API RP 581 Risk-Based Inspection Methodology – Basis For Thinning Probability of Failure Calculations

by L. C. Kaley, P.E.

A Joint Industry Project for Risk-Based Inspection (API RBI JIP) for the refining and petrochemical industry was initiated by the American Petroleum Institute (API) in 1993. 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 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 a Third Edition release in 2014.

Like the Second Edition, the Third Edition will be a three volume set, Part 1 – Inspection Planning Methodology, Part 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 forms the basis for the original table approach it will replace. This paper provides the background for the technology behind the thinning model as well as step-by-step worked examples demonstrating the methodology for thinning in this new edition of API RP 581.