Risk Based Inspection

Risk Based Inspection


Risk Based Inspection defines the risk of operating equipment using a prioritization process which combines two separate factors: the likelihood and the consequence of failure. The AllAssets process utilizes a method that is a blend of qualitative and quantitative analytical techniques. Because of this blend of techniques, the method is labeled as semi-quantitative. The semi-quantitative approach allows the AllAssets Program to take advantage of qualitative methods to simplify work processes while preserving calculation rigor where necessary through quantitative models.
The semi-quantitative method of the AllAssets Program uses a 5 x 5 matrix. Each level in the matrix is calibrated to order-of-magnitude steps. This method has been demonstrated to be relatively simple to implement while still providing adequate discrimination between the items being evaluated.

The core of Risk Based Inspection is a closed loop process that uses feedback of results for continuous improvement. The process consists of developing a PLAN for each equipment item and scheduling the planned work, PERFORMING the work defined in the Inspection Plan, EVALUATING the results of the tasks, and UPDATING the system based on task results and other reasons.

 

Plan

  • Developing detailed inspection plans based on criticality rating to accomplish the tasks that will ensure the integrity and reliability of equipment.
  • Scheduling the planned work which may include preventive maintenance, predictive maintenance such as inspection or tests, and repairs.

 

Perform

The planned work is executed using the Task Procedures that define step-by-step instructions that ensure tasks are performed in the same manner each time. The data recorded is the data needed to measure equipment performance and feed the key business indicators.

 

Evaluate

The inspection results are evaluated to determine if:

  • The equipment has a deficiency and operation cannot continue until the deficiency has been resolved.
  • The equipment has deteriorated but is safe and adequate for continued operation until repairs are made.
  • The equipment has not deteriorated and no further consideration is needed.

 

Update

Update causes continuous improvement of Risk Based Inspection program toward optimum especially for improving Inspection Plans. The update process is normally initiated by the person who performed the task, and may result in:

  • Eliminating a task from the Inspection Plan because a suspected failure mechanism was not present.
  • Adding a task to the Inspection Plan because an unsuspected failure mechanism was detected.
  • Revising the Inspection Procedure(s) used.
  • Correcting errors in drawings or data.
  • Revising the maintenance work plan that documents the additional work necessary to perform a task such as permits, scaffold, and equipment clearing.
  • Improving other parts of Risk Based Inspection process.

In order to maintain the quality of the RBI program, each RBI assessment should be updated and revalidated using the following criteria:

  • Whenever a change occurs in a critical variable in the RBI assessment.
  • Whenever a process or hardware change is implemented that could have a significant impact on risk.
  • Whenever a new deterioration mechanism is introduced to an equipment item in the RBI program.
  • Prior to the expiration date of the previous RBI assessment.
  • A maximum interval of five years since the publication of the previous RBI assessment for an equipment item.
  • Every five years, the Inspection Strategies should be reviewed and evaluated for their effectiveness in managing inspectable risk.

Unless otherwise noted, all AllAssets assessments are applicable for normal, steady-state operations. AllAssets assessments are not intended to manage the risks of start-up, shutdown, or process upsets.

 

Risk Factors

AllAssets is not intended to address the risks caused by unknown - or poorly understood - damage mechanisms in pressure equipment.
The damage mechanisms that are covered by probabilistic models in the AllAssets software are listed below:

Thinning – generalized wall loss, typically from the following causes of thinning:

  • Internal corrosion
  • External corrosion
  • Corrosion Under Insulation
  • Sand Erosion

Environmental Cracking – Examples include the following cracking mechanisms:

  • Amine Cracking
  • Stress Corrosion Cracking (SCC)
  • Hydrogen Induced Cracking (HIC)
  • Stress-Oriented Hydrogen Induced Cracking (SOHIC)

All “Other Damage Mechanisms” are reviewed and assigned damage potential and probability by the Corrosion Engineer. The following are just a few examples:

Mechanical Damage

  • Thermal Fatigue
  • Cyclic fatigue
  • Dents, gouges, external impact

Material Properties Degradation

  • High-Temperature Hydrogen Attack
  • Low Temperature embrittlement
  • Sensitization of Stainless Steels

AllAssets is only dealing with “Inspectable Risk”, some of the Risk factors NOT addressed are the following:

  • Human error
  • Natural disasters
  • External events (e.g., vehicle collisions)
  • Deliberate acts (e.g., sabotage)
  • Design errors
  • Fundamental limitations of the inspection method
  • Unknown deterioration mechanisms

