15 7 方法对城市燃气管道风险管理An Approach to Risk Management of City Gas Pipeline

AN APPROACH TO RISK MANAGEMENT OFCITY GAS PIPELINEK.-S.PARK*,J.-H.LEE and Y.-D.JOInstitute of Gas Safety R&D,Korea Gas Safety Corporation,Gyeonggi-do,Republic of Korea A computer program has been developed with which we can assess and manage riskfor city gas pipeline in Korea.Risk factor of a facility was expressed in terms of con-sequence and frequency,and then the risk was correlated to cost.The consequence of incidents was analyzed in the case of small-scale leak,large-scale release and rupture of gas pipeline,and then correlated to cost.The consequences were evaluated in terms of fatalities, burning,damage to building and gas release.In each case,frequency was estimated in terms of major causes of consequences:excavating by third-party work,corrosion,welding defects and ground movement.Keywords:risk of city gas facility;consequence;frequency;risk correlated to cost.INTRODUCTIONAs the cities expanded and people became aware of clean energy,the consumption of gas in cities increased dra-matically from the1980s in Korea.In order to meet the increased demand,the gas pipeline network carried larger quantities of gas at higher pressures,which implies that city gas facilities,including the gas pipelines,have to be operated more carefully than ever.Operation conditions such as pressure and temperature change little for the city gas pipeline,while the design con-ditions and environmental conditions under which the pipe-lines are buried greatly influence the safety of the pipeline. Until now,a semi-quantitative approach has been applied to assess pipeline risks,depending on the engineer’s empi-rical decision,or the so-called index-based assessment (IBA).The scoring pipeline checklist(SPC)is one of the IBA methods and has been applied to most of city gas pipe-lines in Korea.The Office of Pipeline Safety(OPS)in the USA developed the‘Risk Management Program Standard’in1997and recommend that pipeline companies should use this standard(Courtney et al.,1977).The standard defines the procedure of hazard identification,risk management and risk and environment monitoring according to the prop-erties of the pipeline,and12companies agreed to assess risk by applying the standard.In this work,the risk for the city gas company facility is expressed in terms of loss.Possible incidents of gas release are elucidated by event tree analysis(ETA).The conse-quences of gas release are estimated in terms of material loss andfire damage.The frequency of incidents is esti-mated in terms of major causes of consequence:excavating by third-party work,corrosion,welding defects and ground movement.CONSEQUENCE ANALYSISRisk-based inspection of API581suggested four kinds of hole size,1/4inch(6.4mm),1inch(25.4mm)and 4inch(101.6mm),and rupture(API,2000).In this work, Fearnehough’s suggestion was selected and is listed in Table1,considering easier accessibility to historical data (Jones and Fearnehough,1986).Then consequences and frequency of the suggested scenario were estimated.ETA was conducted to define events developed with time,as shown in Figure1(Stephens,2000).Without ignition,only material loss is considered.On the other hand,if there is an ignition source,a vertical jetflame is assumed,implying consequences of fatality,injury and building damage.In each case,thefinancial loss was esti-mated in terms of material loss by gas release,supply inter-ruption,facility recovery(maintenance),injury,fatalities, and building damage.Figure1illustrates events of city gas incidents with rela-tive probabilities provided by the European Gas Pipeline Incident Data Group(EGIG)(EGIG,1993).Since gas is lighter than air,the released gas seldom forms clouds to cause aflashfire or unconfined vapor cloud explosion (UVCE).Gas Release Rate AnalysisIn applying release model,steady state is assumed,con-sidering progressed friction resistance offluid.The release*Correspondence to:Dr K.