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Thescaleandcomplexityofcomputer-basedsafetycriticalsystems,likethoseusedinthetransportandmanufacturingindustries,posesignificantchallengesforfailureanalysis.

Overthelastdecade,researchhasfocusedonautomatingthistask.Inoneapproach,predictivemodelsofsystemfailureareconstructedfromthetopologyofthesystemandlocalcomponentfailuremodelsusingaprocessofcomposition.Analternativeapproachemploysmodel-checkingofstateautomatatostudytheeffectsoffailureandverifysystemsafetyproperties.Inthispaper,wediscussthesetwoapproachestofailureanalysis.WethenfocusonHierarchicallyPerformedHazardOrigin&

PropagationStudies（HiP-HOPS）–oneofthemoreadvancedcompositionalapproaches–anddiscussitscapabilitiesforautomaticsynthesisoffaulttrees,combinatorialFailureModesandEffectsAnalyses,andreliabilityversuscostoptimisationofsystemsviaapplicationofautomaticmodeltransformations.WesummarisethesecontributionsanddemonstratetheapplicationofHiP-HOPSonasimplifiedfueloilsystemforashipengine.Inlightofthisexample,wediscussstrengthsandlimitationsofthemethodinrelationtootherstate-of-the-arttechniques.Inparticular,becauseHiP-HOPSisdeductiveinnature,relatingsystemfailuresbacktotheircauses,itislesspronetocombinatorialexplosionandcanmorereadilybeiterated.Forthisreason,itenablesexhaustiveassessmentofcombinationsoffailuresanddesignoptimisationusingcomputationallyexpensivemeta-heuristics.

1.Introduction

Increasingcomplexityinthedesignofmodernengineeringsystemschallengestheapplicabilityofrule-baseddesignand

classicalsafetyandreliabilityanalysistechniques.Asnewtechnologiesintroducecomplexfailuremodes,classicalmanual

analysisofsystemsbecomesincreasinglydifficultanderrorprone.Toaddressthesedifficulties,wehavedevelopedacomputerisedtoolcalled‘HiP-HOPS’（HierarchicallyPerformedHazardOrigin&

PropagationStudies）thatsimplifiesaspectsoftheengineeringandanalysisprocess.ThecentralcapabilityofthistoolistheautomaticsynthesisofFaultTreesandFailureModesandEffectsAnalyses（FMEAs）byinterpretingreusablespecificationsofcomponentfailureinthecontextofasystemmodel.Theanalysisislargelyautomated,requiringonlytheinitialcomponentfailuredatatobeprovided,thereforereducingthemanualeffortrequiredtoexaminesafety;

atthesametime,theunderlyingalgorithmscanscaleuptoanalysecomplexsystemsrelativelyquickly,enablingtheanalysisofsystemsthatwouldotherwiserequirepartialorfragmentedmanualanalyses.Morerecently,wehaveextendedtheaboveconcepttosolveadesignoptimisationproblem:

reliabilityversuscostoptimisationviaselectionandreplicationofcomponentsandalternativesubsystemarchitectures.HiP-HOPSemploysgeneticalgorithmstoevolveinitialnon-optimaldesignsintonewdesignsthatbetterachievereliabilityrequirementswithminimalcost.Byselectingdifferentcomponentimplementationswithdifferentreliabilityandcostcharacteristics,orsubstitutingalternativesubsystemarchitectureswithmorerobustpatternsoffailurebehaviour,manysolutionsfromalargedesignspacecanbeexploredandevaluatedquickly.Ourhopeisthatthesecapabilities,usedinconjunctionwithcomputer-aideddesignandmodellingtools,allowHiP-HOPStofacilitatetheusefulintegrationofalargelyautomatedandsimplifiedformofsafetyandreliabilityanalysisinthecontextofanimproveddesignprocess.Thisinturnwill,wehope,addressthebroaderissueofhowtomakesafetyamorecontrolledfacetofthedesignsoastoenableearlydetectionofpotentialhazardsandtodirectthedesignofpreventativemeasures.Theutilisationoftheapproachandtoolshasbeenshowntobebeneficialincasestudiesonengineeringsystemsintheshipping[1]andoffshoreindustries[2].Thispaperoutlinesthesesafetyanalysisandreliabilityoptimisationtechnologiesandtheirapplicationinanadvancedandlargelyautomatedengineeringprocess.

2.Safetyanalysisandreliabilityoptimisation

3.SafetyanalysisusingHiP-HOPS

HiP-HOPSisacompositionalsafetyanalysistoolthattakesasetoflocalcomponentfailuredata,whichdescribeshowoutputfailuresofthosecomponentsaregeneratedfromcombinationsofinternalfailuremodesanddeviationsreceivedatthecomponents’inputs,andthensynthesisesfaulttreesthatreflectthepropagationoffailuresthroughoutthewholesystem.Fromthosefaulttrees,itcangeneratebothqualitativeandquantitativeresultsaswellasamultiplefailuremodeFMEA

[35].AHiP-HOPSstudyofasystemdesigntypicallyhasthreemainphases:

_Modellingphase:

systemmodelling&

failureannotation.

_Synthesisphase:

faulttreesynthesis.

