Page 110 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

4/88 Corrosion Index

Unexpected rise
Unexpected rise
in signal strength
in signal strength
(possible coating damage area)
(possible coating damage area)
..
.. Normal signal
Normal
signal
5
P
e
c slope of signal *. .' .'
slope
of
signal
UJ *.
attenuation
m attenuation
c
z
0
~~~ ~~
Distance from signal source (along pipeline)
Figure 4.9 Example of coating survey results.
schedule such as that shown in Table 4.9 could be used. This Example 4.6: Scoring coating
type of scale would depend on soil corrosivity. Again, lowered
electric current requirements mean less exposed metal and bet- A buried oil pipeline in dry, sandy soil is cathodically pro-
ter electrical isolation from the electrolyte. tected by sacrificial anodes attached to the line at a spacing of
A special situation, such as evidence of high microorganism about 500 ft. A pipe-to-soil voltage measurement is taken
activity or unusually low pH that promotes steel oxidation, twice each year over the whole section to ensure that cathodic
should be accounted for by reducing the point value (but not protection is adequate. Records indicate that the line was ini-
below 0 points). Not knowing the soil corrosion potential tially coated 22 years ago with a plant-applied coal tar enamel
would conservatively warrant a score of0 points. material that was applied over the sand-blasted and primed
A more detailed approach to scoring coating conditions pipe. An inspector supervised the coating process. The pipe-
would be to use inspection results directly. Adjustments are to-ground potential has not changed measurably since origi-
applied to account for age of the inspection-it is conserva- nal installation. This section of line has not been exposed for
tively assumed that coating deteriorates until inspection veri- 10 years.
fies actual condition. In the case of a visual inspection, a zone The evaluator assesses the coating condition as follows:
of influence might be appropriate. Even if only a short length of
pipe is excavated and inspected, this provides some evidence of Coating Condition Parameter Max Weighting Score
coating condition for some distance past the excavation if there
are no known environmental changes. Zone of influence appli- Coating (good) 25% 20%
cations are discussed in Chapter 8. Application (good) 25% 20%
Table 4.10 shows inspection-related risk variables from a Inspection (fair) 25% 10%
scoring system that considers three types of inspection: visual, Defect correction (fair) 25% 10%
nondestructive testing (NDT), and destructive testing (DT).
Each variable can be weighted andor can be hrther divided 100% 60%
into specific measurements.
The coating type in this environment has a good track record.
However, no confirmatory information is available. The evalu-
ator feels that the semiannual pipe-to-soil voltage readings give
a good indirect indication of coating condition. Full points
Table 4.9 Quantitative measure of coating condition would be awarded if this was confirmed by visual inspection
(cracked or disbonded coating may not be found by the poten-
CP Current requirements Coating condition tial readings alone). Defect correction is an unknown at this
point. Three points are awarded based on the thoroughness with
0.0003 mA/sq ft Good which the operator runs other aspects of his operations-in
ft
0.003 Msq Fair other words, some benefit of the doubt is given here. Coating
0.1 MsqA Poor
l.OmA/sqft Absent selection and application processes appear to be high quality,
based on records and conversations with the operator, but