RESEARCH PAPERS FACULTY OF MATERIALS SCIENCE AND TECHNOLOGY IN TRNAVA SLOVAK UNIVERSITY OF TECHNOLOGY IN BRATISLAVA

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1 RESEARCH PAPERS FACULTY OF MATERIALS SCIENCE AND TECHNOLOGY IN TRNAVA SLOVAK UNIVERSITY OF TECHNOLOGY IN BRATISLAVA 2015 Volume 23, Number 36 THE FTA METHOD AND A POSSIBILITY OF ITS APPLICATION IN THE AREA OF ROAD FREIGHT TRANSPORT Adela POLIAKOVÁ Ing. Adela Polaková, PhD., Slovak Unversty of Technology Bratslava, Faculty of Materals Scence and Technology n Trnava, Insttute of Safety, Envronment and Qualty, Paulínska 16, Trnava, Slovak Republc, e-mal: adela.polakova@stuba.sk Abstract The Fault Tree process utlzes logc dagrams to portray and analyse potentally hazardous events. Three basc symbols (logc gates) are adequate for dagrammng any fault tree. However, addtonal recently developed symbols can be used to reduce the tme and effort requred for analyss. A fault tree s a graphcal representaton of the relatonshp between certan specfc events and the ultmate undesred event (2). Ths paper deals to method of Fault Tree Analyss basc descrpton and provdes a practcal vew on possblty of applcaton by qualty mprovement n road freght transport company. Key words Fault Tree Analyss, fault tree, qualty, transport servce, qualty mprovement, logc gate INTRODUCTION Fault Tree Analyss FTA s based on relablty of dffcult systems and comes from specfed key problem dangerous pont, the process that s dvded nto the partal problems, events or prmary acts. The fundamental concept n fault-tree analyss s the translaton of a physcal system nto a structured logc dagram (fault tree), n whch certan specfed causes lead to one specfed TOP event of nterest (4). By usng the possblty theory and statstc survey results, t s possble to apply ths method also n the area of road freght transport. Fault tree progresses systematcally from symptoms of problem to ther reasons and provdes summary model of the faults reasons n dfferent levels. 151

2 The two basc unts nvolved are the AND and OR gates. Another, less often used, element s the NOT gate. TOP events are taken from a prelmnary hazard analyss, however nformal t was; these events are usually strongly undesred system states that can occur as a result of sub-system functonal faults. A fault-tree analyss conssts of the followng four steps: 1. System defnton 2. Fault-tree constructon 3. Qualtatve evaluaton 4. Quanttatve evaluaton (4). Applcaton of the method leads to the system relablty ncrease, because t makes possble to study the faults reasons orgn and, on the bass of the smple prmary events appearance possblty, t defnes the possblty of the analysed key problem appearance. On the bass of ths method results, t s possble to decrease the dangerous act appearance. There s also an factual advantage by usng ths method that deductons can be made also wthout the quanttatve event possblty nterpretaton, what s an unreplaceable value n case of smaller companes performng n the area of road freght transport, where there s always a lack of the qualfed personnel that understands the mathematcal statstcs and possblty area. METHODOLOGY OF APPLICATION FTA s one of the classcal methods for dentfcaton of danger that s classfed nto deducton methods. It s specally used n the faults combnaton determnaton that could lead to fault and errors. There s a lot of versons of ths method wth the common symbols for descrbng the damage reason. Fault-tree analyss begns wth the statement of an undesred event, eg. a faled state of a system. To perform a meanngful analyss, the followng three basc types of system nformaton are usually needed: 1. Component operatng and falure modes: A descrpton of how the output states of each component are nfluenced by the nput states and nternal operatonal modes of that component. 2. System chart: A descrpton of how the components are nterconnected. A functonal layout dagram of the system must show all functonal nterconnectons and dentfy each component. 3. System boundary condtons: These defne the stuaton for whch the fault tree s to be drawn. Top event, ntal condtons exstng or not-allowed events, and the tree top are system boundary condtons (4). The method can be used for qualtatve also quanttatve analyses. It allows relatvely easy fndng the system s weak ponts and uncovers the mportant aspects from the relablty pont of vew. It s a relable good regenerated process useable n the area of projectng and the technologcal processes operaton. It s very mportant to use ths method to choose the top event properly and to comple the fault tree responsbly. Process of ths method can be dvded nto four or fve phases. In the frst phase, the top event s chosen and the possble reasons are generally dentfed, for example by the morphologcal fndng or the data obtaned from the earler chroncled damages studes. 152

