John T. Reynolds
Shell Oil Co.
Houston
Refiners and petrochemical producers spend millions of dollars each year trying to improve the mechanical-integrity record of the processing industry. But the challenge lies in determining where to focus the industry's limited resources to effect the largest impact on equipment safety.
A new risk-based inspection program has been developed to help processors:
- Determine the highest-risk equipment
- Design an inspection program that not only finds corrosion, but also reduces the risk of equipment failure.
With the help of this risk-based inspection (RBI) method, companies can cost-effectively reduce the risk of a catastrophic event resulting from the failure of pressurized equipment.
Engineers know that all equipment contains flaws; in a pragmatic sense, there is no perfect fabricated equipment. Fortunately, most flaws are innocuous and comprise what might be referred to as a "fortuitous collection" of flaws.
The processing industry's equipment also contains many flaws: most are harmless, a few might lead to leaks, and extremely few may lead. to catastrophic failures. The challenge is to cost-effectively find those few critical flaws that could lead to major failures.
RISK-BASED INSPECTION
RBI is an integrated methodology that factors risk into inspection and maintenance decision-making. It is considered "integrated" because it is a qualitative and quantitative process for combining both the likelihood of failure and the consequence of failure, systematically, to establish a prioritized list of pressure equipment using total risk as a basis. From that list, the user can design an inspection program that manages the risk of equipment failures by either reducing risk or maintaining an acceptable level of risk.
HYPOTHETICAL CASE
An RBI analysis of a refinery gas-fractionation plant might produce a prioritized list of, for example, 600 pieces of pressure equipment, including all piping circuits. The list indicates that 80% of the risk of failure is associated with only 20% of the pressure equipment, and the analysis determines which of the individual equipment items are in that top 20%. These results tell the refiner where to focus inspection and testing resources to avoid the highest-risk failures.
As an example, the RBI analysis might reveal that the 12-in. line on the overhead of the depropanizer ranks eighteenth on the prioritized risk list, and that, if it should fail, 25,000 sq ft of the plant might be at risk of destruction from a possible vapor-cloud explosion. With this information, the refiner can concentrate more resources on preventing the failure of this overhead line or on those other pieces of equipment that are "high-risk," driven by potentially serious consequences.
As good as an existing inspection program might be, an RBI analysis may reveal that its focus has been mostly on equipment with higher likelihood of a leak, rather than on equipment with a high combined total risk, driven also by greater consequence potential.
INDUSTRY RECORD
According to one report, of the 170 major property losses in the processing industry during the last 30 years, more than half were caused by mechanical failure of equipment (Fig. 1) (11178 bytes).1 More than 80% of those losses were caused by the failure of pressurized equipment such as piping, vessels, columns, reactors, tanks, pumps, and heat exchangers (Fig. 2) (11037 bytes).
Risk-based inspection methodology is focused on that pressure envelope, including the potential for pump-seal failure.
The frequency and cost of those 170 largest losses was increased significantly over the last 30 years (Fig. 3) (8824 bytes). Application of RBI should reduce those losses attributed to mechanical failure.
METHOD DEVELOPMENT
Risk-based inspection technology is being developed by a group of 16 petrochemical companies which is jointly sponsoring the development through the API. The API has let a contract for Phase I of RBI development to Det Norske Veritas Industry Inc. (DNV), Houston.
DNV is developing a base resource document that the API will utilize to create a recommended practice (API RP 580).2 Initial publication of the recommended practice is expected in 1997, at which time the details of the method will be available to anyone in the industry. The methodology described in this article is being developed by DNV under the API contract, and was first reported at the API annual meeting, Nov. 13-14, 1994, in Los Angeles.
QUALITATIVE RANKING
The first step in the application of RBI is the qualitative risk-ranking of process units, or segments thereof, utilizing a risk matrix like that shown in Fig. 4 (9893 bytes). A 17-page workbook has been developed to rank the risks associated with each process unit, based on the consequence and likelihood of an event. The workbook will be available when the recommended practice is published.
Fifty different aspects of health, safety, and mechanical integrity of process units are covered in the workbook; and only 2-4 hr are required to complete the workbook for each process unit, depending on the availability of the required information. The workbook is available only to participating companies at this time.
In the analysis, each process unit is given a potential likelihood ranking (1-5) and a potential consequence ranking (A-E). This combination of ranking then places the unit in one of the 25 segments of the risk matrix shown in Fig. 4 (9893 bytes). After all process units or segments at a given site have been ranked, the user gains appreciable insight as to where to conduct a full, quantitative RBI analysis.
