DCVG- and CIP-Surveys
A CP-system is a secondary protection device meant for prolonging the operational life of a pipeline. The coating is always the primary system. At the spot where the coating has been damaged by exposure to an electrolyte (in this case the soil), the cathodic protection will in most cases prevent the pipeline from corroding.So at those particular places where the coating is so worn that the pipeline makes contact with the soil, the primary protection system has failed and the CP will take over. Of course this situation is not desirable; the primary system should stay intact. However, below-ground monitoring is a complicated task.
DCVG-measurements enable the above-ground detection of coating damage with an accuracy of 30 cm below the surface. Additionally, the extent of the damage can be measured. The DCVG-method is often used in the following situations:
- Prior to the final acceptance of new pipeline parts.
- To detect large damage to the coating which decreases the CP protective current.
- To provide an insight into the general condition of the coating.
- As a control measurement after (illegal) excavation activities.
- As an inspection within the guarantee period (for example 1 year after the final acceptance), depending on the agreement.
In addition to the DCVG-survey, a CIP-measurement can be conducted to determine possible differences of potential within the soil. In case of damage to the coating, a slight difference of potential will occur (very) locally between the pipeline and the soil; after all, there is contact between the two potentials. As a result, the pipeline will create a potential funnel within the soil, which can be measured above-ground by conducting a CIP-survey.
The difference of potential within the soil is measured by making a comparison with a location that is not affected. Since it is unknown where the damage is, and especially how many weak spots the pipeline contains, a measurement is conducted at intervals of 50 cm or 1 metre. This again depends on the age of the pipe and the type of coating.
An additional advantage of a CIP-survey is that it also picks up disturbances within the soil caused by other conductors. For example, if there is an above-ground connection with an earthing system, the earthing will form a large funnel within the soil.