|Close Interval Potential Survey
Cathodically protected pipelines are equipped with
permanent test stations where electronic leads are attached to the pipeline to measure the
pipe-to-soil potential. This potential should be sufficiently cathodic to ensure adequate
corrosion protection but not excessively cathodic to produce coating damage and/or
The potentials measured at permanent test stations only originate from a
small fraction of the total pipeline length. One proposed rule of thumb estimates that the
measured potential is associated with a relatively short length of pipeline - about 2x the
depth of pipeline burial. The Close Interval Potential Survey (CIPS) technique is aimed at
assessing the CP effectiveness over the entire length of the pipeline, in between the
permanent test stations.
In CIPS the operator establishes an electrical connection to the pipeline
by means of a trailing wire. This coated copper wire unwinds from a spool as the operator
walks the length of the pipeline. The pipeline potential is measured with a set of
reference electrodes at ground level, positioned directly over the pipeline, at intervals
of about 1 meter.
In order to obtain a better indication of the "true"
pipe-to-soil potential, the IR error in the potential readings associated with CP current
flow through the soil has to be minimized. This is achieved by interrupting CP current
flow for an instant-off potential measurement. In practice, this means that that all
sources of significant CP current need to be interrupted synchronously. Typically the
current output from several influencing rectifiers (and also foreign sources of current)
needs to be interrupted synchronously. Increasingly, GPS timing
devices are used for synchronous switching devices.
CIPS technique, schematic
click on image to enlarge
The selection of a suitable interruption time and timing of the potential
measurement in the interruption ("off") cycle has been subject to some debate.
In practice, the interruption cycles tend to vary from a second to fractions of a second,
depending on the instrumentation used. The selected "on" cycle is longer than
the "off" cycle to limit depolarization of the pipeline during the surveys.
Commercial interrupter with GPS timing installed at a
click on image to enlarge
Clearly, in principle, this technique is based on relatively simple
potential measurements. However, in practice, surveys can be very demanding on field
crews, require a high degree of operator skill and experience and demand extensive support
logistics for physical measurements and for computerized data management. Close
collaboration between the client and survey contractor is an important consideration.
|Some cited advantages of the CIPS technique include:
- Simple in principle and widely used.
- Assessment extends along the entire length of the pipeline.
- Complete pipeline right-of-way can be inspected as part of the walk along the pipeline.
The CIPS technique is increasingly used in combination with GPS technology.
R.L. Pawson: "Close Interval Potential Surveys - Planning, Execution,
Results", Materials Performance, February 1998, pp.16-21.
Pipeline corrosion monitoring