Page 237 - Pipeline Rules of Thumb Handbook

224 Pipeline Rules of Thumb Handbook

Signal attenuation coating survey number of categories relating to “point” anomalies or “general
area” anomalies. For example, the following categories may
Results are presented in graphical form although quick be used.
analysis may be performed in the field from raw data. Areas
of poor coating quality may be readily identified from final Classification Definition
graphs. Type I Both the “ON” and “OFF” potentials are
Following the production of graphs and histograms an more positive than preset criteria. There is
evaluation system employed in pipeline coating quality little or no “IR” voltage. The potentials are
assessment may assign values to the level of signal loss and unsatisfactory and the locations are not
the measurement distance interval. The sum of the two val- cathodically protected.
ues referred to as the “dB factor” and the “km factor” may be Type II The “OFF” potential is more positive than a
used to indicate the future action required and the level of preset limit, the “ON” potential more nega-
priority for each section surveyed. tive. There is a reduced level of protection
The evaluation of the coating quality is illustrated in Figure indicated at “point” locations which may be
1. It should be noted, however, that gradings assigned to one improved by increased output from the
particular pipeline cannot be compared to those assigned to nearest cathodic protection station, although
another. where this occurs over long pipeline lengths,
it indicates high resistance coating or soils.
Type III Positive change in the “ON” and “OFF”
potential plot where both remain more
Close interval potential survey
negative than the selected criteria. This
indicates increased current demand at these
Results are shown in graphical form. Defect location is
undertaken after the survey and requires experienced per- locations but that protection is maintained at
a satisfactory level.
sonnel to interpret the results. Type IV Unusual potential variations due to outside
The achievement of cathodic protection on the pipeline is
indicated by the “ON” and “OFF” potential both being more influences, e.g., stray currents, station earth-
ing, AC induced voltages.
negative than the accepted criteria. Type V A change in level of the “ON” potential at a
Pipeline locations with inadequate cathodic protection will
be indicated by the graph “ON” and “OFF” potentials shift- test post due to the change in the length of
that part of the measuring circuit passing
ing to a less negative level and become cause for concern if current, i.e., increasing “IR” volt drop due
the “OFF” potentials are more positive than say -0.85V. to increasing pipe length in circuit.
It is considered that a localized coating defect would nor-
mally be indicated by a sharp positive shift of the “ON” poten- Of the five categories listed above the first three are
tial plot coupled with a plot shift of smaller amplitude of the indicative of coating defects. Types IV and V are commonly
“OFF” potential, i.e., that the potential difference (IR drop) observed on close interval potential survey results and require
between the “ON” and “OFF” reading decreases, indicating careful interpretation.
a reduced coating resistance at that point. It is important that these considerations are borne in mind
The “OFF” potential plot change would not be as large as when selecting the survey or combination of surveys to be
that of the “ON” potential, as the “OFF” potential would be employed and in addition the choice of which survey to
the polarized (or partially polarized) potential of the bare employ and when to employ it is dependent on a number of
steel. This differs from the case where insufficient current for factors, namely,
the achievement of polarization is available.
In all these cases, the IR drop measured between the “ON” • the purpose of the survey,
potentials and the “OFF” potentials includes, along with the • the condition of the pipeline,
resistance of the coating, the resistance of the soil. The resis- • the access over the pipeline,
tance of the coating is assumed to be the largest component • the time allowed.
of this resistance value.
Current is also a factor of the IR value. The current den- As the protection system relies on both the external coating
sity impressed on the pipeline will increase as the cathodic and a cathodic protection system, these may be considered as
protection stations are approached, giving the largest “I” first and second lines of defense against corrosion. It is pos-
value of the IR drop near the location of the negative sible to take the position that concern over damaged coating
connection. is not warranted as back up from the cathodic protection
In order to interpret the data obtained fully, the anomalies system is available and therefore the integrity of the pipeline
observed in the plotted results may be classified into a is not at risk. If this attitude is taken, then obviously close