Fault Tree Analysis (FTA)
FTA is another tool in the reliability engineering toolkit. The general purpose of FTA is to identify the technical reason for the specified unwanted events and to estimate or predict the system reliability performance. FTA logically represents all possible failure modes of a system or package.
FTA is a systematic and deductive method for defining a single undesirable event and determining all possible reasons that could cause that event to occur. The undesired event constitutes the top event of a fault tree diagram, and generally represents a complete or catastrophic failure of a product or process. As well as a FMECA, an FTA can also be used for identifying product safety concerns. Contrary to a FMECA, which is a bottom-up analysis technique, a FTA takes a top-down approach to assess failure consequences. An FTA can be applied to analyze the combined effects of simultaneous, noncritical events on the top event, to evaluate system reliability, to identify potential design defects and safety hazards, to simplify maintenance and trouble-shooting, to identify root causes during a root cause failure analysis, to logically eliminate causes for an observed failure, etc. It can also be used to evaluate potential corrective actions or the impact of design changes.
FTA is best applied during the front-end engineering design (FEED) phase as an evaluation tool for driving preliminary design modifications. Once a product is already developed or even on the market, an FTA can help to identify system failure modes and mechanisms.
Input Data Requirements
Intimate product knowledge of the system logic is required for tree construction, and reliability data for each of the basic units/events are required by quantitative analyses.
Strengths and Weaknesses
API RP 17N illustrates the strengths and weaknesses of FTA: Strengths
- Can support common cause failure analysis;
- Can predict the probability of occurrence of a specific event;
- Can support root cause analysis;
- Compatible with event trees for cause/consequence analyses;
- Supports importance analysis.
- Complex systems may become difficult to manage and resolve manually;
- Not suited to the consideration of sequential events.
Reliability Capability Maturity Model (RCMM) Levels
The reliability capability maturity model provides a means of assessing the level of maturity of the practices within organizations that contribute to reliability, safety, and effective risk management.
Overview of Reliability Capability Maturity Levels : 1 No understanding of reliability concepts. 2 Prescriptive procedures that are repeatable but do not directly relate to reliability. 3 Understanding of historical achievements in reliability but with limited capability to learn from lessons and improve reliability. 4 Understanding of design for availability and how to correct designs to improve reliability given the observation of failure. 5 Understanding of design for availability and implementation into a proactive continuous improvement program (both managerial and operational).
these will lead to higher levels of reliability capability.
Reliability-Centered Design Analysis (RCDA)
Reliability centered design analysis is a formalized methodology that follows a step-by-step process. RCDA lowers the probability and consequence of failure, resulting in the most reliable, safe, and environmentally compliant design. The direct benefits of using RCDA in FEED are as follows:
- Higher mechanical availability, which results in longer operating intervals between major outages for maintenance, significantly increasing revenue.
- Reduced risk. RCDA results in designs that lower the probability and consequence of failure.
- RCDA is a functional-based analysis. It focuses on maximizing the reliability of critical components required to sustain the primary functions for a process.
- Shorter maintenance outages. Reduced downtime results in fewer days of lost production, significantly increasing revenue.
- Safer, more reliable operations, better quality control, more stable operation with the ability to respond to transient process upsets.
- Lower operating expenses. RCDA results in designs that cost less to maintain over the operating life of the asset.
- Optimized preventive and predictive maintenance programs and practices. A comprehensive program is created during RCDA. Training to these practices is performed in advance, so assets are maintained from the minute the project is commissioned.
- Emphasis on condition-based maintenance practices. Equipment condition is continuously monitored, maximizing the full potential of the assets, and avoiding unnecessary inspections and costly overhauls.
- RCDA can be used as a training tool for operators and maintenance personnel. RCDA documents the primary modes of failure and their consequences and causes for failure well in advance of building the platform.
- Spare parts optimization. Because the dominant failure causes are identified for each piece of equipment, the spare parts requirements are also known. Because this analysis is performed on the entire platform, stock levels and reorder levels can also be established.
The RCDA process is integrated into project management stages, that is, FEED. As a process, it follows a uniform set of rules and principles.
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