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.