Risk Assessment Data Directory
Report No. 434 – 6
March 2010
Ignition
probabilities
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RADD – Ignition probabilities
contents
1.0
Introduction ........................................................................ 1
2.0
2.1
2.2
Summary of Recommended Data ......................................... 1
Ignition Probability Curves ......................................................................... 1
Blowout Ignition Probabilities .................................................................. 15
3.0
3.1
3.2
Guidance on use of data .................................................... 16
General Validity .......................................................................................... 16
Alternative Approaches............................................................................. 16
3.2.1
3.2.2
Releases addressed by datasheets in Section 2.0 ............................................ 16
Other releases ....................................................................................................... 19
3.3
Uncertainties .............................................................................................. 19
4.0
Review of data sources ..................................................... 19
5.0
Recommended data sources for further information .......... 21
6.0
References ........................................................................ 21
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RADD – Ignition probabilities
Abbreviations
FPSO
LPG
NAP
NUI
QRA
UKOOA
2
Floating Production Storage and Offloading (Installation)
Liquefied Petroleum Gas
Normal Atmospheric Pressure
Normally Unmanned Installation
Quantitative Risk Assessment
United Kingdom Offshore Operators Association
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RADD – Ignition probabilities
1.0
Introduction
The data presented in section 2 provide estimates of the probabilities of hydrocarbon
releases igniting to result in an explosion and/or a sustained fire. These data may be
applied to any on the leak types described in the Process Release Frequencies
datasheet1.
The values presented relate to “total” ignition probability, which can be considered as
the sum of the probabilities of immediate ignition and delayed ignition. Immediate
ignition can be considered as the situation where the fluid ignites immediately on
release through auto-ignition or because the accident which causes the release also
provided an ignition source. Delayed ignition is the result of the build-up of a
flammable vapour cloud which is ignited by a source remote from the release point. It
is assumed to result in flash fires or explosions, and also to burn back to the source
of 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 detailed
approach 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 drawn
from the UKOOA look-up correlations (see section 4.0) which relate ignition
probabilities in air2 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 cloud
size 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 the
scope of this datasheet.
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RADD – Ignition probabilities
Table 2.2 and Table 2.4. The functions themselves are given in both tabular and
graphical 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 the
immediate ignition probability is 0.001 and is independent of the release rate. As a
result, 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 ignition
probabilities by subtracting 0.001 from the total ignition probability, e.g. an ignition
probability value of 0.004 obtained from the look-up correlations can be considered as
an immediate ignition probability of 0.001 and a delayed ignition probability of 0.003.
2
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RADD – Ignition probabilities
Table 2.1 Onshore Ignition Scenarios
Scenario
No.
1
2
3
4
5
6
7
8
9
10
Look-up Release Type
Application
Pipe Liquid Industrial
(Liquid Releases from onshore
pipeline in industrial area)
Pipe Liquid Rural
(Liquid Releases from onshore
pipeline in industrial area)
Pipe Gas LPG Industrial
(Gas or LPG release from
onshore pipeline in an
industrial area)
Pipe Gas LPG Rural
(Gas or LPG release from
onshore pipeline in a rural area)
Small Plant Gas LPG
(Gas or LPG release from small
onshore plant)
Small Plant Liquid
(Liquid release from small
onshore plant)
Small Plant Liquid Bund Rural
(Liquid release from small
onshore plant where the spill is
bunded)
Large Plant Gas LPG
(Gas or LPG release from large
onshore plant)
Large Plant Liquid
(Liquid release from large
onshore plant)
Large Plant Liquid Bund Rural
(Liquid Released from large
onshore plant where spill is
bunded)
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.
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.
Releases of flammable gases, vapour or liquids significantly above their normal (Normal
Atmospheric Pressure (NAP)) boiling point from onshore cross-country pipelines running
through industrial or urban areas.
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point from onshore cross-country pipelines running through rural areas.
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point from small onshore plants (plant area up to 1200 m2, site area up to 35,000 m2).
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) and
which are not bunded or otherwise contained.
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) and
where the liquid releases from the plant area are suitably bunded or otherwise contained.
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point from large onshore outdoor plants (plant area above 1200 m2, site area above 35,000
m2).
