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

Editor's note: This article is Part 3 of a 3-Part series. The articles in this series provide a step-by-step example of utilization of API RP 581 and build off of each other. It's important that you have read Parts 1 and 2 prior to reading this article. Please refer to Part 1 for an introduction and necessary background information for this worked example, including mechanical design information and process fluid properties of the example equipment and inspection planning details for the example facility.

Part 1 covered Probability of Failure (POF), including modeling of thinning and stress corrosion cracking and demonstrated how API RBI provides credits for past inspections, considering their effectiveness in detecting those specific damage types and the number of inspections performed.

The previous article (Part 2) guided the reader through the health, safety, and environmental and financial consequence calculations, including calculation of Consequence of Failure (COF).

This article (Part 3) will cover the final risk analysis, inspection planning, provide a very helpful table of calculated results for each step to check your calculations against, and a glimpse of the future direction of API RP 581.

Abstract

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.

Risk Analysis

Determination of Risk

The calculation of risk can be determined as a function of time by combining probability of failure and the consequence of failure, as shown in Equation (1.5):

Note that probability of failure, P_f(t), is a function of time since damage factor increases as the damage in component due to thinning, cracking or other damage mechanisms accumulate with time. Consequence of failure, C(t), is assumed to be invariant with time. Therefore, Equations (1.6) and (1.7) show the determination of risk, expressed in area or in financial terms.