5.5 Assessing Criticality – Risk Analysis
Criticality is a risk-based process. The risk is determined by the probability of failure and the consequence of failure.
The assets that have the greatest probability of failure and the greatest consequences associated with failure will be the assets that are the highest risk and therefore most critical. The next most critical assets will fall into three main categories:
- Assets that have a very high probability of failure with low consequence
- Assets that have a low probability of failure with a very high consequence
- Assets that have a medium probability and medium consequence of failure
A technique such as the ranking table presented below can be a good place to start in assessing criticality. Appendix C contains copies of this table for use in criticality analyses at your utility.
To use this table, estimate the probability of failure for a specific asset from 1 to 5, with 5 being very high probability of failure and 1 being a very low probability of failure. Then assess the consequence of failure from 1 to 5 in the same manner. Using the number for probability of failure, move across the row until the column associated with the number for consequence of failure is reached. Locate the number that is in the box where the row and column intersect. That is the criticality score for that asset.
Consider the following scenario.
Asset: 10 inch Cast Iron pipe; constructed in 1950, (61 years old in 2011)
Service History: Numerous breaks in the past 5 years
Service Area: Serves 3 major subdivisions, but there are loop lines available and only residential customers are served
Probability of failure: 4 - because pipe has broken many times, but when repaired it was still in reasonable condition
Consequence of failure: 2 - because there are loop lines so not all customers will be out of water. Repair costs are moderate. Line isn't in a critical roadway so repair is relatively easy.
Using the chart, move across the row for 2, until the column for 4 is reached. The number in the box is 8. Therefore, 8 is the criticality factor for this asset. (See the table below.)