Interestingly, when the API sponsor group companies each conducted qualitative ranking of two selected refining units from their sites, the group was surprised to see how similar process units ranked so differently on the matrix.
For example, a catalytic cracking unit could rank quite differently from another of similar design, based on such issues as:
- Proximity to community
- Record of unscheduled shutdowns and operability problems
- The presence of safety systems and management systems to improve safety
- Total inventory of flammable hydrocarbons or H2S
- Existence of isolation valves and mitigation systems
- Proximity to other high-value capital assets.
Having qualitatively ranked all process units at a given site, the user then is ready to select a higher-risk unit for a quantitative risk-based inspection analysis.
QUANTITATIVE RANKING
A simplified block flow diagram of the quantitative RBI methodology is shown in Fig. 5 (10265 bytes). A glance at the diagram quickly reveals why RBI is referred to as an integrated methodology.
The analysis looks not only at inspection, equipment design, and maintenance records, but also at numerous process safety management issues and all other significant issues that can affect the overall mechanical integrity and safety of a process unit. The analysis does not look solely at inspection programs to establish risk.
The likelihood of failure is calculated for each piece of pressure equipment in the process unit. Starting with generic failure frequencies gleaned from several sources of available data, an adjusted probability of failure (POFA) is calculated by modifying the generic failure frequency (GFF) to produce a failure frequency specific to each piece of a plant's equipment.
The calculation is represented by a simple formula:
POFA = GFF x FE x FMwhere:
- FE is the equipment modification factor-a calculated adjustment based on the quality of a given plant's mechanical integrity program
- FM is the management modification factor-a calculated adjustment based on the quality of a given plant's total process hazards management program.
The Fm is obtained using a workbook that evaluates the effectiveness of a process safety management program, which will be available when the recommended practice is published.
The adjusted probability of failure then is combined with the consequence analysis in a model that produces the risk ranking for all pieces of pressure equipment. Some of the issues that are assessed (quantitatively) to calculate the specific equipment modification factors include:
- Type and rate of damage expected
- Quality and scope of inspection program
- Maintenance and repair quality control program
- Design and construction standards utilized
- Equipment and process histories
- Preventive maintenance programs.
Issues that are assessed to calculate the specific management factor come directly from API RP 750, "Management of Process Hazards."
CONSEQUENCE
A simplified block diagram showing how failure consequences are assessed is show in Fig. 6 (10070 bytes).
Equipment size and installed isolation devices play a big part in the calculation of available inventory for potential events. The size of the leak or rupture, and the likelihood of a release being instantaneous or continuous, will have a great effect on the size and type of any potential event.
Hole-size calculations for four events are performed and summed, ranging from a 1/4-in. leak to a full rupture. For flammable events, calculations are made to determine whether the event is likely to be a vapor-cloud explosion, flash fire, jet fire, liquid-pool fire, or safe dispersion (no ignition).
The effect of business interruption, in terms of dollar loss, is included when capital assets might be lost or shut down for a period of time after an event. The cost of catastrophic environmental effects can be included, especially when a potential liquid release might flow off site; for instance, into a water resource. Potential human toxic effects also are assessed.
The final report from each RBI contains not only the prioritized risk ranking (combined likelihood and consequence of failure), but also a prioritized list of equipment, by likelihood of failure only, and by consequence of failure only. This allows the user to focus on the specific issues that drive up total risk, and to understand whether total risk is driven primarily by likelihood of failure or consequence of failure. That understanding is vital to making decisions about how to reduce risk levels associated with each piece of equipment.
INSPECTION
The final RBI report on a particular process unit will contain a prioritized ranking of each piece of equipment for three levels of inspection activity:
- A minimal inspection plan
- The current level of inspection
- An optimized level of inspection.
These printouts yield an understanding of how different inspection programs with different levels of inspection activity affect total risk levels by changing the likelihood of a failure.
Having calculated a total risk for each piece of equipment, the next step is to decide what to do with the risk prioritized equipment list. Once a refiner identifies its highest-priority pressure equipment, it then can determine very specifically where its inspection and testing efforts should be focused to reduce risk.
First, and most obviously, the frequency of inspection can be adjusted. But also, the methods and tools used for inspecting and testing can be changed. In addition, the scope, quality, and extent of the inspection and data gathering can be adjusted.
