Consequences of a risk based approach for natural gas pipelines

Transcription

1 Consequences of a risk based approach for natural gas pipelines G.M.H. Laheij 1, C.J. Theune 2 1 National Institute for Public Health and the Environment, the Netherlands 2 Ministry of Housing, Spatial Planning and the Environment, the Netherlands Abstract In the Netherlands natural gas is transported through an underground pipeline network with a combined length of about kilometers. In preparation of new legislation, in which zoning distances will be based on a probabilistic approach, the quantitative risk analysis methodology for underground transmission natural gas pipelines is revised to reflect new understandings in the risk scenarios, failure frequencies and effects. In order to get a complete overview of third party risks in the Netherlands, all pipeline owners are obligatory to provide pipeline data to a national risk register. Based on these data and taking into account the new zoning distances, an analysis of the consequences of these new zoning distances for land use planning is carried out. The aim of this analysis is to identify potential bottlenecks where dwellings are situated within the new zoning distances of these pipelines and to identify where, based on the spatial planning plans available up to 2030, in the future possible bottlenecks may appear. Additional measures should reduce the risk if for example dwellings are situated within the new zoning distances or if the guidance value for the societal risk is exceeded. As there are no measures available for reducing the effects of a pipeline rupture, the additional measures focus mainly on reducing the probability of pipeline ruptures. Because external interference is the main cause of pipeline ruptures, the additional measures focus on this cause. Proposed measures for reducing the risk of high pressure natural gas pipelines are for example the use of concrete slabs or warning tapes and agreements with landowners about land utilization. Also, the introduction of a statutory one-call system is an important generic measure for reducing the probability of pipeline failures. 1. Introduction In the Netherlands natural gas is transported through underground pipelines with a combined length of about kilometers. In a circular letter issued in 1984 landuse planning guidelines and generic zoning distances are laid down for underground natural gas transmission pipelines. In preparation of new legislation, in which zoning distances will be based on a probabilistic approach, the quantitative risk analysis methodology for underground transmission pipelines for natural gas is revised. New understandings in the risk scenarios, failure frequencies and effects are now included. This paper gives an overview of the reviewed risk methodology for natural gas pipelines. Also, the consequences for land-use planning will be given together with measures that can be taken to reduce the risk of these pipelines.

2 1.1 Zoning policy The new zoning policy is part of a two-track policy for preventing major accidents. Firstly, the frequency of accidents occurring and their effects when they do occur are reduced as much as reasonably possible by taking measures at the source of risk. Secondly, the number of persons exposed to effects, should an accident occur, is reduced by the zoning policy. Two measures are used in defining these policies: the individual risk as a measure of the level of protection to each individual member of the public, and the societal risk as a measure of the disaster potential for the society as a whole. The individual risk is expressed as the risk of fatality per year; this is defined as the probability that an unprotected person residing permanently at a fixed location will be killed as a result of an accident occurring at a source of risk. The societal risk is defined as the probability that a certain number of deaths will be exceeded during a single accident; it is expressed as the relationship between the number of people killed (N) and the frequency per year (F) that this number will be exceeded. For both the individual risk and societal risk, criteria limits will be set for pipelines [1]. For dwellings and vulnerable objects like schools and hospitals, the individual risk limit is set at 10-6 per year. For less vulnerable objects like small office buildings, restaurants, shops and recreation facilities, the individual risk contour of 10-6 per year is a guidance value. The limit for the societal risk is an indicative limit. For transport routes, the limiting frequency (F lim ) per kilometer of pipeline for the occurrence of an accident with N or more deaths is: F lim N 2 = 10-2 (1) The number of deaths (N) must be larger than 10 to be incorporated into societal risk. In zoning policy, the individual and societal risks complement each other. The individual risk creates a distance between the source of risk and its surroundings. The societal risk limits the population density around the source of risk. 2. Quantitative risk methodology For each scenario, important parameters that should be determined are the relevant failure modes and the failure frequencies. For flammable substances, the consequences are determined by the mass flow rate of each scenario, the probability of ignition, the characteristics of the subsequent fire and the corresponding heat radiation profile. Also, the population in the surroundings of the pipeline must be identified in order to determine the consequences in terms of the number of deaths. These parameters are discussed in this paper. The methodology is described in more detail in [2,3]. 2.1 Scenarios and failure frequencies For underground pipelines, all possible release scenarios are divided over two scenarios, namely a rupture and a leakage [4]. Since leakages do not significantly contribute to the risk of pipelines with flammable substances they are not taken in to account in the risk calculations. The failure frequencies and consequences are therefore only determined for pipeline ruptures. Generic failure frequencies for underground pipelines are given in Part II of the Purple Book [4], which describes quantitative risk assessment guidelines for transportation activities. The general failure frequency for steel pipelines has been set at 6.1x10-4 per kilometer per year. The failure frequency is split into two scenarios, a leakage and a rupture, with a probability of 0.75 and 0.25 respectively. Therefore, for a rupture the failure

