API RP 581 : 2025

This recommended practice, API RP 581, Risk-based Inspection Methodology, provides quantitative procedures to establish an inspection program using risk-based methods for pressurized fixed equipment including pressure vessel, piping, tankage, pressure relief devices (PRDs), and heat exchanger tube bundles. API RP 580, Risk-based Inspection, provides guidance for developing risk-based inspection (RBI) programs on fixed equipment in refining, petrochemical, chemical process plants and oil and gas production facilities. The intent is for API RP 580 to introduce the principles and present minimum general guidelines for RBI while API RP 581 provides quantitative calculation methods to determine an inspection plan.

The calculation of risk outlined in API RP 581 involves the determination of a probability of failure (POF) combined with the consequence of failure (COF). Failure is defined as a loss of containment from the pressure boundary resulting in leakage to the atmosphere or rupture of a pressurized component. Risk increases as damage accumulates during in-service operation as the risk tolerance or risk target is approached and an inspection is recommended of sufficient effectiveness to better quantify the damage state of the component. The inspection action itself does not reduce the risk; however, it does reduce uncertainty and therefore allows more accurate quantification of the damage present in the component.

The API RP 581 methodology is presented in a five-part volume:
1) Part 1—Introduction to Risk-Based Inspection Methodology,
2) Part 2—Probability of Failure Methodology,
3) Part 3—Consequence of Failure Methodology,
4) Part 4—Inspection Planning Methodology, and
5) Part 5—Special Equipment.