Risk-based inspection (RBI): inspect more where the risk is higher

In one line

RBI (Risk Based Inspection) allocates inspection effort according to risk: whatever has a high probability of failing and serious consequences is inspected first and more often; everything else, less.

What it is

The paper from the Universidad Nacional Mayor de San Marcos presents risk-based inspection as an approach to prioritize and plan inspections, used mainly in oil and gas (platforms, refineries, chemical and mining facilities). It analyzes the probability of failure and its consequences to build an inspection plan that says what to inspect, when and how, based on the expected or observed degradation mechanisms (for example, corrosion and fractures from excessive stress) that endanger the integrity of pressurized or structural equipment. It draws on the recommended practices API RP 580 and API 581, which the paper cites.

The central idea is that risk is the combination of probability and consequence. Items with high probability and high consequences get a higher inspection priority than those with low probability or low consequences, which allows a rational investment of inspection resources. The paper lists what it delivers: reducing the risk of high-consequence failures, improving the cost-effectiveness of inspection and maintenance resources, shifting resources from lower-risk to higher-risk equipment, and measuring the risk associated with ongoing inspection programs.

What it is for

It is a specific application of criticality analysis: instead of just ranking equipment, it defines the inspection plan for each item. According to the paper, the output of an RBI plan is the type and frequency of inspection (generally with non-destructive testing) for industrial piping, process systems, pipelines, structures and other assets.

How it is applied

The paper describes the methodology in eight steps: data and information gathering, risk analysis, consequence assessment, assessment of the probability of failure (times per year), risk assessment using a risk matrix, risk ranking, review of the inspection plan and reassessment of the plan.

It also describes three levels of analysis in the API RBI procedure:

• Level I: a screening tool that quickly highlights the highest-risk equipment, so it can be evaluated in more detail.
• Level II: one step closer to a quantitative analysis; it delivers most of the benefit of level III with less input data.
• Level III: a quantitative approach, the most detailed.

Real example

The paper develops the case of a hydrofluoric acid (HF) alkylation unit at the Shell CAPSA refinery in Argentina, which has more than 1,200 static equipment items and 300 storage tanks, and where the inspection tasks demand a lot of resources. The numerical results are confidential and are not published, but the procedure is:

1. A database is built with process data, design data, degradation mechanisms and inspection history for each piece of equipment and piping.
2. Corrosion circuits are defined (sections with similar materials and process conditions, and therefore exposed to similar degradation mechanisms): in this case, 17.
3. Failure modes are assigned to each piece of equipment (each part is called a TAG): the unit has 18 heat exchangers, 10 vessels, 5 fractionating columns, 1 furnace and many pipes, 120 TAGs in total, and with the failure modes this gives 175 TAG-failure mode combinations to analyze.
4. The probability of failure is assessed with questionnaires that depend on the failure mode. The consequences are independent of the failure mode and are assessed with questionnaires in three areas: economic, health and safety, and environment. The most severe one defines the consequence class that goes into the matrix.
5. The combination gives six criticality levels: N (negligible), L (low), M (medium), MH (medium-high), H (high) and E (extreme). If the criticality is N, in principle no formal inspection or monitoring plan is needed (as long as it complies with local regulations). If it is H or E, a more detailed analysis is done that evaluates mitigation options and a cost-benefit analysis.
6. A confidence index (which depends, for example, on how many previous inspections the equipment has had) is combined with the criticality in a two-way table to obtain an interval factor: multiplied by the remaining life, it gives the maximum interval between inspections.
7. Each inspection program defines the location, the technique, the scope and the interval. And the analysis is reviewed when there are plant shutdowns (planned or not), deviations in operating conditions or changes in the plant.

Huerta Mendoza's criticality analysis (frequency times consequence) uses the same reasoning structure, applied to all types of assets and to maintenance decisions in general; RBI takes it into the realm of mechanical integrity and inspections.

How to set it up

Equipment TAG	Corrosion circuit	Failure mode	Probability (class)	Consequence (class)	Criticality (N, L, M, MH, H, E)	Inspection technique and scope	Interval	Next inspection

For a first pass without detailed data, the criticality matrix on this site works as a simple-level tool (frequency times worst consequence on a scale of 1 to 5); it does not replace an API-based RBI study.

Benefits

• It concentrates inspection on the highest-risk equipment and frees up resources from the lowest-risk equipment.
• It gives an objective, reviewable basis to justify what is inspected, with which technique and how often.
• It forces you to consider the consequences (economics, safety, environment) and not just the condition of the equipment.

Limitations to keep in mind

• It is designed mainly for static equipment subject to degradation (vessels, piping, tanks), not for rotating equipment, which is usually handled with RCM and predictive maintenance.
• It requires process data, design data and inspection history; with poor data the analysis loses confidence (hence the confidence index).
• The API RP 580 and 581 standards are paid documents: this site does not reproduce them and relies on an academic paper that describes them.
• The results must be reviewed when the plant or the operating conditions change.

In summary

RBI = probability × consequence applied to inspection: eight steps, three levels of detail and a matrix that separates what has to be inspected most rigorously from what hardly needs it. It is the integrity-engineering version of criticality analysis.