Risk-Based Inspection (RBI) is an analysis methodology and process that involves condition-based inspection & requires qualitative or quantitative assessment of the probability of failure (PoF) and the consequence of failure (CoF) associated with each equipment item, piping circuits included, in a particular process unit. A properly-implemented RBI program categorizes individual pieces of equipment by their risks and prioritizes inspection efforts based on this categorization.
Probability of Failure (PoF)
Probability of Failure (POF) is likelihood that a piece of equipment will fail at a given time and an important part of effective risk analyses. POF is half of the equation when determining risk as part of Risk Based Inspection (RBI) methodology. The POF, calculated together with the Consequence of Failure (COF), helps operators establish the risk level for a particular piece of equipment and set inspection intervals based on the calculated risk.
POF is calculated for individual pieces of equipment by looking at the potential damage mechanisms it could be susceptible to, a general frequency of failures, and management system factors.
Consequence of Failure (CoF)
Consequence of Failure (COF) is one part of the equation to determine risk as part of Risk Based Inspection (RBI) methodology. COF is calculated by reviewing and ranking the potential consequences for the equipment, personnel, environment, etc. in the event of equipment failure.
Basic Concepts of Operational Risk Management
In risk based inspection, risk is calculated as the product of the probability of failure and the consequence associated with a failure:
Risk = Probability of Failure x Consequence of Failure
Risk is usually considered a better measure for prioritization than either the probability of failure alone or the consequence of failure alone, because it is more descriptive of the actual damage/loss caused. As an example, if you need to prioritize two assets where one asset has a high probability of failure but low consequence of failure, and the other asset has a low probability of failure but a high consequence of failure, the analysis would yield completely opposite results if you considered only one factor or the other. The use of risk eliminates this ambiguity.
Importance of the use of RBI
RBI is used to identify & understand the risks, the risk drivers & where the equipment is in its lifecycle. RBI can indicate whether an inspection is needed, however it needs to have additional data that is extremely targeted to reduce the underlying uncertainties associated with the risks about the current & future predicted damage state of the equipment. But RBI should not be used to recommend any inspection when it will not improve knowledge about the damaged state.
RBI can be used to prioritize inspection-related activities, usually by means of nondestructive testing (NDT), in order to reduce the uncertainties around the true damage state of the equipment and the dynamics leading to such. The resulting inspection plan may outline the type and scheduling of inspection for an asset. In addition to NDE, additional risk mitigation activities identified by an RBI assessment might include a change in material of construction, installation of corrosion resistant liners, operating condition changes, injection of corrosion inhibition chemicals, etc.
Consistency and repeatability of analysis are critical to producing an effective RBI program, as RBI is based on relative risks. Caution should be used when mixing RBI platforms (e.g., using a qualitative method to perform the initial screening and quantitative methods to conduct the final analysis).