Ignition probabilities

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Risk Assessment Data Directory

Report No. 434 6.1March 2010

I n t e r n a t i o n a l A s s o c i a t i o n o f O i l & G a s P r o d u c e r s

Ignitionprobabilities


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contents

1.0 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 1

2.0 Summary of Recommended Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 1 2.1 Ignition Probability Curves......................................................................... 1 2.2 Blowout Ignition Probabilities .................................................................. 16

3.0 Guidance on use of data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .17 3.1 General Validity.......................................................................................... 17 3.2 Alternative Approaches ............................................................................ 17 3.2.1 Releases addressed by datasheets in Section 2.0 ............................................ 17 3.2.2 Other releases ............................. .................................. ................................. ....... 20 3.3 Uncertainties.............................................................................................. 20

4.0 Review of data sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .20

5.0 Recommended data sources for further information . . . . . . . . . . .22

6.0 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22


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Abbreviations FPSO Floating Production Storage and Offloading (Installation)LPG Liquefied Petroleum GasNAP Normal Atmospheric PressureNUI Normally Unmanned InstallationQRA Quantitative Risk AssessmentUKOOA United Kingdom Offshore Operators Association


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1.0 Introduction

The data presented in section 2 provide estimates of the probabilities of hydrocarbonreleases igniting to result in an explosion and/or a sustained fire. These data may beapplied to any on the leak types described in the Process Release Frequencies datasheet 1.

The values presented relate to total ignition probability, which can be considered asthe sum of the probabilities of immediate ignition and delayed ignition. Immediateignition can be considered as the situation where the fluid ignites immediately onrelease through auto-ignition or because the accident which causes the release alsoprovided an ignition source. Delayed ignition is the result of the build-up of aflammable vapour cloud which is ignited by a source remote from the release point. Itis assumed to result in flash fires or explosions, and also to burn back to the sourceof the leak resulting in a jet fire and/or a pool fire.

These probabilities are considered appropriate for use in QRA studies where a

relatively coarse assessment is acceptable. Section 3.2 refers to a more detailedapproach for QRAs where this is considered to be required.

2.0 Summary of Recommended Data

2.1 Ignition Probability Curves

Data presented in this section come in the form of 28 mathematical functions drawnfrom the UKOOA look-up correlations (see section 4.0) which relate ignitionprobabilities in air 2 to release rates for typical scenarios both onshore and offshore.The various scenarios are summarised in Table 2.1 ,

1 With the exception of zero pressure releases, where the limited inventory and hence cloudsize would result in a lower ignition probability than would be predicted using this approach.2 Ignition probabilities in other atmospheres, e.g. oxygen enriched or chlorine, are outside thescope of this datasheet.


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Table 2.2 and Table 2.3 . The functions themselves are given in both tabular andgraphical form in the data sheets which follow.

The curves of ignition probability vs. release rate comprise between two and four sections, each a straight line when plotted on log-log axes.

These curves represent total ignition probability. The method assumes that theimmediate ignition probability is 0.001 and is independent of the release rate. As aresult, all the curves start at a value of 0.001 relating to a release rate of 0.1 kg/s.Users of the data may wish to adopt this value and to obtain delayed ignitionprobabilities by subtracting 0.001 from the total ignition probability, e.g. an ignitionprobability value of 0.004 obtained from the look-up correlations can be considered asan immediate ignition probability of 0.001 and a delayed ignition probability of 0.003.


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Table 2.1 Onshore Ignit ion Scenarios

ScenarioNo . Look-up Release Type Application

1 Pipe Liquid Industrial(Liquid Releases from onshorepipeline in industrial area)

Releases of flammable liquids that do not have any significant flash fraction (10% or less) if released from onshore cross-country pipelines running through industrial or urban areas.

2 Pipe Liquid Rural(Liquid Releases from onshorepipeline in industrial area)

Releases of flammable liquids that do not have any significant flash fraction (10% or less) if released from onshore cross-country pipelines running through rural areas.

3 Pipe Gas LPG Industrial(Gas or LPG release fromonshore pipeline in anindustrial area)

Releases of flammable gases, vapour or liquids significantly above their normal (NormalAtmospheric Pressure (NAP)) boiling point from onshore cross-country pipelines runningthrough industrial or urban areas.

4 Pipe Gas LPG Rural(Gas or LPG release fromonshore pipeline in a rural area)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from onshore cross-country pipelines running through rural areas.

