Event trees, hazard analysis

Design Process design checks Unit processes Unit operations Plant equipments Pressure systems Instrument systems Hazard and operability studies (fine scale) Failure modes and effects analysis Fault trees and event trees Hazard analysis Reliability assessments... 

A starting point of the Fault-Event Tree (FET) analysis is always the Hazard (Goble, 1998), (Hessami, 1999a), (RSSB, 2007). 

Methods for performing hazard analysis and risk assessment include safety review, checkhsts, Dow Fire and Explosion Index, what-if analysis, hazard and operabihty analysis (HAZOP), failure modes and effects analysis (FMEA), fault tree analysis, and event tree analysis. Other methods are also available, but those given are used most often. 

Recognized systematic approaches include hazard operability study (HAZOP) event tree analysis fault tree analysis. 

A hazard tree is constructed by first identifying potential hazards. Starting with the hazard itself, it is possible to determine the conditions necessary for this hazard to exist. For these conditions to exist, a source that creates that condition must exist and so forth. Using this reasoning, a hierarchy of events can be drawn, which becomes the hazard tree. In a hazard analysis an attempt is made, starting at the lowest level in the tree, to see if it is possible to break the chain leading to the hazard by elimi-... 

Tliis cliapter is concerned willi special probability distributions and tecliniques used in calculations of reliability and risk. Tlieorems and basic concepts of probability presented in Cliapter 19 are applied to llie determination of llie reliability of complex systems in terms of tlie reliabilities of their components. Tlie relationship between reliability and failure rate is explored in detail. Special probability distributions for failure time are discussed. Tlie chapter concludes with a consideration of fault tree analysis and event tree analysis, two special teclmiques lliat figure prominently in hazard analysis and llie evaluation of risk. 

In Section 20.2, equations for tlie reliability of series and parallel systems are established. Various reliability relations are developed in Section 20.3. Sections 20.4 and 20.5 introduce several probability distribution models lliat are extensively used in reliability calculations in hazard and risk analysis. Section 20.6 deals witli tlie Monte Carlo teclinique of mimicking observations on a random variable. Sections 20.7 and 20.8 are devoted to fault tree and event tree analyses, respectively. 

A fault tree is a grapliic teclmique used to analyze complex systems. The objective is to spotlight conditions tliat cause a system to fail. Fault tree analysis attempts to describe how and why an accident or otlier undesirable event lias occurred. It may also be used to describe how and why an accident or otlier undesirable event could take place. Thus fault tree analysis finds wide application in hazard analysis and risk assessment of process and plant systems. ... 

Tliis particular case study does illustrate how event tree and fault tree analysis can be used in a hazard operability (HAZOP) study. 

Three hazard evaluation procedures using logic diagrams are (1) fault-tree analysis (FTA), (2) event-tree analysis (ETA), and (3) cause-consequence analysis (CCA). Appropriate references are [2,3,251,261]. 

Process hazard analysis (PHA) Any of a number of techniques for understanding and managing the risk of a chemical process or plant. Examples of PHA techniques include HAZOP, checklists, what-if methods, fault tree analysis, event tree analysis, and others. 

The facility is subjected to a process hazard analysis commensurate to the level of hazard the facility represents (i.e., Checklist, PHA, HAZOP, What-If review, Event Tree, FMEA, etc.). The results of these analyses are fully understood and acknowledged by facility management. Where high risk events are identified, quantifiable risk estimation and effects of mitigation measures should be evaluated and applied if productive. 

The outputs of an event tree from a post-release frequency analysis are a number of outcomes ranging from more to less hazardous. An event tree highlights failure routes for which no protective system can intervene and where additional protective systems/mitigative action may be contemplated. The quantitative output is the frequency of each event outcome. These outcomes (which might specify BLEVE, flash fire, pool fire, jet fire) are used to determine individual and societal risk. 

Other examples of inductive tools that have limited application in incident investigation include failure mode and effects analysis (FMEA), hazard and operability study (HAZOP), and event tree analysis (ETA). These are detailed in the CCPS book, Guidelines for Hazard Evaluation Procedures... 

