Rsk analyss of natural gas ppelne Y.-D. Jo 1, K.-S. Park 1, J. W. Ko, & B. J. Ahn 3 1 Insttute of Gas Safety Technology, Korea Gas Safety Corporaton, South Korea Department of Chemcal Engneerng, Kwangwoon Unversty, South Korea 3 Graduate School of Knowledge Based Technology and Energy, Korea Polytechnc Unversty, South Korea Abstract Unlke other hazardous plant, the transmsson ppelnes carryng natural gas are not wthn a secure ndustral ste, but are routed across land not owned by the ppelne company. If the natural gas s accdentally released and gnted, the hazard dstance assocated wth these ppelnes to people and property s known to range from under 0 m for a smaller ppelne at lower pressure, up to over 300m for a larger one at hgher pressure. Therefore, ppelne operators and regulators must address the assocated publc safety ssues. Ths paper focuses on a method to calculate explctly the ndvdual rsk of transmsson ppelne carryng natural gas wth reasonable accdent scenaros for route plannng n relaton to the ppelnes proxmty to surroundng buldng. The mnmum proxmty dstances between ppelnes and buldngs s based on the rupture of the ppelne and the dstances where chosen to correspond to a radaton level of approxmately 3 kw/m. In the desgn crtera of steel ppelnes for hgh pressure gas transmsson(ige/td/1), the mnmum buldng proxmty dstances for rural area are located between 10-5 and 10-6 of ndvdual rsk. Therefore, the rsk from natural gas transmsson ppelne s very low comparng wth other rsk from traffc or chemcal ndustres. Keywords: quanttatve rsk analyss, transmsson gas ppelne, ndvdual rsk, mnmum proxmty, fatal length, jet fre. Rsk Analyss IV, C. A. Brebba (Edtor) 004 WIT Press, www.wtpress.com, ISBN 1-8531-736-1
544 Rsk Analyss IV 1 Introducton Transmsson ppelnes carryng natural gas are not on secure ndustral ste as a potentally hazardous plant, but are routed across the land.e., busy cty or a network of superhghways. Consequently, there s the ever-present potental for thrd partes to nterfere wth the ntegrty of these ppelnes. In addton, the combnaton of thrd-party nterference and ppelne route mght suggest that people around the ppelnes are subject to sgnfcant rsk from ppelne falure. If the natural gas s accdentally released and gnted, the hazard dstance assocated wth these ppelnes to people and property s known to range from under 0 m for a smaller ppelne at lower pressure, up to over 300m for a larger one at hgher pressure[1]. Therefore, ppelne operators and regulators must address the assocated publc safety ssues. Ths paper focuses on a smplfed method to calculate explctly the ndvdual rsk of transmsson ppelne carryng natural gas wth reasonable accdent scenaros. Indvdual rsk The ndvdual rsk at a locaton near a natural gas ppelne can be estmated by ntegratng along the ppelne the lkelhood of accdent multpled by the fatalty at the locaton from all accdent scenaros and can be wrtten as the followng equaton. IR = L 0 ϕ PdL (1) where the subscrpt denotes the accdent scenaros, ϕ falure rate per unt length of the ppelne assocated wth the accdent scenaro, L ppelne length, and P lethalty assocated wth the accdent scenaro. By assumng constant falure rate of the ppelne, the ndvdual rsk s; + l IR = ϕ PdL () The lethalty of above eqn depends on operatng pressure, ppe dameter, accdent scenaro, and dstance from a specfed pont of nterest to the ppelne. By defnng the ntegraton part as fatal length, the ndvdual rsk s expressed n terms of the fatal length and the falure rate of the ppelne. The fatal length means a weghted length of ppelne wthn whch an accdent has the fatal effect on the person at a specfed locaton. l Rsk Analyss IV, C. A. Brebba (Edtor) 004 WIT Press, www.wtpress.com, ISBN 1-8531-736-1
