This failure lesson describes a case where burn-through risk was successfully managed during an in-service hot tap operation, but delayed hydrogen-assisted cracking in the HAZ developed after weld completion and caused a leakage event. The case illustrates that burn-through assessment addresses only one of the critical risks in in-service welding, and that hydrogen cracking risk requires its own independent engineering assessment and inspection protocol.
Engineering Relevance
In-service welding failures from delayed HAZ cracking are underreported because the connection between the welding event and the subsequent leak is not always immediately recognized. The delay between weld completion and cracking initiation can range from hours to weeks depending on material, hydrogen content, restraint, and operating conditions.
⚠️ Case / Failure Lesson
Scenario
A hot tap fitting was welded onto an operating carbon steel pipeline for a new branch connection. The burn-through risk assessment showed that minimum remaining wall thickness, operating pressure, heat input, and flow conditions were within acceptable parameters for the welding procedure. The weld was completed successfully, visual inspection showed acceptable appearance, and immediate post-weld NDE was completed with no relevant indications. The system was returned to service.
What Happened
Approximately 36 hours after weld completion, a small leak developed at the weld toe in the pipe HAZ. Investigation revealed a circumferential crack in the HAZ adjacent to the hot tap fitting weld, consistent with hydrogen-assisted cold cracking (HACC). The crack had initiated after the immediate post-weld inspection was completed.
Root Cause
The base material had a carbon equivalent near the upper range of the applicable weld procedure qualification. The in-service cooling effect from product flow in the pipeline created a faster cooling rate than would occur in normal atmospheric pipe welding at the same ambient temperature. This produced a harder HAZ microstructure with increased susceptibility to hydrogen cracking. Despite the use of low-hydrogen electrodes, sufficient hydrogen remained in the weld metal and HAZ. Under the restraint provided by the fitting geometry and operating pipeline, delayed cracking initiated in the hard HAZ after the immediate inspection.
What Was Missed
Assessment of the base material carbon equivalent against hydrogen cracking susceptibility models for in-service welding conditions. Recognition that in-service cooling creates faster weld cooling rates than ambient-condition cooling calculations might suggest, increasing HAZ hardness and hydrogen retention risk. Specification of a mandatory delayed inspection hold period at 24 to 48 hours after weld completion, with TOFD or PAUT inspection at that time to detect any delayed HAZ cracking before returning the connection fully to service. HAZ hardness verification at an appropriate post-weld timing.
Lessons Learned
In-service welding risk assessment must address burn-through and hydrogen cracking as separate and independent risks. Each requires its own engineering assessment, procedural controls, and inspection requirements. Delayed hydrogen cracking can occur after immediate post-weld NDE is completed and may not be apparent during visual inspection. For higher-risk materials or configurations, a mandatory delayed inspection hold period should be specified in the in-service welding procedure. HAZ hardness monitoring provides an additional indicator of hydrogen cracking susceptibility. Carbon equivalent should be assessed before initiating in-service welding on materials where the base material specification allows a wide composition range.
Applicable Standards
CSA Z662 — Oil and Gas Pipeline Systems: in-service welding provisions. API RP 2201 — Safe Hot Tapping Practices. API 1104 — Welding of Pipelines and Related Facilities. PRCI research on in-service welding: burn-through and hydrogen cracking.
TES Canada Perspective
TES Canada requires that hydrogen cracking risk be independently assessed and addressed in all in-service welding engineering work. We specify delayed inspection requirements where material assessment indicates elevated hydrogen cracking susceptibility, and we document the basis for this requirement in the engineering package.
Standards & References
CSA Z662 — Oil and Gas Pipeline Systems — In-service welding provisions
API RP 2201 — Safe Hot Tapping Practices in the Petroleum and Petrochemical Industries
API 1104 — Welding of Pipelines and Related Facilities
ASME PCC-2 — Repair of Pressure Equipment and Piping
PRCI — In-service welding research — burn-through and hydrogen cracking guidance
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