Page 92 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

4/70 Corrosion Index
pipeline area foreman. No formal specifications exist. The defects exist, and yet, insertion of the pipeline into the casing is
foreman requests paint work whenever he feels it is needed an easy way to damage the coating and create defects. End seals
(based on his personal inspection of a facility). The evaluator are used to keep water, mud, and other possible electrolytes out
awards points as follows: of the casing annular space, but are easily defeated by poor
design and/or installation or by minor ground movements. The
Coating-fair 2.0 pts presence of electrolyte in the annular space can lead to corro-
Application-fair 1.8 sion cells between the casing and the pipeline, as well as inter-
Inspection-fair 2.2 ference problems with the cathodic protection system. Vent
Defect correction-poor 1 .o pipes are often installed to release leaked products, but these
vents allow direct communication between the casing annular
Average 1.75 pts space and the atmosphere--consequently, moisture is almost
always present in the annular space.
Note: In this example, the evaluator wishes to make distinc- Cathohc protection is usually employed to protect buried
tions between the evaluation scores, so she uses decimals to rate steel pipelines. The casing pipe can shield the pipeline from the
items a little above or a little below the normal rating. This may protective currents if there is no electrical bond between the
be appropriate in some cases, but it adds a level of complexity casing and the pipeline. If there is such a bond, the casing usu-
that may not be warranted, given the low point values. ally not only shields the pipeline from the current, but also
The evaluator feels that choice of paint is probably appro- draws current from it, effectively turning the pipeline into an
priate though not specified. Application is slightly below anode that is sacrificed to protect the casing pipe, which is now
fair because no specifications exist and the contractor’s work- the cathode!
force is usually subject to regular turnovers. Inspection is Several mitigative measures can be employed to reduce cor-
slightly above fair because the foreman does make specific rosion problems in casings. These were illustrated earlier in
inspections for evidence of atmospheric corrosion and is trained Figure 4.3 and are described below:
in spotting this evidence. Defect correction is poor because
defect reporting and correction appear to be sporadic at best. Test leads. By comparing the pipe-to-soil potentials (volt-
ages) of the pipeline versus the casing pipe, evidence of
bonding between the two is sought. Test leads allow the volt-
The case fodagainst casings age measurements to be made.
Nonconductive spacers. These are designed to keep the
Buried casings show up at several points in this risk assess- pipeline physically and electrically separated from the casing
ment-sometimes as risk reducers, sometimes as risk creators. pipe. They also help to protect the pipe coating during inser-
The following information provides a general discussion of the tion into the casing.
use of pipeline casings. End seals. These are designed to keep the annular space
Oversized pipe, called casingpipe, is sometimes placed over free of substances that can act as an electrolyte (water, mud,
the carrier pipeline to protect it from external loadings and/or to etc.).
facilitate repairs to the carrier pipe. Casings have long been Filling the annular space. Use of a dielectric (nonconduc-
used by the pipeline industry. They are generally placed under tive) substance reduces the potential for electrical paths
highways, roads, and railroads where higher external loadings between the casing and the pipeline. Unfortunately, it also
are anticipated or where pipeline leaks might cause structural negates some ofthe casing benefits listed earlier.
damages to a structure (see Figure 4.3 earlier).
A casing also allows for easier replacement of the pipeline if Reflecting the trade-off in benefits, casings can be risk
a problem should develop. Instead of digging up a roadway, the reducers (protection from external loads, including third-party
pipeline can simply be pulled out of the casing, repaired, and damages and land movements) yet at the same time be risk
reinstalled without disrupting traffic. adders in the corrosion index (promoting atmospheric and sub-
A third potential benefit from casings is that a slow pipeline surface corrosion of metal). It would be nice to say that one
leak can be contained in the casing and detected via the casing will always outweigh the other, but we do not know that this is
vent pipe rather than slowly undermining the roadway or form- always the case. A risk costhenefit analysis for casings can be
ing underground pockets of accumulated product. performed by using a risk model to quantify the relative advan-
An industry controversy arises because the benefits casings tages and disadvantages from a risk standpoint.
provide are at least partially offset by problems caused by their Other factors must be considered in casing decisions. Often
presence. These problems are primarily corrosion related. It is regulatory agencies leave no choice in the matter. The owner of
probably safe to say that corrosion engineers would rather not the crossing (railroad, highway, etc.) may also mandate a cer-
have casings in their systems. It is more difficult to protect an tain design. Economics, of course, always play an important
encased pipe from corrosion. The casing provides an environ- role. The costs of casings must include ongoing maintenance
ment in which corrosion can proceed undetected and preven- costs, but the costs of not using casing must include pipe strong
tion methods are less effective. Because the pipeline cannot be enough to carry all loads and damages to the crossing, should
directly inspected, indirect methods are used to give indications pipeline replacement be needed.
of corrosion. These techniques are not comprehensive, some- As an additional benefit of applying a risk management sys-
times unreliable, and often require expert interpretation. tem such as this one to the problem of casings, the pipeline
Several dilemmadproblems are typically encountered operator and designer have a rational basis for weighing the
with casings. Atmospheric corrosion can occur if any coating benefits of alternate designs.