11FMECAworksheet:risk rankingTheriskrelatedtothevariousfailuremodesisoftenpresentedeitherby:Riskmatrix,orFrequencyccnseuenRiskprioritynumber(RPN)CatastrophicCrticalMajoMinoO=therankoftheoccurrenceofthefailuremodeS=the rank of the severity of thefailure modeD=therankofthe likelihoodthefailurewill bedetectedbeforethesystemreachestheenduser/customer.All ranksaregivenona scalefrom1to10.The riskprioritynumber (RPN)isdefinedas RPN =SxOXDNTNU-TrondheimNorwegianUniversityofScienceand Technologywww.ntnu.edu
11 FMECA worksheet: risk ranking The risk related to the various failure modes is often presented either by: • Risk matrix, or • Risk priority number (RPN) O = the rank of the occurrence of the failure mode S = the rank of the severity of the failure mode D = the rank of the likelihood the failure will be detected before the system reaches the end‐ user/customer. All ranks are given on a scale from 1 to 10. The risk priority number (RPN) is defined as RPN = S × O × D
12FMECAprosandconsPros:FMECA is a very structured and reliablemethod for evaluatinghardwareand systemsTheconceptandapplicationareeasytolearn,evenbyanoviceTheapproachmakesevaluatingevencomplexsystemseasytodoCons:TheFMEcAprocessmaybetedious,time-consuming(andexpensiveTheapproachisnotsuitableformultiplefailuresItistooeasytoforgethumanerrors intheanalysisNTNU-TrondheimNorwegian University ofScienceandTechnologywww.ntnu.edu
12 FMECA pros and cons Pros: • FMECA is a very structured and reliable method for evaluating hardware and systems • The concept and application are easy to learn, even by a novice • The approach makes evaluating even complex systems easy to do Cons: • The FMECA process may be tedious, time‐consuming (and expensive) • The approach is not suitable for multiple failures • It is too easy to forget human errors in the analysis
13Faulttreeanalysis(FTA)Fault treeanalysis (FTA) is a top-down approach to failureanalysis,startingwithapotential undesirableevent(accident)calledaTOPevent,andthendeterminingall theways itcanhappen.TheanalysisproceedsbydetermininghowtheTOPeventcanbecausedbyindividual orcombined lowerlevel failuresorevents.Thecausesof theTOPevent are"connected"through logicgatesInthiscourseweonlyconsiderAND-gatesandOR-gatesFTAisthemostcommonlyusedtechniqueforcausalanalysisinriskandreliabilitystudies.NTNU-TrondheimNorwegian University ofScienceandTechnologywww.ntnu.edu
13 Fault tree analysis (FTA) Fault tree analysis (FTA) is a top‐down approach to failure analysis, starting with a potential undesirable event (accident) called a TOP event, and then determining all the ways it can happen. The analysis proceeds by determining how the TOP event can be caused by individual or combined lower level failures or events. The causes of the TOP event are “connected” through logic gates In this course we only consider AND‐gates and OR‐gates FTA is the most commonly used technique for causal analysis in risk and reliability studies
14FTAmainsteps1.Definition ofthe system,theToPevent (thepotential accident),andtheboundaryconditionsConstructionofthefaulttreeldentificationoftheminimalcutsets3.QualitativeanalysisofthefaulttreeQuantitativeanalysisofthefaulttreeReportingofresults6.NTNU-Trondheim莎NorwegianUniversityofScienceand Technologywww.ntnu.edu
14 FTA main steps 1. Definition of the system, the TOP event (the potential accident), and the boundary conditions 2. Construction of the fault tree 3. Identification of the minimal cut sets 4. Qualitative analysis of the fault tree 5. Quantitative analysis of the fault tree 6. Reporting of results
15FTApreparationThe starting point of anFTA is often existing FMECAanda system block diagramTheFMECAisanessentialfirststep inunderstandingthesystemThedesign, operation,andenvironmentof thesystemmustbeevaluatedThecauseand effect relationships leadingto theTOoP event must beidentifiedandunderstoodNTNU-TrondheimNorwegian University of梦ScienceandTechnologywww.ntnu.edu
15 FTA preparation The starting point of an FTA is often existing FMECA and a system block diagram The FMECA is an essential first step in understanding the system The design, operation, and environment of the system must be evaluated The cause and effect relationships leading to the TOP event must be identified and understood