premium digest december 2010 consequences of a risk-based approach for natural gas pipelines
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Consequences of a risk-basedapproach for natural gas pipelinesBy G.M.H.Laheij, National Institute for Public Health and the Environment, Netherlands; and,
C.J.Theune, Ministry of Housing, Spatial Planning and the Environment, Netherlands
In the Netherlands natural gas is transported through an underground pipeline network with a combined length of about14,000 km. In preparation of new legislation, in which zoning distances will be based on a probabilistic approach, thequantitative risk-analysis methodology for underground natural gas transmission pipelines has been revised to reectnew understandings in the risk scenarios, failure frequencies and effects.
1. MinistryofVROM,2010.BesluitExterneVeiligheidBuisleidingen(inDutch,inprep.).
In order to get a complete overview of third-party risks in
the Netherlands, all pipeline owners are obliged 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-useplanning 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, 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 eects 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 onthis cause. Proposed measures for reducing the risk of high-
pressure natural gas pipelines include the use of concrete slabs
or warning tapes, and agreements with landowners about land
utilisation. Also, the introduction of a statutory one-call system
is an important generic measure for reducing the probability of
pipeline failure.
IntroductionIn the Netherlands, natural gas is transported through
underground pipelines with a combined length of approximately
14,000 km. In a circular letter issued in 1984, land-use planning
guidelines and generic zoning distances were laid down for
underground natural gas transmission pipelines. In preparationof 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 has been revised. New understandings in the risk scenarios,
failure frequencies, and eects are now included.
This article 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.
Zoning policy
The new zoning policy is part of a two-track policy forpreventing major accidents. Firstly, the frequency of accidents
occurring and their eects 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 eects,
should an accident occur, is reduced by the zoning policy. Two
measures are used in dening these policies: the individualrisk 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 dened as the
probability that an unprotected person residing permanently at
a xed location will be killed as a result of an accident occurring
at a source of risk. The societal risk is dened 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 dwellingsand vulnerable buildings such as schools and hospitals, the
individual risk limit is set at 10-6 per year. For less vulnerable
building such as small oce 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 (Flim) per
kilometre of pipeline for the occurrence of an accident with N or
more deaths is:
Flim N2
= 10-2
(1)
The number of deaths (N) must be larger than 10 to be
incorporated into societal risk. In zoning policy, the individualand 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.
Quantitative risk methodologyFor each scenario, important parameters that should be
determined are the relevant failure modes and the failure
frequencies. For ammable substances, the consequences are
determined by the mass ow rate of each scenario, the probability
of ignition, the characteristics of the subsequent re and the
corresponding heat radiation prole. Also, the population inthe surroundings of the pipeline must be identied in order to
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determine the consequences in terms of the number of deaths.
These parameters are discussed in this article. The methodology
is described in more detail in references 2 and 3.
Scenarios and failure frequenciesFor underground pipelines, all possible release scenarios are
divided over two scenarios, namely a rupture and a leakage4.
Since leakages do not signicantly contribute to the risk of
pipelines with ammable 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 Book4, which describes quantitative risk
assessment guidelines for transportation activities. The general
failure frequency for steel pipelines has been set at 6.1 x 10-4
per kilometre 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 frequency
will be 1.5 x 10-4
per kilometre per year. For high-pressure naturalgas, 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 interference3. The failure
frequency is derived using the PipeSafe methodology5. Firstly, the
probability (fd in km/a) that the pipeline is hit is determined as a
function of the depth of cover (d in metres) of the pipeline6:
fd = 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 metre 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 calculated7. 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 12,000 km 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 19772005. In this period 12
pipeline ruptures occurred, with no pipeline ruptures during thelast 11 years. Based on this data it was determined that there is a
statistical signicant trend in the number of pipeline ruptures2.
Therefore only the last 11 years (with no pipeline ruptures) were
used to compare the model predictions with the historical data.
Based on the last 11 years the upper bound of the 95 per cent
condence 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 not currently apply
to other shippers of (raw) natural gas, as it is believed that the
signicant trend is due to specic 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 the eect of a statutory one-call system on the
failure frequency is also included8. This system laid down
by law replaced the voluntary one-call system in 2008. The
statutory one-call system requires not only that all digging
activities are notied, but also additional rules for the follow-up
of a notication are introduced. The National Institute for PublicHealth and the Environment (RIVM) has estimated the inuence
of the statutory one-call system, and the derivation of this
estimate is described in references 2 and 9. In co-operation with
N.V. Nederlandse Gasunie (the Dutch natural gas transmission
company), the voluntary one-call system was reviewed and
investigations were made why, despite an activity was notied,
incidents still occurred10. Based on this review the rules of the
statutory one-call system were evaluated on how they aect
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 into account in the risk calculations as it commits itself 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 factor is not reached in practice, additional rules
should be put in place. This is possible under the statutory one-
call system.
