FUNCTIONAL SAFETY SERVICES
RAMS STUDY:
RELIABILITY, AVAILABILITY, MAINTAINABILITY, AND SAFETY
Understand how your critical systems will perform across their operating life. Identify failure modes before they become operational problems, and build the evidence base for design decisions that balance safety, reliability, and cost.
Safety and reliability are related but distinct properties of a system. A safety instrumented system may be designed to achieve a specific SIL target, but questions about how often it will demand maintenance, how quickly it can be restored after a failure, and how its availability changes as components age and proof test intervals pass are answered by a different set of analyses: RAMS.
RAMS studies provide a systematic, quantitative assessment of four interdependent system properties. Reliability is the probability that a system performs its required function without failure over a defined period. Availability is the proportion of time the system is in a state capable of performing its function when demanded. Maintainability is the ease and speed with which the system can be restored to a functioning state after failure. Safety is the property that ensures hazardous failure modes are controlled to a tolerable level throughout the operating life.
These four properties interact in ways that design decisions directly influence. A redundant architecture may improve reliability and availability but increase maintenance complexity. A short proof test interval improves safety availability but increases operational burden. RAMS analysis makes these trade-offs visible so that engineering, operations, and management teams can make informed decisions at the point in the lifecycle when changes are still practical.
For organizations operating high-consequence industrial systems, RAMS studies also provide the quantitative evidence base required by regulators, insurers, and internal governance frameworks for demonstrating that safety and operability targets have been rigorously assessed.
Reliability analysis identifies which components dominate the failure behavior of the system, where redundancy or improved component selection would have the greatest reliability benefit, and how reliability changes as a function of operating time and environmental stress.
Availability analysis determines whether a proposed maintenance and testing regime will deliver the required operational performance, identifies the maintenance and logistics factors that most significantly affect system availability, and supports decisions about spare parts holding, maintenance team sizing, and repair time targets.
Maintainability analysis informs decisions about equipment layout and access design, spare parts and tooling requirements, maintenance procedure development, and the trade-off between planned preventive maintenance frequency and corrective maintenance response time.
For safety instrumented systems, safety analysis within RAMS also addresses dangerous undetected failure accumulation between proof tests, the effect of diagnostic coverage on the detected versus undetected failure balance, and the relationship between proof test interval and safety availability.
A Functional Safety Assessment is an investigation to judge the functional safety achieved by one or more systems, carried out at defined lifecycle stages. IEC 61511 Clause 8 requires FSAs to be planned, resourced, and carried out by persons with appropriate independence and competence, and the results to be documented and tracked to closure.
Each FSA examines the outputs of the lifecycle phases completed to that point, assessing whether:
- ✔ The required activities have been carried out in accordance with the FSM Plan and the applicable standard.
- ✔ The outputs of each activity are complete, correct, and consistent with the inputs they were derived from.
- ✔ The safety case as developed to that point is coherent and traceable from hazard identification through to the current lifecycle stage.
- ✔ Identified gaps, errors, or non-conformances are documented and have a defined path to closure.
FSA findings are categorized by severity, typically as non-conformances requiring closure before the lifecycle can advance, observations that represent improvement opportunities, and positive findings that confirm activities have been carried out to a high standard.
Failure Mode and Effects Analysis (FMEA) and FMECA
FMEA is a bottom-up analysis that systematically identifies the failure modes of each component, their effects at the subsystem and system level, and the means by which they are detected. FMECA extends FMEA by adding a criticality assessment, ranking failure modes by their risk contribution to prioritize design and maintenance attention. FMEA and FMECA are foundational inputs to reliability, availability, and safety analysis.
Fault Tree Analysis (FTA)
FTA is a top-down analysis that starts with an undesired top event, such as a system failure or a hazardous condition, and systematically identifies the combinations of component failures and human errors that could cause it. FTA produces a quantitative probability for the top event, identifies the minimal cut sets that represent the most significant failure combinations, and reveals where single-point failures or common cause failures could dominate system risk.
Reliability Block Diagram (RBD)
An RBD models the system as a logical network of components in series and parallel configurations, representing the functional dependence of system reliability on each component. RBD analysis calculates system-level reliability and availability metrics from component failure data, and is particularly effective for analyzing redundant architectures and identifying the configuration that best meets performance targets.
Markov Analysis
Markov analysis models the system as a set of defined states, such as operational, failed, under repair, or in proof test, and calculates the probability of being in each state over time. It is particularly suited to systems with multiple failure modes, repair processes, and diagnostic states, where the interactions between these factors make simpler methods insufficient. For safety instrumented systems, Markov models are often used to calculate PFDavg for complex architectures.
Monte Carlo Simulation
Monte Carlo simulation models system behavior through a large number of random simulations of failure and repair events, producing statistical distributions of reliability and availability metrics. It is used where the complexity of the system or the variability of input parameters makes analytical methods impractical, and provides confidence intervals around key outputs rather than single-point estimates.