Rsk Analyss IV 545.1 Falure rate The falure rate n a partcular secton of ppelne depends on many varables, such as sol, coatng, desgn, cathodc protecton, age of ppelne, depth of cover, hydrostatc test, survey, patrol, tranng, and so on. It s very dffcult to nclude the effects of those varables on the falure rate because accdent data may not be suffcent for statstcal analyss. Generally for the rsk analyss, the falure rate of ppelne s estmated smply by usng some varables from hstorcal data. The falure rate of major gas ppelnes n Western Europe s reported by the European Gas Ppelne Incdent Data Group(EGIG)[]. It s currently based on the experence of 1.5 mllon klometre-years n eght countres of Western Europe. As shown n table 1, the external nterference by thrd party actvty s the leadng cause to major accdents wth the medum or great hole generated. The total falure rates for small, medum and great hole are.76 x 10-4,.43 x 10-4 and 7.475 x 10-5 1/yr.km, respectvely. These values are an order of magntude hgher than the values estmated from DOT data or Brtsh Gas Transco data. In ths work, we adapt the EGIG data conservatvely[3]. Table 1: Falure frequences based on falure causes and hole sze(egig, 1993). Falure causes External nterference Constructon defects Corroson Ground movement Others/unknown Falure frequency [1/yr.km] 3.0 x 10-4 1.1 x 10-4 8.1 x 10-5 3.6 x 10-5 5.4 x 10-5 Percentage of total Percentage of dfferent hole sze[%]* falure rate Small Medum Great 51 % 5 56 19 19 % 14 % 6 % 10 % 69 97 9 74 5 3 31 5 6 <1 40 <1 Total falure rate 5.75 x 10-4 100 % 48 39 13 * The hole szes are defned as follows: Small hole: hole sze s lower than cm; Medum hole: hole sze ranges from cm up to the ppe dameter; Great hole: Full bore rupture or hole sze s greater than the ppe dameter.. Fatal length Domnant hazards from natural gas ppelne are the effect of sem-confned exploson and the effect of thermal radaton from a sustaned jet fre, whch may be preceded by a short-lved freball[1]. When a person s afflcted from the two events at the same tme, the death probablty should be consdered for the Rsk Analyss IV, C. A. Brebba (Edtor) 004 WIT Press, www.wtpress.com, ISBN 1-8531-736-1
546 Rsk Analyss IV ntersecton of both events to avod overestmaton. The hazard dstance from the exploson s shorter than that from the jet fre whch may follow the exploson[1]. It mples that death probablty by the exploson should be ncluded n that of jet fre followng t. The death probablty at a specfed locaton from an accdent of natural gas ppelne can be estmated then smply by consderng only the thermal effect of jet fre. The heat flux at a certan dstance from a jet fre depends on the shape of flame. A jet flame can be dealzed as a seres of pont source heat emtters spread along the length of the flame. The total heat flux reachng a gven pont s obtaned by summng the radaton receved from each pont source emtter. By collapsng the set of heat emtters nto a sngle pont source emtter located at ground level, the total heat flux receved by ground level damage receptor s estmated conservatvely. Ths assumpton has advantage to avod tedous calculaton and t gves very smple equaton for rsk assessment, even though the result has some error. Wth the heat of combuston of the natural gas at room 7 temperature, H c = 5.00 10 J/kg, humdty assumed as 65% and the atmospherc transmssvty assumed as the unty, τ a = 1, heat flux at a certan dstance from a pont fre source, whch s defned by the recever per unt area, can be calculated by usng the followng equaton [1]. 5.09 I = 8.11 10 Qeff r (3) where r s the dstance from a specfed locaton to the fre and Q eff s effectve release rate of natural gas. The probablty of death from an accdent can be estmated as the followng equaton[3]. P = 1 π Pr 5 s / e ds (4) The argument of the functon s the probablty unt, Pr, characterzng the dose-effect relatonshp between the doses of such concrete harmful load as pressure, heat, or toxcty, and such recpent categores as death or njures. Probablty unt of death from the heat flux can be wrtten as followng equaton[4]. Pr 4 / 3 ti 14.9 +.56 ln (5) 10 = 4 where t s the exposure tme and I s the radatonal heat flux at a specfed locaton of nterest. The duraton of exposure depends on so many crcumstances that t would not be possble n fact to establsh any specfc rule to evaluate the degree of harm. Rsk Analyss IV, C. A. Brebba (Edtor) 004 WIT Press, www.wtpress.com, ISBN 1-8531-736-1
Rsk Analyss IV 547 Rausch recommends a value of 30 seconds as exposure tme[5]. Therefore, the Probt equaton for death at a specfed locaton from a jet flame of natural gas can be estmated conservatvely by usng eqns (4) and (5)..09 Pr = 16.67 + 3.4ln( / r ) (6) The gas release rate from a hole of ppelne can be estmated by usng the followng equaton[6,7]. For an accdent near the gas supplyng staton; Q eff Q eff, 3 1.783 10 AP α p0 =, L α 410 (7) 3 1+ 4.196 10 α L / d d Fatal length[m] (Release rate of natural gas[kg/s]) 1/.09 1 8 4 0 0 4 6 8 Dstance from gas ppelne[m] (Release rate of natural gas[kg/s]) 1/.09 Fgure 1: Fatal length at a specfed locaton. For an accdent far away from the gas supplyng staton; Q eff, = 4 5.349 10 AP α p0 L, α > 410 (8) d where A s cross-secton area of ppelne, p p 0 operatng pressure, L dstance from gas supplyng staton to release pont and α effectve hole area dvded by A. The subscrpt denotes small, medum, or large hole on the ppelne. p Rsk Analyss IV, C. A. Brebba (Edtor) 004 WIT Press, www.wtpress.com, ISBN 1-8531-736-1
548 Rsk Analyss IV The fatal length for a gven pont can be solved by usng eqns (), (6), (7) and (8). The fatal length scaled wth the effectve release rate depends only on the scaled dstance of a specfed pont as shown n Fg. 1[7]. 3 Rsk analyss and dscussons The ndvdual rsk can be estmated explctly by usng the eqn (7), table 1, and Fg. 1 for a secton of ppelne whch s far away from gas supplyng staton. The dstances from the ppelne for so-ndvdual rsk contours are depended drectly to the dameter of ppelne and square root of the operatng pressure as shown n Fg.. H = a pd + b (9) where the a and b are the coeffcents of the above eqn whch are lsted n table. 400 Mnmum Proxmty of IGE/TD/1 Dstance of so-rsk(m) 300 00 100 IR = E-6 IR = E-5 IR = E-4 0 6 10 14 [P(bar)d(m) ] 1/ Fgure : Indvdual rsk from natural gas transmsson ppelne. In the desgn crtera of steel ppelnes for hgh pressure gas transmsson(ige/td/1), the mnmum buldng proxmty dstances for rural area are showed n Fg. [8]. It s located between 10-4 and 10-5 of ndvdual rsk. Ths mnmum proxmty dstances between ppelnes and buldngs s based on the rupture of the ppelne and the dstances where chosen to correspond to a radaton level of approxmately 3 kw/m. Comparng the data of ppelne falure rate n UK and EGIG, the mnmum proxmty les between 10-5 and 10-6 Rsk Analyss IV, C. A. Brebba (Edtor) 004 WIT Press, www.wtpress.com, ISBN 1-8531-736-1
