Risk Based Inspection
Every now and then, the world is shocked by incidents, such as explosions, emissions of toxic gases, or large fires. For instance, you will recall the giant explosion of 2,750 tons of ammonium nitrate in the Beirut harbor on August 4, 2020. Such incidents are nightmares that must be prevented at all times.
This article focuses on one aspect of risk reduction in the process industry, namely the risks of static asset failure. It presents a methodology that helps companies to focus on the most critical static assets. That focus is necessary because in an average plant there simply are too many static assets to always keep an eye on.
Risk Based Inspection is an effective and efficient method because it contributes to the integrity and reliability of static assets in industrial facilities. It helps to properly allocate the inspection resources to the static assets with the highest risk profiles that need the most attention.
Risk Based Inspection (RBI) is an effective and efficient inspection management & planning methodology. RBI contributes to the integrity and reliability of static assets in industrial facilities. It helps to properly allocate the inspection resources to the static assets with the highest risk profiles that need the most attention.
Therefore, it is recommended to use RBI as a default method, especially during large maintenance projects, rather than waiting for static asset failures, with related unsafe situations and consequential damage.
RBI is considered as part of the Risk Based Maintenance (RBM) approach and is also seen as a way of working towards Condition Based Maintenance (CBM).
Static assets and risks
Static and mechanical assets There is distinction between static and mechanical assets. The simple fact that mechanical parts contain rotating or moving parts usually leads maintenance departments to pay more attention to these. Generally speaking, mechanically energized rotating or moving parts often break down relatively fast during operation. The movement of parts tends to create friction, heat and vibrations that cause wear.
However, these are not a valid reason to neglect the static parts in industrial facilities, because these are subject to different deterioration mechanisms, for example corrosion under insulation (see textbox). Static assets are typically used for the construction of plants or for storage and transportation of fluids. These fluids are often dangerous, especially in chemical installations.
The integrity and reliability of mechanical assets is the domain of Reliability Centered Maintenance (RCM), while static assets are the subject of Risk Based Inspection. Despite the similarities in approach of RCM compared to RBI, for example with regard to the risk matrix, RCM is beyond the scope of this document.
Static assets are parts of industrial facilities that do not contain rotating or moving parts. An average plant can easily number more than a hundred such static assets, for example:
- piping systems
- storage tanks
- (pressure) vessels
- heat exchangers
- asset housing or casing
- load-bearing structures.
Examples of mechanical assets are pumps, rotating shafts, conveyor belts, turbines, engines, and robot arms.
Failures of static assets
The process industry faces some major issues that imply that Risk Based Inspection is part of the solution to a broader problem than the failure of static assets:
• Corrective maintenance is often carried out too late (after the outage), preventive maintenance often too early (better safe than sorry)
• Cost-inefficiencies and low uptime / availability levels due to unplanned stops
• Inadequate degradation models, limited standardized guidelines, lack of specialized (technical) tools.
Corrosion under insulation (CUI)
Corrosion under insulation (CUI) is a degradation mechanism that occurs in insulated pipes and appliances and it is one of the major threats to the aging assets of our contemporary industry. It is a difficult phenomenon to control because the locations where it occurs are difficult to detect. The rate of degradation depends on many factors and is difficult to predict. Corrective action is on average necessary after 20-30 years of operation. Potentially, the degradation of steel pipes and other equipment by CUI can lead to major incidents due to loss of integrity. To prevent this, corrective measures worth billions of euros are being taken.
In order to reduce the risks associated with failure mechanisms, these mechanisms must be well understood, while control measures must be defined to ensure the integrity and reliability of the installation. The control measures consist of inspections, monitoring, adjustments & repairs.
Improved system integrity, fewer unplanned stops, lower maintenance costs, and higher production are examples of the benefits and positive results of Risk Based Inspection.
Deliverables of Risk Based
RBI results in five related deliverables (see Figure 1):
- Prioritization of high-risk components: WHAT to inspect
- Determination of inspection intervals: WHEN to inspect
- Expected damage mechanisms: WHERE to inspect
- Selection of best inspection method: HOW to inspect
- Data requirements for continuous improvement: WHAT to report.
