Two Items With Identical Risk Scores but Different Uncertainty
Two equipment items have the same calculated risk score of 62 on a 100-point scale. Item A is a carbon steel column with 0.5 mm/yr general corrosion, four UT points per shell course, and a 2019 inspection showing 60% of minimum thickness remaining. Item B is a stainless steel reactor in wet chloride service with no cracking inspection history and one small external corrosion patch. The site's risk criterion says any item above 50 is 'high' and must have an inspection plan with a documented effective method. The turnaround window is 18 months.
1. Are the two items equally risky? How should the ranking be interpreted?
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No. Equal numerical scores do not mean equal risk or equal urgency. Item A has a well-characterized thinning mechanism, repeated thickness measurements, and a known remaining margin. Its uncertainty is relatively low, so the score is reasonably supported by data. Item B has a credible cracking mechanism in wet chloride service and no cracking inspection history; its score is based on limited evidence and could be non-conservative.
A defensible ranking should carry an uncertainty or confidence dimension. When two items have similar scores but different data quality, the item with the greater uncertainty and the more rapid potential failure mode should receive earlier attention. The ranking should not be used mechanically to defer Item B because its number equals Item A's.
2. What inspection methods and timing would you specify for each item?
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For Item A, continue UT thickness surveys at the established points, with enough coverage to confirm the 0.5 mm/yr rate and the remaining margin. Because the mechanism is general thinning and the data are consistent, the inspection can follow the planned interval, provided the remaining life and risk target are met.
For Item B, thickness readings and external visual inspection are not effective for chloride stress corrosion cracking. Specify an effective cracking inspection, such as penetrant or wet fluorescent magnetic particle testing on stainless steel, or ultrasonic shear-wave examination where applicable, at susceptible welds and cold-worked areas. Do this before the 18-month turnaround if the risk criterion requires it. The inspection plan should document why the chosen method is effective for the damage mechanism.
3. The owner proposes deferring Item B to the 18-month turnaround. What is a defensible response?
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A deferral cannot be justified by the risk score alone. The owner must show that the deferral maintains risk below the acceptance criterion, considering the cracking mechanism, consequence, and inspection uncertainty. Because Item B has no cracking inspection history, a deferral leaves a high-uncertainty failure mode unexamined.
A defensible response is to require either an interim effective inspection before deferral or documented risk mitigation that reduces the uncertainty, such as selective cracking inspection, insulation removal at suspect locations, or operating controls. The deferral should be approved through the management-of-change or risk-acceptance process, with the basis, duration, and interim measures recorded. If neither inspection nor mitigation is feasible, the item should not be deferred solely because its score matches Item A's.