3 frequency will be 1.5x10-4 per kilometer per year. For high pressure natural gas, it has been reviewed whether this failure frequency is still valid. For natural gas pipelines, the failure frequency of pipeline ruptures is determined by external interference [3]. The failure frequency is derived using the PipeSafe methodology [5]. Firstly, the probability (f d in kilometers per year) that the pipeline is hit is determined as a function of the depth of cover (d in meters) of the pipeline [6]: f d = e -2.4 d-3.5 (2) This function was derived combining the number of incidents in a pipeline depth class with the overall years of experience in the depth class. For every meter of extra depth of cover, the hit frequency decreases by about a factor of 10. Secondly, using historical damage data and fracture mechanics the probability of a pipeline rupture is calculated [7]. Pipeline parameters used in the calculations are the diameter, pressure, depth of cover, wall thickness, yield strength and Charpy energy. Using this model, the probability of a pipeline rupture for the total cross-country pipeline network of Gasunie was calculated. The Gasunie network is about kilometers in length and represented in the calculations by about 1.2 million data points. From this analysis 0.7 rupture per year is predicted. Historical failure data for the Gasunie network were available for the period In this period 12 pipeline ruptures occurred, with no pipeline ruptures during the last 11 years. Based on this data it was determined that there is a statistical significant trend in the number of pipeline ruptures [2]. Therefore only the last 11 years (with no pipeline ruptures) were used to compare the model predictions with the historical data. Based on last 11 years the upper bound of the 95% confidence interval is equal to 0.25 pipeline rupture per year. As the model predicts 0.7 pipeline rupture per year for the Gasunie network, it was decided to reduce the model predictions by a factor of 2.8 (=0.7/0.25). The factor of 2.8 does currently not apply to other shippers of (raw) natural gas as it is believed that the significant trend is due to specific measures taken by Gasunie. Also, for pipelines with raw natural gas the additional wall thickness included for internal corrosion has to be excluded from the calculation of the failure frequency. In this review also the effect of a statutory one-call system on the failure frequency is included [8]. This by law laid down system replaced the voluntary one-call system in The statutory one-call system requires not only that all digging activities are notified, but also additional rules for the follow-up of a notification are introduced. The National Institute for Public Health and the Environment (RIVM) has estimated the influence of the statutory one-call system. The derivation of this estimate is described in [2,9]. In cooperation with N.V. Nederlandse Gasunie (a Dutch natural gas transmission company), the voluntary one-call system was reviewed and investigations were made why, despite an activity was notified, incidents still occurred [10]. Based on this review the rules of the statutory one-call system were evaluated on how they affect the chance that a pipeline is hit due to external interference. This evaluation leads to a reduction factor of 2.5 for spillages caused by external interference. The Ministry of Housing, Spatial Planning and the Environment (VROM) decided to take this factor already into account in the risk calculations as they commit themselves to a result achievement. Whether in practice the factor of 2.5 will be established must be monitored in the forthcoming years. If the risk reducing

4 factor is not reached in practice additional rules should be put in place. This is possible under the statutory one-call system. 2.2 Release and Effect Calculations In the release scenarios for pipelines with flammable substances only pipeline ruptures are taken into account, as leaks don t contribute to the individual risk contour of 10-6 per year. For pipelines with flammable substances the effects are determined by heat radiation. The probability of death due the exposure to heat radiation is calculated with the use of a probit function. The probit function for death due to heat radiation is given by [4,11]: Pr = ln(q 4/3 t) (3) Where Q is the heat radiation (W m -2 ) and t is the exposure time (s). The maximum exposure time is 20 seconds. Overpressure effects don t contribute significantly to the risk and are therefore not included in the calculations. A pipeline rupture of an underground natural gas pipeline results in a vertical jet. In the calculations, two separate jets are taken into account. The first jet is based on the average release during the first 20 seconds of the accident. The second jet is based on the release between 120 and 140 seconds. This approach is chosen as the maximum exposure time for flammable effects is set to 20 seconds, see Equation 3 [4,11]. Also, from historical data it was determined that in 75% of the incidents ignition takes place in the first 30 seconds. In 25% of the incidents ignition takes place after at least 120 seconds. In the release calculations also the effect of the crater on the momentum of the jet is included. The methodology was initially set-up for pipelines with processed natural gas [3]. It was also investigated whether for pipelines transporting raw natural gas the same methodology as for processed gas could be used [12]. Raw natural gas pipelines contain, among other by products, water and condensate. For raw natural gas pipelines the pipeline pressure drops over time. It was therefore first investigated which production case gives the highest release rate and results in the highest heat of combustion in case of a pipeline rupture; the low production case with relatively high liquid hold-up or the high production case with relatively small liquid hold-up in the pipeline. Investigating a pipeline transporting gas with a high condensate gas ratio (CGR) of 80 [m 3 condensate per million Nm 3 of gas] and using computed fluid dynamics it was concluded that the high production case results in the highest release rates. In both cases no rain-out of condensate droplets occurred. It was also concluded that the highest heat of combustion occurs in the high production case. Furthermore, the calculated effect distances for the high production case where almost equal to the effect distances of a similar pipeline with processed natural gas. Therefore it was concluded that for the release and effect calculations of pipelines with raw natural gas (CGR < 80) the same models as for processed gas can be used without the need to adapt them specifically for raw natural gas. 2.3 Probability of ignition The probability of ignition is subdivided into direct ignition and delayed ignition. For natural gas pipelines, ignition results in a jet fire. From historical data it was

