Integrity Operating Windows: The Missing Link Between RBI and Operations

What Are Integrity Operating Windows?

IOWs are defined ranges or limits for process variables that, if exceeded, may affect equipment integrity by changing damage mechanism susceptibility, corrosion rate, cracking risk, fouling, thermal cycling, pressure protection risk, or other integrity-related behaviour.

IOWs are different from routine process control limits. Process alarms may focus on production stability or safety trips, while IOWs focus on long-term equipment integrity and degradation control.

Why RBI Needs IOWs

RBI is built on assumptions about process chemistry, temperature, pressure, flow regime, water content, contaminants, corrosive species, inhibitor performance, operating duty, start-up/shutdown frequency, and relief scenarios. When these assumptions change, PoF and inspection planning may no longer be valid.

Examples of IOW Variables

• Water cut or free water presence — changes from dry to wet service dramatically affects corrosion risk
• pH — controls H2S corrosion, amine corrosion, and carbonate cracking susceptibility
• H2S and CO2 partial pressures — affect SOHIC, HIC, SSC, and wet corrosion rates
• Chloride concentration — affects stainless steel SCC risk and carbon steel pitting
• Temperature — affects CUI susceptibility ranges, creep, HTHA, and coating performance
• Flow velocity — affects erosion-corrosion risk at elbows, injection points, and tees
• Oxygen ingress — affects MIC and general corrosion rates in water-containing systems
• Inhibitor residual — affects corrosion rate protection in corrosive service
• Operating pressure — affects PSV set pressure margin, relief demand, and fatigue

IOW Exceedance Response

When an IOW limit is approached or exceeded, a defined engineering response should be triggered: review of the damage mechanism assumptions in the RBI model, update of corrosion rate estimates, review of the inspection plan and intervals, acceleration of specific inspections where risk may have increased, process correction or mitigation, and formal management of change documentation where operating conditions change permanently.

IOWs and CUI Management

For CUI programs, operating temperature changes, shutdown frequency, and insulation system condition directly affect CUI susceptibility. Facilities that increase shutdown/start-up frequency or extend ambient temperature exposure may require updated CUI risk ranking even if no significant equipment changes have occurred.

Integration with RBI Evergreening

IOWs should be formally linked to the RBI evergreening process. When IOW exceedances are recorded, the RBI model should be reviewed and updated. This ensures the integrity management program remains defensible as operating conditions evolve.