Inspectioneering Journal

API RP 581 Risk-based Inspection Technology Demonstrating the Technology Through a Worked Example Problem

By Lynne Kaley at Trinity Bridge LLC / Trinity Bridge Digital. This article appears in the January/February 2009 issue of Inspectioneering Journal

The Joint Industry Project for Risk-Based Inspection (RBI JIP) was initiated and managed by API within the refining and petrochemical industry in 1992. The work from the JIP resulted in two publications, API 580 Risk-Based Inspection released in 2002 and API 581 Base Resource Document – Risk-Based Inspection originally released in 1996. The concept behind these publications was for API 580 to introduce the principles and present minimum general guidelines for RBI while API 581 was to provide quantitative RBI methods. The API RBI JIP has made major advances in the technology since the original publication of these documents and released the second edition of API 581 - Recommended Practice for Risk-Based Inspection Technology in September 2008. The second edition is a three volume set, Part 1 – Inspection Planning Using API RBI Technology, Part 2 – Probability of Failure in API RBI, and Part 3 – Consequence Modeling in API RBI. This paper provides a step-by-step worked example that demonstrates the technology documented in API 581, Second Edition.


The API Risk-Based Inspection (API RBI) methodology has been used in the Refining and Petrochemical industries to manage the overall risk of a plant since the mid-1990’s as a methodology for focusing inspection efforts on the process equipment with the highest risk. API RBI provides the basis for making informed decisions on inspection frequency, the extent of inspection, and the most suitable type of NDE. In most processing plants, a large percent (80-90%) of the total unit risk will be concentrated in a relatively small percentage (10-20%) of the equipment. These potential high-risk components require greater attention, often through a more extensive inspection or by more advanced inspection methods. The cost of this increased inspection effort often is offset by reducing inspection efforts on the larger percentage of lower risk equipment.

API 581 provides quantitative RBI methods to establish an inspection program. The worked example presented in this paper follows the step-by-step procedure outlined in the document to demonstrate use of the technology developed by the API JIP. This will enable practitioners to better understand the methodology and facilitate effective peer review.

The second edition of API 581 was published in September 2008 and presents the API RBI methodology in a three part volume:

  • Part 1 – Inspection Planning Using API RBI Technology
  • Part 2 – Determination of Probability of Failure in an API RBI Assessment
  • Part 3 – Consequence Analysis in an API RBI Assessment

Calculation of risk in API RBI involves the determination of a probability of failure (POF) combined with the consequence of failure (COF). Failure in API RBI is defined as a loss of containment from the pressure boundary resulting in leakage to the atmosphere or rupture of a pressurized component. As damage accumulates in a pressurized component during in-service operation the risk increases. At some point, the risk tolerance or risk target is exceeded and an inspection of sufficient effectiveness to determine the damage state of the component is recommended. It is important to note that the inspection itself does not reduce risk; however, it reduces the uncertainty in the component condition by allowing better quantification of the damage present.

The following worked example follows API RBI’s step- by-step procedure to calculate risk for a drum that is susceptible to internal corrosion and in-service stress corrosion cracking. API RBI typically involves evaluating the risk associated with releases from four representative hole sizes. This example demonstrates the calculation of POF for each of these representative hole sizes. COF will be calculated using the Level 1 consequence model for one of the representative hole sizes; however consequence results for all four hole sizes will be used in the final consequence and risk calculations. Inspection planning will be demonstrated using a simplified approach to demonstrate the technology but without the iterative calculations typically required to plot risk over time and to determine the impact of inspection on risk.

All figure, table and equation numbers used in this paper correspond to the actual numbers in API 581 for an easy cross reference with the document.

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