Girth Weld Repair on a High-Strength Line with Marginal Toughness
A 30-inch, grade X70 gas transmission line is being welded with a cellulose-electrode downhill procedure qualified under API 1104. During production, a girth weld is rejected by radiographic inspection for a 40 mm elongated slag inclusion located in the root region. The construction engineer proposes grinding the defect out from the outside and re-welding only the cap passes, arguing that the root is inaccessible and the defect is small relative to the pipe diameter. The line operates at 80% SMYS, and the qualified WPS specifies a minimum preheat of 100 °C with a maximum interpass temperature of 250 °C. Ambient temperature at the tie-in is 2 °C with wind gusts, and the repair welder's qualification covers only the downhill root-and-hot-pass sequence, not partial-section repairs. The inspector must decide whether a partial-thickness repair is permissible and how to control the repair.
1. Evaluate whether the proposed partial-thickness repair (cap-only re-weld after grinding from outside) is acceptable, and state what the inspector should require before any repair weld begins.
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A partial-thickness repair is not acceptable as described. The defect is in the root region, so grinding from the outside to remove it would either fail to eliminate the defect or create an unacceptable wall-thickness reduction; more importantly, re-welding only the cap does not restore the root geometry or the through-thickness properties the original qualification established.
The inspector should require:
- Full removal of the defect confirmed by surface NDT (MT or PT) after grinding, and verification that remaining wall thickness meets the minimum specified in the applicable code and project specification.
- A repair procedure qualified for the actual configuration - full-thickness repair or a qualified partial-thickness repair with documented extent limits.
- Welder qualification covering the repair type, position, and process being used; the existing downhill root/hot-pass qualification does not cover a cap-only repair.
- Reinstatement of preheat to at least 100 °C, measured on the pipe adjacent to the repair, with wind protection and a preheat check after grinding exposes cooler metal.
- Interpass temperature monitoring to stay below 250 °C, especially on a small repair volume where heat builds quickly.
- Post-repair NDT appropriate to the repair thickness, and documentation of the repair location and extent in the as-built record.
The reasoning is that repair qualification is configuration-specific: the question is not whether the defect is 'small' but whether the qualified procedure and personnel cover the geometry actually being welded.
2. The welder argues that because the ambient temperature is only 2 °C, preheat is unnecessary since the WPS already specifies 100 °C. How should the inspector respond, and what measurement controls apply?
Read the worked answer
The welder's reasoning is inverted: the 100 °C minimum preheat is a requirement, not a default that ambient conditions can waive. At 2 °C ambient with wind, the pipe surface will lose heat rapidly, so preheat must be applied and verified rather than skipped.
The inspector should require:
- Preheat applied over a band wide enough on both sides of the joint to prevent rapid heat loss, with the temperature measured by contact thermometer or calibrated device at multiple points around the circumference.
- Measurement taken after the heating source is removed and immediately before welding, because surface temperature can drop quickly in wind.
- Wind shielding erected so the preheat and interpass temperatures are maintained; without shielding, the measured preheat may not represent conditions at the arc.
- A re-check if welding is interrupted, since a long stoppage at 2 °C will drop the joint below the minimum.
- Interpass temperature checked before each pass, keeping below 250 °C, which is a maximum, not a target.
The underlying judgement is that preheat serves two purposes - slowing cooling rate to avoid hard, crack-susceptible microstructure, and driving off moisture - and both are more critical, not less, in cold windy conditions. The inspector should also confirm the WPS preheat value is appropriate for the wall thickness and grade, and that the welder understands the difference between minimum preheat and maximum interpass.