To obtain these deliverables efficiently, RBI follows a structured process.
The RBI process
The RBI process as shown in Figure 2 consists of a loop with six steps:
- Data and information collection
- Risk assessment
- Risk ranking
- Inspection plan
Step 1: Data and information collection
The RBI process is initiated with a data and information collection phase. This is needed to understand the characteristics of the primary processes, especially the damaging effect of the process mediums (chemicals) in the static equipment. This step provides accurate and up-to-date information for the next steps in the RBI process. The primary goal is to convert all that data into a suitable risk-based inspection plan for continuous condition monitoring, periodic inspections, and larger turnarounds.
To enhance the failure-forecasting capability, the RBI database must include the following up-to-date information:
- Description of failure mechanisms
- Corrosion studies, especially of corrosion under insulation (CUI), a notorious equipment killer
- Degradation models per process.
Data is typically collected through detailed process analysis in conjunction with long-term corrosion and degradation studies for each part of the static equipment involved. Collecting and processing this data can take a lot of effort, requiring a long-term perspective, especially when different information systems are involved. Many large companies have already built a library of degradation models.
Data for RBI analysis
Data normally required for an RBI analysis may include, but is not limited to:
- Type of equipment (original parts, replacements, or modifications; remaining lifetime)
- Materials of construction
- Inspection, repair, and replacement records
- Process fluid compositions
- Inventory of fluids
- Operating conditions
- Safety & detection systems
- Deterioration mechanisms, rates, and severity (e.g., corrosion, corrosion under insulation, metal fatigue, stress, or chemical attack)
- Personnel densities
- Coating, cladding, and insulation data
- Business interruption costs
- Equipment replacement costs
- Environmental remediation costs.
A library of degradation models is the cornerstone of all maintenance strategies. Such a library is a prerequisite to switch from rule-based or time-based inspection to risk-based inspection.
Step 2: Risk assessment
For each part of the static equipment the probability of failure and the consequences of failure are assessed, based on data from the RBI database. The probability of failure analysis should address all deterioration mechanisms to which the equipment being studied is susceptible.
The following consequences of failure must be considered:
- Financial aspects
- Health aspects
- Environmental aspects
- Regulatory consequences.
RBI addresses the concerns of many plant managers:
- Declining integrity of installations
- Failure of static assets and its consequences (massive repairs, unplanned stops)
- Insufficient availability and reliability of installations
- Consequences of failure for business (costs, HSE, etc.)
- Safety of employees and local residents (getting injured, burned, or poisoned)
- Clean environment (hazardous leaks, powerful explosions, emissions of toxic gases)
- Requirements imposed by the authorities (high fines, plant closure).
The probability is typically expressed in terms of frequency (categories ranging from 1-5), while the consequences are ranging from “A” (minor) to “E” (severe). Next, the risk of failure is calculated, which results in risk categories, “high”, “medium”, “low”.
Risk of failure
probability of failure x
consequences of failure
RoF = PoF x CoF
The assessment of overall plant risk is highly complex. That's why a multidisciplinary team with a broad expertise is needed. The RBI process involves multiple stakeholders and various engineering backgrounds. The judgment of old hands in the profession is to be considered particularly valuable, but even for them it remains quite difficult to estimate the risks in the first place.
Step 3: Risk ranking
To prioritize the risks and to communicate the results of the analysis a risk matrix can be used, like the example shown in Figure 3 (a 7x7 matrix is also common). The risk categories (RoF) are plotted in the matrix of probability categories (PoF) by consequence categories (CoF).
The analysis, as depicted in the risk matrix and substantiated by quantitative data, is used to optimize priorities and intervals for the inspection planning. Equipment items residing towards the right upper corner of the matrix should take priority, because these items have the highest risk. These are the most probable failures with the most severe consequences. In contrast, items residing towards the left bottom corner of the matrix will tend to take lower priority, because these items have the lowest risk.
Like for many other phenomena, the Pareto principle is applicable for RBI, as it turns out that a large percentage of the total unit risk will be concentrated in a relatively small percentage of the equipment items. So, from all equipment items that are competing for attention, the idea is to review the inspection plan focusing on those components with the highest risk.
