Page 235 - Pipeline Rules of Thumb Handbook
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222 Pipeline Rules of Thumb Handbook
Methods of locating coating defects
Pearson survey are indicated in Figure 1. Any area indicating high signal
losses may then be resurveyed using the same procedures to
The Pearson survey is an above ground technique used to more closely locate the coating defects(s).
locate coating defects on buried pipelines. An AC signal is
injected onto the pipeline from a power source connected
between a cathodic protection test post or valve and an earth Close interval potential survey
pin. The AC frequency selected is dependent upon the type
of pipeline coating in order to minimize current loss from the The close interval potential survey is an above ground tech-
pipeline through sound coating. At coating defects, the AC nique used to provide a detailed profile of the pipe to soil
signal leaks to earth and flows via the soil to the earth spike potential with distance. This profile can be used to determine
to complete the circuit. The localized current flow from the detailed information concerning the performance of the
pipeline at the defect creates a potential gradient through the cathodic protection system, the coating system and to a lesser
soil and this is detected by measurement at a ground surface. degree, interaction effects. The advent of portable micro-
In practice, this is achieved by two surveyors walking in processor based measuring equipment and computer assisted
tandem over the entire pipeline, or section of pipeline. Earth data handling, together with accurate synchronized high
contacts, either ski poles or foot cleats, are connected to a speed current interruption devices have allowed these surveys
to provide accurate, practical data and collection methods.
receiver via a cable harness. As the first surveyor approaches To eliminate errors in the pipe to soil potentials recorded,
a coating fault, an increased voltage gradient is observed, arising from the inclusion of volt drops caused by the flow of
either audibly through headphones, or visually on the receiver cathodic protection current in the soil, synchronized current
signal level meter. As the lead surveyor passes the defect, the interruptors are employed in the cathodic protection units.
signal fades, and then increases again as the rear surveyor’s Pipe to soil potentials are then measured instantly after the
contacts pass over the defect. current is interrupted, before significant chemical depo-
The defect may be recorded on a preprepared record sheet
complete with a measured distance from a fixed reference larization has occurred thus providing an “error-free” polar-
ized potential.
point or indicated by a marker peg or non-toxic paint. A In practice, the precision current interruptors are best
recent development in Pearson equipment has been the fitted in the DC output circuit of the cathodic protection
introduction of a system that continuously records, through power supplies, prior to commencing the survey to interrupt
data logging equipment, the potential difference between the cathodic protection current for short cyclic periods. The
each earth contact. In this way, data may be graphically cycle time is chosen with regard to pipeline characteristics
presented.
although a range of “ON” and “OFF” periods and “ON/OFF”
ratios are available.
Signal attenuation coating survey
For this survey a variety of purpose built voltmeter/micro-
processors are available that record and store the pipe to soil
This method is similar in many respects to the Pearson
survey, the essential difference being the method of defect potential data with an additional facility to store coded entries
of topographical or pipeline features. This information is
detection. The AC current injected onto the pipeline creates transferred to floppy disc storage from which graphs of ON
a magnetic field around the pipe, the strength of this field and OFF pipe to soil potentials against pipeline chainage are
being proportional to the amplitude of the AC signal. produced. A typical close interval potential graph is illustrated
Measurements of the field strength are made by recording the in Figure 2.
voltage generated in a sensing coil and indicated on a dB It will be apparent from the description of the pipeline
scale, when the coil is held over the pipeline. Loss of current surveys that the form of results are different for each survey:
from a coating defect creates a change in the field strength
due to a change in the pipe current and this is detected as a
change in output from the sensing coil. Pearson
In practice, this is achieved by one surveyor measuring field
strengths at discrete, preset intevals. At each location the field Results are not generally presented in graphical form.
strength and pipeline chainage are recorded. A graph of dis- Coating defects are marked in the field; in addition the detec-
tance versus field strength may then be produced, sections of tion of defects is dependent upon operator skill. Only if the
pipeline having poor coating quality are indicated by the recording Pearson technique is used can data be presented in
steepest gradient on the line plot. The results may also be graphical form. Experience of this technique is at present
expressed as histograms to aid interpretation. Typical results limited.