RBI Concepts, Principles, and Definitions
Why RBI exists
Risk-based inspection uses risk as the basis for deciding where inspection effort should be spent. Instead of treating every item in a unit equally, RBI allocates inspection resources in proportion to the risk each item carries, so that effort concentrates on the equipment most likely to cause harm or loss.
The core equation
At the heart of everything is risk = probability of failure (POF) × consequence of failure (COF). POF is normally expressed per unit of time (e.g., per year) for a defined failure mode; COF expresses the safety, environmental, and economic outcomes of the release scenario being considered. A change in either term changes risk by the same proportion.
Vocabulary to own precisely
- Damage mechanism – the physical process that degrades a component (general corrosion, chloride stress corrosion cracking, erosion).
- Failure mode – how the loss of function occurs: small leak, large leak, rupture.
- Failure – loss of containment or loss of function.
- Likelihood/probability – dimensionless or per-unit-time chance of that failure mode occurring.
- Inspection – activity that determines current condition (visual, NDE, pressure test).
- Mitigation – any activity that reduces POF or COF.
Traps in questions
Questions frequently swap inspection with mitigation. Inspection only finds damage; risk falls once a repair, coating, inhibitor program, or inventory reduction follows. Another favorite: equating corrosion rate with probability of failure, when rate is only one input. Also remember RBI ranks equipment within a defined scope - it does not replace engineering judgment or fitness-for-service evaluation.
Rule of thumb
In a five-by-five risk matrix, moving one order of magnitude in POF shifts the item one column; moving one consequence category shifts it one row. Both are equally powerful levers.