Reliability-Centered Maintenance is structured around seven key questions that guide the development of effective maintenance strategies. Understanding functions, functional failures, and failure modes represents the foundation for subsequent analyses. By clearly defining what assets are expected to do, how they can fail to meet those expectations, and the ways in which failures occur, we establish a structured path toward evaluating consequences, identifying degradation patterns such as the P–F curve, and ultimately selecting appropriate maintenance tasks. These elements ensure that maintenance decisions are grounded in both operational performance and risk management.
Functions and Functional Failures
The process unit should be divided into Systems, with clearly defined expected performance for each system. These defined Systems should support an effective Failure Mode and Effect Analyses, enabling assessment of how equipment failures impact System functions.
Functions and functional failures should be defined in relation to production performance. Key aspects to consider include:
Production Reference Plan
Production Targets, Reliability, Budget, Growth Plan
License-to-operate requirements
Environmental, Health, and Safety requirements
Compliance with environmental regulations and statutory nuisance limits
Production processes
Process phases, recycle loops and treating steps.
Main streams, auxiliary streams, and utilities
Production storage capability
Product Quality Requirements (off-specification effects)
Equipment redundancy
TA strategy and intervals
Maintenance strategy
Operational arrangements and shift patterns (onshore/offshore)
Repair time (maintenance response time), logistics, and travel considerations
Spare parts availability, lead times, and logistics
Economic factors, such as Production Loss Equation (PLE), margin, and throughput
Environment and seasonal conditions
Failure Modes
The analysis typically focuses on dominant failure modes (FMs) that have been observed over time. The following steps are recommended to identify relevant failure modes.
Review equipment images to understand external features and condition
Examine cross-sectional drawings of the equipment to identify wearable or degradable parts
Review the equipment in the P&ID to verify:
the tag numbers
possible process conditions
indicators and instruments
auxiliary equipment.
Note
Equipment should always be referenced using a diagram.
Applicable diagrams include P&IDs, EFDs, electrical diagrams, or safeguarding diagrams.
In some cases, diagrams may be available only in maintenance manuals.
Tag names should be verified against the asset register to ensure consistency.
Failure modes are defined assuming the equipment operates under design conditions.
What happens when each failure occurs (P-F Curve)
For each failure mode, the degradation process should be described under the assumption that no maintenance is performed. This description should define:
Where degradation begins.
How degradation progresses over time.
Where degradation reaches a stable failed state.
This progression can be expressed using the P-F curve. The P-F curve represents the degradation Interval between:
Potential failure (P) - the point at which degradation becomes detectable.
Functional failure (F) - the point at which the asset can no longer perform its intended function.
The P–F curve illustrates the cause-and-effect relationship leading to failure and may include multiple detectable points (P points). Each P point is critical for defining appropriate maintenance tasks.
Historical “as-found” maintenance data should be used to validate how degradation occurs in practice.
Failure Consequences (Does the failure matter?)
The probability and consequence of each failure are elevated in relation to:
Asset impact (economic loss)
People (health and safety)
Environment.
These factors are assessed using a criticality matrix with six defined risk levels (from N to E).
For economic evaluation, criticality is also expressed as Annual price of non-conformance (k$/yr).
Maintenance Task Selection (What can be done to predict or prevent each failure?)
Maintenance tasks should be determined based on the failure effects and degradation patterns described by the P–F curve.
The characteristics of each failure mode determine the appropriate maintenance strategy:
Age-related failures - time-based maintenance.
Non-age-related failures - condition-based maintenance.
To ensure proper economic balance between the “no maintenance” scenario and the “maintenance performed” scenario - the maintenance tasks should include a restoration task.
What if a suitable proactive task cannot be found?
If no suitable proactive maintenance task can be identified, the decision should be evaluated based on suitability criteria.
Risk-Based Evaluation
Does the task provide sufficient risk reduction?
Is the Residual Risk acceptable?
Residual risk levels classified as medium-high (MH), high (H), or extreme (E) are typically unacceptable and require further risk reduction measures.
Economic Evaluation
Does the task provide economic benefit?
Is the MEI high enough?
The MEI is defined as:
MEI = Risk Reduction / Maintenance Cost
Where:
Risk Reduction = Base Risk − Residual Risk
An MEI value below 1 indicates that the maintenance task is not cost-effective and may support a run-to-failure strategy.
This evaluation ensures that maintenance activities are both economically justified and effective in reducing risk.