Remaining Life
Definition
The calculated time until the wall thickness at a CML or the minimum measured thickness on a component reaches the retirement thickness, based on the current measured thickness and the applicable corrosion rate. Remaining life is the inspection-data-driven equivalent of Remaining Useful Life (RUL), specifically calculated from thickness data for static equipment.
Guide
Context
Remaining life is the direct output of CML data that drives inspection interval calculations per API 510 and API 570. The maximum inspection interval cannot exceed half the remaining life (for general corrosion) or one-quarter of the remaining life (for localized corrosion). In Reliatic, remaining life is calculated per CML and the minimum across all CMLs determines the asset's remaining life.
Formula
Remaining Life = (Measured Thickness - Retirement Thickness) / Corrosion Rate
Examples
- Measured thickness 10.8mm, retirement thickness 4.8mm, corrosion rate 0.40 mm/yr: Remaining Life = (10.8-4.8)/0.40 = 15.0 years.
- The minimum remaining life across all CMLs on a vessel is 6.2 years (at CML-S2-03), setting the maximum inspection interval at 3.1 years (half remaining life per API 510).
Applicable Standards
- API 510
- API 570
Related Terms
The estimated time from the present until an equipment item reaches a condition where it can no longer perform its intended function safely and must be repaired, replaced, or retired. For static equipment, RUL is typically calculated from the current measured wall thickness, the minimum required thickness, and the corrosion rate. For rotating equipment, RUL may be estimated from degradation models fitted to condition monitoring data.
Corrosion RateThe rate of material loss due to corrosion or erosion, typically expressed in millimeters per year (mm/yr) or mils per year (mpy). Short-term corrosion rate is calculated between the two most recent thickness measurements; long-term corrosion rate is calculated from the original (or earliest) thickness to the most recent measurement. The more conservative (higher) rate is typically used for remaining life calculations.
Retirement ThicknessThe minimum acceptable wall thickness below which equipment must be retired from service, repaired, or re-rated. Retirement thickness equals the minimum required thickness for pressure containment plus any additional thickness required by the applicable code (such as structural minimum for external loads, corrosion allowance for future service, or minimum for NDE reliability). When measured thickness reaches retirement thickness, the equipment has exhausted its design margin.
Minimum Required Thickness(t_min)The minimum wall thickness required by the applicable design code to safely contain the operating pressure at the design temperature, considering the allowable stress of the material, weld joint efficiency, and applicable code margins. This is the absolute minimum thickness for pressure containment — below this value, the equipment exceeds its design basis and must be retired, repaired, or de-rated.
Inspection IntervalThe time period between successive inspections of an equipment item, determined by the RBI risk ranking, the remaining life, and applicable code requirements. API 510/570 set maximum intervals but allow extension based on RBI assessments. The inspection interval must not exceed half the remaining life (for general thinning) and should be set at less than half the P-F interval for condition monitoring tasks.
Where this appears in Reliatic
This concept is used in the Inspections module.