5 Small Plant Gas LPG(Gas or LPG release from smallonshore plant)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from small onshore plants (plant area up to 1200 m2, site area up to 35,000 m2).

6 Small Plant Liquid(Liquid release from smallonshore plant)

Releases of flammable liquids that do not have any significant flash fraction (10% or less) if released from small onshore plants (plant area up to 1200 m2, site area up to 35,000 m2) andwhich are not bunded or otherwise contained.

7 Small Plant Liquid Bund Rural(Liquid release from smallonshore plant where the spill isbunded)

Releases of flammable liquids that do not have any significant flash fraction (10% or less) if released from small onshore plants (plant area up to 1200 m2, site area up to 35,000 m2) andwhere the liquid releases from the plant area are suitably bunded or otherwise contained.

8 Large Plant Gas LPG(Gas or LPG release from largeonshore plant)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from large onshore outdoor plants (plant area above 1200 m2, site area above 35,000m2).

9 Large Plant Liquid(Liquid release from largeonshore plant)

Releases of flammable liquids that do not have any significant flash fraction (10% or less) if released from large onshore outdoor plants (plant area above 1200 m2, site area above 35,000m2) and which are not bunded or otherwise contained.

10 Large Plant Liquid Bund Rural(Liquid Released from largeonshore plant where spill isbunded)

Releases of flammable liquids that do not have any significant flash fraction (10% or less) if released from large onshore outdoor plants (plant area above 1200 m2, site area above 35,000m2) and where the liquid releases from the plant area are suitably bunded or otherwisecontained.


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11 Large Plant Congested GasLPG(Gas or LPG released from alarge confined or congestedonshore plant)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from large onshore plants (plant area above 1200 m2, site area above 35,000 m2), wherethe plant is partially walled/roofed or within a shelter or very congested.

12 Tank Liquid 300m x 300m Bund(Liquid release from a largeconfined or congested onshoreplant)

Releases flammable liquids that do not have any significant flash fraction (10% or less) if released from very large onshore outdoor storage area 'tank farm' (e.g. spill in a large multi-tank bund over 25,000 m2 area).See curve No. 30 Tank Liquid diesel, fuel oil if liquids are stored at ambient conditionsbelow their flash point.

13 Tank Liquid 100m x 100m Bund(Liquid release from onshoretank farm where spill is limitedby small or medium sized bund)

Releases of flammable liquids that do not have any significant flash fraction (10% or less) if released from onshore outdoor storage area 'tank farm' (e.g. spill in a large tank bundcontaining four or fewer tanks, or any other bund less than 25,000 m2 area).See curve No. 30 Tank Liquid diesel, fuel oil if liquids are stored at ambient conditionsbelow their flash point.

14 Tank Gas LPG Plant(gas or LPG release fromonshore tank farm within theplant)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from onshore outdoor storage tanks located in a 'tank farm' entirely surrounded byplants. For tank farms adjacent to plants use curve No. 15 Tank Gas LPG Storage Industrialor Curve No. 16 Tank Gas LPG Storage Only Rural look-up correlations. Releases fromprocess vessels or tanks inside plant areas should be treated as plant releases.

15 Tank Gas LPG StorageIndustrial(Gas or LPG released fromonshore tank farm sitedadjacent to a plant or away fromthe plant in an industrial area)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from onshore outdoor storage tanks located in a 'tank farm' adjacent to plants or situated away from plants in an industrial or urban area.

16 Tank Gas LPG Storage OnlyRural

(Gas or LPG released fromonshore tank farm sitedadjacent to a plant or away fromthe plant in an industrial area)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from onshore outdoor storage tanks located in a 'tank farm' adjacent to plants or

situated away from plants in a rural area.

Source: Energy Institute [1]


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Table 2.2 Offshore Ignit ion Scenarios


17 Offshore Process Liquid(Liquid release from offshoreprocess module)

Releases of flammable liquids that do not have any significant flash fraction (10% or less) if released from within offshore process modules.

18 Offshore Process Liquid NUI(Liquid release from offshoreprocess area on NUI)

Releases of flammable liquids that do not have any significant flash fraction (10% or less) if released from within offshore process modules or decks on NUIs.

19 Offshore Process Gas OpenDeck NUI(Gas release from offshoreprocess open deck area on NUI)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from an offshore process weather deck/ open deck on NUIs. Can also be used for open/uncongested weather decks with limited process equipment on larger attendedintegrated platforms.