_Analysisphase:

faulttreeanalysis&

FMEAsynthesis.

Althoughthefirstphaseremainsprimarilymanualinnature,theotherphasesarefullyautomated.Thegeneralprocessin

HiP-HOPSisillustratedinFig.2below:

Thefirstphase–systemmodelling&

failureannotation–consistsofdevelopingamodelofthesystem（includinghydraulic,electricalorelectronic,mechanicalsystems,aswellasconceptualblockanddataflowdiagrams）andthenannotatingthecomponentsinthatmodelwithfailuredata.ThisphaseiscarriedoutusinganexternalmodellingtoolorpackagecompatiblewithHiP-HOPS.HiP-HOPShasinterfacestoanumberofdifferentmodellingtools,includingMatlabSimulink,Eclipse-basedUMLtools,andparticularlySimulationX.Thelatterisanengineeringmodelling&

simulationtooldevelopedbyITIGmbH[36]withafullyintegratedinterfacetoHiP-HOPS.Thishastheadvantagethatexistingsystemmodels,oratleastmodelsthatwouldhavebeendevelopedanywayinthecourseofthedesignprocess,canalsobere-usedforsafetyanalysispurposesratherthanhavingtodevelopanewmodelspecifictosafety.Thesecondphaseisthefaulttreesynthesisprocess.Inthisphase,HiP-HOPSautomaticallytracesthepathsoffailurepropagationthroughthemodelbycombiningthelocalfailuredataforindividualcomponentsandsubsystems.Theresultisanetworkofinterconnectedfaulttreesdefiningtherelationshipsbetweenfailuresofsystemoutputsandtheirrootcausesinthefailuremodesofindividualcomponents.Itisadeductiveprocess,workingbackwardsfromthesystemoutputstodeterminewhichcomponentscausedthosefailuresandinwhatlogicalcombinations.ThefinalphaseinvolvestheanalysisofthosefaulttreesandthegenerationofanFMEA.Thefaulttreesarefirstminimisedtoobtaintheminimalcutsets–thesmallestpossiblecombinationsoffailurescapableofcausinganygivensystemfailure–andthesearethenusedasthebasisofbothquantitativeanalysis（todeterminetheprobabilityofasystemfailure）andtheFMEA,whichdirectlyrelatesindividualcomponentfailurestotheireffectsontherestofthesystem.TheFMEAtakestheformofatableindicatingwhichsystemfailuresarecausedbyeachcomponentfailure.ThevariousphasesofaHiP-HOPSsafetyanalysiswillnowbedescribedinmoredetail.

4.DesignoptimisationusingHiP-HOPS

HiP-HOPSanalysismayshowthatsafety,reliabilityandcostrequirementshavebeenmet,inwhichcasetheproposedsystemdesigncanberealised.Inpractice,though,thisanalysiswilloftenindicatethatcertainrequirementscannotbemetbythecurrentdesign,inwhichcasethedesignwillneedtoberevised.Thisisaproblemcommonlyencounteredinthedesignofreliableorsafetycriticalsystems.Designersofsuchsystemsusuallyhavetoachievecertainlevelsofsafetyandreliabilitywhileworkingwithincostconstraints.Designisacreativeexercisethatreliesonthetechnicalskillsofthedesignteamandalsoonexperienceandlessonslearntfromsuccessfulearlierprojects,andthusthebulkofdesignwork

iscreative.However,webelievethatfurtherautomationcanassisttheprocessofiteratingthedesignbyaidingintheselectionofalternativecomponentsorsubsystemarchitecturesaswellasinthereplicationofcomponentsinthemodel,allofwhichmayberequiredtoensurethatthesystemultimatelymeetsitssafetyandreliabilityrequirementswithminimalcost.Ahigherdegreeofreliabilityandsafetycanoftenbeachievedbyusingamorereliableandexpensivecomponent,an

alternativesubsystemdesign（e.g.Aprimary/standbyarchitecture）,orbyusingreplicatedcomponentsorsubsystemstoachieveredundancyandthereforeensurethatfunctionsarestillprovidedwhencomponentsorsubsystemsfail.Inatypical

systemdesign,however,therearemanyoptionsforsubstitutionandreplicationatdifferentplacesinthesystemanddifferent

levelsofthedesignhierarchy.Itmaybepossible,forexample,toachievethesamereliabilitybysubstitutingtwosensors

inoneplaceandthreeactuatorsinanother,orbyreplicatingasinglecontrollerorcontrolsubsystem,etc.Differentsolutionswill,however,leadtodifferentcosts,andthegoalisnotonlytomeetthesafetygoalsandcostconstraintsbutalsotodosooptimally,i.e.finddesignswithmaximumpossiblereliabilityfortheminimumpossiblecost.Becausetheoptionsforreplicationand/orsubstitutioninanon-trivialdesignaretypicallytoomanytoconsidermanually,itisvirtuallyimpossiblefordesignerstoaddressthisproblemsystematically;

asaresult,theymustrelyonintuition,oronevaluationofafewdifferentdesignoptions.Thismeansthatmanyotheroptions–someofwhicharepotentiallysuperior–areneglected.Automationofthisprocesscouldthereforebehighlyusefulinevaluatingalotmorepotentialdesignalternatives