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 hydrogen embrittlement.

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 rectifier
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.

The CIPS technique is increasingly used in combination with GPS technology.