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 35,000
m2) and which are not bunded or otherwise contained.
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 35,000
m2) and where the liquid releases from the plant area are suitably bunded or otherwise
contained.
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RADD – Ignition probabilities
Scenario
No.
11
12
Look-up Release Type
Application
Large Plant Congested Gas
LPG
(Gas or LPG released from a
large confined or congested
onshore plant)
Tank Liquid 300m x 300m Bund
(Liquid release from a large
confined or congested onshore
plant)
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point from large onshore plants (plant area above 1200 m2, site area above 35,000 m2), where
the plant is partially walled/roofed or within a shelter or very congested.
13
Tank Liquid 100m x 100m Bund
(Liquid release from onshore
tank farm where spill is limited
by small or medium sized bund)
14
Tank Gas LPG Plant
(gas or LPG release from
onshore tank farm within the
plant)
15
Tank Gas LPG Storage
Industrial
(Gas or LPG released from
onshore tank farm sited
adjacent to a plant or away from
the plant in an industrial area)
Tank Gas LPG Storage Only
Rural
(Gas or LPG released from
onshore tank farm sited
adjacent to a plant or away from
the plant in an industrial area)
16
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 multitank bund over 25,000 m2 area).
See curve No. 30 “Tank Liquid – diesel, fuel oil’ if liquids are stored at ambient conditions
below their flash point.
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 bund
containing 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 conditions
below their flash point.
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point from onshore outdoor storage tanks located in a 'tank farm' entirely surrounded by
plants. For tank farms adjacent to plants use curve No. 15 “Tank Gas LPG Storage Industrial”
or Curve No. 16 “Tank Gas LPG Storage Only Rural” look-up correlations. Releases from
process vessels or tanks inside plant areas should be treated as plant releases.
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point from onshore outdoor storage tanks located in a 'tank farm' adjacent to plants or
situated away from plants in an industrial or urban area.
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point 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]
4
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RADD – Ignition probabilities
Table 2.2 Offshore Ignition Scenarios
Scenario
No.
17
18
19
20
Look-up Release Type
Offshore Process Liquid
(Liquid release from offshore
process module)
Offshore Process Liquid NUI
(Liquid release from offshore
process area on NUI)
Offshore Process Gas Open
Deck NUI
(Gas release from offshore
process open deck area on NUI)
Offshore Process Gas Typical
(Gas release from typical
offshore process module)
21
Offshore Process Gas Large
Module
(gas release from typical
offshore process module)
22
Offshore Process Gas
Congested or Mechanical
Vented Module
(Gas released from a
mechanically ventilated or very
congested offshore process
module)
Offshore Riser
(Gas release from typical
offshore riser in air gap)
23
Application
Releases of flammable liquids that do not have any significant flash fraction (10% or less) if
released from within offshore process modules.
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.
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point 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 attended
integrated platforms.
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point from within offshore process modules or decks on integrated deck / conventional
installations). Process modules include separation, compression, pumps, condensate
handling, power generation, etc. If the module is mechanically ventilated or very congested –
see curve No. 22 “Offshore Process Gas Congested or Mechanical Vented Module”.
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point from within large offshore process modules or decks on integrated deck / conventional
installations (module greater than 1000 m2 floor area). Process modules include separation,
compression, pumps, condensate handling, power generation, etc. If the module is
mechanically ventilated or very congested – see curve No. 22 'Offshore Process Gas
Congested or Mechanical Vented Module'.
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point from within offshore process modules or decks on integrated deck / conventional
installations: applies where the module is enclosed and has a mechanical ventilation system
or is very congested (volume blockage ratio => 0.14 and less than 25% of area of the end
walls open for natural ventilation)
Releases from offshore installation risers in the air gap area where there is little chance of the
release entering process areas on the installation (e.g. solid decks, wind walls). Applies to
partial flashing oil or gas releases.
May also be used for blowouts with well positioned diverters directing any release away from
the installation (see also curve No. 27 “Offshore Engulf – blowout riser”).
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RADD – Ignition probabilities
Scenario
No.
