Page 177 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

7/154 Leak Impact Factor
A drawback to this scale appears for the special case in which and/or dispersion. The effectiveness varies depending on the
a low-penetration soil promotes a wider spill-surface area and type of system being evaluated. Emergency response and leak
hence places additional receptors at risk. In very general terms, detection evaluation methods are more fully discussed later in
a spill of a more acutely hazardous product might generate less this chapter.
risk with greater soil penetration. This is especially true when
the product is less persistent in the soil, as is often the case with Leak detection and vapor dispersion Leak detection plays a
higher flammability products. The counter to this reasoning is relatively minor role in minimizing hazards to the public in
the increased cleanup costs and decreased volatility as soil most scenarios of gas transmission pipelines. Therefore, many
penetration increases. vapor dispersion analyses will not be significantly impacted by
Note also that natural factors such as wind strength and any assumptions relative to leak detection capabilities. This is
direction or topography can protect a receptor from damage, especially true when defined damage states (see Chapter 14)
even if that receptor is fairly close to the leak site. use short exposure times to thermal radiation, as is often
warranted.
Spill and leak mitigation Reference [83] illustrates that gas pipeline release hazards
depend on release rates which in turn are governed by pressure.
There are sometimes opportunities to reduce the volume or dis- In the case of larger releases, the pressure diminishes quicHy-
persion of released pipeline contents after a failure. The more quickly than would be affected by any actions that could
pipeline operator’s ability to seize these opportunities can be be taken by a control center. In the case of smaller leaks, pres-
included in the risk assessment. Secondary containment and sures decline more slowly but ignition probability is much
emergency response, especially leak detectionireaction, are lower and hazard areas are much smaller. In general, there are
considered to be risk mitigation measures that minimize poten- few opportunities to evacuate a pressurized gas pipeline more
tial consequences by minimizing product leak volumes and/or rapidly than occurs through the leak process itself, especially
dispersion. The effectiveness of each varies depending on the when the leak rate is significant. A notable exception to this
type of system being evaluated. case is that of possible gas accumulation in confined spaces.
This is a common hazard associated with urban gas distribution
Secondary containment systems and is covered in Chapter 1 1.
Another, less common exception would be a rather remote
Opportunities to fully contain or limit the spread of a liquid scenario involving the ignition of a small leak that causes
release can be considered here. These opportunities include immediate localized damages and then more widespread dam-
ages as more combustible surroundings are ignited over time as
Natural harriers or accumulation points the fire spreads. In that scenario, leak detection might be more
Casingpipe useful in minimizing potential impacts to the public.
Linedtrench
Berms or levees Leak detection and liquid dispersion Leak detection capabili-
Containment systems. ties play a larger role in liquid spills compared to gas releases.
Long after a leak has occurred, liquid products can be detected
Most secondary containment opportunities are found at because they have more opportunities for accumulation and are
stations and are discussed in Chapter 13. usually more persistent in the environment. A small, difficult-
Although waterways are often areas of special environmen- to-detect leak that is allowed to continue for a long period of
tal and population concern, they also sometimes offer an envi- time can cause widespread contamination damages, especially
ronment in which a liquid release is readily isolated. This may to aquifers. Therefore, the ability to quickly locate and identify
be the case when the spill occurs in a stable water body such as a even small leaks is critical for some liquid pipelines.
pond or lake, which offers limited transport mechanisms and in
which the spilled product is relatively immiscible and insolu-
ble. This can enable more rapid and complete cleanup, includ- Scoring releases
ing the possible (and controversial) choice to bum off a layer of
spilled hydrocarbon from the water surface. A more damaging Once she has an understanding of release mechanisms and risk
scenario involves water bodies with more rapid transport mech- implications, the evaluator will next need to model potential
anisms and spills that reach the more sensitive receptors that are releases for the risk assessment. This is often done by assigning
typically found on shorelines. a score to various release scenarios. To score the relative dnper-
Where secondary containment exists, or it is recognized that sion area or hazard zone of a spill or release, the relative meas-
special natural containment exists, the evaluator can adjust the ures of quantity released and dispersion potential can be
spill score accordingly. combined and then adjusted for mitigation measures. When the
A system for evaluating secondary containment for pipeline quantity and dispersion components use the same variables, it
stations is shown in Chapter 13. might be advantageous to score the two components in one
step.
Emergency response As more and more variables are added to the assessment
in order to more accurately distinguish relative consequence
Emergency response and especially leak detection and reac- potential, the benefits of the scoring approach diminish.
tion, is appropriately considered as a mitigation measure to Eventually the evaluator should consider performing the
minimize potential consequences by minimizing spill volumes detailed calculations-estimating actual hazard zones using