DCVG coating defect survey of steel pipelines

The DC Voltage gradient technique is a relatively new development to the Water industry, but is extensively used in the Oil and Gas industries in order to locate coating defects in the protective coating on large diameter steel pipelines.

The DCVG technique is the most accurate technique available to the water, oil & gas industry in order to locate the faults in the protective coating on buried coated steel pipelines. Townley Brothers have extensive experience using this technology for survey of coal tar coated steel water and effluent pipelines, where external corrosion only occurs at coating defect locations.

Steel pipelines suffer from pin-hole bursts following external corrosion of unprotected pipe wall steel, where the protective coating has been damaged. A DC Voltage Gradient survey can locate every coating defect to an accuracy of 15cm from above the buried pipeline without need for excavation, indicating the approximate size (surface area of exposed, unprotected steel) of each defect, even those as small as 10mm in size.

Depending on the condition of the coaltar coating, a steel pipeline may generally have between 1 and 100 coating defects per kilometre. Without cathodic protection each defect may eventually represent a burst depending on rate of external corrosion, in turn depending upon soil aggressivity.

Combined with soil resistivity and pH surveys, defects can be prioritised for selective inexpensive repair, rather than rely on the reactive maintenance of potentially hazardous failures.

Another unique advantage of the DCVG technique, is that when conducted on pipelines with Cathodic Protection, it can identify exactly where and how much of the available CP current is being lost via bare steel contact through the surrounding soils. Selective re-coating of the larger size coating defects would enable the CP system to halt future external corrosion.

The DCVG survey technique is non-obtrusive and is carried out from above the buried pipeline. An operator will walk along the length of the pipeline and measure the location of defects as a distance from the nearest valve or connection point or field/road boundary etc. A connection to the steel of the trunk main at various intervals is essential and is usually made at Valve locations or exposures where present. Coating Defect positions  are captured using Trimble Differential GPS to sub-metre accuracy, allowing data to be associated directly to the pipeline asset if required.

Once a survey has been carried out, defects are entered onto a database and their approximate size is calculated (IR% - representing the approximate area of exposed steel). This then allows non-destructive inspections to be located at defect locations to determine the degree of external corrosion and remaining service life of the pipeline.

As corrosion on steel pipelines only usually occurs where there is a breakdown in the protective coating, DCVG is a proven tool for locating potential and actual corrosion problems.

The technique is very versatile and can be used in complex pipeline networks, in city streets, across rivers and lakes.

We have carried out major surveys, over many kilometres, on both clean water and wastewater pipelines in Wales , Scotland and England . The technique is used extensively in the oil and gas market overseas where it is often the survey technique of choice.