Maintenance, Repair, and Remaining-Life Assessment
What this domain tests
Remaining-life and repair questions test API 579-1/ASME FFS-1 logic, NBIC repair rules, weld procedures, PWHT and inspection after repair. You must calculate remaining life from measured thickness and corrosion rate, then judge fitness for continued service.
Core rules and concepts
- Remaining life = (t_actual - t_required)/corrosion rate, using the same units and a verified rate. t_required is the minimum required thickness for pressure, not the nominal thickness.
- A short inspection interval can give a misleading rate; use at least two credible readings over a known period.
- API 579 provides assessment levels: Level 1 is conservative screening; Level 2 is more detailed; Level 3 uses advanced analysis.
- Weld repairs follow NBIC and the original construction code. Critical repairs may need PWHT, especially for thick carbon steel or cracking-susceptible service.
- After repair, perform the required NDE and hydrotest; do not return to service on visual inspection alone.
Worked example
Measured thickness = 15.0 mm, required thickness = 10.5 mm, verified corrosion rate = 0.40 mm/y. Remaining life = (15.0 - 10.5)/0.40 = 11.25 years. But the next inspection is 6 years away: predicted thickness = 15.0 - 0.40(6) = 12.6 mm, above 10.5 mm, so the interval is acceptable. If localized pitting is found, use the pit depth and API 579 local assessment, not the average thickness.
Traps that cost marks
Using nominal thickness; dividing by the wrong time interval; averaging away pitting; skipping PWHT on a code-required repair; assuming a hydrotest alone proves remaining life.