Corrosion Cluster on an In-Service Gas Line
An in-line inspection of a 16-in. OD, 0.375-in. wall X60 gas transmission line finds a corrosion cluster on the pipe body. MAOP is 1,000 psi and the location is Class 3. The cluster is 8.0 in. long, 2.0 in. wide, and 0.140 in. deep at the deepest point. There is no gouge, no seam weld, and the area is accessible in an agricultural field. The operator's rule requires action when d/t exceeds 0.25 unless a remaining-strength calculation shows predicted failure pressure at least 1.25 times MAOP. The line cannot be out of service more than 8 hours without a bypass. The nearest launcher is 3 miles away. A Type B sleeve, a 12-in. stopple, and a 6-ft pup joint are available. The integrity engineer asks the field mechanic to help choose the repair and explain the basis.
1. Calculate d/t and estimate whether the anomaly meets the 1.25 × MAOP strength criterion.
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d/t = 0.140 / 0.375 = 0.373, so the wall loss is about 37%. For an approximate remaining-strength check, use flow stress = SMYS + 10,000 = 70,000 psi. The Folias factor is M = sqrt(1 + 0.6275 × L²/(Dt) − 0.003375 × (L²/(Dt))²). Here L²/(Dt) = 8² / (16 × 0.375) = 10.67, giving M = 2.70. Then P_f ≈ (2 × 0.375 × 70,000 / 16) × (1 − 0.85 × 0.373) / (1 − 0.85 × 0.373 / 2.70) = 3,281 × 0.683 / 0.883 = 2,540 psi. The criterion is 1.25 × 1,000 = 1,250 psi, so the estimated failure pressure is about twice the required value. The cluster is strength-acceptable, but d/t above 0.25 still triggers the operator's action rule and an integrity assessment.
2. Given the 8-hour outage limit and available equipment, what repair method do you recommend and why?
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Because the anomaly is strength-acceptable, an immediate cutout is not required by pressure alone. A full-encirclement Type B sleeve can reinforce the area and is often installed with the line in service under a qualified welding procedure and engineering approval. It avoids a long shutdown and fits the available equipment. A qualified composite wrap is another option. A stopple and pup cutout would require a bypass and more than 8 hours, and the launcher 3 miles away makes pigging isolation impractical. Recommend an engineering-approved Type B sleeve, with a verification dig and direct UT to confirm the actual depth and length before welding. If the excavation shows a gouge, crack, or deeper metal loss than reported, stop and recalculate; cutout may then be required.
3. What if the ILI depth tolerance is ±10%? How does that change your decision?
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Worst-case depth is 0.140 × 1.1 = 0.154 in., so d/t = 0.154 / 0.375 = 0.411. Recomputing with the same method: 0.85 × 0.411 = 0.349, and P_f ≈ 3,281 × (1 − 0.349) / (1 − 0.349 / 2.70) = 3,281 × 0.651 / 0.871 = 2,452 psi. That is still well above 1,250 psi, so the strength criterion is still met. However, the larger assumed depth makes the action and reassessment intervals more conservative and increases the importance of direct UT verification at the dig. Do not rely on the ILI alone. If the actual depth exceeds the worst-case value, recalculate again and consider a stronger repair or cutout.