Release and effect calculationsIn the release scenarios for pipelines with ammable
substances only pipeline ruptures are taken into account, as leaks
dont contribute to the individual risk contour of 10-6
per year. For
pipelines with ammable substances the eects are determined
by heat radiation. The probability of death due to the exposureto heat radiation is calculated with the use of a probit function.
2. G.M.H.Laheij,A.A.C.vanVliet,andE.S.Kooi,2008.Achtergrondenbijherzienezoneringsafstandenhogedrukaardgastransportleidingen.RIVMreport620121001/2008(inDutch).
3. M.Gielisse,M.T.Drge,andG.R.Kuik,2008.Risicoanalyseaardgastransportleidingen.GasuniereportDEI2008.R.0939(inDutch).
4. CommitteeforthePreventionofDisasters,1999.Guidelinesforquantitativeriskassessment,CPR18E.
5. M.R.Acton,P.J.Baldwin,T.R.Baldwin,andE.Jager,1998.ThedevelopmentofthePipeSaferiskassessmentpackageforgastransmissionpipelines.Proc.Int.PipelineConference,Calgary,ASMEInternational.
6. E.Jager,G.R.Kuik,G.Stallenberg,andJ.Zanting,2002.AqualitativeriskassessmentofthegastransportservicespipelinesystemnetworkbasesonGISdata,ICTPrague.
7. I.Corder,1995.Theapplicationofrisktechniquestothedesignandoperationofpipelines,IMechE,C502/016.
8. Staatsblad2008,Wetvan7februari2008,houdenderegelsoverdeinformatie-uitwisselingbetreendeondergrondsenetten(Wetinformatie-uitwis-
selingondergrondsenetten)Stb.2008,120,Sdu(inDutch).9. G.M.H.Laheij,G.R.Kuik,R.vanElteren,andA.A.C.vanVliet,2008.Inuenceofastatutoryone-callsystemontheriskofnaturalgaspipelines.PSAM9,9thInt.Conf.onProbabilisticSafetyAssessmentandManagement,HongKong,China,18-23May(Eds.Tsu-MuKao,EnricoZioandVincentHo).
10. R.vanElteren,M.H.vanAgteren,K.H.Kutrowski,G.G.J.Achterbosch,G.R.Kuik,2004.BepalingeectiviteitKLIC-procestenaanzienvanaardgastrans-portleidingen,GasuniereportRT04.R.0694(inDutch).
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The probit function for death due to heat radiation is given by:4, 11
Pr = -36.38 + 2.56 ln(Q4/3t) (3)
where Q is the heat radiation (Wm-2
) and t is the exposure time (s).
The maximum exposure time is 20 seconds. Overpressure eects
dont contribute signicantly 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 rst jet is based on the average release
during the rst 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 ammable eects is
set to 20 seconds, see Equation 34, 11. Also, from historical data
it was determined that in 75 per cent of the incidents ignition
takes place in the rst 30 seconds. In 25 per cent of the incidents
ignition takes place after at least 120 seconds. In the release
calculations also the eect of the crater on the momentum of thejet is included.
The methodology was initially set-up for pipelines with
processed natural gas3. It was also investigated whether for
pipelines transporting raw natural gas the same methodology
as for processed gas could be used12. Raw natural gas pipelines
contain, among other byproducts, water and condensate. For raw
natural gas pipelines the pipeline pressure drops over time. It
was therefore rst 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 [m3 condensate per million Nm3 of gas] and using computed
uid 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 eect distances for the high
production case where almost equal to the eect distances of
a similar pipeline with processed natural gas. Therefore it was
concluded that for the release and eect 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 specically for
raw natural gas.
Probability of ignitionThe probability of ignition is subdivided into direct ignition and
delayed ignition. For natural gas pipelines, ignition results in a jet
re. From historical data it was determined that the probability of
ignition (Pign) is related to the diameter and pressure of the pipeline3:
Pign = a + bpD2 (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 diameter natural
gas pipeline at 40 bar the probability of ignition equals 0.08; in
the case of a 48 inch diameter pipeline at 80 bar, the probability
of ignition equals 0.8. As the inuence 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 separately2. 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:
Sparks induced by the impact of debris on house bricks; and,
The ignition of gas inltrated into buildings.