HAZOP and What-If reviews are two of the most common petrochemical industry qualitative methods used to conduct process hazard analyses. Up to 80% of a company s process hazard analyses may consist of HAZOP and What-If reviews with the remainder 20% from Checklist, Fault Tree Analysis, Event Tree, Failure Mode and Effects Analysis, etc. An experienced review team can use the analysis to generate possible deviations from design, construction, modification, and operating intent that define potential consequences. These consequences can then be prevented or mitigated by the application of the appropriate safeguards. 

Extrapolation of historical data to larger scale operations may overlook hazards introduced by scale up to larger equipment Limitation of fault tree tlieory requires system simplification Incompleteness in fault and event tree analysis Uncertainties in data -... 

Thus, the use of event trees is sometimes limiting for hazard analysis because it usually does not quantify the potential of the event occurring. It may also be incomplete if all the initial occurrences are not identified. Its use is beneficial in examining, rather than evaluating, the possibilities and consequences of a failure. For this reason, a fault tree analysis should supplement this model to establish the probabilities of the event tree branches. This topic is introduced in the next problem. 

After the serious hazards have been identified with a HAZOP study or some other type of qualitative approach, a quantitative examination should be performed. Hazard quantification or hazard analysis (HAZAN) involves the estimation of the expected frequencies or probabilities of events with adverse or potentially adverse consequences. It logically ties together historical occurrences, experience, and imagination. To analyze the sequence of events that lead to an accident or failure, event and fault trees are used to represent the possible failure sequences. 

A systems hazards analysis (SHA) is a systematic and comprehensive search for and evaluation of all significant failure modes of facility systems components that can be identified by an experienced team. The hazards assessment often includes failure modes and effects analysis, fault tree analysis, event tree analysis, and hazards and operability studies. Generally, the SHA does not include external factors (e.g., natural disasters) or an integrated assessment of systems interactions. However, the tools of SHA are valuable for examining the causes and the effects of chemical events. They provide the basis for the integrated analysis known as quantitative risk assessment. For an example SHA see the TOCDF Functional Analysis Workbook (U.S. Army, 1993-1995). 

There are various types of analyses that are used for a process hazard analysis (PHA) of the equipment design and test procedures, including the effects of human error. Qualitative methods include checklists, What-If, and Hazard and Operability (HAZOP) studies. Quantitative methods include Event Trees, Fault Trees, and Failure Modes and Effect Analysis (FMEA). All of these methods require rigorous documentation and implementation to ensure that all potential safety problems are identified and the associated recommendations are addressed. The review should also consider what personal protective equipment (PPE) is needed to protect workers from injuries. 

The methodology of PSA is in principle a combination of event tree and fault tree analyses. The target of the analysis decisively determines the selection and the definition of modes in which the technical system is at the end of an event sequence. These modes comprise both the safe ones resulting from successful safety measures and also those unwanted ones characterized by a certain release of hazardous materials. A PSA is extended to a PRA, if the consequences of the different system modes are determined and linked with the event frequencies of those modes. 

Three hazard analysis techniques are currently used widely Fault Tree Analysis, Event Tree Analysis, and HAZOP. Variants that combine aspects of these three techniques, such as Cause-Consequence Analysis (combining top-down fault trees and forward analysis Event Trees) and Bowtie Analysis (combining forward and backward chaining techniques) are also sometimes used. Safeware and other basic textbooks contain more information about these techniques for those unfamiliar with them. FMEA (Failure Modes and Effects Analysis) is sometimes used as a hazard analysis technique, but it is a bottom-up reliability analysis technique and has very limited applicability for safety analysis. 

Accident investigations, when the events and physical causes are not obvious, often make use of a hazard analysis technique, such as fault trees, to create scenarios to consider. STPA can be used for this purpose. Using control diagrams of the physical system, scenarios can be generated that could lead to the lack of enforcement... 

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