Linking IOWs to RBI – Staying Evergreen

Introduction

Understanding the impact of operating conditions on equipment health is fundamental for achieving sustained success with risk-based inspection (RBI) or conventional MI programs. However, updating RBI analyses to account for changes in operating conditions during service is challenging. RBI calculates risk by multiplying the probability of failure (POF) by the consequence of failure (COF). The POF is often directly affected by the operating conditions. This means changes to relevant operating parameters can, and often do, affect the onset and progression of damage mechanisms and damage rates. The effects can include the development of new damage mechanisms and/or changes in damage rates. Furthermore, changes in operating conditions can also affect the consequences of failure (e.g., flammability or environment). These changes can impact the risk analysis, driving risk up or down, depending upon the conditions.

Integrity operating windows (IOWs) are a powerful tool that can be used to monitor and provide awareness of changes in operating conditions. The changes, if relevant, should be considered as to their impact on potential damage to ensure your RBI program is effective. Accordingly, RBI reanalysis should be required every time an IOW limit that contributes to a change of probability when it comes to damage presence or rates is exceeded, and such an exceedance is deemed consequential (i.e., it will affect the POF so that fitness for purpose and time to leak changes so that another inspection, repair, or replacement in sufficient time to address is done prior to reaching unacceptable risk or other criteria). This article proposes a framework for consideration, based on experience, that helps practitioners in utilizing IOWs to enable a more effective, sustainable, and dynamic RBI program. We present the concept of evergreen RBI and highlight what we have experienced as potential areas to improve the effectiveness of IOWs as per API RP 584. Finally, we present a framework for consideration as a guideline for implementing IOWs as part of evergreening an RBI program.

IOW Concept

The main purpose of implementing an IOW program is to have a system in place that can reduce and better control the risk of an unexpected loss of containment due to damage mechanisms arising from a change in operating conditions. API RP 584 suggests owner-operators:

• Identify all potential damage mechanisms.
• List all reasonable operating parameters which may affect the identified damage mechanisms.
• Set limits for each operating parameter.
• Assign criticality levels to each parameter.
• Define an action plan for each parameter when they are exceeded.

Accordingly, the identified damage mechanisms may become relevant if/when an associated condition exists and when parameters are exceeded. Therefore, the defined action plan should be implemented to return the operating condition to within accepted limits, if possible. As per API RP 584, the defined action plan is to be executed within an appropriate response time determined after considering the limitations of instrumentation, communications, and the process system design. However, API RP 584 does not specify when an identified damage mechanism, for which the IOW limits were set, should be considered as applicable or unacceptable. Is that after exceeding the response time? Or right after the IOW limits are exceeded?

Note that there will be times when nothing can be done to affect the IOW condition/level, such as a change in crude slate or chemical composition of the purchased feed. It is still important to be keenly aware of the impact of the now potential deleterious environment on the anticipated equipment damage state and rate so the appropriate actions can be taken prior to exceeding acceptable risk or other targets such as conventional corrosion half-life and/or FFS calculations.