EmiratesGreen Electrical & Mechanical Trading 
Abu Dhabi 
(UAE)

Oil & Gas, Petrochemicals, Power Plants and other Industries Process Equipment & Materials, Piping & Valves, Measurement Tools & Devices & others  

CIPS Survey

DCVG Survey - DC-Voltage Gradient (DCVG) Surveys

Introduction

DCVG surveys are typically performed on coated pipelines with a view to determining the location of holidays (coating defects) or other defects. As discussed below, such surveys can be used to not only pin-point the location of defects, but they can also be used to provide a measure of the “size” (severity) of a particular defect.

The effectiveness of CP system highly depends upon coating of the pipelines, as the Coating of the Pipelines can be damaged after certain period time. Therefore for integrity of assets it is vital to investigate coating condition of the pipeline. DCVG survey is the best technique to detect coating defects and faults in buried piplines. DCVG Surveyor walks over the soil and points the place of defect. EmiratesGreen has highly experienced DCVG Survey Team and the Equipment’s. We are capable to conduct DCVG survey on pipeline and pipeline network.

Since DCVG surveys are close-interval in nature, they can be considered complementary to pipe-to-soil close interval surveys (CIS) and both types of surveys are typically performed on the same pipeline sections as part of the ECDA (External Corrosion Direct Assessment) protocol regarding “Indirect Inspections”. With regard to the ECDA protocol, at least 2 types of close-interval “Indirect Inspection” surveys are required to be performed on all sections of a buried pipeline and typically DCVG and CIS surveys are employed to satisfy this requirement on coated-pipelines.

PHYSICAL PRINCIPLES

Potential (Voltage) Gradients

When a buried pipeline is under cathodic protection, the DC rectifier current that flows to (and in) the pipeline (impressed current) causes the pipeline to be negatively-charged with respect to the surrounding soil. In fact, as illustrated in Figure 1 below, a potential gradient exists between the pipeline and the surrounding soil, the potential being largest at the pipe itself (largest negative value) and dropping off rapidly with distance away from the pipe. The potential at “remote earth” is zero and the potential gradient is the potential difference between the pipe potential (negative potential at the pipe) and remote earth.

Figure 1: Potential Gradient Associated with a Cathodically-Protected

Pipeline (Equipotential Lines are closer together near the pipe, indicating that the potential drops quickly (initially) with distance away from the pipe).

In the case of a well-coated section of pipeline, the potential at ground level in the vicinity of the pipe will be close to zero (close to remote earth potential).

However, when coating defects (holidays) are present, the local potential at the defect locations increases significantly, since the impressed current flows through the soil primarily to the defects. This situation is illustrated schematically in Figure 2 below.


Figure 2: The Influence of Coating Defects on Impressed-Current Flow Distribution along a Pipeline (locally high potentials occur at defect locations due to locally large currents flowing to the defects).

A small amount of current will also flow to the pipeline through the coating (the coating will not be a perfect electrical insulator), however, this current will be negligible compared to that flowing to the coating defect areas.

Since the pipe potential can be high at the site of a defect (due to large impressed-current flow to the defect), the potential at ground level above the pipe (above the defect site) can be significantly higher than zero (the remote earth potential). Consequently, the measurement of a significant “above the defect” soil potential (voltage) with respect to remote earth, is a good indication that a coating-defect has actually been located, especially if the soil potential above the pipe away from the defect location is essentially zero with respect to remote earth.

Since the defect site is surrounded by soil, a voltage gradient will exist between the defect site and the soil. A voltage gradient will actually exist in 3 dimensions. Consider the defect site as being the center of a series of concentric spheres with the surface of each sphere being a surface of constant potential. Near the defect, the concentric spheres will be closely-spaced which means that the potential will drop-off quickly with distance from the epicenter (the defect site). The spheres (constant potential surfaces) will be spaced further and further apart, implying a slower potential drop-off rate, with distance away from the defect site.

If we consider the surface of the ground above the pipeline to be a “plane”, the ground will have an “in-plane” potential gradient in the region above the site of the defect. Imagine taking a slice through the ball of concentric spheres (spherical surfaces of constant potential). The slice would represent the surface of the ground, with the distance from the epicenter of the concentric spheres to the slice being the distance from the defect to the ground, ie, the depth of the pipeline.

illustrate this point, the constant potential circles have been labeled as percentages. It is assumed that the potential on the surface of the ground directly above the defect is 100% of the maximum potential, ie, the largest potential measurable for this defect.

Figure 3: Potential (Voltage) Gradient at the Surface of the Ground

associated with a Coating Defect on a Buried Pipeline (concentric circles represent constant potential lines). The “X” symbol represents location of coating defect.

If we imagine pushing a pole vertically from the surface of the ground directly above the defect down to the defect itself, and then walking on the surface of the ground away from the pole, we’ll encounter lower potentials (with respect to our maximum potential at the pole) as we move further and further away from the pole. The percentages indicated in Figure 3, would represent percentages of the maximum potential measured at the pole location.

Since such surface potential (voltage) gradients exist above pipeline defects, we can perform soil-to-soil potential difference surveys to detect the defects. Such surveys are know as DCVG surveys (Direct-Current Voltage Gradient surveys).

DCVG surveys can be performed in two different ways: Perpendicular Mode and In-Line (or Parallel) Mode. In the case of Perpendicular DCVG surveys, an imaginary line drawn between the reference electrodes on the surface of the ground would be perpendicular to the direction of the pipeline and, in the case of In-Line DCVG surveys, an imaginary line drawn between the reference electrodes on the surface of the ground would (ideally) be in-line with the center of the pipeline (directly above the pipeline).


II. 2 Perpendicular DCVG Survey Technique
:

II. 10 In-Line (Parallel) DCVG Survey Technique: