Leaders in Pipelin
e Survey Systems
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SYSTEM MANUAL Version 6.03i
6
PRINCIPLES OF OPERATION
An electrical current applied to a well coated buried metal pipeline will decrease gradually
with increasing distance from the current injection point, as the current escapes to earth
through the coating.
If the coating has a uniform thickness and separates the pipeline from the surrounding soil
at all points, the strength of the signal current on the pipeline will decline logarithmically,
and the rate of decline will be dependant primarily on the electrical resistivity of the wrap or
coating in use, and the area of coating in contact with the soil per unit length of pipe (i.e.
for a given coating, the decline is proportional to the circumference of the pipe). [Note:
because of the relative magnitude of the resistances involved, local changes in soil
resistance can usually be ignored].
If there is a low resistance electrical path from the pipeline direct to the soil at any point,
there will be a substantial local increase in the rate of loss of signal current. Such a low
resistance path could arise from: incorrectly applied coating, mechanical damage to the
coating before, during or after installation, decay of the coating due to soil conditions,
disbonding of the coating from the pipeline (provided that ground water has penetrated into
the gap to provide an electrical path to earth), or a leak in the pipeline itself causing the
coating to fail at the leak point.
Because the resistance of such a path is likely to be several orders of magnitude less than
the resistance of the undamaged coating, the resultant loss of current, even from a single
small fault less than 0.125
2
in, can usually be detected by a significant increase in the
apparent rate of current decline over quite a long length of pipeline.
In practice, the existence of one or two small faults on a section of pipeline several
hundred feet in length can usually be tolerated because the Cathodic Protection System
can be expected to prevent serious corrosion developing quickly. In this case it may not
be necessary to locate the specific faults immediately but the rate of logarithmic decline of
current between two specific points can be logged for future reference so that any
deterioration of the condition of the pipeline can be monitored. [Note: the logarithmic rate
of decline of the current (attenuation), which is always measured in millibels per metre, is
effectively independent of the applied current and is only marginally affected by seasonal
changes in soil resistivity, so that it is virtually an absolute indication of the average
condition of the coating between two given points at the date of the survey.].
If the attenuation level over a given section of pipeline is particularly high, serious
anomalies (or a large number of small anomalies) in the protective coating are indicated.
Intermediate readings of attenuation levels can be taken to identify the worst sections, and
the precise location of these anomalies (generally to an accuracy of 3-6ft) can be
determined by recording and plotting the actual current levels at close intervals (6-30ft)
over the suspect section and noting those places where the rate of current decline is
steepest. The most serious anomalies can then be excavated and repaired.