-S.Park,Institute of Gas Safety R&D,Korea Gas Safety Corporation,332-1,Daeya-dong,Shiheung-shi,Gyeonggi-do, 429-712,Republic of Korea.E-mail:kspark@kgs.or.kr4460957–5820/04/$30.00+0.00#2004Institution of Chemical EngineersTrans IChemE,Part B,November2004 Process Safety and Environmental Protection,82(B6):446–452rate of gas is dependent on gas pressure and friction resistance,and can be expressed by equation (1),assuming isentropic expansion (Jo,2001):Q p ¼p 4ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiÀr 0P 0d 52f F L Ág þ1P eP 0 (g þ1)=g À1"#v u u t (1)wheref F ¼14½1:14À2log (1=d ) 2(2)Since most city gas pipelines operate at pressures lower than 1kg cm 22,subsonic gas release could be assumed and the rate is given by equation (3):Q h ¼C D A C ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigr e P e2g þ1 (g þ1)=(g À1)s (3)whereC ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2À1 g þ12 (g þ1)=(g À1)P a P e 2=g Â1ÀP a P e(g À1)=g "#v u u uu u u u t (4)If the gas released through pinhole,C D is taken 0.85for subsonic release (API,2000).Fire AnalysisThe fire of gas released from pipeline can be defined as jet flame.In this work,the jet flame is treated as a point source radiator.Although the radiated heat is also a func-tion of water,carbon dioxide and soot (Lees,1996),the thermal load was estimated by the method of Hymes (1983)for simple calculation.The heat intensity from heat source is expressed by equation (5):I ¼h X g Q h H c 4p r 2(5)The thermal load is expressed by:L p ¼tI n(6)The fatality by thermal load is listed in Table 2,as has been suggested by Hymes (1983).The effect of thermal load to building is expressed by the following equation and listed in Table 3with empirical results (Bilo and Kinsman,1997):L b ¼(I ÀI x )t n(7)The 20min is referred to as the minimum time required for emergency measures,including time needed to shut down the block valve or put out the fire.Figure 2shows a typical model to evaluate the consequences to buildings.Most of the city gas pipelines in Korea pass along roads or walkways,keeping a distance of at least 1.5m from build-ings.The consequence area is expressed by:A conseq ¼r 2cosÀ1x rÀx ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi(r 2Àx 2)p (8)where r represents the consequence radius within which heat intensity effects are 15.7–26.2kW m 22.Table 1.Size of hole at gas pipeline according to the scenario suggested by Jones and Fearnegough (1986).Hole sizeAverage size Standard deviationSmall-scale leak 0–20mm 10mm 5.77mm Large-scale release 20–80mm 50mm 17.3mmRuptureDiameter of pipeDiameter of pipe—Table 2.Fatalities by thermal radiation from fire source.Burning 1%fatality 50%fatality 100%fatality Thermal load,(kW m 22)s 210–700106023003500Thermal intensity,kW m 224.3–10.714.626.135.8Figure 1.The ETA of city gas incidents.Table 3.The thermal intensity that causes buildings to catch fire.Pilot ignitionImmediate ignition Thermal load (I 214.7)*t 0.667¼118.6(I 225.6)*t 0.8¼167.6Thermal intensity in 20min ignition15.7kW m 2226.2kW m 22Trans IChemE,Part B,Process Safety and Environmental Protection ,2004,82(B6):446–452RISK MANAGEMENT OF CITY GAS PIPELINE447FREQUENCY ANALYSISAccording to the Department of Transportation (DOT)of the USA,the major causes of gas release from pipeline are excavation by third parties,corrosion and ground move-ment (Courtney et al.,1977).The most frequent incident of gas pipeline is excavation by third parties.Since the probability of pipeline damage by corrosion is time-dependent,its probability is estimated by manipulating data obtained by the direct current voltage gradient (DCVG)method.In case that inspection data are not suffi-cient,historical failure data analysis (HFDA)is applied for simple evaluation.The probability of damage by ground movement that may affect welds was analyzed by HFDA.Damage by Excavation by a Third PartyThe basic events were obtained by fault tree analysis (FTA)and are shown in Figure 3.The top event is mecha-nical damage of gas pipeline.This may occur when there is excavation around a pipeline (B 1)and protecting measures fail to work (E 2or E 3).According to Meadows and Sage (1985),the frequency of excavation in an urban area is 0.2/km .year.Seoul is a densely populated area and the fre-quency of B 1was modified based on the data from the city gas company as about 0.4days /km .year.The protecting measures may fail to work (E 2)when excavation depth exceeds the buried depth of the pipeline (B 2),and a protecting plate is not installed or fails to work (B 3).The event B 2refers to the probability of exca-vation damage to the pipeline and depends on the buried depth of the gas pipeline,as shown by equation (9)in Kief-ner et al.