Page 326 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Ignition probabilities 141303
Table 14.14 Estimates of ignition probabilities for natural gas, based Table 14.17 Estimates of ignition probabilities for various products
on offshore data. above and below grade
Release Ignition probahilir); Ignition probability Pi)
Minor (<I kg/sec) 0.01 Above and belousground Belowgmundonl)
Massive (>50 kgisec) 0.3 ~~ ~
Major (1-50 kg/sec) 0.07 Crude oil 3.1 2
Diesel oil 1.8 1.5
Source: Office of Gas Safety, "Guide to Quantitative Risk Assessment Fuel oil 2 0
(QRA)," Standards Australia ME-038-01 (Committee on Pipelines: Gasoline 6 3.1
Gas and Liquid Petroleum), Risk and Reliability Associates Pty Ltd.. Kerosene 0 0
April 2002.. Jet fuel 4.5 38
Oil and gasoline 3.4 0
All 3.6 2.1
Table 14.15 Estimates of ignition probabilities of natural gasfora
range of hole sizes (European onshore pipelines)
Failure mode Ignition probabilrr); mulation) x (70% chance ofnot near a building) x (1 5% chance
of ignition when not near building) = 3.1%.
Pinhole/crack(dim. < 20 mm) 0.027 For ruptures, the ignition probabilities nominally range from
Hole (20 mm < diam. c 200 mm) 0.019 about 4 to 15% with the higher probability occurring when
Rupture (200 mm < diam. < 400 mm) 0.099 ignition occurs immediately at the rupture location. Given a
Rupture (diam. > 400 mm) 0.235 rupture, the probability of subsequent ignition at the rupture
location is given a value 15%. If ignition does not occur at the
Source. Office of Gas Safety, "Guide to Quantitative Risk Assessment
(QRA)." Standards Australia ME-038-01 (Committee on Pipelines: rupture (85% chance of no ignition at rupture), then the proba-
Gas and Liquid Petroleum), Risk and Reliability Associates Pty Ltd., bility of subsequent ignition is 5%. So, the latter leads to a prob-
April 2002. ability estimate of 85% x 5% = 4.3%.
Derived from the European Gas pipeline incident data Group (EGIG) In both the leak and rupture scenarios, these estimates are
foronshore pipelinesfrom 1970 to 1992. referred to as base caseprobabilities. They can be subsequently
Note that these findings are based on hole size and not on release rate, adjusted by the factors shown inTables 14.19 and 14.20. These
which will vary with pipeline pressure. probabilities are reportedly derived from U.S. gas transmission
pipeline incident rates (U.S. Department of Transportation,
One study uses 12% as the ignition probability ofNGL (natural
gas liquids, referring to highly volatile liquids such as propane) Table 14.18 Estimates of ignition probabilities for below-grade
based on U.S. data [43]. Another study concludes that the over- gasoline pipelines
all ignition probability for natural gas pipeline accidents is
about 3.2% [95]. A more extensive model of natural gas risk Ignition probabiliy (9%
assessment, called GRI (Gas Research Institute) PIMOS [33],
estimates ignition probabilities for natural gas leaks and rup- Location Rupture Hole Leak
tures under various conditions. This model is discussed in the ~~~
following paragraphs. Overall Rural 3.1 3.1 0.62
6.2
Urban
1.24
6.2
In the GRI model, the nominal natural gas leak ignition prob- Immediate Rural 1.55 1.55 0 31
abilities range from 3.1 to 7.2% depending on accumulation Urban 3.1 3. I 0.62
potential and proximity to structures (confinement). The higher Delayed Rural 1.55 1.55 0.31
range occurs for accumulations in or near buildings. There is a Urban 3.1 3.1 0.62
30% chance of accumulation following a leak and a 30%
chance of that accumulation being in or near a building, given Source: Morgan, B., et al., "An Approach to the Risk Assessment of
that accumulation has occurred and an 80% chance of ignition Gasoline Pipelines," presented at Pipeline Reliability Conference,
Houston, TX, November 1996.
when near or in a building, given an accumulation. Hence, that Notes: US. experience is approximately 1.5 times higher than CON-
scenario leads to a 7.1% chance of ignition (30% x 30% X 80% CAWE (data shown above are from CONCAWE). Assumes the urban
= 7.1%). The other extreme scenario is (30% chance of accu- is 2x base rates and that base rates reflect mostly rural experience.
Leak ignition probability is 20% of that for ruptures or holes. Immediate
and delayed ignitions occur with equal likelihood. Rupture is defined as
Table 14.16 Estimates of ignition probabilitiesfor various products 0.5 diameter or larger. Hole is >IO mm, but less than the rupture Leak
is<lOmm.
Product lgnition probahiliiy (5%) Notes: US. experience is approximately 1.5 times higher than CON-
CAWE (data shown above are from CONCAWE). Assumes the urban
Gasoline 4-6 is 2x base rates and that base rates reflect mostly rural experience.
Gasoline and crude oil 3 Leak ignition probability is 20% of that for ruptures or holes. Immediate
and delayed ignitions occur with equal likelihood. Rupture is defined as
Source- Table created from statements in Ref. [86], which cites various 0.5 diameter or larger. Hole is 210 mm, but less than the rupture. Leak
sources for these probabilities. is < 10 mm.