Page 255 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

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biological impacts. In practice, most modem plastics are resist- Testing. A program is in place to test buried pipe for corro-
ant to such factors [2]. sion damage. The rate of corrosion should be a factor in the
This category of corrosion can be scored by assessing the program design. The test time interval should be specified so
material susceptibility in general and then looking at preventive that all potentially harmful corrosion will be detected by the
measures and actual conditions. Note that a high susceptibility test before the line can fail in service.
can be mostly, but not entirely, offset by preventions and the 0 Barrier-type protection. Some means of separating the pipe
presence of rather benign conditions. from a potentially harmful environment has been employed.
The evaluator should award full points when she is ensured
Material susceptibility The pipe wall material susceptibility that the design, installation, and maintenance of such pro-
to any form of buried pipe external corrosion in a reasonably tection will indeed eliminate the corrosion potential. Ideally,
foreseeable environment should first be assessed. Where possi- a testing or monitoring program will verify the bamer
ble contact with corrosive substances would be rare, score the effectiveness.
material as less susceptible. A qualitative rating scale can be set
up to facilitate scoring.
Soil corrosivity When the material susceptibility variable
High identifies a potential for external corrosion, soil corrosivity
The pipe material is relatively incompatible with some environ- should be scored to reflect the presence of conditions that
ments that it can reasonably be expected to contact. In such enhance the threat.
incompatible environments, corrosion damage or leaks have As the environment that is in direct contact with the pipe, soil
occurred in the past. Damage can occur relatively quickly. characteristics that promote corrosion must be identified. The
Without preventive measures, pipe failures are common. evaluator should list those characteristics and score the section
Corrosive mechanisms might have the potential for highly appropriately. Minimum points are awarded when there is a
localized, rapid damage. high presence of potentially damaging characteristics in the
Medium soil. Maximum points would indicate a benign soil condition.
Some corrosion is expected, but serious damage is improbable. See pages 76-78 for more discussion on soil corrosivity.
Perhaps the formation of a protective layer or film of corro-
sion by-products precludes continuation of the damage. Mechanical corrosion This risk variable is more fully dis-
Several potentially damaging reactions are possible, but cussed in Chapter 4. Note that nonmetal materials are also sus-
damage would be slow and not severe. When the pipe is of ceptible to mechanical-corrosion mechanisms such as stress
the age where chemical or physical changes have caused a corrosion cracking (SCC). While the environmental parame-
minor reduction in its structural properties, this score may be ters that promote SCC in nonmetals are different than in metals,
appropriate. there are some similarities. When a sensitizing agent is present
Low on a sufficiently stressed pipe surface, the propagation of
There is a remote chance of corrosion mechanisms under minute surface cracks accelerates. This mirrors the mechanism
somewhat rare conditions. Perhaps rare weather conditions seen in metal pipe materials. For plastics, sensitizing agents can
causing changes in the soil or rare spill of chemicals occa- include detergents and alcohols. The evaluator should deter-
sionally seen in the area could promote damage. Only rare mine (perhaps from the material manufacturer) which agents
substances not normally found in the soil can corrode the may promote SCC. A high stress level coupled with a high pres-
pipe wall, or corrosion mechanisms might be so slow as to be ence of contributing soil characteristics would score the lowest
virtually no threat. point levels.
None Score this item as discussed on page 78 or by comparing the
No known corrosive mechanisms exist for the pipe in any stress level in the pipe wall with the aggressiveness of the envi-
foreseeable environment. ronment (as captured in variables such as the product corrosiv-
ity score and the soil corrosivity score).
Preventive measures Where preventive measures are External erosion is also considered here as a potential corro-
employed to eliminate or reduce a known threat of corrosion, sion mechanism. For instance, an exposed concrete pipe in a
those measures can be evaluated and scored based on how flowing stream can be subject to erosion as well as mechanical
effective they are in reducing the potential damage. When more forces (assessed in the design index). See page 77 for more
than one technique is used, points may be added up to the speci- information on erosion potential.
fied maximum. When preventive measures are absolutely By this scoring, maximum points are awarded for the safest
unnecessary, this variable can receive maximum points. The conditions, that is, when no external corrosion mechanisms are
following are examples of common preventive measures and present. Increasing material susceptibility andor more threat-
some sample criteria for their assessment. ening conditions will lower the score.
0 Monitoring. A program is in place to reliably detect (and Generalized subsurface corrosion potential
take appropriate action for) all potentially harmful corro-
sion. The inspection might be based on statistically sam- Subsurface corrosivity is more problematic for a risk model
pling sections of pipe. Full points should only be awarded assessing and comparing many different pipe materials. Each
when all pipe is examined or when the statistically driven material might have different sensitivities to different soil char-
program can be demonstrated to reduce the inspection acteristics. Soil resistivity is widely recognized as a variable
error. that generally correlates with corrosion rate of a buried metal.