(1990).P ¼0:71d2(9)Corder’s (1995)study shows that the protecting plate is up to 80%effective in protecting the pipeline.Thus the prob-ability of pipeline damage is 1without the protecting plate and 0.2with the plate (Leeds,2000).Additionally,the protecting measures fail to work (E 3)in the case of illegal excavation (B 4)or failure to recognize the correct position of the pipeline (E 4).If the third-party worker is aware of the position of the pipeline,according to the City Gas Law in Korea,he /she must dig manually to protect the pipeline.Illegal excavation is where the worker neglects the guideline and digs with a mechanical excavator.The probability of pipeline damage is 0.2with position marking only,and 0.01in the presence of a pipe-line manager.The pipeline manager is a person who is in charge of safety of a certain section of pipeline and patrols it periodically,once or twice a day.The city gas pipeline must be covered by yellow or red resin tape to warn of its position,depending on the gas pressure in Korea.Additionally,pipeline operated at more than 4kg cm 22pressure must be covered by steel plate to protect it.Third-party workers fail to recognize proper position of pipeline when the warning tape is not in the right position or missing (B 5),and the position of the pipeline is not clear (E 5).The warning tape is known to be up to two-thirds as effective as the protecting plate (Corder,1995).Assuming that the rate of missing is 10%,the probability that the tape will fail to be noticed by the worker is assumed to be 0.4(¼120.9Â2/3).Figure 2.A typical illustration to estimate consequences of fire for nearby buildings.Damage to (a)humans and (b)buildings.Figure 3.The fault tree analysis of city gas incidents.Trans IChemE,Part B,Process Safety and Environmental Protection ,2004,82(B6):446–452448PARK et al.Event E 5is defined as unclear position of buried pipeline.This event occurs when the pipeline manager does not know the correct alignment of the pipeline (B 6)or when the patrol does not recognize the excavation (E 7).This case implies the following:the pipeline position is different from the drawing,leading to misreading,or changes have not updated due to inappropriate management of drawings.This event becomes negligible for a city gas company where the city gas company manages pipeline drawings very well.Event E 6occurs when the patrol fails to see the exca-vation (E 7)and the third-party worker has not reviewed the pipeline position prior to excavation work (E 8).Event E 7occurs when the pipeline manager does not patrol during excavation (B 8)or the pipeline manager fails to find out about the excavation (B 7).Event B 8is dependent on the patrol period,i.e.,frequent patrols may discover more illegal excavations.Event B 9represents the possi-bility that the patrol fails to discover illegal excavation owing to carelessness,or human error while patrolling.The possibility is assumed to be 0.01with patrol by a car.Event E 8occurs when the third-party worker fails to notify the excavation to the patrol (B 9),or does not commu-nicate with the pipeline manager prior to work beginning (E 9).This event is referred to as system error and can be negligible.Event E 9occurs when the worker has not communicated with the patrol prior to moving to the excavation site (B 11).This event occurs where the excavation worker does not know that he /she must communicate with the city gas company about the position of buried pipelines.According to the DOT statistics,about 54%of communications prior to excavation were made (Courtney et al.,1977),while this figure is 20%in Korea (Korea Gas Safety Corp.,2001).Event E 10occurs when the worker fails to find the indi-cation line-marker (B 12)or neglects it (B 13).If a worker has not communicated prior to the excavation,he /she can inquire to the city gas company.The line-mark is installed every 500m in Korea and the probability of the worker failing to recognize it is 0.4.If the line-marker is recog-nized,the probability of the workers neglecting it is 0.64(Korea Gas Safety Corp.,2001).Corrosion AnalysisThe frequency of coating defects was obtained from inspection data of the DCVG at inspected sections.Thefrequency of uninspected part is calculated by the Baysian method.Not all coating defects found by applying DCVG are susceptible repair considering the costs.Recent work shows that,with good protection,more than 99%of coating faults do not induce metal corrosion.Digging points for rehabilitation were selected by corrosion factor,including DC potential,soil resistance,pH of soil and other factors (Leeds,2000).Corrosion rate can be estimated by the size of the corrosion defect and the burial period,by assuming constant corrosion rate.As has been studied,the corrosion rate has Weibull distribution (Sheikh et al.,1990).The probability of a small-scale leak is estimated by distribution of defect