DCVG Defect Excavations & Inspection

A DCVG survey will find bare steel/coating holidays in contact with the soil or water (the electrolyte) and receiving Cathodic Protection (CP). It will also find anodes or other items which may be attached to the pipeline and receiving CP current, for example, anodes.

The DCVG technique is also handy to identify third-party pipeline damage. It will Identify the coating loss, which may when excavated, expose mechanical damage.

The technique will not identify.

• Areas shielded from CP current by, for example, rocks, plastic pipes or cables on or against the pipeline, disbonded coating, i.e., loose heat shrinks.

• Coating holidays on pipes inside casings, in valve pits or any other areas where the electrolyte is not touching the pipe.

Dry ground conditions, Tarseal and concrete pavement reduce the survey method's effectiveness because of the substrate's high resistance. This can typically be overcome by having a water truck in front of you while you survey. An ACVG survey works better in high resistivity conditions.

The technique does not indicate the pipe condition, i.e., any corrosion. AS 4827.1 2008 is the standard with which the DCVG survey is performed in accordance with. There are many examples of DCVG identifying coating defects resulting from mechanical damage. Here is a selection of high-profile ones.

DCVG surveys are also used for

• New pipelines as the final acceptance criteria, asset owners stipulate that all DCVG defects greater than 0.2% IR are repaired.

• Where a new road is going over a pipeline, the pipeline owners specify that a DCVG survey is performed and repairs carried out. When the work is completed, any new defects are to be repaired at the contractor's cost.

• Where it is suspected that a contractor has been working near a pipeline, a DCVG survey of the area will show if the coating has been damaged.

• As a routine inspection tool to identify the amount of coating breakdown, Excavating these areas is the ultimate proof that CP is preventing corrosion. Typical time spans are 5 yearly for critical lines and 10 yearly for non critical lines.

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Example 1 - The Refinery to Marsden point line rupture

Issue: The pipe was damaged in 2014 but did not rupture till 2017. If the pipeline had been DCVG surveyed between these dates, this would have shown up as a new indication and hopefully been excavated and repaired (welded sleeve?).

Note: I had previously (when working for NGC) DCVG surveyed this line and the off takes to the compressors, but this I think, occurred in between the routine ten-yearly cycle.

The pipeline is flown by helicopter every month, looking for signs of soil disturbance. If a disturbance is identified, the area is DCVG surveyed to determine if the pipeline has been damaged.

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Example 2 - The 2" High-pressure Natural gas pipeline feeding New Plymouth City.

Issue: 2" line was hit by a mole plough. The Mole plough damaged the coating. A DCVG survey identified the damaged coating. When excavated, the pipe was found to have been severely damaged and bent upwards at almost 90 degrees creasing the pipe. This section of pipe had to be cut out and replaced.

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Example 3 - Fuel pipeline in Wellington

Issue: Excavator scrap over the pipeline. A DCVG survey identified the coating defect. The damage location was near the office, which shows things can happen "right under your nose".

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