Page 159 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

7/136 Leak Impact Factor
severe potential consequences are often used as a surrogate for narios, and it is also has the potential to cause short- and long-
the distribution in a relative risk model. When absolute risk term environmental damages.
values are sought, the consequence distribution must be better Many products will have some acute hazard characteristics
characterized as is described in later chapters. and some chronic hazard characteristics. The evaluator should
A comprehensive consequence assessment sequence might imagine where his product would fit on a scale such as that
follow these steps: shown in Figure 7.3, which shows a hypothetical scale to illus-
trate where some common pipeline products may fit in relation
1. Determine damage states of interest (see Chapter 14) to each other. Aproduct’s location on this scale depends on how
2. Calculate hazard distances associated with damage states of readily it disperses (the persistence) and how much long-term
interest hazard and short-term hazard it presents. Some product hazards
3. Estimate hazard areas based on hazard distances and source are almost purely acute in nature, such as natural gas. These are
(burning pools, vapor cloud centroid, etc.) location (see shown on the left edge of the scale. Others, such as brine, may
particle fmce element inTable 7.6) pose little immediate (acute) threat, but cause environmental
4. Characterize receptor vulnerabilities within the hazard areas harm as a chronic hazard. These appear on the far right side of
the scale.
Limited modeling resources often requires some short cuts
to this process-leading the use of screening simplifications Al. Acute hazards
and detailed analyses at only critical points. Such simplifica-
tions and the use of conservative assumptions for modeling Both gaseous and liquid pipeline products should be assessed
convenience, are discussed in this chapter. in terms oftheir flammability, reactivity, and toxicity. These are
the acute hazards. One industry-accepted scale for rating prod-
uct hazards comes from the National Fire Prevention
A. Product hazard Association (NFPA). This scale rates materials based on the
threat to emergency response personnel (acute hazards).
The primary factor in determining the nature of the hazard is If the product is a mixture of several components, the mix-
the characteristics of the product being transported in the ture itself could be rated. However, a conservative alternative
pipeline. It is the product that to a large degree determines the might be to base the assessment on the most hazardous compo-
nature of the hazard. nent, because NFPA data might be more readily available for
In studying the impact of a leak, it is often useful to make a the components individually.
distinction between acute and chronic hazards. Acute can mean Unlike the previous point scoring systems described in this
sudden onset, or demanding urgent attention, or of short dura- book, the leak impact factor reflects increasing hazard with
tion. Hazards such as fire, explosion, or contact toxicity are increasing point values.
considered to be acute hazards. They are immediate threats
caused by a leak. Flammabili& Nr
Chronic means marked by a long duration. A time variable is
therefore implied. Hazards such as groundwater contamina- Many common pipeline products are very flammable. The
tion, carcinogenicity, and other long-term health effects are greatest hazard from most hydrocarbons is from flammability.
considered to be chronic hazards. Many releases that can cause The symbol N, is used to designate the flammability rating of
damage to the environment are chronic hazards because they a substance according to the NFPA scale. The five-point scale
can cause long-term effects and have the potential to worsen shows, in a relative way, how susceptible the product is to com-
with the passage of time. bustion. The flash point is one indicator of this flammability.
The primary difference between acute and chronic hazards is
the amount of time involved. An immediate hazard, created
instantly upon initiation of an event, growing to its worst case
level within a few minutes and then improving, is an acute haz- Methane Ammonia Brine
ard. The hazard that potentially grows worse with the passage
of time is a chronic hazard. Ethane Gasoline Diesel
For example, a natural gas release poses mostly an acute haz-
ard. The largest possible gas cloud normally forms immedi- Propane Fuel oil
ately, creating a fire/explosion hazard, and then begins to shrink Propylene Toluene
as pipeline pressure decreases. If the cloud does not find an Ethylene Benzene
ignition source, the hazard is reduced as the vapor cloud
shrinks. (If the natural gas vapors can accumulate inside a Styrene
building, the hazard may become more severe as time passes- Oxygen
it then becomes a chronic hazard.)
The spill of crude oil is more chronic in nature because the 1
potential for ignition and accompanying thermal effects is Immediate t
more remote, but in the long term environmental damages are only 1
likely.
A gasoline spill contains both chronic and acute hazard char- Figure 7.3 Relative acute-chronic hazard scale for common pipeline
acteristics. It is easily ignited, leading to thermal damage sce- products.