Risk-based inspection (RBI) is an accepted methodology for managing the risk of failure and the related consequences in the oil and gas industry. RBI is synonymous with risk-prioritized inspections, risk-informed inspections and with inspection planning using risk-based methods. As stated in API RP 580, the expected outcome from the application of the RBI process should be the linkage of risk with the appropriate inspection, process control or other risk mitigation activities. The RBI process is capable of generating a ranking by relative risk of all equipment evaluated, and a detailed description of the inspection plan to be employed for each equipment item including:
• Inspection methods that should be used (e.g., visual, ultrasonic, radiography, magnetic particle inspection and acoustic emission).
• Extent of application of the inspection method or methods (e.g., extent of total area to be examined or specific locations).
• Timing of inspections (inspection intervals and due dates).
RBI represents the next generation of inspection approaches and interval/due date setting, recognizing the ultimate goal of inspection is the safety and reliability of the operating facility. RBI, as a risk-based approach, focuses attention specifically on the equipment and the associated damage mechanisms representing the most risk to the facility.
With the release of the fourth edition of API Standard 653, RBI is the primary methodology for determining the internal inspection interval for aboveground storage tanks. As a result, new and innovative RBI methods for evaluating the internal condition of aboveground storage tanks are available. These new RBI methods use an equivalent risk concept for managing the inspection interval for internal inspections of aboveground storage tanks to meet API 653 requirements. The methods make use of simple in-service measurements of the bottom thinning of the actual tank being evaluated rather than relying on the use of a control tank to predict the failure mechanism of similar tanks. These methods safely extend the time between inspection intervals without increasing the risk of failure. While the foundations of the models are statistical in nature, the implementation is empirical and uses actual data from the tank being evaluated. Unlike the typical application of the RBI methodology to evaluate an entire facility, this new RBI methodology allows for the models to be used on individual tanks and produces accurate results without having to go through the iterative process of implementation, evaluation and revision that is typically associated with the application of the RBI process to entire facilities. The immediate results associated with these new risk-based methodologies provide a method of maintaining regulatory compliance for inspections intervals on tanks critical to operations without the need for extensive out-of-service periods to perform the typical operations associated with an API 653 internal inspection. In addition to eliminating the need for taking a tank out of service, these new RBI methodologies can provide significant cost savings compared to the typical out-of-service internal inspection.
The National Institute for Storage Tank Management (NISTM) will hold its 16th Annual International Aboveground Storage Tank Conference & Trade Show in Orlando on April 23-25.
For more information on the National Institute for Storage Tank Management, visit www.nistm.com or call (800) 827-3515.
For more information on this topic, contact Stephen Ford at sford@vista-precision.com or call (937) 669-5875
