Last updated on Aug 6, 2024

How do you allocate reliability targets for a complex system using fault tree analysis?

Reliability allocation is a process of assigning reliability goals to different components or subsystems of a complex system, based on their functions, importance, and failure modes. It helps to ensure that the system meets its overall reliability requirements and minimizes the cost and risk of failures. One of the methods for reliability allocation is fault tree analysis (FTA), a graphical technique that models the logical relationships between failures and their causes. In this article, you will learn how to use FTA for reliability allocation in six steps.

1 Step 1: Define the system and its reliability requirement

The first step is to define the system and its reliability requirement, which is usually expressed as a probability of failure or a mean time between failures (MTBF) for a given time period. You should also identify the system boundaries, the level of detail, and the assumptions and constraints for the analysis.

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Sagar More

🧑💼Site Reliability Engineering Evangelist 🚀 Tackling Unknown Unknowns 🔍 Architecting Resilient Systems 🛠️ Accelerating Digital Transformation 🌐 Driving Business Growth through Innovative Technology Solutions 💡📈
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In my experience with fault tree analysis, defining the system and its reliability requirements is the linchpin of successful target allocation. It’s like drawing a blueprint for a high-stakes project—understanding every component and its role ensures precision in setting realistic reliability goals. This clarity not only sharpens the focus of our analysis but also paves the way for achieving unparalleled system performance.

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2 Step 2: Construct the fault tree

The next step is to construct the fault tree, which is a diagram that shows how the system can fail due to different combinations of component failures or events. The fault tree has a top event, which is the system failure of interest, and a series of lower-level events, which are the causes or contributors to the top event. The events are connected by logical gates, such as AND, OR, or NOT, which indicate how they affect the occurrence of the higher-level event.

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3 Step 3: Quantify the basic events

The basic events are the lowest-level events in the fault tree, which represent the failures or conditions of individual components or subsystems. To quantify the basic events, you need to assign a failure rate or a probability of failure to each one, based on data from testing, field experience, or expert judgment. You can also use reliability distributions, such as exponential, Weibull, or lognormal, to model the failure behavior of the basic events.

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4 Step 4: Calculate the top event probability

The top event probability is the probability of the system failure that you are analyzing. To calculate it, you need to use the fault tree logic and the basic event probabilities to propagate the failure information from the bottom to the top of the tree. You can use different methods, such as cut sets, minimal cut sets, or binary decision diagrams, to simplify the fault tree and reduce the computational effort.

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5 Step 5: Allocate reliability targets

The reliability targets are the reliability goals that you want to achieve for each component or subsystem in the system. To allocate them, you need to use the fault tree and the top event probability to determine how much each basic event contributes to the system failure. You can use different criteria, such as equal contribution, proportional contribution, or criticality, to allocate the reliability targets based on the importance or impact of each basic event.

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Jim Cencula

Sr. Engineering Consultant at RMS
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In my experience one needs to understand to be clear that reliability allocations are based on a flow down of the risk based on the major systems and failure modes and then flowed to each subsystem or the key sub failure modes.

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6 Step 6: Evaluate and optimize the reliability allocation

The final step is to evaluate and optimize the reliability allocation, which means to check if the allocated reliability targets are feasible, consistent, and cost-effective. You should also compare the allocated reliability targets with the actual or expected reliability performance of the components or subsystems, and identify any gaps or discrepancies. You can use different techniques, such as sensitivity analysis, trade-off analysis, or design of experiments, to optimize the reliability allocation and improve the system reliability.

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Oseghale Okohue BEngr. Msc. MBA. DBA. Prince2®Practitioner

Accredited NEC4 ECC Project Manager & Snr. P6 Planner | EPICM, Offshore Engineering,Renewable Energy (HVDC & HVAC Power Cables, Wind, Solar, BESS, Grid Code Compliance (G99, G5/5, P28) & Integration, GIS/AIS Sub-Station)
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Allocate reliability targets for complex systems using fault tree analysis by identifying critical components and failure modes. In renewable energy, for offshore wind farms, fault tree analysis prioritizes reliability targets for key components like turbines, ensuring uninterrupted power generation. Grid code compliance studies employ fault tree analysis to allocate reliability targets for transmission infrastructure, preventing grid failures. In manufacturing, it identifies critical failure points in machinery to meet production reliability goals. For battery storage, fault tree analysis sets targets for system components to ensure consistent energy supply.

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How do you allocate reliability targets for a complex system using fault tree analysis?

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1 Step 1: Define the system and its reliability requirement

2 Step 2: Construct the fault tree

3 Step 3: Quantify the basic events

4 Step 4: Calculate the top event probability

5 Step 5: Allocate reliability targets

6 Step 6: Evaluate and optimize the reliability allocation

Reliability Engineering

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