Page 298 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Modeling ideas I 131275
both of these “systems” are employed, the spill quantity to be of the former, the greater the protection offered by secondary
used in the model is 2000 bbl - [1/2 x (3000 bbl)] + (3000 - containment, the smaller the spill size to be used in modeling
2000) = 1500 bbl. If the first spill volume is 4500 bbl, then the spill consequences:
model value is 2000 - [ 112 x (4500 bbl)] + (4500 - 2000) = 2000
bbl (since the primary score should not be worsened by this exer- Spill size reduction percentage = [(secondary containment Oh)
cise). The value of 50% is rather arbitrary-as is the mathemati- (adjustment factor)]
cal relationship used-and can be replaced by any value or where
scoring approach more suitable to the evaluator. Consistency is
more critical than the absolute value, in this case. Recall that Secondary containment YO =portion of total facility volume
“penalties,” in the form of increased surface area, are also that can be held
assigned to portions of the facility that are hidden from view Adjustment factor = obtained by adding all conditions
(buried) and therefore have less opportunity for leak detection that apply to the secondary con-
by some methods. tainment, up to the value of the
Added to the detection time is the reaction time, which is secondary containment YO, as
generally defined as the amount of additional time that will showninTable 13.8. Inthis table,
probably elapse between the strong leak indication and the iso- items are detractors from sec-
lation of the leaking facility (including drain downtime). Here, ondary containment effective-
consideration is given to automatic operations, remote opera- ness, except the first.
tions, proximity of shutdown devices, etc. As a simple way to
account for various reaction times in the aforementioned scor- Limited secondary containments such as pump seal vessels and
ing protocols, the following rationale can be used: A spill vol- sumps are designed to capture specific leaks. As such they pro-
ume equal to (a leak rate of 1000 gal/day) x (the most probable vide risk reduction for only a limited range of scenarios. Risk
reaction time) is added to the original spill volume. Benefits of reduction credit can be given for secondary containment pro-
remote and automatic operations as well as staffing levels are portional to the size of the effective area it protects. Using this
captured here. approach in one recent application, the credit was capped at a
This is thought to fairly represent the value of reaction maximum of go%, regardless of the mathematical results, as
time. Of course, for a large leak, this value is probably under- shown inTable 13.9.
stated and for a small leak it is probably overstated, but, over
the range of model uses and for a relative assessment, this
approach might be appropriate. In one application of a V. Modeling ideas I
methodology similar to the one outlined here, a sensitivity
analysis showed that changes in leak detection and reaction Dow Chemical Company’s Fire and Explosion Index [26] is a
capabilities from 5,000 to 10,000 gallons changed the spill well-regarded loss estimation system for process plants. It is an
score and also the overall risk by 2 to 3%. This seemed reason- indexing type assessment used for estimating the damage that
able for the resolution level of that risk assessment. In a situa- would probably result from an incident in a chemical process
tion where the spill score is less dominated by the leak-volume plant. The F&EI system is not designed for public safety evalu-
component of the calculation and/or where the range of the ations or environmental risk assessments, but provides some
spill calculation is smaller, the impact on the spill score and the useful concepts that can be used in such assessments.
risk would be greater. The process plant incidents addressed in this evaluation include

Secondary containment With any spill size scenario, the A blast wave or deflagration
presence of secondary containment can be considered as an Fire exposure
opportunity to reduce (or eliminate) the “area of opportunity” Missile impact
for consequences to occur. Secondary containment must be Other releases as secondary events.
evaluated in terms of its ability to
The secondary events become more significant as the type and
Contain the majority of all foreseeable spills. storage amounts of other hazardous materials increase. The
Contain 100% of a potential spill plus firewater, debris, or F&EI is directly related to the area of exposure.
other volume reducers that might compete for containment In performing F&EI calculations, the nature of the threat is
space-largest tank contents plus 30 minutes of maximum assessed by examining three components: a material safety
firewater flow is sometimes used [26].
Contain spilled volumes safely-not exposing additional
equipment to hazards. Table 13.8 Secondary containment sample adjustment faclors
Contain spills until removal can be effected-no leaks.
Condition Adjustmentfactor (?A)
Note that ease of cleanup of the containment area is a secondary
consideration (business risk). Impervious liner 15
Risk is reduced as secondary containment improves. The risk Semipervious liner 40
No immediate fill indication
S
“credit” can be in the form of a reduced spill size rating or eval- No overtlow alarms 5
uated as an independent variable assessing the dispersion Additional equipment exposed to spilled product IO
potential, when secondary containment is present. In the case