Step 4: Inspection plan
After the risk ranking, engineers try in collaboration with corrosion engineers to design an inspection plan that gives priority to components with the highest total risk. Material degradation and failure mechanisms will continue to develop, no matter if the act of inspecting is carried out or not. Inspection serves to identify, monitor, and measure these mechanisms before becoming critical. The inspection serves as a continual risk-updating and reduction effort, merely in terms of knowing what is currently going on with the static assets that are being utilized.
Limited resources and manpower prevent thorough inspections of all static assets, especially when costly inspection methods must be used in a relatively short period of time. Because the inspection plan allocates the inspection resources to the static assets with the highest risk profiles, while avoiding unnecessary regular inspections on non-critical items, RBI is actually a cost-cutting maintenance strategy. The potential savings on inspection cost are 20-40 percent by implementing RBI.
Categories of inspection
To identify, classify, analyse, and evaluate failure mechanisms, RBI uses three categories of inspection:
- Visual inspection: external inspection.
- Invasive inspection: opening-up assets to take samples and examining CUI.
- Non-destructive inspection: endoscope, Eddy-current, acoustic emission and vibration analysis.
The risk ranking will not provide a straightforward indication of the type of inspection; this needs to be determined per item by choosing the inspection method that is sufficient for detecting the deterioration mechanisms and its severity.
Typical situations where risk management through inspection may have little or no effect are:
- Corrosion rates well established, and equipment is nearing end of life
- Instantaneous failures related to operating conditions such as brittle fracture
- Inspection technology that is not sufficient to detect or quantify deterioration adequately
- Too short a time frame from the onset of the deterioration to final failure for periodic inspections to be effective (e.g., high-cycle fatigue cracking)
- Event-driven failures (circumstances that cannot be predicted).
Step 5: Mitigation
If completed inspections have shown that the inherent overall risk of a static item is acceptable or relatively low when compared to other evaluated static items, no further mitigation measures may be necessary. However, since the whole idea revolves around tackling static items with the highest risk, more often than not some form of mitigation has to be considered. For effective risk reduction you can devise measures that limit the consequences and probability. To prevent future failures, it may be necessary to repair, modify, or renew parts of the installation, or to shorten the time interval between turnarounds or regular inspections.
RBI can potentially be used as a steppingstone to condition-based monitoring as it provides an organization with the ability to gain valuable insight into their highest risk items of static equipment, which is routinely inspected. The initial investments in condition-based monitoring, including complex sensors and software packages, can be quite high, but can also be seen as a form of mitigation.
Step 6: Reassessment
The previous steps are all based on particular moments. As time goes by, changes that could affect the probability or consequences of failure are inevitable. Therefore, it’s important that the facility has an effective Management of Change process that identifies when a reassessment is necessary. The RBI reassessment concerns:
- Process and hardware characteristics
- Maintenance (strategies / approaches).
Many deterioration mechanisms are time dependent. Typically, the RBI assessment will project deterioration at a continuous rate. These rates may vary over time. Through inspection activities, the average rates of deterioration may be better defined. Some deterioration mechanisms are independent of time, e.g., they occur only when there are specific conditions present. These conditions may not have been predicted in the original assessment but may have subsequently occurred. Inspection activities will increase information on the condition of the equipment and the results should be reviewed to determine if a RBI reassessment is necessary.
By creating and using predictive degradation models, combined with routine inspections and testing as efficiently and effectively as possible, RBI allows for long-term monitoring of static assets. In the context of continuous improvement being embedded in the RBI approach, the RBI process should be repeated after the cycle has taken place and the necessary investments in risk reduction have been made. Keep in mind that there will always be some degree of residual risk as all risks can never be completely eliminated. The aim of RBI is to reduce this residual risk to an acceptable level.
Visual inspection is one of the most basic and oldest maintenance interventions. Skilled technicians traverse facilities to identify structural flaws, surface wear and tear, cracks, leakages, corrosion, buckling of welds and other structural members. The main goal of visual inspection is to identify defects that may impede the performance of an asset. After recognizing the flaws, technicians take the initiative to fix them before they worsen and result in asset damage or accidents.
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