Page 104 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

4/82 Corrosion Index
scored accordingly (0 points, by the example scales). Table 4.5 case, the annual, on-reading survey where all readings show
shows an example of a scoring system for CP effectiveness. An adequate CP, will show point values of (Maximum CP
age-of-survey adjustment would be used in arriving at final Effectiveness Points) x (Annual on survey, option 3) x (test lead
point values. adj) yielding point values of 15 points x 10% x 100% = 1.5
According to the above scoring rules, the evaluator has three points for portions of the pipeline within 1 mile of the test lead
options for scoring the annual test lead survey. As one option, and 15 x 10% x 50% = 0.75 for portions of the pipeline 1.5
he can consider each test lead reading to be a pipe-to-soil read- miles from a test lead.
ing representing about 5 ft along the pipeline either side of the This may appear to be a rather complicated scoring scheme,
reading location. This means that very short sections of pipe but it does reflect the reality of the complex corrosion control
would receive 20, 30, or 55% of the CP effectiveness points choices commonly encountered in pipeline operations. It is not
(depending on what type of survey is used), where test lead uncommon for the corrosion specialist to have results of vari-
readings show adequate CP levels. All pipe sections in between ous types of surveys of varying ages and be faced with the
the reading locations would be penalized for having no pipe-to- challenge of assimilating all of this data into a format that can
soil voltage information at all4 points. For example, in an support decision making. The previous scenarios discount
annual on-reading survey (current applied), where all readings additional adjustments for age of survey and equipment mal-
show adequate CP, the risk assessment will show point values functions. Such adjustments should play a role in scoring
of (Maximum CP Effectiveness Points) x (Annual on survey (even though they are not illustrated here) because they are
weighting, option 1) or 15 points x 30% = 4.5 points for the 10 important considerations in evaluating actual CP effective-
ft of pipe around the test lead location, and 0 points elsewhere. ness. The scoring scheme is patterned after the decision
This indicates that the evaluator has no evidence that CP levels process of the corrosion control engineer, but is of course con-
are adequate between test lead locations. sidering only some of the factors that may be important in any
In another option, the evaluator feels that the test lead read- specific situation.
ing does yield useful information on CP levels between test
lead reading locations, even at distances thousands of feet away.
The weighting, however, must be far less than for a CIS, where Inte r$erence potential (weighting 10% of corrosion
the reading locations are very closely spaced. So, only 1% of threat)
the maximum possible points are awarded, but the points apply
to all locations between test lead locations. In this case, the Corrosion Index (1 00 pts)
annual, on-reading survey where all readings show adequate Atmospheric
CP, will show point values of (Maximum CP Effectiveness Internal
Points) x (Annual on survey weighting, option 2) or 15 points x Subsurface
1% = 0.15 points everywhere. Subsurface environment
In yet another option, the evaluator feels that the test lead { Coating
reading yields information about surrounding lengths ofpipe in Cathodic protection
proportion to their distance from the test lead location. In this Effectiveness

Table 4.5 Sample of more detailed scoring for CP effectiveness
Weight (to be
multiplied by
maximum CP
Information source (survey type) effectivenesspoints) Comments and directions for scoring
CIS polarization 55% Polarization survey usually gets to 100% since other survey types are done as part of
the polarization survey.
CIS on (current applied) 30% CIS readings with current applied. Ifpipe-to-soil criteria are met and survey is recent.
then points are 15 x 30% = 4.5 points.
CIS off (current is interrupted) 20% Establishes static line the first time; can reuse static line with subsequent CIS
interrupted surveys; CIS-interrupted also gains credit for CIS-on survey, so
30 + 20 = 50%
30% or20% Use survey type weighting (20%. 30%, or combination = 50%) and apply to 5 ft either
side of a test lead location
Annual on or intempted 1 % Apply to half the distance to next test lead
(at test lead locations only) 1 0% Multiply also by test lead adjustment factor
Annual polarization 55% Same as above. Test is done at test lead stations by interrupting rectifier and using a
(at test lead locations only) 4% static polarization survey measure for comparison.
15%
Test lead spacing Adjustment 100% when all parts of the pipeline segment are within 1 mile oftest lead; 0% when any
part of segment is greater than 2 miles. lfany part ofthe segment is > 1 mile from the
test lead, then degrade to 0 points when distance reaches 2 miles;
Rectifier out of service Adjustment Penalties for equipment outages in any year plus cumulative outages over several years.
Penalties removed after ILI or visual confirmation that no damage occurred.