© 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Oil and Hydrocarbon Spills III, CA Brebbia (Editor). ISBN 1-85312-922-4
Natural gas pipeline accident consequence analysis
P.L. Metropoloi & A.E.P. Brown2 ‘Unidade de Pat.his, Paulinia/SP, Brazil ~EscolaPolitecnica, USP, Rua Jacatirlo, 240, S. Paulo/SP, Brazil
Abstract
Industry licensing activities require safety andenvironmental studies to enhance safe~and enviromental prevention andprotection applied to the pipeline, Afler World War II, industry activities have given a great jump in progress but, associated to it, potential accident risk has also been enlarged. Most common accidents like toxic emissions, fire and explosion must be avoided. Risk analysis is a world wise technique used to forseen those undesired events to occur, Accident scenarios consequence analysis wasprepared and results presented, as well people vulnerability totheinvestigated event's consequences,
1 Introduction
With the outcome of natural gas fuel exploitation in South America and the increase of energy costs, industry is ready to use this new source of energy. This source of energy uses natural gas as fhel. Natural gas comes from Bolivia field towards Brazil in an 18“ pipeline up to the consumers. The total confirmed reserves of the Bolivian natural gas is ca. 108 billions m3, Within the Brazilian Program to thermoelectrical units installation in the next 20 years, assuming a capacity factor of 70Y0,the units will consume ca, 16 billions m3 of natural gas. Considering preventive and protection concepts, risk analysis was performed, aiming gas pipeline licensing by the federal environmental agent y (IBAMA). Hazard identification techniques [1,2,3], such as preliminary hazard analysis (PHA) and event tree analysis (ETA), were utilized. Consequences and vulnerability of potential hazard scenarios were simulated with the software CHEMS-PLUS [4] that also determined the hazard damages to people and materials. Finally, process safety measures were recommended to increase pipeline risk acceptability. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Oil and Hydrocarbon Spills III, CA Brebbia (Editor). ISBN 1-85312-922-4
2 Frequency occurrence estimation
Event tree analysis (ETA) shows graphically the possible sequence of a potential accident that can result from an initiating event [1,2,3]. ETA considers the responses of safety systems and operators to the initiating event to be analyzed. ETA requires a good knowledge of potential initiating failures or systems upsets (that can potentially cause a non desired event), safety systems fmctions or emergency procedures, which can potentially mitigate the consequences of each initiating event. It also depends of the initiating events complexity and safety systems. Delphi technique was used to solve ETA’s [1,3,5]. In the study, it was constructed one quantitative graphical model of ETA, as shown Figure 1, From to PHA results, it was decided to model ETA to the event sequence of a leakage of natural gas from the pipeline, because it was considered as a moderate risk. Frequencies occurrence estimation resulted in jet fue and flash fue probabilities of 3.3 E-7 occurrences/yr and natural gas dispersion of 3.2 E-05 occur. /yr.
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3 Consequence and vulnerability analyses
Computer simulations used the CHEMS-PLUS [4] software considering flammable product release from the gas pipeline, followed by fire or explosion. From the hypothetical scenario studied, it was considered jet fire scenario as the worst to people, materials and to the enviromnent. To accident modeling calculations, these hypotheses were considered: Natural gas was considered similar to methane; Product toxicity was not considered; Turbulent flow of the jet fire.
3,1 Accident scenario
Accident scenario of jet fue due natural gas pipeline failure was studied. This scenario considers a flammable or explosive atmosphere formation due gas © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Oil and Hydrocarbon Spills III, CA Brebbia (Editor). ISBN 1-85312-922-4
oil a)zdHvdt-ocarbon Spills 111 457 pipeline failure. So, gas leakage also carries air through the gas effluent stream. The gas dilution starts at the leaking point, because of the high pressure of the gas pipeline. Therefore, a turbulent free jet is formed, wherein the gas concentration and velocity along its axis depend on the orifice diameter. The mass of this turbulent jet, composed by a vapor and air mixture, can be or not within the flammability limits, In case of an immediate ignition, a typical flame, called jet f~e, is formed, which depends directly on the gas leakage duration. The flame is extinguished when the gas leakage ceases. The velocity along the axis of the jet decreases progressively with the distance flom the leakage point. The point where this velocity gets equal to the wind velocity is considered as the transition point between the dispersion due to the jet effect and the dispersion due to the vertical turbulence of the atmosphere. In case of a non-immediate ignition, two accidental events may occur: - Flash fue that is the delayed ignition of a gas cloud, with short time duration and without overpressure effects, but presenting thermal effects. - UVCE (unconfined vapor cloud explosion). According to Wiekema [8], more than one ton of methane within the flammability limits is required to the occurrence of this event. Consequence study to a possible flash f~e was not performed because there is no consequence model (thermal radiation) associated to it, considering that, unless there is a person within the area occupied by the flammable cloud, this event will not take to any injuries to any person, The analysis of the results of the consequences simulations concluded that in fracture and crack scenarios of the gas pipeline, the mass of natural gas existing within the flammability limits is lower than one ton, since this product is rapidly dispersed into air. Therefore, UVCE was only considered for the collapse scenario. The input data to simulate accident scenario with CHEMS-PLUS was: A natural gas leakage occurs at 18” carbon steel pipeline, at a minimum depth of 1 meter fi-om the ground level, The operational pressure of the pipeline is 100 kg/cm2, and the temperature 25”C, considered as constants during the whole leakage period. The analysis supposes that the gas can be released through the soil (1 meter) with no resistance to its flow, as a conservative condition; Gas leakage cannot be interrupted quickly and the yield of the methane in the UVCE was assumed as 2Yo; Meteorological data assumed was: amual atmosphere temperature of 26°C, prevailing wind direction: SW, with average velocity of 3 rrds, and air relative humidity of 70’%o; The rupture classes assumed in the study are in accordance with the standards of the World Bank [2], where D is the internal diameter of the gas pipeline, equivalent diameter and released gas quantities are also shown in Table 1. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Oil and Hydrocarbon Spills III, CA Brebbia (Editor). ISBN 1-85312-922-4
458 01[~lttliH\~irocc]rbol~ Spills Ill
Table 1: Pipeline Rupture Classes
RuptureClass IntervaloftheDiameterof Equivalent ReleasedQuantity theRuptureEquivalentLine Diameter (m) (Kg/s) Fracture Upto5%D 0.023 7 Crack Between 5?’oD and 20% D 0.092 112 Collapse More than 20% D 0.46 2,800
Tables 2 and 3 present computer simulation results considering gas pipeline rupture.