5 determined that the probability of ignition (P ign ) is related to the diameter and pressure of the pipeline [33]: P ign = a + b p D 2 (4) where a, b are constants, p is the pipeline (barg) and D is the pipeline diameter (mm). The maximum ignition probability equals 0.8. For example, using Equation 4, for a 4 inch natural gas pipeline at 40 bar the probability of ignition equals In case of a 48 inch pipeline (80 bar), the probability of ignition equals 0.8. As the influence of the built-up area on the probability of ignition is most likely not included in Equation 4, the contribution of the built-up area to the probability of ignition was evaluated separately [2]. From a literature study, it was concluded that the most important contribution of the built-up area to the ignition probability comes from two sources: 1) sparks induced by the impact of debris on house bricks and 2) the ignition of gas infiltrated into buildings. From release calculations it could be concluded that the built-up area can only influence the ignition probability for releases of pipelines with a diameter smaller than 18 inch. For pipelines with a diameter equal to or larger than 18 inch, the ignitable part of the jet, defined by its 50% Lower Explosive Limit (LEL) contour, will not be present at a height lower than 20 meter. For these pipelines it was therefore concluded that there is no contribution of the built-up area. For pipelines with a diameter smaller than 18 inch it was estimated that ignition probability as a result of the impact of debris was increased by 0.07 and as a result of the infiltration of gas in buildings by 0.03 [2]. For these pipelines the probability of ignition used in the calculations is now: P ign = a + b p D (5) Using Equation 5, for a 4 inch natural gas pipeline at 40 bar the probability of ignition equals In case of a 48 inch pipeline (80 bar), the probability of ignition still equals Results Using the risk methodology as described in the above paragraphs, both individual risk and societal risk calculations can be performed. For natural gas pipelines the PipeSafe program is used to calculate the individual and societal risk [5]. The risk is not only dependent on the diameter and pressure of the pipeline but also on the depth of cover, wall thickness, yield strength and Charpy energy. Therefore, the distance to the individual risk contour of 10-6 per year lies between 0 meters and the maximum effect distance of a pipeline. The pipelines maximum effect distance, defined as the distance to 1% lethality is only dependent on the diameter and pressure of the pipeline. For 48 inch pipelines the maximum effect distance can be up to 600 meters. Whether the indicative limit of the societal risk will be exceeded, also strongly depends on the above mentioned parameters. 3. Consequences for land-use planning In order to get a complete overview of third party risks in the Netherlands, pipeline owners are required to provide pipeline data to a national risk register [13]. Based on

