dcvg technique

DCVG technique:

The 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.

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