Leaders in Pipeline Survey Systems
OPERATOR’S MANUAL
Issue 4.04 - 1207 23
4. OPERATION
4.1. Introduction
This section describes the C-Scan Pipeline Survey System model 2010 in normal operation by
using a detailed familiarisation practice session in the System’s built-in DEMO MODE.
Please refer to the accompanying SURVEY PROCEDURE manual for more detailed information
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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