Page 183 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

7/160 Leak Impact Factor
The role of leak detection can be evaluated either in the rate of leakage multiplied by the time the leak continues is often
determination of spill size and dispersion or as a stand-alone the best estimate of total leak quantity. However, some potential
element that is then used to adjust previous consequence spill sizes are more volume dependent than leak-rate depend-
estimates. The former approach is logical and consistent with ent. Spills from catastrophic failures or those occurring at
the real-world scenarios. The benefit ofleak detection is indeed pipeline low points are more volume dependent than leak-rate
its potential impact on spill size and dispersion. The latter dependent. Such spill events are not best estimated by leak rates
approach, evaluating leak detection capabilities separately, because the entire volume of a pipeline segment will often be
offers the advantage of a centralized location for all leak detec- involved, regardless of response actions.
tion issues and, therefore, a modeling efficiency.
As discussed on pages 142-146, leak size is at least partially Detection methodologies
dependent on failure mode. Small leak rates tend to occur due
to corrosion (pinholes) or some design (mechanical connec- Pipeline leak detection can take a variety of forms, several of
tions) failure modes. The most damaging leaks occur below which have been previously discussed. Common methods used
detection levels for long periods of time. Larger leak rates tend for pipeline leak detection include
to occur under catastrophic failure conditions such as external
force (e.g., third party, ground movement) and avalanche crack Direct observation by patrol
failures. 0 Happenstance direct observation (by public or pipeline
Larger leaks can be detected more quickly and located more operator)
precisely. Smaller leaks may not be found at all by some meth- SCADA-based computational methods
ods due to the sensitivity limitations. The trade-offs involved 0 SCADA-based alarms for unusual pressure, flows, tempera-
between sensitivity and leak size are usually expressed in terms tures, pumpicompressor status, etc.
of uncertainty. Flow balancing
The method of leak detection chosen depends on a variety of Direct burial detection systems
factors including the type of product, flow rates, pressures, the Odorization
amount of instrumentation available, the instrumentation char- Acoustical methods
acteristics, the communications network, the topography, the Pressure point analysis (negative pressure wave detection)
soil type, and economics. Especially when sophisticated instru- Pig-based monitoring
mentation is involved, there is often a trade-off between the sen-
sitivity and the number of false alarms, especially in “noisy” Each has its strengths and weaknesses and an associated
systems with high levels oftransients. spectrum of capabilities.
At this time, instrumentation and methodology designed to Despite advances in sophisticated pipeline leak detection
detect pipeline leaks impact a relatively narrow range of the technologies, the most common detection method might still be
risk picture. Detection of a leak obviously occurs after the leak direct observation. Leak sightings by pipeline employees,
has occurred. As is the case with other aspects of post-incident neighbors, and the general public as well as sightings while
response, leak detection is thought to normally play a minor patrolling or surveying the pipeline are examples of direct
role, if any, in reducing the hazard, reducing the probability of observation leak detection. Overline leak detection by hand-
the hazard, or reducing the acute consequences. Leak detection held instrumentation (sntffeevs) or even by trained dogs (which
can, however, play a larger role in reducing the chronic conse- reportedly have detection thresholds far below instrument
quences of a release. As such, its importance in risk manage- capabilities) is a technique used for distribution systems.
ment for chronic hazards may be significant. Pipeline patrolling or surveying can be made more sensitive by
This is not to say that leak detection benefits that mitigate adjusting observer training, speed of survey or patrol, equip-
acute risks are not possible. One can imagine a scenario in ment carried (may include gas detectors, infrared sensors, etc.),
which a smaller leak, rapidly detected and corrected, averted altitudeispeed of air patrol, topography, ROW conditions, prod-
the creation of a larger, more dangerous leak. This would theo- uct characteristics, etc. Although direct observation techniques
retically reduce the acute consequences of the potential larger are sometimes inexact, experience shows them to be rather con-
leak. We can also imagine the case where rapid leak detection sistent leak detection methods.
coupled with the fortunate happenstance of pipeline personnel More sophisticated leak detection methods require more
being close by might cause reaction time to be swift enough instrumentation and computer analysis. A mainstay of pipeline
to reduce the extent of the hazard. This would also impact the leak detection includes SCADA-based capabilities such as
acute consequences. These scenarios are obviously unreliable monitoring of pressures, flows, temperatures, equipment sta-
and it is conservative to assume that leak detection has limited tus, etc. For instance, (1) procedures might call for a leak detec-
ability to reduce the acute impacts from a pipeline break. tion investigation when abnormally low pressures or an
Increasing use of leak detection methodology is to be expected abnormal rate of change of pressure is detected; and (2) a flow
as techniques become more refined and instrumentation rate analysis, in which flow rates into a pipeline section are
becomes more accurate. As this happens, leak detection may compared with flow rates out of the section and discrepancies
play an increasingly important role. are detected, might be required. SCADA-based alarms can be
As notedpreviously, leak quantity is a critical determinant of set to alert the operator of such unusual pressure levels, differ-
dispersion and hence of hazard zone size. Leak quantity is ences between flow rates, abnormal temperatures, or equip-
important under the assumption that larger amounts cause more ment status (such as unexplained pumpicompressor stops).
spread of hazardous product (more acute impacts), whereas Alarms set to detect unusual rates of changes in measured flow
lower rates impact detectability (more chronic impacts). The parameters add an additional level of sensitivity to the leak