20 Offshore Process Gas Typical(Gas release from typicaloffshore process module)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from within offshore process modules or decks on integrated deck / conventionalinstallations). Process modules include separation, compression, pumps, condensatehandling, power generation, etc. If the module is mechanically ventilated or very congested see curve No. 22 Offshore Process Gas Congested or Mechanical Vented Module.

21 Offshore Process Gas LargeModule(gas release from typicaloffshore process module)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from within large offshore process modules or decks on integrated deck / conventionalinstallations (module greater than 1000 m2 floor area). Process modules include separation,compression, pumps, condensate handling, power generation, etc. If the module ismechanically ventilated or very congested see curve No. 22 'Offshore Process GasCongested or Mechanical Vented Module'.

22 Offshore Process GasCongested or MechanicalVented Module(Gas released from amechanically ventilated or verycongested offshore processmodule)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from within offshore process modules or decks on integrated deck / conventionalinstallations: applies where the module is enclosed and has a mechanical ventilation systemor is very congested (volume blockage ratio => 0.14 and less than 25% of area of the endwalls open for natural ventilation)

23 Offshore Riser (Gas release from typicaloffshore riser in air gap)

Releases from offshore installation risers in the air gap area where there is little chance of therelease entering process areas on the installation (e.g. solid decks, wind walls). Applies topartial flashing oil or gas releases.May also be used for blowouts with well positioned diverters directing any release away fromthe installation (see also curve No. 27 Offshore Engulf blowout riser).


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24 Offshore FPSO Gas(Gas release from offshoreFPSO process module)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from within offshore process modules or decks on FPSOs. See curve No. 25 offshoreFPSO Gas Wall if the release is from an area downwind of a transverse wall across the FPSOdeck.

25 Offshore FPSO Gas Wall(Gas release from offshoreFPSO process module behind atransverse solid wall)

Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boilingpoint from within offshore process modules or decks on FPSOs. This correlation applies if the release is from an area downwind of a transverse wall across the FPSO deck.

26 Offshore FPSO Liquid(Liquid release from typical

offshore FPSO processmodule)

Releases of flammable liquids that do not have any significant flash fraction (10% or less) if released from within offshore process modules or decks on FPSOs

27 Offshore Engulf blowout riser (Major release which can engulf an entire offshore installation)

Releases from drilling or well working blowouts or riser failures under open grated deckareas where the release could engulf the entire installation and reach into platform areas:applies to partial flashing oil or gas releases. (see also curve No. 23 Offshore Riser for riser releases and blowouts with divertors)

Source: Energy Institute [1]Note. Curve Nos. 28 and 29 related to Cox, Lees and Ang formulation which were included in the document for comparison

Table 2.3 Special (Derived) Ignit ion Scenarios

ScenarioNo .

Look-up Release Type Application

30 Tank Liquid diesel fuel oil(Liquid Release from onshoretank farm of liquids below their flash point, e.g. diesel or fueloil)

Releases of combustible liquids stored at ambient pressure and at temperatures below their flash point (e.g. most gas, oil, diesel and fuel oil storage tanks) from onshore outdoor storagearea tank farm. This look-up correlation can be applied to releases from tanks and lowpressure transfer lines or pumps in the tank farm/ storage area. However, it should not beused for high-pressure systems (over a few barg): in these situations use curve No. 12 TankLiquid 300m x 300m Bund or curve No. 13 Tank Liquid 100 x 100m Bund



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Data Sheet 1: Scenarios 1 4


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Data Sheet 4: Scenarios 12, 13 & 30


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Data Sheet 6: Scenarios 17 & 18


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Data Sheet 9: Scenarios 23 & 27


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Notes:

1. A flammable substance above its auto-ignition temperature is likely to ignite onrelease and should be modelled as having an ignition probability of one.

2. Very reactive substances are unlikely to found in oil and gas processingoperations but if present it is suggested that the values given in the look-upcorrelations are doubled, subject to a maximum of 1. Such substances includehydrogen, acetylene, ethylene oxide and carbon disulphide.

3. High flash point (>55C) liquids stored at or near ambient conditions aresignificantly less likely to ignite than suggested in the look-up correlations. It issuggested that an ignition probability from the look-up correlations is multiplied bya factor of 0.1 subject to a minimum of 0.001 and taking account of the 0.001immediate ignition probability.

