Inspection
Best Inspection Method for CUI (Corrosion Under Insulation) — Expert Answer | Riis Energy Services
Omar Al-Haddad, Integrity Manager, Middle East · Published · Updated
Quick answer
No single method finds all CUI. The most cost-effective approach is risk-based screening with pulsed eddy current (PEC) or profile radiography across the insulated system, followed by phased-array ultrasonic testing (PAUT) only where screening flags wall loss. This typically limits insulation removal to 5–10% of piping.
Why CUI is hard to find
Corrosion under insulation develops where water is trapped against carbon or low-alloy steel operating between roughly -4 °C and 175 °C. It is hidden by cladding and insulation, so visual inspection alone misses most of it.
Screening methods
- Pulsed eddy current (PEC): measures average wall thickness through insulation up to about 200 mm and cladding; fast, no insulation removal.
- Profile radiography: good at supports, dead legs and small-bore piping.
- Infrared thermography: finds wet insulation, which is the precondition for CUI.
Follow-up methods
At flagged locations, insulation is removed and wall thickness is mapped with PAUT or automated UT C-scan, giving data suitable for a fitness-for-service assessment to API 579.
Recommended approach
- Rank lines by susceptibility (temperature, coating age, insulation condition, location) per API 583.
- Screen high- and medium-risk lines with PEC and profile RT.
- Quantify flagged areas with PAUT.
- Calculate remaining life and set re-inspection intervals.