Page 51 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

2/30 Risk Assessment Process
rDepth cover

I I I 1

Conditions *-------------------------------------
+ Actions
Figure 2.3 Example items on attributes-preventions scale.

the environment of the section-harsh conditions require more The range should include the smallest to largest values in sys-
preventions to meet the standard. tems to be studied as well as future systems to be acquired or
other systems that might be used as comparisons.
Model scope and resolution Given the difficulties in predicting future uses of the model,
a more generic model-widely applicable to many different
Assessment scope and resolution issues further complicate pipeline systems-might be appropriate.
model design. Both involve choices of the ranges of certain risk
variables. The assessment of relative risk characteristics is Special Risk Factors
especially sensitive to the range of possible characteristics in
the pipeline systems to be assessed. If only natural gas trans- Two possible risk factors deserve special consideration since
mission pipelines are to be assessed then the model does not they have a general impact on many other risk considerations.
necessarily have to capture liquid pipeline variables such as
surge potential. The model designer can either keep this vari- Age as a risk variable Some risk models use age as a risk
able and score it as “no threat” or she can redistribute the variable. It is a tempting choice since many man-made systems
weighting points to other variables that do impact the risk. experience deterioration that is proportional to their years in
As another example, earth movements often pose a very local- service. However, age itself is not a failure mechanism-at
ized threat on a relatively few stretches of pipeline. When the vast most it is a contributing factor. Using it as a stand-alone risk
majority of a pipeline system to be evaluated is not exposed to any variable can detract from the actual failure mechanisms and can
land movement threats, risk points assigned to earth movements also unfairly penalize portions of the system being evaluated.
will not help to make risk distinctions among most pipeline seg- Recall the discussion on time-dependent failure rates in
ments. It may seem beneficial to reassign them to other variables, Chapter 1, including the concept of the bathtub failure rate
such as those that warrant full consideration. However, without curve. Penalizing a pipeline for its age presupposes knowledge
the direct consideration for this variable, comparisons with the of that pipeline’s failure rate curve.
small portions of the system that are exposed, or future acquisi- Age alone is not a reliable indicator ofpipeline risk, as is evi-
tions of systems that have the threat, will be difficult. denced by some pipelines found in excellent operating condi-
Model resolution-the signal-to-noise ratio as discussed in tion even after many decades of service. A perception that age
Chapter I-is also sensitive to the characteristics of the systems always causes an inevitable, irreversible process of decay is not
to be assessed. A model that is built for parameters ranging an appropriate characterization ofpipeline failure mechanisms.
from, say, a 40-inch, 2000-psig propane pipeline to a 1-inch, 20- Mechanisms that can threaten pipe integrity exist but may or
psig fuel oil pipeline will not be able to make many risk distinc- may not be active at any point on the line. Integrity threats are
tions between a 6-inch natural gas pipeline and an 8-inch natural well understood and can normally be counteracted with a
gas pipeline. Similarly, a model that is sensitive to differences degree of confidence. Possible threats to pipe integrity are not
between a pipeline at 1 100 psig and one at 1200 psig might have necessarily strongly correlated with the passage of time,
to treat all lines above a certain pressure/diameter threshold as although the “area of opportunity” for something to go wrong
the same. This is an issue ofmodeling resolution. obviously does increase with more time.
Common risk variables that should have a range established The ways in which the age of a pipeline can influence the
as part of the model design include potential for failures are through specific failure mechanisms
such as corrosion and fatigue, or in consideration of changes in
Diameter range manufacturing and construction methods since the pipeline
Pressure range was built. These age effects are well understood and can
Products to be included normally be countered by appropriate mitigation measures.