Our RAMS studies are structured in accordance with:
- IEC 60300-3-1: Dependability management – Analysis techniques for dependability, covering FMEA, FTA, RBD, and Markov analysis methods
- IEC 60812: Failure mode and effects analysis (FMEA and FMECA) methodology
- IEC 61025: Fault tree analysis
- IEC 61511: Reliability and safety analysis requirements for safety instrumented systems in the process industry, including PFDavg calculation and SIL verification
- IEC 61508: Reliability and safety analysis requirements for component and system-level functional safety assessment
- EN 50126: Railway applications RAMS standard, applied for clients in transportation and rail-adjacent industrial sectors
- ISA/IEC 62443: Where RAMS analysis addresses the reliability and availability of OT/ICS infrastructure that supports or interacts with safety systems
A RAMS study is only as useful as its inputs and as practical as its outputs. We invest in both rigorous analysis built on accurate failure data and system knowledge, and results presented in a way that engineering and operations teams can act on.
Scope Definition and Data Collection
We define the system boundary, identify the functions to be assessed, and collect the failure rate data, repair time data, and maintenance regime information needed to support the analysis. Where generic failure rate databases are used, we apply appropriate source selection and justification. Where site-specific failure history is available, we incorporate it to improve the accuracy of results.
Failure Mode Identification (FMEA or FMECA)
We conduct a structured FMEA or FMECA for the system in scope, identifying all relevant failure modes, their causes, their detection mechanisms, and their effects at the subsystem and system levels. The FMEA output forms the foundation for the quantitative reliability and safety analysis that follows.
Quantitative RAMS Analysis
We apply the appropriate combination of FTA, RBD, Markov analysis, and simulation methods to produce quantitative reliability, availability, and safety metrics for the system. Where the study is linked to a functional safety program, RAMS outputs are aligned with SIL assessment calculations to ensure consistency.
Results Interpretation and Recommendations
We interpret RAMS results against the design targets and operational requirements, identify the failure modes and design features that most significantly affect performance, and develop practical recommendations for design improvement, maintenance strategy optimization, and spare parts planning. Outputs are structured so that design teams, operations managers, and safety engineers can each use the results relevant to their decisions.
- A quantitative understanding of how your system will perform across its operating life, replacing assumptions and engineering judgment with evidence
- Identification of the failure modes and design features that most significantly affect reliability, availability, and safety, so that design effort and maintenance resources are directed where they have the greatest effect
- A maintenance strategy grounded in failure mode analysis and availability modeling, rather than fixed time-based schedules that may be either too conservative or insufficient
- Quantitative evidence for design decisions, procurement trade-offs, and spare parts investments that can be documented and defended to internal and external stakeholders
- Alignment between safety system reliability analysis and the SIL assessment and functional safety case, avoiding inconsistencies between different parts of the technical safety record
- Audit-ready RAMS documentation that supports regulatory submissions, insurance assessments, and internal engineering governance requirements
- RAMS study report covering scope, methodology, input data, analysis results, and recommendations
- FMEA or FMECA worksheet with failure mode register, effects analysis, detection mechanism, and criticality ranking
- Fault tree diagrams and quantitative FTA results for key top events, including minimal cut set analysis
- Reliability block diagrams and quantitative RBD results for reliability and availability metrics
- Markov model outputs where applicable, including state transition diagrams and steady-state availability calculations
- Sensitivity analysis identifying the input parameters and design features that most significantly affect key RAMS metrics
- Maintenance strategy recommendations derived from FMEA and availability analysis
- Action list with prioritized design and maintenance recommendations, with owner and rationale
Industrial systems are increasingly software-intensive and network-dependent. The reliability and availability of a modern safety instrumented system depend not only on the failure rates of its sensors and final elements but also on the stability of its programmable logic controller platform, the robustness of its OT network connections, and the integrity of its configuration management. RAMS analyses that treat the SIS as a purely electromechanical system miss a significant part of the reliability picture.
- Integrated RAMS and OT/ICS perspective: our team understands both classical reliability analysis methods and the ways in which software, firmware, and networked components contribute to system reliability and safety risk
- Rigorous quantitative analysis built on appropriate failure data sources, clearly justified and documented, so that results can be independently verified.
- Practical, actionable outputs that give engineering and operations teams clear guidance on where to invest design effort and maintenance resources
- Alignment with the functional safety program, ensuring RAMS outputs and SIL assessment calculations are consistent and mutually supporting
- Deep operational experience across high-consequence sectors, including oil and gas, energy, pharmaceuticals, and process manufacturing
We do more than produce reliability numbers. We help teams understand what those numbers mean for design decisions, maintenance planning, and the long-term safety performance of their systems.
Ready to Assess the Reliability and Safety of Your Critical Systems?
Reach out to our functional safety team. We will confirm the scope of your RAMS study, the analysis methods most appropriate for your systems, and the outputs your engineering, operations, and governance teams need