Many industrial assets cannot easily be taken out of service for every repair, reinforcement, branch connection, hot tap, or modification. In-service welding can provide a practical integrity solution, but only when supported by engineering assessment, qualified procedures, experienced execution, and appropriate inspection. It should not be presented as a shortcut to avoid shutdown — it is an engineered option appropriate when risk is properly assessed and controlled.
For pipeline operators and facility owners, in-service welding enables repairs and modifications that would otherwise require costly shutdowns, production interruptions, or product displacement. The engineering value depends entirely on the quality of the assessment and execution.
Technical Context
What Is In-Service Welding?
In-service welding is welding performed on equipment, piping, or pipelines that remain pressurized, operating, or containing product. Common applications include full-encirclement repair sleeves, split sleeves and reinforcement sleeves, hot tap fittings and branch connections, line stopping support, local reinforcement, repair of corrosion or mechanical damage where permitted, and modifications to operating piping or pipeline systems.
Hot tapping includes both welding on equipment in service and cutting through the pressure boundary, introducing additional hazards beyond welding alone.
Why In-Service Welding Requires Engineering Assessment
The process involves simultaneous welding, heat transfer, pressure containment, metallurgy, product safety, inspection, and operational risk. Decisions cannot be based only on welder skill or routine WPS documents. The engineering assessment must review material type and weldability, minimum remaining wall thickness, corrosion and defect morphology, internal pressure and operating temperature, contents and flammability/reactivity, flow rate and cooling conditions, and regulatory and owner requirements.
Main Technical Risks
Burn-Through
Burn-through occurs when the wall beneath the weld pool loses sufficient strength to contain pressure due to local heating. Risk is influenced by wall thickness, heat input, pipe diameter, pressure, flow condition, product, welding process, travel speed, and joint configuration. No universal thickness rule is sufficient for all cases; each situation requires engineering evaluation.
Hydrogen-Assisted Cracking
In-service welding often involves rapid cooling because the flowing contents remove heat from the weld area. This can create hard HAZ microstructures and increase hydrogen cracking risk. Low-hydrogen consumables, preheat where feasible, heat input control, procedure qualification, material assessment, and inspection timing are important. Hydrogen cracking may occur after welding, so delayed inspection or defined inspection hold points may be required.
Product and Process Safety
Contents may be flammable, reactive, toxic, corrosive, hydrogen-containing, or susceptible to decomposition under elevated temperature. Hot work permitting, gas testing, isolation planning, pressure/flow control, emergency planning, and operational coordination are essential.
Existing Degradation
Corrosion, laminations, dents, gouges, weld defects, cracking, or local wall loss can change weldability and pressure containment. Inspection before welding is critical.
Engineering Workflow for In-Service Welding Support
- Define repair/modification objective and confirm asset data
- Perform pre-job inspection and wall thickness verification
- Characterize defects and degradation
- Assess burn-through and hydrogen cracking risk
- Review product/process hazards and safety requirements
- Select repair concept and fitting/sleeve design where applicable
- Develop or review qualified WPS/PQR and welding controls
- Define inspection/NDT plan before, during, and after welding
- Coordinate operations, safety, and field execution
- Document limitations, acceptance criteria, and post-repair monitoring
Relationship with FFS, ECA, and RBI
FFS determines whether damage can remain in service temporarily or permanently, or whether repair/replacement is required. ECA may assess crack-like flaws or weld-related indications. RBI helps prioritize assets and identify where repair or inspection resources should be focused. Advanced NDT provides reliable data for engineering decisions.
TES Canada approaches in-service welding as an integrated engineering service. We support clients through engineering assessment and repair feasibility, burn-through and hydrogen cracking risk evaluation, WPS/PQR review or development support, retained CWB welding engineering support, and advanced NDT planning for pre- and post-weld inspection.
Standards & References
- CSA Z662 — Oil and Gas Pipeline Systems
- 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
- API 579-1 / ASME FFS-1 — Fitness-for-Service
- BS 7910 — Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures
- CSA B51 — Boiler, Pressure Vessel, and Pressure Piping Code
Need support with this type of technical challenge?
TES Canada can help you assess the issue, select the right inspection or engineering approach, and develop a practical integrity management solution.
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