4. For liquids with flash fractions above 10% it is suggested that the ignitionprobability is estimated by combining the relevant liquid ignition probability with asuitable gas/LPG ignition probability. The appropriate release rates should be

obtained from the flash fraction, e.g. a 10 kg/s release with a 20% flash fractionshould give rise to an equivalent 2 kg/s gas release and 8 kg/s liquid release.

The two probabilities can be combined using the following equation;

Alternatively the higher of the two ignition probabilities can be used on the basisthat the areas covered by the liquid and gas are likely to have considerableoverlap.

5. Since the correlations are based on typical combinations of ignition sources, itfollows that they should not be used in situations where particularly strongsources such as fired heaters are present. In this case the full UKOOA ignitionmodel is more appropriate.

2.2 Blowout Ignition Probabilities

An alternative to the blowout ignition probabilities given by the UKOOA look-upcorrelations can be obtained from Scandpowers interpretation of the blowout dataprovided by SINTEF 2. This is given in Table 2.4. The most significant category is thatfor deep blowouts which indicates an early ignition probability of 0.09. For thepurposes of QRA studies this can be taken as occurring immediately on release. Thereport also gives a delayed ignition probability of 0.16 although all of these are takento occur more than one hour after the start of the release. Conservatively, this could

be taken as occurring shortly after the initial release and result in an explosion.

Table 2 .4 Igni t ion Probabi l i t ies for Blowouts and Wel l Releases onPlat forms

Release Type Early ignition(< 5 min)

Delayedignition

(5 60 min)

Very Delayedigni t ion (> 60 min)

Shallow Gas Blowout 0.07 0.11 0.07Deep Blowout 0.09 - 0.16

Deep Well Release 0.03 - -


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3.0 Guidance on use of data

3.1 General Validity

The correlations are considered to provide an acceptable approach for use in typicalQRA studies. For more detailed analysis it is recommended that the full spreadsheetUKOOA ignition model is used so that the specific circumstances with regard tolayout and ignition sources can be more accurately represented.

The correlations were developed for UKOOA member companies with the intention of providing representative probabilities for installations operating in UK waters. Theymay be applied to the analysis of hydrocarbon releases in other regions which complywith recognised industry good practice, as it is applied in the UKCS.

The forward to the Energy Institute report states that the model and look-upcorrelations are not suited to the ignition probability assessment of refrigeratedliquefied gases, vapourising liquid pools, sub-sonic gas releases, or non-momentumdriven releases, such as those following catastrophic storage vessel failure.

Despite this note, flashing liquid releases are covered by a number of the correlationsand analysts may further modify them by combining them with a gas or LPG ignitionprobability in suitable proportions as suggested in note 4 of section 2.1. Atmosphericstorage tanks are dealt with in the Storage Incident Frequencies data sheet. Lowmomentum and sub-sonic gas releases are uncommon in process systems. Anapproach to the scenarios for which the correlations are not valid is suggested inSection 3.2.2.

3.2 Alternative Approaches

3.2.1 Releases addressed by datasheets in Section 2.0

The initial task for the analyst is to determine which of the scenarios given in Table 2.1to


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Table 2.2 and Table 2.3 best matches the scenario under consideration. There may besituations where the scenario under consideration lies between two of the describedscenarios, in which case the analysts may attempt to interpolate between two curves.

The data presented in the tables in Section 2.0 can be used in three ways:

1. Estimate from the graphs

2. Obtain probability based on the tabulated values

3. Use values in Table 3.1 to calculate the probability. Note that, in interpolatingbetween the data points, it is necessary to take logarithms of the release rate andprobabilities, interpolate between these to find the logarithm of the requiredprobability and then obtain the value itself, i.e.:

where P ign is the required ignition probability corresponding to release rate Q is the ignition probability at a release rate of Q lower (the lower bound of

the relevant curve section), and

is the ignition probability at a release rate of Q upper (the upper bound of

the relevant curve section)

The third of these options is the recommended approach and the analyst may find itconvenient to construct a spreadsheet or some other computer programme to carrythis out.

The data used to generate the lines on the graphs in the datasheets (Section 2.1) are

shown in Table 3.1. This has been derived from Table 2.9 in the Institute of Energyreport 1, which provides further explanation on the derivation of the lines. Thisspecifies the release rates and ignition probabilities relating to each of the pointsbounding the segments as indicated in Figure 3.1. Some information on the timing of ignitions is also available in 1.

