Close Interval Potential Survey (CIPS) technique

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, schematicclick 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 rectifierclick 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.

 

References/Literature:

R.L. Pawson: "Close Interval Potential Surveys - Planning, Execution, Results", Materials Performance, February 1998, pp.16-21.

DCVG techniqueThe Direct Current Voltage Gradient (DCVG) technique is a "newer" technique for coating surveys on buried pipelines. It has been used for not only locating but also sizing coating defects. The technique is fundamentally based on measuring the voltage gradients in the soil above a cathodically protected pipeline. A distinctive feature of this technique is that even small defects can be located accurately, with a claimed accuracy of about 10 cm (4 inches).

    

click on images to enlarge(courtesy of Pipeline Performance Technologies, PPT, South Africa )

The diagram below shows that a voltage gradient is established in the soil surrounding a defect, in the coating of a pipeline with an impressed current CP system. The highest gradient is recorded in close proximity to the defect. In the DCVG methodology, the DC input signal used to measure the voltage gradient is pulsed. A sensitive milli-voltmeter and two copper-copper sulfate reference electrodes (placed about one meter apart by the operator) are typically used for measuring purposes. The nature of the milli-voltmeter signal during a DCVG survey is illustrated in the diagram below, in relation to the defect epicenter.

click on image to enlarge

Apart from locating defects, their sizing is also important in order to prioritize excavation and repair. Based on DCVG measurements it is possible to compute a so-called %IR value. Four defect categories, related to the %IR computation, have been proposed. This defect classification is empirical and is also dependent on the soil resistivity. Hence, excavation of actual defects may be advisable.

Some cited advantages of the DCVG technique include:

Suitability to complex piping arrangements and in congested city areas, where a "modest" amount of stray current interference can be expected. (In the case of "heavy" interference the instrumentation may not perform satisfactorily.)

High accuracy in locating defects. Usually involves no trailing wires. Can be used in combination with other techniques.

In principle requires only a single operator.

 

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

 

References/Literature:

J.M. Leeds and J. Grapiglia: "The DC Voltage-Gradient Method for Accurate Delineation of Coating Defects on Buried Pipelines", Corrosion Prevention and Control, Vol.42, No.4, 1995, pp.77-86.

From South Africa: two references submitted by Dr. Chris Ringas on the DCVG technique:

C. Ringas, J.M. Leeds and P. Oosthuizen: "The application of DC voltage-gradient technology to accurately determine buried pipeline rehabilitation requirements." Pipeline Risk Assessment, Rehabilitation and Repair Conference, Houston, Texas, 12-15 September 1994, organized by PipeLine Industry and Pipes & Pipelines International.

Z. Masilela and J. Pereira: "Using the DCVG technology as a quality control tool during construction of new pipelines", Engineering Failure Analysis, Vol. 5, No. 2, pp. 99-104, 1998.