At a rough estimate, 10% of the equipment in a plant contributes 90% of the risk. Therefore, if this 10% of the equipment is identified, testing and inspection activities can be focused meaningfully, rather than wasting effort on low-risk items. This results in improved safety; fewer forced failures, and reduced operational costs
The inspection normally includes routine and preventative maintenance developed to reduce the risk but RBI is intended to reduce the operating risk of process plant, Provide strategies to achieve the desired level of mechanical integrity (MI) and reliability of the plant. Inspection should therefore be a proactive, strategic planning process focused on the risk factors that impact safety, reliability and integrity, as well as performance and profitability.
Risk-based inspection (RBI) assists industry and considered as the best practice for Plant integrity management. RBI assists the safety managers, site inspectors and others involved in industrial risk assessment. The RBI methodology combines risk assessment and risk management techniques with inspection activities, such as planning, inspecting, documentation and data analysis, to develop inspection plans that govern inspections toward the areas of highest risk.
Risk-Based Inspection (RBI) is an inspection method that requires a qualitative or quantitative assessment of the probability of failure (PoF) and the consequence of failure (CoF) associated with each equipment item, structural components, piping circuits included, in a particular process unit.
The risk level for a particular piece of equipment is determined by calculating the PoF together with the Consequence of Failure (CoF), Probability of Failure (PoF) is the likelihood that equipment or a system will fail at a given time which makes up an important part of effective risk analyses. POF is half of the equation when determining risk as part of Risk-Based Inspection (RBI) methodology.
Probability of Failure (PoF) x Consequence of Failure (CoF) = Risk.
The consequence of Failure (CoF) is another part of the equation to determine risk as part of Risk-Based Inspection (RBI) method. CoF is calculated by categorizing the potential consequences for the equipment, personnel, environment, etc. in case of any equipment/ system failure.
The risk of failure combines the probability of failure with a measure of the consequences of that failure. Then the risk is defined as the product of the probability of failure rate and the measured consequence when evaluated numerically.
RBI schemes can be broadly divided into two types: those that rely predominantly on qualitative methods and incorporate probabilistic methods / quantitative). Qualitative methods are more common than quantitative methods. Quantitatively, the RBI programme is a hybrid technique between risk analysis and mechanical integrity. In its elemental form, a risk analysis is comprised of five tasks:
- System definition
- Hazard identification
- Consequence assessment
- Probability assessment
- Risk results.
CAUSES OF FAILURE
Root causes of failure include:
- faulty design,
- material defects,
- processing errors,
- assembly or installation defects,
- maintenance deficiencies
- Improper operation.
- Corrosion and Erosion
- External Loading
- Impact
- Pressure
- Temperature
- Wrong Equipment
- Defective Equipment and
- Human Error
Risk-Based Inspection involves planning, implementation and evaluation of examinations to determine the physical and mechanical integrity of equipment or structural integrity in terms of fitness-for-service (FFS). Examination methods include visual surveys and NDT techniques designed to detect the defects, such as ultrasonic testing and radiography.
The inspection programme is intended to reduce the risk by establishing the steps like:
1) What type of damage to look for.
2) Where to look for damage.
3) How to look for damage.
4) When to look for damage.
What and Where is known from reviewing the design data, process data and the equipment history, How to look for the damage is decided by reviewing the extent of damage, validity of inspection sample, sample size, methods of detection based on past observations. When to look for damage is related to the estimated remaining life of the component.
RBI Inspection covers the following equipment:
- Pressure Vessels
- Piping systems
- Storage Tanks
- Pressure Relief Valves
- Rotating Equipment
- Heat Exchangers
Implementing an RBI Program
Implementation of an RBI program normally requires a competent multi-disciplinary team. The team needs to have the right mix of knowledge and experience and normally the team members are from five core disciplines:-
- RBI team study Facilitator (RBI and Fitness-for-Service assessment expertise)
- Inspection Engineer
- Plant Mechanical (or Maintenance) Engineer
- Plant Operations and Process Engineer
- Metallurgist or Corrosion Engineer
Areas covered in RBI include-
- Pressure vessels and storage tanks
- Emergency shutdown and fire protection systems
- Process safeguards such as relief and vent systems and devices, controls, interlocks, sensors and alarms
- Pumps and piping systems (including components such as valves)
- Quality assurance, materials of construction and reliability engineering
- Maintenance and preventive maintenance programmes.
The RBI team should use risk assessment, which may be either qualitative or quantitative, to prioritise plant for inspection. The risk assessment can be broken down into stages,
- Identification of potential modes of failure and effects
- Assessment of the probability of failure for each mechanism/mode
- Assessment of the consequences of equipment failure, which will involve the identification of the accident scenarios involving the failure of the equipment
- Determination of the risk from equipment failure
- Risk ranking and categorization
Benefits of implementing the RBI Program
- Reducing the operating risk of process plant
- Provides effective strategies to achieve the desired level of mechanical integrity (MI) and plant reliability
- Reducing downtime by avoiding unplanned failures
- Increases the reliability of equipment
- Lowers the safety risks
- Optimizes inspection and maintenance costs
- Focuses the resources on areas where maintenance is most critical
- Enables better budgeting and planning for future inspection, maintenance, and run-repair-replace decisions
- Results in more effective strategic planning
RBI prioritizes inspection-related activities, usually by means of Non-Destructive Testing (NDT), in order to reduce the uncertainties and the damage state of the equipment. The resulting inspection plan may outline the type and scheduling of inspection for an asset. In addition to NDT, additional risk mitigation activities identified by an RBI assessment might include a change in the material of construction, installation of corrosion-resistant liners, changes in operating condition, injection of corrosion inhibition chemicals, etc.
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