Inspectioneering Journal

Examining the Latest Changes to API RP 581 Risk-Based Inspection Methodology Thinning and the Probability of Failure Calculations

By Lynne Kaley, Director of Reliability Strategy at Pinnacle. This article appears in the November/December 2016 issue of Inspectioneering Journal.

Editor's Note: This article is an overview, distilled from a detailed technical report. The detailed technical report, with explanations and equations, may be accessed at Please note that this article and the referenced technical paper are focused on thinning. The term “base case” is used throughout this article. It always refers to the data basis that was used to establish thinning damage factors in the rst two editions of the API RP 581 technical basis.


A Joint Industry Project for Risk-Based Inspection (API RBI JIP) for the refining and petrochemical industry was initiated by the American Petroleum Institute in 1993. The project was conducted in three phases:

  1. Methodology development Sponsor Group resulting in the publication of the Base Resource Document on Risk-Based Inspection in October 1996.
  2. Methodology improvements documentation and software development User Group resulting in the publication of API RP 581 Second Edition in September 2008. API Software User Group split from methodology development through the creation of an API 581 task group in November 2008. The document is now managed in an API task group under the oversight of the API Sub-Committee on Inspection instead of the former JIP structure.
  3. Methodology improvements documentation resulting in the publication of API RP 581 Third Edition in April 2016.

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 implementing and maintaining a risk-based inspection (RBI) program while API 581 was to provide specific quantitative RBI methods for calculating risk. The API RBI JIP made improvements to the technology since the original publication of these documents and released API RP 581, Second Edition in September 2008. Since the release of the Second Edition, the API 581 task group has been improving the methodology and revising the document for the Third Edition release in 2016.

Like the Second Edition, the Third Edition is a three volume set, Part 1: Inspection Planning MethodologyPart 2: Probability of Failure Methodology, and Part 3: Consequence of Failure Methodology.

Among the changes incorporated into the Third Edition of API RP 581 is a significant modification to the thinning Probability of Failure (POF) calculation. The methodology documented in the Third Edition provides the basis for the original Art table approach it will replace. This paper provides the background for the technology behind the Third Edition thinning model as well as step-by-step worked examples demonstrating the methodology for thinning in this new edition of API RP 581. This paper is a revision to a previous publication: API RP 581 Risk-Based Inspection Methodology – Basis for Thinning Probability of Failure Calculations published in November 2013.

The project started in May 1993 as a JIP to develop practical methods for implementing RBI, the API RP 581 methodology focuses inspection efforts on process equipment with the highest risk. This sponsor group was organized and administered by API and included the following members at project initiation: Amoco, ARCO, Ashland, BP, Chevron, CITGO, Conoco, Dow Chemical, DNO Heather, DSM Services, Equistar, Exxon, Fina, Koch, Marathon, Mobil, Petro-Canada, Phillips, Saudi Aramco, Shell, Sun, Texaco, and UNOCAL.

The stated objective of the project was to develop a Base Resource Document (BRD) with methods “aimed at inspectors and plant engineers experienced in the inspection and design of pressure-containing equipment.” The BRD was specifically not intended to become “a comprehensive reference on the technology of Quantitative Risk Assessment (QRA).” For failure rate estimations, the project was to develop “methodologies to modify generic equipment item failure rates” via “modification factors.” The approach that was developed involved specialized expertise from members of the API Committee on Refinery Equipment through working groups comprised of sponsor members. Safety, monetary loss, and environmental impact were included for consequence calculations using algorithms from the American Institute of Chemical Engineers (AIChE) Chemical Process Quantitative Risk Assessment (CPQRA) guidelines. The results of the API RBI JIP and subsequent development were simplified methods for estimating failure rates and consequences of pressure boundary failures. The methods were aimed at persons who are not expert in probability and statistical methods for Probability of Failure (POF) calculations and detailed QRA analysis.

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