From release calculations it could be concluded that the built-
up area can only inuence the ignition probability for releases of
pipelines with a diameter smaller than 18 inches. For pipelines
with a diameter equal to or larger than 18 inches, the ignitable part
of the jet, dened by its 50 per cent lower-explosive-limit (LEL)
contour, will not be present at a height lower than 20 m. For these
pipelines it was therefore concluded that there is no contribution
of the built-up area. For pipelines with a diameter smaller than18 inches 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
inltration of gas in buildings by 0.032. For these pipelines the
probability of ignition used in the calculations is now:
Pign = a + bpD2 + 0.1 (5)
Using Equation 5, for a 4 inch natural gas pipeline at 40 bar
the probability of ignition equals 0.18; in the case of a 48 inch
pipeline (80 bar), the probability of ignition still equals 0.8.
ResultsUsing the risk methodology as described in the aboveparagraphs, 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 risk8. 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 m and the maximum eect
distance of a pipeline. The pipelines maximum eect distance,
dened as the distance to 1 per cent lethality, is only dependent
on the diameter and pressure of the pipeline. For 48 inch
diameter pipelines the maximum eect distance can be up
to 600 m. Whether the indicative limit of the societal risk willbe exceeded, also strongly depends on the above mentioned
parameters.
Consequences for land-use planningIn 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 register13. Based on 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 was to identify potential bottlenecks
where dwellings are situated within the new zoning distances
and to identify where, based on land-use plans available up
11. RIVM.ReferenceManualBeviRiskAssessments.Version3.2.2009.
12. K.Beijer,2009.Technicalnote:Mogelijkeverschillenin(externeveiligheid)risicotussendeoperatievannatgasendrooggastransportleidingsystemen,NAMEP200702210020,revision3.March(inDutch).
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to 2030, future possible bottlenecks may appear. The analysis
has been performed using data from 11 pipeline owners with in
total about 14,000 km 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 Netherlands14. 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 approximately 4050 km. Due to the land-use plans up to
2030 additional 80 km of pipeline could become a bottleneck. It
depends strongly on how these new plans are nally developed
whether the potential identied bottlenecks appear or not.
Additional measuresAdditional 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 measuresavailable for reducing the eects 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 ruptures3,
the additional measures for natural gas pipelines focus on this
cause2, 15. Proposed measures for reducing the risk of natural
gas pipelines can be categorised into two groups. Measures in
the rst group prevent (partly) that the pipeline is actually hit
during digging activities. Measures in the second group prevent
or control digging activities in the neighbourhood 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 dened and all
preconditions should be met before the subsequent reduction
factor can be used. In Table 1, proposed measures with their eect
on probability of a pipeline rupture due to external interference
are given. The eectiveness of the measures in practice must be
monitored in the forthcoming years.
Harmonisation with other transmission pipelinesIt is the objective to harmonise the methodologies for
underground transmission pipelines with ammable liquids
and other chemical substances. However, it is noted that other
failure mechanism than external interference, such as corrosion
or mechanical failure, are of importance, and the above measures
are therefore only partly eective for risk reduction. The risk
methodologies and risk reduction measures for these substancesare under development.
ConclusionsIn 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 has been
revised to reect new understandings in the risk scenarios, failure
frequencies and eects. 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 optimised land use. It also
will allow for tightened compliance checks by governmental
competent bodies.
13. J.P.vantSant,H.J.Manuel,A.vandenBerg,2008.Dutchregistrationofrisksituations.ESRELEuropeanSafetyandReliabilityConference,Valencia,Spain,22-25September(eds.S.Martorelletal.).
14. NetherlandsInstituteforPlanningandHousing(NIROV),2009.DeNieuweKaartvanNederland,January.
15. G.M.H.LaheijandA.A.C.vanVliet,2009.Measuresforreducingtheprobabilityofrupturesofhighpressurenaturalgaspipelines.EuropeanSafetyandReliabilityconference2009,Prague,7-10September.
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 1.6 100
Table1:Eectofadditionalmeasuresontheprobabilityofa
pipelinerupture.
This paper was presented at the Pipeline Technology
Conference held in Hannover, Germany, in April 2010, and
organised by EITEP.