Pipeline Coating Defects Detector DDRC technique.

Introduction:

The DDRC technique (Détection de Défauts de Revêtement de Canalisations) is a technique for coating surveys on buried pipelines.

Theory:

It is used not only for 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 less than 10 cm.
Diagram above shows that a voltage gradient is established in the soil surrounding a coating defect of a pipeline with an impressed current CP system. The highest gradient is recorded in close proximity to the defect. In the DDRC methodology, the direct current input signal used to measure the voltage gradient is pulsed. The DDRC is made up of a millivoltmeter and two copper - copper sulphate reference electrodes (about one meter from one another on pipeline axis)
This method only applies for pipelines electrically continuous (not for cast iron pipes).
Apart from locating defects, their sizing is also important in order to prioritize excavation and repair.

Equipment:

Transmitter (ON - OFF switch):
It’s an interrupter to switch alternatively ON and OFF the CP system (1 Hz frequency).

Measurement rods (copper-copper sulphate reference electrode):
They enable to measure the voltage gradient created by the coating defects.

Receiving:
It is constituted by an analogical millivoltmeter with central zero.

Starting:




Principle of measurements:

The system uses the CP direct current. The operator measures the sense and the amplitude of the voltage gradients in soil.

The receiving is synchronized with the transmitter. The receiving, constituted by an analogical millivoltmeter at central zero, makes the difference permanently between the ON and OFF signal (1 Hz frequency). Thus the applied signal is separated from all stray direct current influences such as tellurics, direct current traction, etc. It limits stray direct current interferences.

The operator puts the measurement electrodes on the soil, his arms stretched in the axis of the shoulders parallel to the pipeline. The measurement electrodes are placed about one meter apart by the operator. The measurements are spaced out 5 meters to 10 meters apart. The regular beating of the millivoltmeter needle indicates the presence of a coating defect. The inversion of the beating indicates that the operator has passed the coating defect. The exact location of the coating defect is at the reversing voltage gradient point.

A correct interpretation depends on :
- soil resistivity
- soil moisture content
- electric interferences (VHV lines, DC-traction systems, …)

Estimation of coating defect size:

It is possible to size coating defects by rule of three.

It is not possible to have metallic surface informations :
- corrosion
- loss of weight
- hollow

Informations :
- European Standard (number 13509 / october 2003) : cathodic protection measurement techniques
- ADCA Électronique, D.D.R.C. instructions for use