Single-point failure analysis

Several catastrophic fire incidents in the petroleum industry have been the result of the facility firewater pumps being directly affected by the initial effects of the incident. The cause of these impacts has been mainly due to the siting of the fire pumps in vulnerable locations without adequate protection measures from the probable incident and the unavailability or provision of other backup water sources. A single point failure analysis of firewater distribution systems is an effective analysis that can be performed to identify where design deficiencies may exist. For all high risk locations, fire water supplies should be available from several remotely located sources that are totally independent of each and utility systems which are required for support. 

What is clear is that, while FTA is mostly used to provide a quantitative assessment of a failure condition, it remains fundamentally a quahtative analysis method due to the means that the FTA is developed [see NASA Fault Tree Handbook paragraph 1.2]. Nevertheless, the discipline the analyst goes through to consider each failure path methodically provides an excellent deductive method to provide a reasoned estimate of failure probability. Additionally, the FTA provides more information than simply probability of the top event and can be used even without probability calculation to understand weaknesses in the system design (such as single point failures) and to conduct sensitivity analysis to determine which parts of the system may drive the overall probability of particular failure modes. 

The failure mode and effect analysis (FMEA) is one of the more familiar of the system safety analysis techniques in use. It has remarkable utility in its capacity to determine the reliability of a given system. The FMEA will specifically evaluate a system or subsystem to identify possible failures of each individual component in that system, and, of greater importance to the overall system safety effort, it attempts to forecast the effects of any such failure(s). Because of the FMEA s ability to examine systems at the component level, potential single-point failures can be more readily identified and evaluated (Stephenson 1991). Also, although the FMEA should be performed as early in the product life cycle design phase as possible (see Figure 3.4), based on the availability of accurate data, the system safety analyst can also use this tool, as necessary, throughout the life of the product or system to identify additional failure elements as the system matures. 

The second and more common hardware FMEA examines actual system assemblies, subassemblies, individual components, and other related system hardware. This analysis should also be performed at the earliest possible phase in the product or system life cycle. Just as subsystems can fail with potentially disastrous effects, so can the individual hardware and components that make up those subsystems. As with the functional FMEA, the hardware FMEA evaluates the reliability of the system design. It attempts to identify single-point failures, as well as all other potential failures, within a system that could possibly result in failure of that system. Because the FMEA can accurately identify critical failure items within a system, it can also be useful in the development of the preliminary hazard analysis and the operating and support hazard analysis (Stephenson 1991). It should be noted that FMEA use in the development of the O SHA might be somewhat limited, depending on the system, because the FMEA does not typically consider the ergonomic element. Other possible disadvantages of the FMEA include its purposefiil omission of multiple-failure analysis within a system, as well as its failure to evaluate any operational interface. Also, in order to properly quantify the results, a FMEA requires consideration and evaluation of any known component failure rates and/or other similar data. These data often prove difficult to locate, obtain, and verify (Stephenson 1991). 

Software System Hazard Analysis This type of analysis is conducted similar to a hardware system hazard analysis (SHA), analyzing software functional processing steps to determine whether they may have any particular hazardous effect on the system. The analysis utilizes a hazard-risk index to illustrate the severity of each potential failure. The main advantage to this method is in its ability to positively identify safety-critical hardware and software functions as well as consider the effect of the human element in system software operations. The results of the software SHA, which identifies single-point failures or errors within a system, can often be used to assist in the development of a software fault tree analysis or, to some degree, a system FMEA. However, as with the other various SWHA techniques briefly described above, this method is also time-consuming and costly to perform. 

FMEA is an analytical method used to identify potential problems in the product and in its process of development. It is an inductive method used for identification of hazards of a system with single point failure. When criticality analysis is added with FMEA it is known as failure mode effect and criticality analysis (FMECA). It was used as early as 1950 in reliability engineering. FMEA/FMECA is mainly used for manufacturing, product development, etc. 

It is important to remember that a deviation at the ammonia fill station could have consequences in other parts of the plant. The most obvious is that insufficient product is available to other parts of the plant. If the use of ammonia is critical to the process, then the ammonia fill station could be a single-point failure in the process. If the fill station stops operating, it could bring the entire plant to a halt. Section 8.1 (Failure Modes, Effects, and Criticality Analysis) will discuss a lot more about single-point failures and how they impact safety. 

Assure that single point failures do not result in catastrophic failures Increase use of self-analysis Documentation... 

FMEA is an inductive process that examines the effect of a single point failure on the overall performance of a system through a bottom-up approach (Andrews and Moss (2(X)2)). This analysis should be performed iteratively in all stages of design and operation of a system. 

Within aviation, analyzing the exact causes of accidents and incidents is a nontrivial task. Even if detailed flight data from the black box are available, it is usually still difficult to come up with a clear analysis, for the simple reason that the causes of incidents cannot be attributed to a single point of failure of one individual entity. Instead, most incidents in aviation are found to be caused by a complex interplay of processes at various levels of the socio-technical system, involving pilots, air traffic controllers, technical systems, and their interaction. For example the famous accident in 2009 of Air France Flight 447 is stiU under investigation and seems to have been the consequence of a rare combination of factors. On May 31, 2009, this flight disappeared... 

Failure Modes and Effects Analysis systems safety technique that analyzes systems individual components for the purpose of identifying single point hazards. 

In the application of the single failure criterion, any failure which could occur as a consequence of the PIE should be identified and included in the starting point for the single failure analysis. 

Finally, when allocating risk reduction, it is important to remember that one operator equals one response. Multiple alarms generally do not yield higher performance because the operator is the single point of failure for the necessary response. If the team has allocated risk reduction to an operator action in the BPCS layer, additional risk reduction should not be taken for an operator action allocated to the SIS layer for the same hazard scenario unless a detailed analysis is performed. When examining the overall risk reduotion that can be provided by the alarms, it is important to recognize the potential for common-mode failure due to operator or procedural error. 

Silva, M.B., Skjoedt, M., Atkins, A.G., Bay, N. and Martins, RA.F. (2008b) Single point incremental forming formability/failure diagrams. Journal of Strain Analysis for Engineering Design, 43 15-36. 

NOTE 3 If the above estimations are considered too conservative, then a detailed analysis of the failure modes of the hardware element can classify each failure mode into one of the fault classes (single-point faults, residual faults, latent, detected or perceived multiple-point faults or safe faults) wifli respect to the specified safety goal and determine the failure rates apportioned to the failure modes. Annex B describes a flow diagram that can be used to make the fault classification. 

Fault-tree analysis (FTA) focuses on the identification of multiple point failures by using a deductive top-down method to analyze effects of initiating faults and events occurring in complex systems. FTA works very well, showing how complex systems can overcome single or... 

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