3 Alternatvely, t s possble to use the Hazard Tree Analyss, f compled standard fault tree should serve for specfed usage. The Fault Tree Analyss and The Hazard Tree Analyss are correlated together, so the dangerous aspects could be defned n specal ponts of operaton process or operaton nsttuton. The next step s an analyss of dfferent chan faults n operaton (n process or n machnery) that lead nto the top event. By elmnatng the ndvdual component fault, reasons should be defned. By usng the fault tree analyses, we create an operatonal and systematc vsual prevew for dentfcaton of the way n whch ndvdual basc elements lead nto system faults at the frst vew. The basc tool of the FTA method s the known fault events tree that ntroduces the graphcal vew of relaton n ndvdual partal events (partal faults) and the fnal undesrable event. Stable graphcal symbolsm n constructon of ths tree s used. non-elementary event basc elementary event & AND gate 1 OR gate Fg. 1 Basc graphcal symbolsm used n the Fault Tree constructon The most often used graphcal symbolsm s dsplayed n Fg. 1. ACHIEVED RESULTS Fault-tree constructon s generally a complcated and tme-consumng task. Computeraded synthess has attracted consderable attenton and several methodologes have been proposed. They dffer n the modellng of components and n ther objectves (4). The fault trees can be created horzontally or vertcally. In case of vertcal constructon, the top event s dsplayed on the top and the elementary events on the bottom; n case of horzontal constructon, the top event can be on the left or on the rght of the page sde. Relatons between ndvdual elementary events and top event are expressed by the gate junctor dsplayed n Fgure 1. The top event s ndcated as TOP event that s the man undesrable fault event. 153

4 Quanttatve fault-tree analyss conssts of determnng the mnmal cut sets and mnmal path sets and the common cause falures. Two major approaches used for determnng mnmal cast sets for fault trees are Monte Carlo smulaton and determnstc methods (4). In relaton wth ths event, we can use OR GATE for some nput case. OR specfes that the output event can occur n case of any nput case appears. It s a logc summary. Resultng possblty of event appearance at the output from OR gate can be expressed by the formula: P 1- (1- P(C )).(1- P(C )).(1- P(C ))....(1- P(C )), [1] "OR" = n where: P OR probablty of the event n output from OR gate, C events nputtng nto OR gate, P(C) probablty of events nputtng nto OR gate appearance. From the lsted formula, we can deduce that, n case of OR gate, the probablty of outputtng event s hgher or mnmally the same as the most possble event n nput, so: P max P(C ). [2] " OR" = It s obvous from the lsted formula that the OR gate presence n a fault events tree s unfavourable from the pont of vew of possblty of undesred event appearance. It s possble to use also the AND GATE that ndcates that the output event occurs only when all nputtng cases appear. It s a logc collaton of possbltes of ndvdual ndependent events appearance.. Resultng possblty of event appearance at the output from It s possble to formulate the resultng probablty of appearance of the event outputtng from AND GATE can be expressed by the formula: P P(B ).P(B ).P(B ))....P(B ), [3] "AND" = n where: P A probablty of outputtng event from AND GATE appearance, B events nputtng nto AND GATE, P(B) probabltes of events nputtng nto AND GATE appearance. Consderng ndvdual probabltes are defned by value at nterval 0,1, we can deduce from lsted relaton that, n case of AND GATE, the probablty of the outputtng event s lower than or equal to the probablty of the least probable event on nput. Therefore: P mn P(B ). [4] " AND" = 154

5 It s known from ths relaton that AND GATE appearance n the fault events tree s very favourable from the pont of vew of probablty of undesrable event appearance. The hgher lsted formulas of probabltes of event branchng nto partal events va AND or OR GATES are used for the possblty guess of dangerous event appearance and also for sutable actons to decrease the probablty. In the area of road freght transport, t s possble to defne the probablty of poor qualty va ths method applcaton. It s possble to defne also the events when we can consder provded transport servce to be of poor qualty. In my opnon, n smaller transport companes, t s not sutable to deal wth probabltes calculaton. Ths part of FTA method scorng requres basc knowledge of probablty theory. I can recommend endng the FTA analyses wth Fault Tree creaton and acceptance of actons for decreasng the undesrable events appearance. We can apply ths method n a road freght transport company for example n defnng customer dssatsfacton and the reasons that lkely cause ths dssatsfacton. We can show a process n the followng case study. Table 1 lsts ndvdual dscovered dsagreements wth dscovered number and chosen dentfcaton. The table lsts the results of clams observaton n practce of the transport company provdng servce n road freght transport. Table 2 shows relatvely calculated possbltes of undesrable event appearance. INDIVIDUAL DISAGREEMENTS, IDENTIFICATION AND TOTAL TRANSPORTATION IN 2013 Table 1 Indvdual dsagreement Identfcaton Number Delvery term breach B1 36 Fault shpment B2 24 Road traffc accdent B3 6 Transport reservatons B4 15 Unsutable drvers behavour B5 7 Total number of dsagreement 88 Total transportaton BT 512 By usng FTA method, we can determne the ndvdual possbltes for ndvdual dsagreement faults of process of road freght transport servce by usng the formula: where: P(B) possblty of falure event appearance, B ndvdual dsagreement number of appearance, BT total transportaton n followed tme perod. B B P(B 1 ), [5] T 155

