RISK BASED INSPECTION

Identifying and evaluating potential degradation mechanisms are important steps in an assessment of the likelihood of a piping failure. However, adjustments to inspection strategy and tactics to account for consequences of a failure should also be considered. Combining the assessment of likelihood of failure and the consequence of failure are essential elements of risk-based inspection (RBI).

When the owner/user chooses to conduct a RBI assessment it must include a systematic evaluation of both the likelihood of failure and the associated consequence of failure, in accordance with API RP 580. The likelihood assessment must be based on all forms of degradation that could reasonably be expected to affect piping circuits in any particular service. Examples of those degradation mechanisms include: internal or external metal loss from an identified form of corrosion (localized or general), all forms of cracking including hydrogen assisted and stress corrosion cracking (from the inside or outside surfaces of piping), and any other forms of metallurgical, corrosion, or mechanical degradation, such as fatigue, embrittlement, creep, etc. Additionally, the effectiveness of the inspection practices, tools, and techniques utilized for finding the expected and potential degradation mechanisms must be evaluated. This likelihood of failure assessment should be repeated each time equipment or process changes are made that could significantly affect degradation rates or cause premature failure of the piping.

Other factors that should be considered in a RBI assessment conducted in accordance with API RP 580 include: appropriateness of the materials of construction; piping circuit design conditions, relative to operating conditions; appropriateness of the design codes and standards utilized; effectiveness of corrosion monitoring programs; and the quality of maintenance and inspection Quality Assurance/Quality Control programs. Equipment failure data and information will also be important information for this assessment. The consequence assessment must consider the potential incidents that may occur as a result of fluid release, including explosion, fire, toxic exposure, environmental impact, and other health effects associated with a failure of piping.

It is essential that all RBI assessments be thoroughly documented in accordance with API RP 580, clearly defining all the factors contributing to both the likelihood and consequence of a piping failure. (API RP 570)

RELATIVE RISK VS. ABSOLUTE RISK

The complexity of risk calculations is a function of the number of factors that can affect the risk. Calculating absolute risk can be very time and cost consuming and often, due to having too many uncertainties, is impossible. Many variables are involved with loss of containment in hydrocarbon and chemical facilities and the determination of absolute risk numbers is often not cost effective. RBI is focused on a systematic determination of relative risks. In this way, facilities, units, systems, equipment or components can be ranked based on relative risk. This serves to focus the risk management efforts on the higher ranked risks.

It is considered, however, that if a Quantitative RBI study is conducted rigorously that the resultant risk number is a fair approximation of the actual risk of loss of containment due to deterioration. Numeric risk values determined in qualitative and semi-quantitative assessments using appropriate sensitivity analysis methods also may be used to evaluate risk acceptance. (API RP 580)