24
25
26
27
Look-up Release Type
Offshore FPSO Gas
(Gas release from offshore
FPSO process module)
Offshore FPSO Gas Wall
(Gas release from offshore
FPSO process module behind a
transverse solid wall)
Offshore FPSO Liquid
(Liquid release from typical
offshore FPSO process
module)
Offshore Engulf – blowout –
riser
(Major release which can engulf
an entire offshore installation)
Application
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point from within offshore process modules or decks on FPSOs. See curve No. 25 “offshore
FPSO Gas Wall” if the release is from an area downwind of a transverse wall across the FPSO
deck.
Releases of flammable gases, vapour or liquids significantly above their normal (NAP) boiling
point 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.
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
Releases from drilling or well working blowouts or riser failures under open grated deck
areas 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 2.4 Special (Derived) Ignition Scenarios
Scenario
No.
30
Look-up Release Type
Application
Tank Liquid – diesel fuel oil
(Liquid Release from onshore
tank farm of liquids below their
flash point, e.g. diesel or fuel
oil)
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 storage
area “tank farm”. This look-up correlation can be applied to releases from tanks and low
pressure transfer lines or pumps in the tank farm/ storage area. However, it should not be
used for high-pressure systems (over a few barg): in these situations use curve No. 12 “Tank
Liquid 300m x 300m Bund” or curve No. 13 “Tank Liquid 100 x 100m Bund”
Source: Energy Institute [1]
6
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RADD – Ignition probabilities
Data Sheet 1: Scenarios 1 – 4
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RADD – Ignition probabilities
Data Sheet 2: Scenarios 5 – 7
8
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RADD – Ignition probabilities
Data Sheet 3: Scenarios 8 – 11
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RADD – Ignition probabilities
Data Sheet 4: Scenarios 12, 13 & 30
10
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RADD – Ignition probabilities
Data Sheet 5: Scenarios 14 – 16
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RADD – Ignition probabilities
Data Sheet 6: Scenarios 17 & 18
12
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RADD – Ignition probabilities
Data Sheet 7: Scenarios 19 – 22
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RADD – Ignition probabilities
Data Sheet 8: Scenarios 24 – 26
14
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RADD – Ignition probabilities
Data Sheet 9: Scenarios 23 & 27
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RADD – Ignition probabilities
Notes:
1. A flammable substance above its auto-ignition temperature is likely to ignite on
release and should be modelled as having an ignition probability of one.
2. Very reactive substances are unlikely to found in oil and gas processing
operations but if present it is suggested that the values given in the look-up
correlations are doubled, subject to a maximum of 1. Such substances include
hydrogen, acetylene, ethylene oxide and carbon disulphide.
3. High flash point (>55°C) liquids stored at or near ambient conditions are
significantly less likely to ignite than suggested in the look-up correlations. It is
suggested that an ignition probability from the look-up correlations is multiplied by
a factor of 0.1 subject to a minimum of 0.001 and taking account of the 0.001
immediate ignition probability.
4. For liquids with flash fractions above 10% it is suggested that the ignition
probability is estimated by combining the relevant liquid ignition probability with a
suitable 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 fraction
should 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 basis
that the areas covered by the liquid and gas are likely to have considerable
overlap.
5. Since the correlations are based on typical combinations of ignition sources, it
follows that they should not be used in situations where particularly strong
sources such as fired heaters are present. In this case the full UKOOA ignition
model is more appropriate.
2.2
Blowout Ignition Probabilities
An alternative to the blowout ignition probabilities given by the UKOOA look-up
correlations can be obtained from Scandpower’s interpretation of the blowout data
provided by SINTEF 2. This is given in Table 2.5. The most significant category is that
for deep blowouts which indicates an early ignition probability of 0.09. For the
purposes of QRA studies this can be taken as occurring immediately on release. The
report also gives a delayed ignition probability of 0.16 although all of these are taken
to 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 2.5 Ignition Probabilities for Blowouts and W ell Releases on
Platform s
16
Release Type
Early ignition
(< 5 min)
Shallow Gas Blowout
Deep Blowout
Deep Well Release
0.07
0.09
0.03
©OGP
Delayed
ignition
(5 – 60 min)
0.11
-
Very Delayed
ignition (> 60 min)
0.07
0.16
-
RADD – Ignition probabilities
3.0
Guidance on use of data
3.1
General Validity
The correlations are considered to provide an acceptable approach for use in typical
QRA studies. For more detailed analysis it is recommended that the full spreadsheet
UKOOA ignition model is used so that the specific circumstances with regard to
layout 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. They
may be applied to the analysis of hydrocarbon releases in other regions which comply
with recognised industry good practice, as it is applied in the UKCS.
