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Risk-based inspection refers to the application of risk analysis principles to manage inspection programs for plant equipment. RBI has been used in the nuclear power generation industry for some time and is also employed in refineries and petrochemical plant. The ultimate goal of RBI is to develop a cost-effective inspection and maintenance program that provides assurance of acceptable mechanical integrity and reliability.
Risk is defined as the combination of probability and consequence. The highest risk is mostly associated with a small percentage of plant items. Risk based inspection procedures can be based on either qualitative or quantitative methodologies. Qualitative procedures provide a ranking of equipment, based largely on experience and engineering judgment. Quantitative risk-based methods use several engineering disciplines to set priorities and develop programs for equipment inspection.
Risk Based Inspection (RBI) schemes are a planning tool used to develop the optimum plan for the execution of inspection activities. RBI uses the findings from a formal risk analysis, such as a Corrosion Risk Assessment, to guide the direction and emphasis of the inspection planning and the physical inspection procedures. A risk based approach to inspection planning is used to:(reference)
Ensure risk is reduced to as low as reasonably practicable
Optimize the inspection schedule
Focus inspection effort onto the most critical areas
Identify and use the most appropriate methods of inspection
Some of the engineering disciplines include non-destructive examination, system and component design and analysis, fracture mechanics, probabilistic analysis, failure analysis, and operation of facilities. Quantitative analysis methods can be expensive, time consuming, tedious, and are therefore not commonly used. Often, insufficient information is available for conducting a quantitative risk analysis. Two organizations that are currently working on quantitative risk-based analysis procedures for use by the chemical industry are the American Society of Mechanical Engineers (ASME) and the American Petroleum Institute (API). A typical RBI implementation is shown in the following figure.
Where such a Risk Based Inspection scheme is used, it should be noted that the determination of future inspection requirements, by extrapolation of historical trends, is based on an assumption that the conditions in the future are similar to those in the past and that there is no change in degradation mechanism. Any significant change in operating conditions could result in significant changes in corrosion rate and/or corrosion damage, which could in turn lead to different inspection requirements. It is therefore appropriate for the model which is driving the Risk Based Inspection scheme to be re-run either at specific time intervals, or when a process variable exceeds a previously agreed boundary condition.
On new assets or in the absence of good quality historic data on mature assets, it is normally considered good practice to carry out a baseline survey to establish a known condition from which to monitor.
