Page 36 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Risk assessment issues 1/15

Ignition (1/600) Detonation
(1/100) Large (500/600) High thermal
rupture damages
1 (29/30)
No ignition (99/600) Torch fire only

equipment-
contacts (1/20) Corrosion
Third- line
party - (6/10)
[ survey
damage (1 :2 years) - - - - - - - - - -
Reported
@/lo) Cathodic
(4/10) Unreported 1t;;ion
leak
(2/10)

(’/’ O0) No damage - No event
Figure 1.3 Event-tree analysis

how quickly the interrelationships make an event tree very jectivity and judgment and should be carefully documented.
large and complex. especially when all possible initiating While a screening analysis is a logical process to be used
events are considered. The probabilities associated with events subsequent to almost any risk assessment, it is noted here as a
will also normally be hard to determine. For example, Figure possible stand-alone risk tool. As such, it takes on many
1.3 suggests that for every 600 ignitions of product from a large characteristics of the more complete models to be described,
rupture. one will result in a detonation, 500 will result in especially the scoring-type or indexing method.
high thermal damages, and 99 will result in localized fire
damage only. This only occurs after a Ym chance of ignition, VII. Risk assessment issues
which occurs after a Yim chance of a large rupture, and after a
once-every-two-years line strike. In reality, these numbers In comparing risk assessment approaches, some issues arise
will be difficult to estimate. Because the probabilities must that can lead to confusion. The following subsections discuss
then be combined (multiplied) along any path in this diagram, some ofthose issues.
inaccuracies will build quickly.
Absolute vs. relative risks
0 Screening analyses. This is a quantitative or qualitative
technique in which only the most critical variables are Risks can be expressed in absolute terms, for example, “num-
assessed. Certain combinations of variable assessments are ber of fatalities per mile year for permanent residents
judged to represent more risk than others. In this fashion, the within one-half mile of pipeline. . . .” Also common is the use
process acts as a high-level screening tool to identify rela- of relative risk measures, whereby hazards are prioritized
tively risky portions of a system. It requires elements of suh- such that the examiner can distinguish which portions of the