API RP 581
Risk-Based Inspection Methodology
|Publication Date:||1 April 2016|
This recommended practice, API 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 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 580 to introduce the principles and present minimum general guidelines for RBI, while this recommended practice provides quantitative calculation methods to determine an inspection plan.
The calculation of risk outlined in API 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.
In most situations, once risks have been identified, alternate opportunities are available to reduce them. However, nearly all major commercial losses are the result of a failure to understand or manage risk. In the past, the focus of a risk assessment has been on-site safety-related issues. Presently, there is an increased awareness of the need to assess risk resulting from:
a) on-site risk to employees,
b) off-site risk to the community,
c) business interruption risks, and
d) risk of damage to the environment.
Any combination of these types of risks may be factored into decisions concerning when, where, and how to inspect equipment.
The overall risk of a plant may be managed by focusing inspection efforts on the process equipment with higher risk. API 581 provides a basis for managing risk by making an informed decision on inspection frequency, level of detail, and types of nondestructive examination (NDE). It is a consensus document containing methodology that owner-users may apply to their RBI programs. In most plants, a large percent of the total unit risk will be concentrated in a relatively small percent of the equipment items. These potential higher risk components may require greater attention, perhaps through a revised inspection plan. The cost of the increased inspection effort can sometimes be offset by reducing excessive inspection efforts in the areas identified as having lower risk. Inspection will continue to be conducted as defined in existing working documents, but priorities, scope, and frequencies can be guided by the methodology contained in API 581.
This approach can be made cost-effective by integration with industry initiatives and government regulations, such as Process Safety Management of Highly Hazardous Chemicals (OSHA 29 CFR 1910.119), or the EPA risk management programs for chemical accident release prevention.
Organization and Use
The API 581 methodology is presented in a five-part volume:
a) Part 1-Introduction to Risk-Based Inspection Methodology,
b) Part 2-Probability of Failure Methodology,
c) Part 3-Consequence of Failure Methodology,
d) Part 4-Inspection Planning Methodology,
e) Part 5-Special Equipment.
Part 1 provides methods used to develop an inspection plan for fixed equipment, including pressure vessels, piping, atmospheric storage tanks (ASTs), PRDs, and heat exchanger tube bundles. The pressure boundaries of rotating equipment may also be evaluated using the methods in Part 1. The methods for calculating the POF for fixed equipment are covered in Part 1 and Part 2. The POF is based on the component type and damage mechanisms present based on the process fluid characteristics, design conditions, materials of construction, and the original construction code. Part 3 provides methods for computing the COF. Two methods are provided: Level 1 is based on equations with a finite set of well-known variables generated for common fluids or fluid groups found in refinery and petrochemical processing units, while Level 2 is a more rigorous method that can be used for any fluid stream composition.
An overview of the POF and COF methodology calculations, with reference to the associated sections within this document, is provided in Table 1.1.