFE pressure, not the predicted failure pressure. It screens a single area of general metal loss on nominal wall: it does not evaluate cracks, gouges, dents, seam or girth weld anomalies, interacting defects, or corrosion under external loading, and it is not a fitness-for-service assessment. Modified B31G and RSTRENG typically permit more on the same defect.

One published criterion evaluated on measured dimensions.

Estimate. AHJ and licensed professional govern.

Field names used by the API: od_in, wall_in, smys_psi, defect_depth_in, defect_length_in, safety_factor, maop_psig, depth_pct, a_parameter, folias_m, flow_stress_psi, failure_pressure_psi, safe_pressure_psi, margin_psi

• Nominal wall and a single defect interacting defects and clusters need a different treatmentASME B31G
• General metal loss only cracks, gouges, dents and weld anomalies are outside the criteriona fitness-for-service assessment
• Conservative by design RSTRENG with a river-bottom profile usually permits morethe operator's integrity management program