Page 188 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Receptors 7/16
hazard of flammability and the chronic hazards of toxicity Possible pipeline rupture impacts on the surrounding envi-
(high benzene component), residual flammability (from pock- ronmental and population receptors are highly location specific
ets of liquid), and environmental insult. He feels that a 50% due to the potential for ignition andor vapor cloud explosion.
reduction in risk can be attained if the spill size is reduced Variables include the migration of the spill or leak, the sensitiv-
by 50%. if 50% of the spilled product is contained quickly, or if ity of the receptor, the nature of the thermal event, the amount
50% of the potentially affected residents can be evacuated of shelter and barriers, and the time of exposure.
hefbre they are exposed to the acute hazard. Because gaseous product release from a pipeline is a tempo-
He has determined that the leak detection and emergency rary excursion, the pollution potential beyond immediate
response activities are in place to warrant an adjustment of the toxicity or flammability is not specifically addressed here
leak impactfactor. for releases into the air. This discounts the accumulative dam-
The basis for this determination is the following items age that can be done by many small releases of atmosphere-
observed or ascertained from interviews with the operators: damaging substances (such as possible ozone damages from
greenhouse gases). Such chronic hazards are considered in
Automatic valves are set to isolate pipeline sections around the assignment of the equivalent reportable release quantity
the town of Smithville. The valves trigger if a pressure drop ( RQequlvalent) for volatile hydrocarbons.
of more than 20% from normal operating conditions occurs. Ideally, a damage threshold would lead to a hazard area esti-
The valves are thoroughly tested every 6 months and have a mation that would lead to a characterization of receptor vulner-
good operating history. A 20% drop in pressure would occur ability within that hazard area. Damage threshold levels for
very soon after a substantial leak, thermal radiation and overpressure effects are discussed in
Annual emergency drills are held, involving all emergency Chapter 14.
response personnel from Smithville. The drills are well doc-
umented and reflect a high degree of response preparedness. Dl. Population density
The presence of the automatic valves should limit the spill As part ofthe consequence analysis, a most critical parameter is
to 50% of what it would be without the valves. This alone the proximity ofpeople to the pipeline failure. Population prox-
would have been sufficient to adjust the chronic leak impact imity is a factor here because the area of opportunity for harm is
factor. The strong emergency response program should limit increased as human activity occurs closer to the leak site. In
exposure due to residual flammability and ensure proper han- addition to potential thermal effects, the potential for ingesting
dling of the gasoline during cleanup. Containment is not seen contaminants through drinking water, vegetation. fish, or other
as an option, but by limiting the spill size, the environmental ingestion pathways is higher when the leak site is nearby.
insult is minimized also. The evaluator sees no relief from the Less dilution has occurred and there is less opportunity for
acute hazard but feels an adjustment for the chronic hazard is detection and remediation before the normal pathways arc con-
appropriate. taminated. The other pathways, inhalation and dermal contact,
are similarly affected.
A full evaluation of human health effects from pipeline fail-
Exaniple 7.4: Adjustments to the liquid spill score ures is often unnecessary when the pipeline’s products are com-
(Cuse B) mon and epidemiological effects are well known (see
discussion of product hazurd, this chapter). In assessing
The evaluator is assessing a section of brine pipeline in a absolute probabilities of injury or fatality from thermal effects.
remote, unpopulated area. The leak scenario she is using the time and intensity of exposure must be estimated. This is
involves a complete line rupture. The hazards are only chronic discussed earlier in this chapter and methods for quantifying
in nature; that is, there are no immediate threats to public or these effects are shown in Chapter 14. Shielding and ability to
responders. The chronic threat is the exposure to the groundwa- evacuate are critical assumptions in such calculations. Most
ter table, which is shallow in this area. general risk assessment methods will produce satisfactory
The best chance to reduce the chronic risk by SO% is thought results with the simple and logical premise that risk increase as
to be limiting the spill size by 50%. Emergency response will nearby population density increases.
not reliably occur quickly enough to isolate the leaking pipeline Population density can he taken into account by using any of
before line depressurization and pump shutoffs slow the leak the published population density scales such as the DOT Part
anyway. containment in a timely fashion is not possible. 192 class locations I, 2.3, and 4 (see Table 7. IS). These are for
No adjustments to the chronic leak impact factor are made. rural to urban areas, respectively. The class locations are deter-
mined by examining the area 660 ft on either side of the
pipeline centerline, and 1 mile along the pipeline. This I-mile
D. Receptors by 1320-ft rectangle, centered over the pipeline, is the defined
area in which to conduct counts of dwellings.
Of critical importance to any risk assessment is an evaluation of If any 1-mile stretch of pipeline has more than 46 dwellings
the types and quantities of receptors that may be exposed to a inside this defined area, that section is designated a Class 3
hazard from the pipeline. For these purposes, the term receptor area. A section with fewer than 46 dwellings but more than
refers to any creature. structure, land area, etc., that could 10 dwellings in the defined area is a Class 2 area. Each mile
“receive” damage from a pipeline rupture. The intent is to cap- with fewer than 10 dwellings is a Class 1 area. A Class 4 area
ture relative vulnerabilities of various receptors, as part of the exists when the defined area has a prevalence of multistory
consequence assessment. buildings.