6 We can calculate the possbltes as follows: P(B1) = 36/512 = P(B2) = 24/512 = P(B3) = 6/512 = P(B4) = 15/512 = P(B5) = 7/512 = The results of calculaton are lsted n Table 2: POSSIBILITIES OF TRANSPORTATION DISAGREEMENTS Table 2 Dsagreement B Possblty P(B) Delvery term breach B Fault shpment B Road traffc accdent B Transport reservatons B Unsutable drvers behavour B The undesred event Customer s dssatsfacton possblty P(B6) can be defned va formula: P(B6) = 1 (1 P(B1))* (1 P(B2))* (1 P(B3))* (1 P(B4))* (1 P(B5)) P(B6) = 1 ( )* ( )* ( )* ( )* ( ) P(B6) = 1 ( )* ( )* ( )* ( )* ( ) P(B6) = *0.9531*0.9883*0.9707* P(B6) = P(B6) = DISCUSSION The possblty of customer s dssatsfacton n ths case s Therefore, t s necessary that the company n next perod concentrates on the undesrable dsagreements n transportaton decrease. A fault tree s constructed by properly relatng all possble sequences of events that, upon occurrence, result n the undesred event. Begnnng wth the most undesred (top) event, the fault tree graphcally depcts the paths that lead to each succeedng lower level of the dsplay. Ths does not mply that each descendng fault path has a "hgher probablty of occurrence"; n fact, n many nstances, the opposte may be the case. However, a seres of "lttle thngs," each wth a relatvely low probablty of occurrence, may trgger an event at the next hgher level. Ths s depcted n the fault tree as a progresson of events through the logc gates. CONCLUSION To measure the level of safety of an operatonal product, the ntal step must be the defnton of the most undesred event,.e., the event that must be prevented from happenng. Defnton of the most undesred event s not always as smple as t mght appear from a 156

7 superfcal vew of any system, be t a power lawnmower or a supersonc arcraft. Injury to the operator may appear to be the most undesred event to the lawnmower manufacturer. Loss of lfe and destructon of the arplane could well be selected as the most undesred event for the supersonc plane desgner. However, both of these obvous selectons may be napproprate. The safety of any system must be measured for a specfc tme and type of actvty. For ths reason, the system safety engneer must understand the system and ts ntended use. One objectve of the analyst s to determne how the system, ncludng the people nvolved wth system operaton and mantenance, could fal and cause the undesred event (4). An appearance of undesrable event Customer s dssatsfacton can be expanded by Fault Tree that s presented n Fg. 2. Customer s dssatsfacton 1 Delvery term breach Unsutable drvers behavour Transport reservatons Road Traffc Accdent Fault shpment Fg. 2 Undesred event Customer s dssatsfacton Fault Tree Of course, the fault tree can be developed by ths way, but I consder ths example a fully satsfactory to llustrate ts applcaton. By usng the applcaton of Fault Tree Analyses, we found that the most possble faults appearance s caused by the delvery term breach. The transport company should target on ths problem durng establshng the arrangements. Reference: 1. GROSH, D. L., LEE, W. S., LIE, C. H., TILLMAN, F. A Fault Tree Analyss, Methods, and Applcatons A Revew. IEEE Transactons on relablty, R-34(3) HIXENBAUGH, A. F Fault Tree for Safety. Seattle. WA: The Boeng Company. D Retreved PLURA, J Plánování a neustálé zlepšování jakost. 1. vydane. (Plannng and contnuous mprovement of qualty. 1 st edton). Prague: Computer Press. ISBN Avalable on the Internet: 5. Avalable on the Internet: 157

8 6. Avalable on the Internet: 7. Avalable on the Internet: 8. Avalable on the Internet: Revewers: doc. Ing. Vladmír Konečný, PhD. Ing. Ivan Koblen, CSc. 158