Category/Levels of Risk

The AllAssets Program defines four (4) categories or levels of risk:

  • High
  • Medium-High
  • Medium
  • Low

The probability of failure of a specific piece of equipment from inspectable causes is a function of the identified failure mechanisms for that item, the rate of deterioration, and the effectiveness of the prior inspection history in detecting and properly characterizing the damage to the equipment.
The safety consequence of failure of the item is a function of the type of fluid the equipment contains, how much might be released in the event of a failure, and the effect of such a release. Well defined methods have been developed to characterize the safety consequence of a release. A safety consequence of failure category can be determined for flammable effect and for a toxic effect.
Appropriate inspection and maintenance strategies are developed based on the level of risk. An understanding of the identified failure mechanisms and their severity enables the selection of appropriate inspection methods. The inspection frequency and coverage may also be adjusted, based on the results of the risk analysis.

 

Criticality Assumptions

Inspection influences risk by improving the knowledge of the current state and deterioration rate of pressure equipment. The following list of critical assumptions should be reviewed on a periodic basis to assure the validity of the plans and activities generated by the AllAssets assessment. Violation of any of the critical assumptions may invalidate the conclusions and plans generated by the AllAssets assessment.

  • Any deficiencies that could directly lead to loss of containment are addressed and eliminated as soon as practicable. Risk reduction is predicated on the assumption that any deficiencies identified by the inspection process are removed as soon as practicable.
  • Any repairs or modifications made as a result of an AllAssets assessment restore the deficiency to an acceptable condition.
  • The pressure equipment is adequately designed and installed for the current service
  • The pressure equipment is operated within the acceptable design envelope.
  • The selected inspection methods and tools are appropriate for identifying and assessing the expected and potential degradation mechanisms of an item.
  • Risk changes over time. Any AllAssets assessment is time-dependent and results and conclusion may change over time.
  • Data sources are up-to-date and accurate.


Probability Analysis

The probability analysis in the AllAssets program is performed to aid in establishing a relative ranking of equipment items on the basis of risk.
The probability analysis considers the following causes for component failure:

  • Internal corrosion
  • External corrosion including corrosion under insulation
  • Environmental cracking
  • Other Damage Mechanisms (user supplies the Category)
    The probability analysis is the same as for vessels, tank shells, and piping.


Consequence

The consequence analysis in an AllAssets program is performed to aid in establishing a relative ranking of equipment items on the basis of risk. The consequence analysis is a simplified, credible estimate of what might be expected to happen if a loss of containment were to occur in the equipment item being modeled. The estimate is then converted to one of five consequence categories which provide a relative ranking of consequence of failure of the equipment item being analyzed in the AllAssets program.
The consequence analysis method developed in this document has a demonstrated ability to provide the required discrimination between higher and lower consequence equipment items. The method is based upon the Environmental Protection Agency’s RMP OFFISTE CONSEQUENCE ANALYSIS GUIDANCE DOCUMENT published by the U.S. Environmental Protection Agency (EPA) on Mary 24, 1996 for distance to injury models for flammables and toxics.


Criticality and Inspection Planning

The Consequence Category for the Equipment Item becomes the highest (A is highest, E is lowest) Consequence category of the three categories - Flammable, Toxic, or Production Loss.

The Internal Corrosion, External Corrosion, Environmental Cracking, and Other Damage Mechanism Probability Categories are independently determined and tracked, then combined to determine the Combined Probability Category for the asset or equipment component being evaluated. The combined Probability Category is the highest (lowest numerical) Probability Category for the four individual categories. When two or more individual Probability Categories have the same highest value less than “4”, the combined category is one level higher (numerically lower) than the highest individual categories.

Using the Criticality Rating Matrix, the Consequence Category and the Probability Category are combined to determine the Criticality Rating for each piece of equipment. Once the ranking has been identified for a piece of equipment, an appropriate inspection and maintenance strategy can be developed. An understanding of the identified failure mechanisms and their severity will allow the selection of appropriate inspection methods. The inspection frequency and coverage can also be adjusted, based on the results of the risk analysis.
A.2 Reassessments

RBI is a dynamic tool based on a dynamic variable: risk. As time goes by, a facility ages, and changes are inevitable. To maintain a high-quality RBI program, it is important to maintain and update the RBI assessments when necessary.