depths,as shown in Figure 4.The probability is expressed as the shaded area,which is referred to the probability of larger depth of corrosion than the width of the pipeline.The distribution of defect depth is a function of time.The probability of a large-scale leak can be estimated by distribution of remaining strength of the defective part.The distribution function can be obtained by estimated distribution of corrosion defect depth,applying the ASME B31G code,and is illustrated in Figure 5.The distribution of defect depth shifts leftwards with time,as shown in the figure.The probability of a defect which causes a large-scale leak is expressed by the shaded area in Figure 5,assuming constant operating pressure (Kiefner and Vieth,1989).Welding Defects and Ground Movement Analysis The probability of damage to buried pipeline by ground movement could be expressed by equation (10)according to the historical approach:R i ¼R b i M i A i(10)where i represents the failure mode such as small-scale leak,large-scale release or rupture of the pipeline.Table 4lists city gas pipeline accidents resulting from groundTable 4.City gas pipeline accidents resulting from ground movement.Year Total Pipeline defects Welding defects Decomposition of welds1995512319962——219973—12Figure 4.The distribution curve of corrosion defectdepth.Figure 5.The distribution curve of remaining strength of defective part.Trans IChemE,Part B,Process Safety and Environmental Protection ,2004,82(B6):446–452RISK MANAGEMENT OF CITY GAS PIPELINE449movement.The table shows that about three accidents take place every year in Korea,which is equivalent to 3.6Â1025km21year21.Hovey and Framer(1993) reported it to be5.5Â1024km21year21.Ground move-ment affects underground pipeline to give large-scale release or rupture rather than a small-scale leak.The Euro-pean Gas Pipeline Incident Data Group(EGIG)reported the relative failure mode as10–20%for small-scale leaks,35–45%for large-scale releases,and35–45%for ruptures,based on incident data between1970and1998 (EGIG,1993).In this work the failure mode factor,M i,is taken as20%for small-scale leaks,45%for large-scale releases and35%for rupture.The compensation factor is expressed in equation(11)as a function of pipeline shape,welding property and buried depth:A F¼K M(F shapeþF weld)F depth(11) According to the Thomas model,pipeline shape factor, F shape is proportional to the length and diameter of the pipe-line,and is reciprocally proportional to square of pipeline thickness:F shape¼Ldt2(12)The welding factor can be expressed as:F weld¼50Â1:75N C DtþN LL3:14t(13)Effect of ground movement on pipeline damage becomes larger near surface.The factor of buried depth is summa-rized in Table5.ESTIMATION OF LOSS Conventionally,the risk of a facility can be expressed by consequence and frequency and can be correlated in terms of cost as:Risk($=year)¼consequence($=event)Âfrequency(event=year)(14) Loss($=year)¼risk Total¼X1S¼1C SÂF S(15)The total risk is expressed by the summation of each risk estimated by each scenario.In this study,the total cost of incident is expressed in equation(16).The cost of repair, gas-supply interruption and material(gas)loss refers to the case of no ignition and the cost of damage to human and building refers to the case of ignition.The costs described are samples and further modification is recom-mended considering environmental factors:C total¼C repairþC interrþC materialþC humanþC const(16)Table5.Samples of cost of repair.Diameter(mm)Small-scaleleakLarge-scalereleasePipelinerupture ,10025003300500 100–300300037506600 .300330042008400 Unit:US$.Figure6.Cost estimation in terms of ground movement,excavation and corrosion.Trans IChemE,Part B,Process Safety and Environmental Protection,2004,82(B6):446–452 450PARK et al.The cost of pipeline repair depends on diameter of pipeline and condition of road,and summarized in Table5.Cost of gas-supply interruption depends on time of release and repair.The cost of lost gas depends on release rate and time.Release time refers to time to recognize release,to move to the release point, and to shut down the block valve.The recognition time of small-scale leak is assumed to be2h and that of large-scale release and pipeline rupture is assumed to be10min.The time for reacting and shutting down the pipeline is assumed to be30min and5min in each case.If a remote controlled valve is installed,the reaction time is assumed to be zero.The cost of human damage was estimated by the Hoffman method,considering age distribution.The cost of heavy,medium and light injury is assumed to be50, 10and1%of the cost of fatality.The cost of building damage is estimated by the summation of construction and interior repair costs.As has been described,risk can be expressed in terms of consequence and frequency.A software program,PRAS (Pipeline Risk Assessment System),has been developed by a collaboration of the Korea Gas Safety Corp and the Kukdong City Gas to easily analyzerisk.Figure7.Risk estimation in terms of cost with and without inspection.