Premium Digest December 2010 Consequences of a Risk-based Approach for Natural Gas Pipelines

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19 PiPelines international digest | deCeMBer 2010

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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.

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Premium Digest December 2010 Consequences of a Risk-based Approach for Natural Gas Pipelines