Rsk Analyss IV 549 of ndvdual rsk[3]. Therefore, the rsk from natural gas transmsson ppelne s very low comparng wth other rsk from traffc or chemcal ndustres. The eqn (9) can be useful for route plannng of new gas ppelne and selectng hazardous secton of the ppelne to be analyzed n more detal. Table : Coeffcents of eqn (9) wth rsk levels. Indvdual Rsk 1.00E-04 5.00E-05 1.00E-05 5.00E-06 1.00E-06 a 53.8431 0.376 30.67457 3.67509 35.3444 b -467.413-55.8151-6.47-19.1336 -.83841 For transmsson ppelne wth the dameter of 1 m and the operatng pressure of 80 bar, the dstances of the so-rsk contours for 10-5 and 10-6 decrease stffly wth ncreasng the ppelne length from gas supplyng staton as shown n Fg. 3. But for 10-4, the dstances of the so-rsk decrease slowly wth the length. It s related to the effect of medum hole on the rsk. The contrbuton of medum hole s gnorable for 10-5 and 10-6 of ndvdual rsk because the effect of great hole s very hgh comparng wth that of medum hole. But the medum hole wll be sgnfcant relatvely for 10-4 of ndvdual rsk. 500 Dstance of so-rsk(m) 400 300 00 100 0 IR = E-6 IR = E-5 IR = E-4 0 5000 10000 15000 0000 5000 Ppelne Length(m) Fgure 3: The mnmum proxmty dstance between ppelne(d = 1 m, p = 80 bar, h = 500 m) and buldng. Rsk Analyss IV, C. A. Brebba (Edtor) 004 WIT Press, www.wtpress.com, ISBN 1-8531-736-1
550 Rsk Analyss IV By assumng the rsk contour decrease lnearly wth ppelne length untl a certan value, the ndvdual rsk contour along the ppelne can be estmated conservatvely by the followng eqns. For IR = 10-6 4.64 14.06 Pd = ( L 500) + 49.4 Pd 7. 48 H, L 410d (10) 410d 500 For IR = 10-5 1.03 11.91 Pd = ( L 500) + 4.58 Pd 7. 5 H, L 410d (11) 410d 500 For IR = 10-4 334.5 18.3 Pd = ( L 500) + 35.05 Pd 133.16, H L 38, 560d (1) 38,560d 500 The dstance of a gven ndvdual rsk can be estmated explctly wth ppelne dameter, ppelne length, and operaton pressure by usng the eqns (11), (1), and (13) for near feld and by usng the eqn (9) for far feld of gas supplyng staton. These equatons can be usable n safety gudelnes. 4 Conclusons Quanttatve rsk assessment recently has become mportant n controllng the rsk level effectvely n gas ppelne management. Ths work proposes a smple method of ndvdual rsk assessment for natural gas ppelne and ntroduces the parameters of fatal length. Wth currently acceptable crtera taken nto account for ndvdual rsk, the mnmum proxmty of the ppelne to occuped buldngs s approxmately proportonal to the square root of the operatng pressure of the ppelne. And t decreases wth the ppelne length due to resstance of gas flow through the ppelne. The rsk of natural gas transmsson ppelne s lower than that of traffc or chemcal ndustres. The proposed method for ndvdual rsk assessment explctly may be useful for settng safety gudelne. References [1] Jo, Y.-D., & Ahn, B.J., Analyss of Hazard Area Assocated wth Hgh- Pressure Natural-Gas Ppelne, Journal of Loss Preventon n the Process Industres, 15, p179, 00. [] European Gas Ppelne Incdent Data Group, Gas Ppelne Incdents 4 th Report 1970-1998, 1999. [3] John, M., Chrs, B., Andrew, P., & Charlotte, T., An Assessment of Measures n Use for Gas Ppelne to Mtgate aganst Damage Caused by Thrd Party Actvty, Prnted and Publshed by the Health and Safety Executve, C1 10/01, 001. [4] API RP 51, Recommended Practce 51, Thrd Edton, API, Washngton, D.C., November, 1990. Rsk Analyss IV, C. A. Brebba (Edtor) 004 WIT Press, www.wtpress.com, ISBN 1-8531-736-1
Rsk Analyss IV 551 [5] Rausch, A.H., Esenberg, N.A., & Lynch, C.J., Contnung development of the Vulnerablty model(vm), Department of Transportaton, Unted States Coast Guard, Washngton D.C., Report No. CG-53-77, Feb., 1977. [6] Jo, Y.-D., & Ahn, B.J., A Smple Model for the Release Rate of Hazardous Gas from a Hole on Hgh-Pressure Ppelnes, Journal of Hazardous Materals, 97, p31, 003. [7] Jo, Y.-D., & Ahn, B.J., A Method of Quanttatve Rsk Assessment for Transmsson Ppelne Carryng Natural Gas, Submtted to J. of Hazardous Materals. [8] IGE, Steel Ppelne for Hgh Pressure Gas Transmsson, IGE Code TD/1 Edton 4, Communcaton 1670, 001. Rsk Analyss IV, C. A. Brebba (Edtor) 004 WIT Press, www.wtpress.com, ISBN 1-8531-736-1