The forward to the Energy Institute report states that the model and look-up
correlations “are not suited to the ignition probability assessment of refrigerated
liquefied gases, vapourising liquid pools, sub-sonic gas releases, or non-momentum
driven releases, such as those following catastrophic storage vessel failure.”
Despite this note, flashing liquid releases are covered by a number of the correlations
and analysts may further modify them by combining them with a gas or LPG ignition
probability in suitable proportions as suggested in note 4 of section 2.1. Atmospheric
storage tanks are dealt with in the Storage Incident Frequencies data sheet. Low
momentum and sub-sonic gas releases are uncommon in process systems. An
approach to the scenarios for which the correlations are not valid is suggested in
Section 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 2.1
to
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RADD – Ignition probabilities
Table 2.2 and Table 2.4 best matches the scenario under consideration. There may be
situations where the scenario under consideration lies between two of the described
scenarios, 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 interpolating
between the data points, it is necessary to take logarithms of the release rate and
probabilities, interpolate between these to find the logarithm of the required
probability and then obtain the value itself, i.e.:
where Pign
is the required ignition probability corresponding to release rate Q
is the ignition probability at a release rate of Qlower (the lower bound of
the relevant curve section), and
is the ignition probability at a release rate of Qupper (the upper bound of
the relevant curve section)
The third of these options is the recommended approach and the analyst may find it
convenient to construct a spreadsheet or some other computer programme to carry
this 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 Energy
report 1, which provides further explanation on the derivation of the lines. This
specifies the release rates and ignition probabilities relating to each of the points
bounding the segments as indicated in Figure 3.1. Some information on the timing of
ignitions is also available in 1.
Figure 3.1 Typical Ignition Probability Curve
18
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RADD – Ignition probabilities
A more accurate assessment may be obtained by the use of the full UKOOA ignition
model which is described in 1. This has been implemented in a spreadsheet tool
which 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 ignition
sources. However, this requires more effort on the part of the analyst and the
availability of more installation specific data compared with the relative ease with
which the look-up functions can be used.
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RADD – Ignition probabilities
Table 3.1 Data for Look-up Correlations
Scenario
No.
Type
1
2
3
4
5
6
7
8
9
10
Pipe Liquid Industrial
Pipe Liquid Rural
Pipe Gas LPG Industrial
Pipe Gas LPG Rural
Small Plant Gas LPG
Small Plant Liquid
Small Plant Liquid Bund Rural
Large Plant Gas LPG
Large Plant Liquid
Large Plant Liquid Bund Rural
Large Plant Congested Gas
LPG
Tank Liquid 300x300 Bund
Tank Liquid 100x100 Bund
Tank Gas LPG Plant
Tank Gas LPG Storage Only
Industrial
Tank Gas LPG Storage Only
Rural
Offshore Process Liquid
Offshore Process Liquid NUI
Offshore Process Gas Open
Deck NUI
Offshore Process Gas Typical
Offshore Process Gas Large
Module
Offshore Process Gas
Congested or Mechanically
Vented Module
Offshore Riser
Offshore FPSO Gas
Offshore FPSO Gas Wall
Offshore FPSO Liquid
Offshore Engulf – Blowout Riser
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
30
20
Tank Liquid - Diesel and
Fuel Oil
Point 1
Release
Probability
rate
0.1
0.001
0.1
0.001
0.1
0.001
0.1
0.001
0.1
0.001
0.1
0.001
0.1
0.001
0.1
0.001
0.1
0.001
0.1
0.001
Point 2
Release
Probability
rate
70.00
0.07
0.30
0.00
1000.01
1.00
10.00
0.00
1.00
0.00
1.00
0.00
1.00
0.00
1.00
0.00
1.00
0.00
1.00
0.00
Poin
Release
P
rate
70.00
23408.55
3.00
100.00
8.05
260.00
109.99
42.49
0.1
0.001
1.00
0.00
70.00
0.1
0.1
0.1
0.001
0.001
0.001
1.00
1.00
1.00
0.00
0.00
0.00
7.00
7.00
102.84
0.1
0.001
1.00
0.00
100.00
0.1
0.001
1.00
0.00
10.00
0.1
0.1
0.001
0.001
100.00
24.73
0.02
0.01
0.1
0.001
1.00
0.00
31.42
0.1
0.001
3.00
0.01
37.01
0.1
0.001
5.00
0.03
30.00
0.1
0.001
1.00
0.01
92.63
0.1
0.1
0.1
0.1
0.001
0.001
0.001
0.001
38.27
1.00
0.30
100.00
0.03
0.00
0.00
0.03
50.00
10.00
0.1
0.001
100.00