Figure 3.1 Typical Ignit ion Probabil i ty Curve


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A more accurate assessment may be obtained by the use of the full UKOOA ignitionmodel which is described in 1. This has been implemented in a spreadsheet toolwhich is made available on a CD which accompanies the report. This allows the user to input specific data relating to release conditions, platform layout and ignitionsources. However, this requires more effort on the part of the analyst and theavailability of more installation specific data compared with the relative ease withwhich the look-up functions can be used.


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Table 3.1 Data for Look-up Correlations

Point 1 Point 2 Point 3 Point 4Scenario

No .Type Release

rateProbabili ty Release



rateProbabili ty

1 Pipe Liquid Industrial 0.1 0.001 70.00 0.072 Pipe Liquid Rural 0.1 0.001 0.30 0.00 70.00 0.013 Pipe Gas LPG Industrial 0.1 0.001 1000.01 1.004 Pipe Gas LPG Rural 0.1 0.001 10.00 0.00 23408.55 1.005 Small Plant Gas LPG 0.1 0.001 1.00 0.00 3.00 0.01 498.99 0.606 Small Plant Liquid 0.1 0.001 1.00 0.00 100.00 0.107 Small Plant Liquid Bund Rural 0.1 0.001 1.00 0.00 8.05 0.01

8 Large Plant Gas LPG 0.1 0.001 1.00 0.00 260.00 0.659 Large Plant Liquid 0.1 0.001 1.00 0.00 109.99 0.1310 Large Plant Liquid Bund Rural 0.1 0.001 1.00 0.00 42.49 0.05

11 Large Plant Congested GasLPG

0.1 0.001 1.00 0.00 70.00 0.43 325.03 0.70

12 Tank Liquid 300x300 Bund 0.1 0.001 1.00 0.00 7.00 0.00 519.62 0.1213 Tank Liquid 100x100 Bund 0.1 0.001 1.00 0.00 7.00 0.00 49.03 0.0214 Tank Gas LPG Plant 0.1 0.001 1.00 0.00 102.84 1.00

15 Tank Gas LPG Storage OnlyIndustrial 0.1 0.001 1.00 0.00 100.00 0.23 988.11 1.00

16 Tank Gas LPG Storage OnlyRural 0.1 0.001 1.00 0.00 10.00 0.02 52551.35 0.50

17 Offshore Process Liquid 0.1 0.001 100.00 0.0218 Offshore Process Liquid NUI 0.1 0.001 24.73 0.01

19 Offshore Process Gas OpenDeck NUI

0.1 0.001 1.00 0.00 31.42 0.03

20 Offshore Process Gas Typical 0.1 0.001 3.00 0.01 37.01 0.04

21 Offshore Process Gas LargeModule

0.1 0.001 5.00 0.03 30.00 0.05

22 Offshore Process GasCongested or MechanicallyVented Module

0.1 0.001 1.00 0.01 92.63 0.04

23 Offshore Riser 0.1 0.001 38.27 0.0324 Offshore FPSO Gas 0.1 0.001 1.00 0.00 50.00 0.1525 Offshore FPSO Gas Wall 0.1 0.001 0.30 0.00 10.00 0.1526 Offshore FPSO Liquid 0.1 0.001 100.00 0.03

27 Offshore Engulf Blowout -Riser 0.1 0.001 100.00 0.10

30Tank Liquid - Diesel andFuel Oil 0.1 0.001 1.00 0.00 7.00 0.00 25.55 0.00


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3.2.2 Other releases

As noted in Section 3.1, the UKOOA ignition model cannot be considered valid for alltypes of release. In particular, it does not refrigerated releases that form evaporating

liquid pools.Analysis of these and the other scenarios referred to there may require a morefundamental treatment by calculating likely cloud sizes for the given release, materialand weather conditions and estimating the number and strength of ignition sourceswhich the flammable part of the cloud may reach. There is no generally recognizedmethod for determining ignition source strength for use in QRAs. Some values aregiven in the Purple Book [3] but these are estimates based on engineering judgmentand do not have any more scientific basis.

3.3 Uncertainties

The assessment of ignition probability is subject to a large degree of uncertainty. Thespreadsheet model produced under phase I of the joint industry project is itself subject to uncertainties in the analytical approach taken and in the data used. Theadoption of the lookup correlations based on this model introduces moreuncertainties because a compromise has to be made in selecting the most appropriatecurve and these curves themselves are approximations to the curves produced by themodel itself.

Ignition probabilities are influenced by design layout, the number and separation of ignition sources, the quality of maintenance of equipment, and thereby the control of ignition sources.

