# Risk based inspection Metric

Risk Based Inspection is a study to develop inspection plan based on equipment risk ranking. Mathematically, Risk is defined as PoF x CoF. What is PoF ? PoF or Probability of failure is a rank that tell us about how likely a equipment will fail. CoF or Consequence of failure is a rank that tell us about how much failure impacts. In a simple way PoF, CoF and Risk are 3 terms that we used to rank all equipment in a company.

Risk based inspection metric may consist of 3×3 , 4×4, 5×5 or even 6×6. For a company has a small quantity of equipment, they may choose to use 3×3 metric. But for company that have a lot of equipment -e.g. 1,000,000- with wide range of consequence, they may prefer to use 6×6 metric. This metric tell us about how detail we group the equipment. In 6×6 metric, we group probability of failure to 6 categories and group consequence of failure to 6 categories.

## Risk based inspection approach

Risk based inspection may use qualitative approach, quantitative approach or even semi-quantitative approach. In qualitative approach, we likely use “expert” team to qualify the PoF and CoF. There is no exact number involved in this approach. We don’t have a clear, detail and exact parameter that tell us how to rank probability of failure. Equipment A installed near to sea water has higher PoF than equipment B installed at plant far away from sea water. The expert team can justify this rank by using general corrosion study. At this stage, we need a corrosion expert to join Risk Based Inspection study.

At the other side, a quantitative Risk Based Inspection use a set of “number” to describe the rank. For example, equipment A can be classified to PoF = 1 because of its corrosion rate is > 10 mpy. Equipment B can be classified to PoF = 6 because its corrosion rate < 1.0 mpy. Everyone -even he is not a corrosion expert- can use this rule if we have “complete” corrosion rate data for all equipment. At this stage, we need an excellent database to be used for Risk Based Inspection Study.

https://www.researchgate.net/figure/264863512_tbl3_Table-3-Classification-of-General-and-Pitting-Corrosion-Rates