6 these data and taking into account the new zoning distances, an analysis of the consequences of these new zoning distances was carried out. The aim of this analysis is to identify potential bottlenecks where dwellings are situated within the new zoning distances and to identify where, based on land-use plans available up to 2030, in the future possible bottlenecks may appear. The analysis has been performed using data from 11 pipeline owners with in total about kilometers of natural gas pipelines. The data used for the existing vulnerable objects were obtained combining several commercially available databases with coordinates of dwellings, building functions and population numbers. The land-use plans of municipalities were taken from the New Map of the Netherlands [14]. Using these data, the following results were found. The total length of pipelines with already existing vulnerable objects within the individual risk contour of 10-6 per year is about kilometers. Due to the land-use plans up to 2030 additional 80 kilometer of pipeline could become a bottleneck. It depends strongly on how these new plans are finally developed whether the potential identified bottlenecks appear or not. 4. Additional Measures Additional measures should reduce the risk if dwellings are situated within the new zoning distances or if the guidance value for the societal risk is exceeded. As there are no measures available for reducing the effects of a pipeline rupture, the additional measures focus mainly on reducing the probability of pipeline ruptures. Because for natural gas pipelines external interference is the main cause of pipeline ruptures [3], the additional measures for natural gas pipelines focus on this cause [2,15]. Proposed measures for reducing the risk of natural gas pipelines can be categorized into two groups. Measures in the first group prevent (partly) that the pipeline is actually hit during digging activities. Measures in the second group prevent or control digging activities in the neighborhood of a pipeline. Proposed measures for reducing the risk of high pressure natural gas pipelines are for example the use of concrete slabs or warning tapes and agreements with landowners about land utilization. For all measures preconditions are defined and all preconditions should be met before the subsequent reduction factor can be used. In Table 1, proposed measures with their effect on probability of a pipeline rupture due to external interference are given. The effectiveness of the measures in practice must be monitored in the forthcoming years. Table 1: Effect of additional measures on the probability of a pipeline rupture Measure Reduction factor Extra depth of cover See Equation 2 Warning tape 1.67 Concrete slab 5 Concrete slab + warning tape 30 Stringent supervision of digging activities 3 Agreements about land utilization Harmonization with other transmission pipelines It is the objective to harmonize the methodologies for underground transmission pipelines with flammable liquids and other chemical substances accordingly. However, it is noted that other failure mechanism then external interference, such as

7 corrosion or mechanical failure, are of importance. Therewith, the above measures are only partly effective for risk reduction. The risk methodologies and risk reduction measures for these substances are under development. 5. Conclusions In preparation of new legislation, in which zoning distances and the limits for societal risk are based on a probabilistic approach, the quantitative risk analysis methodology for underground transmission pipelines for natural gas is revised to reflect new understandings in the risk scenarios, failure frequencies and effects. An outline of the new risk methodology is given together with the consequences for land-use planning and measures that can be taken to reduce the risk of these pipelines. The new legislation will lead to additional measures, both in design and operation, with a more optimized land-use. It also will allow for tightened compliance checks by governmental competent bodies. References [1] Ministry of VROM. Besluit Externe Veiligheid Buisleidingen (in Dutch, in prep.) [2] Laheij GMH, Vliet van AAC, Kooi ES. Achtergronden bij herziene zoneringsafstanden hoge druk aardgastransportleidingen. RIVM report / (in Dutch) [3] Gielisse M, Dröge MT, Kuik GR. Risicoanalyse aardgastransportleidingen. Gasunie report DEI 2008.R (in Dutch) [4] Committee for the Prevention of Disasters, Guidelines for quantitative risk assessment, CPR 18E, [5] Acton MR, Baldwin PJ, Baldwin TR, Jager E, The development of the PipeSafe risk assessment package for gas transmission pipelines, Proceedings of the International Pipeline Conference, Calgary, ASME International, [6] Jager E, Kuik GR, Stallenberg G, Zanting J, A qualitative risk assessment of the gastransport services pipeline system network bases on GIS data, ICT Prague, [7] Corder I, The application of risk techniques to the design and operation of pipelines, IMechE, C502/016, [8] Staatsblad 2008, Wet van 7 februari 2008, houdende regels over de informatie-uitwisseling betreffende ondergrondse netten (Wet informatieuitwisseling ondergrondse netten) Stb. 2008, 120, Sdu (in Dutch) [9] Laheij GMH, Kuik GR, Elteren van R, Vliet van AAC, Influence of a statutory one-call system on the risk of natural gas pipelines, PSAM9, 9th International Conference on Probabilistic Safety Assessment and Management, Hong Kong, China, May (Eds. Tsu-Mu Kao, Enrico Zio and Vincent Ho). [10] Elteren van R, Agteren van MH, Kutrowski KH, Achterbosch GGJ, Kuik GR, Bepaling effectiviteit KLIC-proces ten aanzien van aardgastransport leidingen, Gasunie report RT 04.R (in Dutch) [11] RIVM. Reference Manual Bevi Risk Assessments. Version [12] Beijer K, Technical Note: Mogelijke verschillen in (externe veiligheid) risico tussen de operatie van natgas en drooggas transportleidingsystemen, NAM EP , revision 3. March 2009 (in Dutch).

8 [13] Sant JP van t, Manuel HJ, Berg van den A, Dutch registration of risk situations. ESREL European Safety and Reliability Conference, Valencia, Spain, September (eds. S. Martorell et al.) [14] Netherlands Institute for planning and housing (NIROV), De Nieuwe Kaart van Nederland, January [15] Laheij GMH, Vliet van AAC, Measures for reducing the probability of ruptures of high pressure natural gas pipelines. European Safety and Reliability conference 2009, Prague, 7-10 September 2009.