Degradation Mode Granularity in RBI: Why Broad Damage Categories Can Distort Probability of Failure

What Is Degradation Mode Granularity?

Degradation mode granularity is the level of technical detail used to distinguish one damage mechanism from another within an RBI program. The difference between broad categories such as corrosion, cracking, or thinning, and more specific mechanisms such as general corrosion, localized pitting, under-deposit corrosion, erosion-corrosion, CUI, chloride SCC, wet H2S cracking, fatigue, or HTHA is not merely semantic — it directly affects inspection planning, CML placement, method selection, and PoF assessment.

Why Broad Damage Categories Distort PoF

PoF depends on mechanism-specific assumptions: damage rate, initiation likelihood, growth behaviour, inspection detectability, location of likely damage, confidence in inspection data, susceptibility to process excursions, and likelihood of interaction with other damage. If mechanisms are grouped too broadly, the RBI may average out the real risk.

Examples of Mechanisms That Should Not Be Averaged Together

General Corrosion vs Localized Pitting

General corrosion may be monitored with representative thickness readings if truly uniform. Localized pitting requires closer location targeting, more sensitive inspection methods, and different remaining life assumptions.

CUI vs Internal Corrosion

CUI and internal corrosion may both produce wall loss, but they occur at different locations, progress differently, and require completely different inspection methods. Treating them as a single "corrosion" mechanism produces an inspection strategy that may address neither effectively.

Injection-Point Corrosion vs Mid-Span Corrosion

Injection-point corrosion is driven by flow turbulence, water dropping out, and chemistry mixing. It typically occurs at the injection nozzle and a short distance downstream. A CML placed mid-span will not capture this damage even if it is performed frequently.

SCC vs General Corrosion

Stress corrosion cracking may occur at nominally low corrosion rates and without significant wall loss, yet lead to sudden brittle failure. Treating SCC with the same PoF model as general thinning can dramatically underestimate risk.

Practical Approach to Defining Degradation Modes

For each circuit, the engineering team should ask: what mechanisms are credible based on material, process, temperature, and operating history? Where are those mechanisms likely to initiate and progress? What inspection method is appropriate for each? Should this circuit be segmented into sub-circuits or separate damage mechanism assessments?

When Granularity Is Too Much

Over-granularity creates administrative burden without engineering benefit. When data quality is poor or damage is genuinely uniform, excessive segmentation may introduce false precision. The objective is fit-for-purpose granularity — enough detail to support defensible inspection planning, not maximum complexity.