Page 259 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

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susceptibility to failure, especially when joint failures are char- and known loadings, provides a margin of safety against
acterized by complete separation of the pipe sections. A rating unanticipated loads as well as an increased survival time when
scheme can be devised to assess pipelines with more problem- corrosion or fatigue mechanisms are active. If nonpipe compo-
atic joints-those that historically have failed more often or nents are in the section being evaluated, their strengths should
more catastrophically in certain environments. Joining designs also be considered in calculating safety margins.
and installation processes are also covered in the incorrect Inspection may reveal areas of wall loss, pinhole corrosion,
operations index. graphitization (in the case of cast iron), and leaks. This infor-
mation should be included in the model to adjust the estimated
Rehabilitatedpipelines wall thickness. When actual wall thickness measurements are
not available, the nominal wall thickness can be adjusted by an
In some portions of distribution systems, replacement of estimated corrosion rate or a conservative assumption based on
pipelines by conventional open-cut methods is impractical, material type, age, and suspected deterioration mechanisms.
extremely costly, and/or unacceptably disruptive to the public. In scoring the safetyfactor, the evaluator should take into
Adverse environmental impact, road closures, traffic delays, account material differences and other pipe design factors
site restorations, and other disruptions to the community are peculiar to distribution systems. This can be done by first scor-
challenges to urban pipeline rehabilitation. Trenchless tech- ing the variable as described on pages 94-102 and then adjust-
niques are now often being used to minimize these impacts. ing this score by material considerations when it is deemed
Common trenchless pipe rehabilitation techniques involve appropriate to do so. Table 7.3 shows the material toughness for
the insertion a liner of some type into an existing pipeline some materials commonly seen in distribution piping. When
whose integrity has become compromised. Liner materials the evaluator feels that the type of material limits its usefulness
include synthetic fibers, polyurethane membranes, textile hose, as extra pipe wall thickness, be can adjust the pipe safety factor
and high-density polyethylene. Sometimes the liner is bound to accordingly.
the existing pipe wall with an adhesive; at other times a friction In deciding whether normal or maximum pressures are to be
fit locks the two systems together. Sometimes, a smaller used in calculating safety margins, special attention should be
pipeline is merely inserted into the line to be rehabilitated, given to the design of pressure regulation for the distribution
where the existing line becomes only a conduit for the new line. system (see also page 94).
To compensate for the reduced diameter, the newer line can be
designed for a higher operating pressure andor have a lower Fatigue
resistance to flow.
From a risk viewpoint, these composite material systems Note that traffic loadings can be a significant source of fatigue
may require special consideration (see Chapter 5). Because on distribution system components. Score this item as
some liner techniques are relatively new, in-service failure described on pages 102-104.
modes are not well defined. Possible gas migration through a
liner (on a molecular level) can pressurize an annular space- Surge potential
between the liner and the original pipe wall-which may not
be intended to contain pressure. Composite systems also bring Score as described on pages 104105. Note that this item
with them challenges for leak pinpointing, should the new applies only to transported fluids that can generate surges. This
liner develop a leak. The evaluator should incorporate failure usually excludes highly compressible fluids (gases).
experience into the evaluation of such systems as it becomes
available. Integrity verifications
We now take a look at the Chapter 5 design inder variables as
they apply to distribution systems. Table 1 1.5 lists the variables In hydrocarbon transmission pipelines, inspection plays a large
and their possible weights for a distribution system risk assess- role in integrity management. For most hydrocarbon transmis-
ment, which are discussed in the following subsections. sion (and increasingly for gathering systems also), it is impera-
tive to ensure that the system integrity will not be compromised
Safety factor and to quickly detect any size leak should system integrity fail.
As such, many inspection techniques have been developed to
Pipeline strength is characterized in this part of the risk model. detect even the most minor flaw in continuously welded steel
Pipe wall thickness, above what is needed for internal stresses pipelines-by far the most prevalent type of high-pressure
pipeline. The application of these techniques and the frequency
of application play large roles in risk management and, in
Table 11.5 Design index possible variables and weights fact, are the basis of some regulatory initiatives. Distribution
system integrity verification includes pressure testing, acoustic
Variable Weight or electrical conductivity testing for reinforced concrete pipe
materials, visual inspections, and others. Where inspection!
Safety factor 30 monitoring techniques are used to verify distribution system
Fatigue 15 integrity, risk reduction can be noted.
Surge potential 15 However, inspection does not usually play a significant
Integrity verifications 20
Land movements 20 role in most nontransmission pipeline systems. Few in situ
Design index total IO0 inspection techniques exist or are practical to accommodate the
complicated configurations of branches, components, and