Table 2: Overpressure Darnages
Overpressure (bar) Distances(m) 0.05 564 0.14 268 0.5 125 1 83 Explosivemass (kg) 172,000
Table 3: Thermal Radiation Effects
Radiation (kW/mz) Distances(m) 0.7 280 20 260 37 190 100 119
The vulnerability model for fire scenarios was based in studies published by TNO [6] that uses the dose concept. The Probit equation (1) defines the probability of fatality [7]:
Pr = -38.48 + 2.561n(Dose) (1)
The vulnerability analysis results concerning jet fue formation and UVCE scenarios are presented in Tables 4 and 5, respectively. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Oil and Hydrocarbon Spills III, CA Brebbia (Editor). ISBN 1-85312-922-4
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Table 4: People Vulnerability in Scenario of Immediate Ignition
Scenario Distance to Distance to Incident Probability of Observer Thermal Radiation to Fatality At Distance Rddiation Observer at (%) Level of Distance XO (%) 1 kW/m2 (KW/m2) (m)
55 270 20 0 Fracture 7 70 100 65 41 270 37 7 Crack 26 270 100 91 260 560 20 24 Collapse 190 560 37 84 710 c~n 1 nn 4 nn
Table 5: People Vulnerability in Scenario of Non-Immediate Ignition –UVCE
Scenarios Type of injury People Overpressure Distance Affected Value (m) (?6) (Bar)
Lung [ 1,019 4 Hemorrhage Fracture Eardrum I 0.17 11 Rupture 90 0.86 4 Lung 1 1.019 16 Hemorrhage Crack Eardrum 1 0.17 45 Rupture 90 0.86 18 Lung ~ 1.019 83 Hemorrhage Collapse Eardrum 1 0.17 237 Rupture 50 044 140 90 0.86 92
4 Conclusions
Computer simulations do not consider obstacles between the event and the observer to the resulted distances. Analyzing the results of consequences and vulnerability analyses, it is concluded that: © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Oil and Hydrocarbon Spills III, CA Brebbia (Editor). ISBN 1-85312-922-4
The worst consequence to the community will be scenario of the gas pipeline collapse inside a residential area. Possible fatality occurrence and material darnage reach distances up to 200 meters. Possible fatalities will occur at distances of 7 meters from the jet fne. Harmfid effects to people will include eardrum rupture at a distance of 4 meters or less of the gas pipeline fracture. Possible fatalities due to the jet fue thermal radiation will occur up to the distance of 26 meters, while harmful effects (eardrum rupture) would happen within a minimum distance of 18 meters considering a crack on the pipeline. Secondary effects of a potential gas explosion, considering impacts on the whole body or head, the surviving possibility is ca. 99% for the studied scenarios, since the minimum distances will be observed in each case. Any l%tureexcavation job at the pipeline area must follows the Pipeline Risk Management Plan.
References
[1] Lees, F.P., Loss Prevention in the Process Industries, 2ud. Ed., v. 1/2, Butterworth, Oxford, 1996. [2] The World Bank, Manual oflndustrial Hazard Assessment Techniques, v. 1, Office of Environment and Scientific Affairs, Washington DC, USA, 1985. [3] Brown, A,E.P,, Risk AnaZysis Course, S. Paulo/SP, Brazil, 2001. [4] Cherns-Plus-TM, v.1,0, Risk Works Sofware, Arthur D’Little, Cambridge, Mass., USA, 1995. [5] Ferries, O. & Andersen, T., Reliability and Economics of Pipelines Operations, De Norske Veritas, Oslo, Norway, 1980. [6] Green Book Methods for the Determination of Possible Damage, CPR16E, The Netherlands Organization for Applied Scientific Research (TNO), 1st, ed, The Hague, Netherlands, 1992. [7] Eisenberg, N,A,, Lynch, C,J, & Breeding, R,J., Vulnerability Model; A Simulation System for Assessing Damage Resulting j%om Marine Spills, Dept. of Transportation, Washington DC, USA, 1979. [8] Wiekema, B.J., Guidelines for Chemical Process Quantitative Risk Analysis, AICHE, NY, USA, 1989.