0.10
0.1
0.001
1.00
0.00
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RADD – Ignition probabilities
3.2.2
Other releases
As noted in Section 3.1, the UKOOA ignition model cannot be considered valid for all
types 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 more
fundamental treatment by calculating likely cloud sizes for the given release, material
and weather conditions and estimating the number and strength of ignition sources
which the flammable part of the cloud may reach. There is no generally recognized
method for determining ignition source strength for use in QRAs. Some values are
given in the “Purple Book” [3] but these are estimates based on engineering judgment
and do not have any more scientific basis.
3.3
Uncertainties
The assessment of ignition probability is subject to a large degree of uncertainty. The
spreadsheet 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. The
adoption of the lookup correlations based on this model introduces more
uncertainties because a compromise has to be made in selecting the most appropriate
curve and these curves themselves are approximations to the curves produced by the
model 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 Energy
Institute Research Report [1], published on behalf of the joint industry project
sponsors UKOOA (Now Oil and Gas UK), the HSE and the Energy Institute. The report
reviews existing models and develops a new model which could be applied to both
onshore and offshore scenarios. The work was undertaken in two phases. The first
phase involved developing a model for assigning ignition probabilities in QRA studies
and to further the understanding of scenario specific ignition probabilities. The work
was undertaken by AEA Technology (now ESR Technology) and co-ordinated by a
joint industry steering group drawn from UKOOA member representatives, the HSE
and 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 the
usefulness of the UK HSE’s hydrocarbon release database as a basis to develop an
improved 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 to
include the capability to assess the ignition probability of most typical onshore
hydrocarbon leak scenarios.
The spreadsheet attempts to model the ignition
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RADD – Ignition probabilities
probability by considering the size of the gas cloud which would be formed by the
release and taking into account the number and type of ignition sources which the
cloud, at sufficient concentration, might reach. As a result of the complexity of the
model, 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 commissioned
to consider representative scenarios which would generate look-up correlations which
could be used in QRA studies without the need for the user to gather the data required
for 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 aerosol
formation 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 maximum
below 1000 kg/s)
The configurations considered are given in Table 2.1 to Table 2.4.
A large number of analyses were carried out to produce graphs of ignition probability
against release rate. Figure 4.1 shows a typical set of curves.
In the final stage of the process, groups of similar curves were considered and
grouped into the scenarios listed in Table 2.1 to Table 2.4. These scenarios were then
examined 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 plotted
on logarithmic axes. It is these curves which are depicted in the data sheets.
Figure 4.1 Exam ple of Ignition Probability Curve Calculated by UKOOA
ignition m odel
Source: Energy Institute [1]
22
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RADD – Ignition probabilities
Prior to the introduction of the UKOOA ignition model approach outlined above, the
formulation attributed to Cox, Lees and Ang 4 was widely used. This gained
acceptance largely because of the proportion of analysts using it rather than because
of the rigour of the theory underlying it. Ignition probabilities predicted by this
method were in excess of what was found to occur in practice and this was partly
responsible for instigating the work which resulted in the UKOOA ignition model.
References in this report to “UKOOA (spreadsheet) model” and “UKOOA look-up
correlations” 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, Research
Report 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: Institution
of Chemical Engineers, ISBN 0 85295 258 9.
©OGP
23
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