Despite these uncertainties, the approach is considered to be an advance on other formulations which relate ignition probability to release rate only with no regard for the presence of ignition sources, the nature of the fluids or the layout of the plant.

4.0 Review of data sources

The data presented in Section 2 are largely a reproduction of data from the EnergyInstitute Research Report [1], published on behalf of the joint industry projectsponsors UKOOA (Now Oil and Gas UK), the HSE and the Energy Institute. The reportreviews existing models and develops a new model which could be applied to bothonshore and offshore scenarios. The work was undertaken in two phases. The firstphase involved developing a model for assigning ignition probabilities in QRA studiesand to further the understanding of scenario specific ignition probabilities. The workwas undertaken by AEA Technology (now ESR Technology) and co-ordinated by ajoint industry steering group drawn from UKOOA member representatives, the HSEand consultants working in the field of onshore and offshore QRA.

The report summarised the current status of knowledge and research in the field of ignition probability estimation in support of QRA. It evaluated this, together with theusefulness of the UK HSEs hydrocarbon release database as a basis to develop animproved ignition model for use in QRA. The end result is a spreadsheet model for estimating the ignition probability of process leaks offshore and also attempts toinclude the capability to assess the ignition probability of most typical onshorehydrocarbon leak scenarios. The spreadsheet attempts to model the ignition


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probability by considering the size of the gas cloud which would be formed by therelease and taking into account the number and type of ignition sources which thecloud, at sufficient concentration, might reach. As a result of the complexity of themodel, users are required to obtain and enter a significant amount of data relating to

the platform configuration and the distribution of ignition sources.Having completed the work to establish a model, a second phase was commissionedto consider representative scenarios which would generate look-up correlations whichcould be used in QRA studies without the need for the user to gather the data requiredfor the full model. The following summarises the release types considered. Gas releases LPG (flashing liquefied gas) releases Pressurised liquid oil releases leading to a spray release with flashing/

evaporation/ aerosol formation

Low pressure liquid oil releases leading to a spreading pool only (no aerosolformation or flashing) Release rates from 0.1 to 1000 kg/s (graphs shown in the data sheets are

extended to 10000 kg/s where the probability function does not reach a maximumbelow 1000 kg/s)

The configurations considered are given in Table 2.1 to Table 2.3.

A large number of analyses were carried out to produce graphs of ignition probabilityagainst release rate. Figure 4.1 shows a typical set of curves.

In the final stage of the process, groups of similar curves were considered andgrouped into the scenarios listed in Table 2.1 to Table 2.3. These scenarios were thenexamined and a representative curve assigned to them. These curves consist of between two and four segments each of which appears as a straight line when plottedon logarithmic axes. It is these curves which are depicted in the data sheets.

Figure 4 .1 Example of Igni t ion Probabi l i ty Curve Calcula ted by UKOOA

igni t ion model



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Prior to the introduction of the UKOOA ignition model approach outlined above, theformulation attributed to Cox, Lees and Ang 4 was widely used. This gainedacceptance largely because of the proportion of analysts using it rather than becauseof the rigour of the theory underlying it. Ignition probabilities predicted by thismethod were in excess of what was found to occur in practice and this was partlyresponsible for instigating the work which resulted in the UKOOA ignition model.

References in this report to UKOOA (spreadsheet) model and UKOOA look-upcorrelations relate respectively to the output from the two phases of the project [1].

5.0 Recommended data sources for further information

For further information, on the ignition probability curves presented in this document,the Energy Institute report 1 should be consulted.

6.0 References

1. Ignition Probability Review, Model Development and Look-Up Correlations , ResearchReport published by the Energy Institute, January 2006. ISBN 978 0 85293 454 8

2. Scandpower Risk Management AS 2006. Blowout and Well Release Frequencies Based on SINTEF Offshore Blowout Database, 2006 , Report No. 90.005.001/R2.

3. Guidelines for quantitative risk assessment (Purple book), Part 1, Establishment ,CPR18 E, Committee for the Prevention of Disasters (CPR), National Institute of Public Health and Environment (RIVM), Ministry of Transport, Public Works &Water Assessment Management, AVIV Adviserend Ingenieurs Save Ingenieurs(Adviesbureau), 1999.

4. Cox, Lees and Ang, 1991. Classification of Hazardous Locations , Rugby: Institutionof Chemical Engineers, ISBN 0 85295 258 9.


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Ignition probabilities

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