Risk based inspection refers to the application of risk analysis principles to manage inspection program 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 fo 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 plantitems. 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 : 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 commontly used. Often, insufficient information is available for conducting a quantitative risk analysis. Two organization 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). Where such a Risk Based Inspection schemes is used, it should be noted that the dermination of future inspection requirements, by extrapolation of historical trends, is based on an assumption that the conditions in the future are similiar 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 spesific time intervals, or when a prcess 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 pratice to carry out a baseline suvey to establish a known condition from which to monitor.