More global inspection and testing techniques (like thermography) can be applied, when appropriate. More on stream inspections can be utilized to assess damage occurring while the unit is in service. And inspections can be focused more toward areas of expected damage.
These changes in inspection activity then are planned into upcoming scheduled inspections-for example, turnaround planning.
The inspections are conducted, the results are analyzed, the equipment is assessed for its continued fitness for service, and the recommended repairs are made. Then the user is ready to feed the information back into the RBI program to determine how the total risk for each piece of equipment was affected by the changes in inspection activity. After a few "turns of the crank," out pops another reprioritized list of equipment, and the user gains a quantitative appreciation for how the risk of a catastrophic event in the process unit was changed.
Lower-risk equipment might have received less inspection resources and activity without its risk of failure being appreciably affected. Higher-risk equipment may have dropped significantly in risk-ranking as a result of having received more inspection and maintenance attention during the turnaround.
Overall, the potential for injury, capital asset loss, and production losses may have been reduced, and it might be possible to accomplish that with fewer total inspection resources (Fig. 7) (13982 bytes).
OPTIMIZATION
One possible outcome of RBI analysis is an effort to optimize a plant's inspection program by obtaining the lowest reasonable risk at the lowest cost. To accomplish this, a company may find that it can shift its limited inspection resources away from low-risk equipment (which may be inspected too often) and toward higher-risk equipment (which may not be inspected often enough).
The changes in risk then can be assessed with RBI analysis and compared to changes in the inspection resources utilized to determine if risk optimization is occurring (if total risk and inspection costs are decreasing). Fig. 7 (13982 bytes) shows, conceptually, how focusing limited inspection resources on the highest-risk equipment might achieve that.
OTHER ACTIVITIES
Because RBI is a fully integrated methodology, the user also has the opportunity to reduce risk by means other than changing the inspection program.
There may be a number of opportunities to strengthen process safety management systems and procedures. The user also can lower risk by installing safety systems, leak-detection systems, isolation valves, and anything else that might mitigate consequences once a release has occurred.
INDUSTRY STANDARDS
Risk-based inspection integrates well with current editions of industry inspection codes and standards, such as: APR-510 (Pressure Vessel Inspection Code), APR-570 (Process Piping Inspection Code), and API-653 (Storage Tank Inspection Standard). Each of these standards sets forth minimum practices for inspection frequencies and many recommended practices for inspection activities associated with pressure equipment.
Certainly, the industry will continue to utilize these codes for the purposes for which they were intended. However, as is the case with the mechanical integrity aspects of process safety management regulations (OSHA 29 CFR 1910.119), these codes offer the user much flexibility and many options relative to the scope and extent of the inspection activities to be conducted.
Risk-based inspection provides a systematic method to guide the user in the selection of those inspection options that optimize the inspection program for the purpose of reducing risk.
THE FUTURE
Once RBI is fully developed and accepted by jurisdictions and insurers as a sound basis for lowering facility risk, there will be further opportunities to improve the methodology. Software will be needed to minimize the resources necessary to conduct an RBI analysis.
There also is a need to closely align, and perhaps integrate, this methodology with reliability-centered maintenance (RCM). RCM focuses on the functionality of equipment to determine what preventive maintenance may be needed to improve the reliability (availability) of process equipment. Clearly, the failure of the pressure boundary of process equipment is the ultimate reliability impact, and can have a large, long-term effect on process unit availability.
Finally, the industry needs to build a failure data base for pressure equipment. An industry-specific failure data base will increase the accuracy of probability-of-failure calculations, as well as the efficiency of the RBI analysis.
There are two opportunities for participating in the development and understanding of RBI in the near future. The National Petroleum Refiners Association is holding an RBI workshop at its 1995 Maintenance Conference in San Antonio, May 23-26. And the API has a task group on RBI, through which refiners can participate, as sponsor members, in developing the technology, or gain access to it before it is published.
REFERENCE
- "Large Property Damage Losses in the Hydrocarbon-Chemical industries," a Thirty-Year Review by M&M Protection Consultants, 14th ed., 1992, Marsh & Mclennan.
- Base resource document on risk-based inspection, American Petroleum Institute.
THE AUTHOR
Reynolds is chairman of the subcommittee on inspection in the API committee on refinery equipment and chairman of the inspection codes task group. He is vice-chair of the API task group on risk-based inspection. He also is a member of the ASME board of pressure technology for the codes/standards task group on post-construction issues.
Copyright 1995 Oil & Gas Journal. All Rights Reserved.