(a)Inspection is desirable considering total cost.(b)Inspection is not desirable considering total cost.Trans IChemE,Part B,Process Safety and Environmental Protection,2004,82(B6):446–452RISK MANAGEMENT OF CITY GAS PIPELINE451Figure6shows a sample of cost estimation in a certain section of pipeline.The risk for the pipeline is expressed in terms of cost.In this section,the cost of excavation by third parties is the highest and must be reduced to satisfy the criteria of the gas company.Figure7shows a cost comparison of inspected and unin-spected pipelines.Costs are estimated with varying the number of inspections by the DCVG,flame ionization detection and gas-tight tests.Without inspection,the risk or the cost increases linearly with time.On the other hand,inspection reduces risk remarkably.In Figure7(a), risk in terms of cost of uninspected pipeline is2Â107 Korean Won km21(equivalent to ca.17,000US$km21), while that of inspected pipeline is8Â106Won km21 (ca.6700US$km21).In Figure7(b),however,inspection does not reduce the total cost considering the cost of inspection.CONCLUSIONSIn this study,an approach has been made to express the risk for a gas pipeline in terms of cost.Total cost consists of cost of repair,supply interruption,material loss and damage to humans and buildings.The PRAS program can also express the loss of a city gas facility by accident in terms of cost,which helps the city gas company optimize its resources.Although the program gives a brief idea on how to correlate risk to cost,it has to be revised further to provide more reliable results such as considering view factor in calculating thermal load.NOMENCLATUREA opening area of ruptured pipelineA conseq consequence area onfireA i consequence area onfireC const reconstruction cost of an incidentC D release coefficientC human compensation cost of an incidentC interr interruption cost of an incidentC material material(gas)cost of an incidentC repair repair cost of an incidentC total total cost of an incidentd diameter of pipelined ref reference depthf F Fanning friction coefficientH c heat of combustion(’50:000kJ=kg for methane)I x heat intensity at infinite time of exposure tofirel length of corrosive defectL length of pipelineL p thermal load in equation(6)M Folius factorM i likelihood depending on failure mode,0.2for small-scale leak,0.45for large-scale release,and0.35for rupturen exponent[ 1.33for equation for equation(6)]Nc number of welding along axial directionN L number of welding along longitude directionP0operating pressureP a atmospheric pressureP e gas pressure at crack of pipelineP f remaining strengthQ release rate of gasQ h heat rariation from jetflamer distance from heat source for equation(5)R i probability of failure by ground movement R bibasic probability of failuret time of exposure tofire for equations(8)and(9)t thickness of pipeline for equation(12)x distance from center of jetflame in equation(8)X g radiation rate( 0.2)Greek Symbolsg specific heat capacity(’1:036for methane)1roughness of pipeline surfacer0gas densityh effectiveness factor(’0:35)d yield minimum yield strengthREFERENCESAPI,2000,Risk Based Inspection Base Resource Document,API Publi-cation581,1st edn(American Petroleum Institute).Bilo,M.and Kinsman,P.R.,1997,Thermal radiation criteria used in pipeline 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Sheikh,A.K.,Boah,J.K.and Hansen,D.A.,1990,Statistical modeling of pitting corrosion and pipeline reliability,Corrosion,46(3):190–196. Stephens,M.J.,2000,A model for sizing high consequence areas associated with natural gas pipelines,GRI Report no.GRI-00/ 0189.ACKNOWLEDGEMENTThe author gives thanks to the KISTEP(Korea Institute of Science and Technology for Evaluation and Planning)for support of this study with NRL(National Research Laboratory)Program.The manuscript was received13March2004and accepted for publi-cation after revision4October2004.Trans IChemE,Part B,Process Safety and Environmental Protection,2004,82(B6):446–452 452PARK et al.。