Leaders in Pipeline Survey Systems
4 - 2
OPERATING MANUAL
4.1.1 Principles of Operation
An electrical current applied to a well wrapped buried metal pipeline will decrease
gradually with increasing distance from the current injection point, as the current
escapes to earth through the wrapping.
If the wrap has a uniform thickness and separates the pipe from the surrounding soil at
all points, the strength of the signal current on the pipe 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 wrap 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 pipe 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 wrapping, mechanical damage to
the wrap before, during or after installation, decay of the wrap due to soil conditions,
disbonding of the wrap from the pipe (provided that ground water has penetrated into
the gap to provide an electrical path to earth), or a leak in the pipe itself causing the
wrap 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 wrap, the resultant loss of current, even from a
single small fault of a few square millimetres, can usually be detected by a significant
increase in the apparent rate of current decline over quite a long length of pipe.
In practice, the existence of one or two small faults on a section of pipeline several
hundred metres 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) measured in millibels per metre
(mB/m), 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 wrap between two given points at the date of the survey].
If the attenuation level over a given section of pipeline is particularly high, serious
faults(or a large number of small faults) in the protective wrap are indicated.
Intermediate readings of attenuation levels can be taken to identify the worst sections,
and the precise location of faults (generally to an accuracy of 1-2 metres) can be
determined by recording and plotting the actual current levels at close intervals (3-5
metres) over the suspect section and noting those places where the rate of current
decline is steepest. The most serious faults can then be excavated and repaired.
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