In engineering and safety-critical systems, understanding failure rates is crucial for ensuring reliability and safety. Failure rates are statistical representations that indicate how often a system or component is likely to fail during a given period or under specific conditions. But these rates are not standalone metrics; they are deeply influenced by the operational context defined by Mission profiles. By analyzing mission profiles, one can gain insights into how various conditions affect system performance and its failure rates, leading to more informed decision-making regarding design, maintenance, and risk management.

Why?

Why is Understanding Failure Rate and Mission Profile Important?

Functional safety revolves around preventing catastrophic failures in industrial processes and systems. By understanding the failure rate and mission profile of safety-critical equipment, industries can predict and mitigate risks, thus avoiding accidents, unplanned downtimes, or financial losses. An example is the 2010 Deepwater Horizon oil spill, which was partly attributed to insufficient safety checks and understanding of system wear over time​.

What?

What Are Failure Rate and Mission Profile?
Failure Rate:
The failure rate quantifies how often a system or component is expected to fail during a specified time, usually expressed in failures per hour (e.g., failures per million hours).
In the context of IEC 61508 and IEC 61511, failure rates help in determining Safety Integrity Levels (SIL). Lower failure rates indicate higher reliability and are essential in achieving a higher SIL rating.
Failure rates are commonly used to predict the reliability of components in various industries, especially in safety-critical sectors like aerospace, automotive, and medical devices. These metrics help engineers estimate how long a system can operate before failure, guiding design improvements, maintenance schedules, and risk assessments. However, failure rates alone do not account for the varying conditions under which a system operates, which is why they are often paired with mission profiles for more accurate predictions.
Mission Profile:
A mission profile outlines the operational conditions a system will face throughout its lifecycle, such as temperature ranges, humidity, vibration, or load cycles.
It is crucial for evaluating how these environmental factors impact failure rates. The mission profile provides a realistic scenario of how a system will operate, ensuring it is tested and validated under the expected conditions.
Methodologies for Integrating Mission Profiles

To effectively utilize mission profiles in evaluating failure rates, organizations can adopt several methodologies. One common approach is to perform reliability testing under simulated mission conditions. This method involves subjecting components or systems to stress tests that mimic real-world scenarios, thereby collecting failure data that is more representative of actual operational performance. Additionally, data analytics and modeling techniques can be employed to analyze historical failure data alongside mission profiles, enabling predictive maintenance and targeted improvements in design and operation.

Example: A gas detector in an offshore platform must endure high humidity, salt spray, and temperature fluctuations. Its mission profile ensures it can reliably function despite these harsh conditions.

How?

How Are Failure Rate and Mission Profile Applied in Practice?
Failure Rate Calculation:
Determine the failure rate of individual components through historical data, reliability studies, or manufacturer specifications.
Use the failure rate data to calculate the overall system’s PFD, which directly influences the SIL rating of a Safety Instrumented System (SIS). This involves summing up the failure rates of components to estimate the system’s likelihood of failure.
Real-world example: In a refinery, if a pressure transmitter has a failure rate of 10 failures per million hours, and this transmitter is part of an emergency shutdown system, its reliability directly impacts the safety function.
Mission Profile Development:
Identify the operational environment (temperature, pressure, exposure to chemicals) and duration for which the system will be used.
Test components under simulated mission conditions to validate that they meet the required reliability before deployment.
Real-world example: A control valve in a petrochemical plant might have a mission profile that includes exposure to corrosive gases and high temperatures for up to 10 years. This profile ensures the materials and design are robust enough to handle such conditions.

Conclusion

By understanding the failure rate and mission profile, industries can design safer systems that prevent catastrophic failures, save lives, and avoid significant financial losses. Properly evaluating these factors is a cornerstone of achieving compliance with functional safety standards like IEC 61508 and IEC 61511.