Description of the DCVG Technique
The DC Voltage Gradient (DCVG) technique is used primarily to locate defects/faults on the pipeline external coating. In addition to the former, the technique enables one to carry out further tests to determine the status of the located defects.
When DC current flows in the soil to or from bare steel/metal in contact with the soil, as in cathodic protection, a voltage gradient is established in the soil. This voltage gradient increases with defect size, pipe depth and soil resistance.
The DCVG method utilizes a sensitive millivoltmeter to measure the voltage difference between two copper/copper sulphate half-cells (probes) placed in the soil across a typical voltage gradient region ie. Near a defect. The half-cells are placed one or two metres apart with one half cell adopting a more positive potential than the other, and as a result the direction of current flow can be established.
Description of the PCM Technique
The Pipe Current Mapper (PCM) technique is used primarily to map current flow on a pipeline with significant benefits in approximating external pipeline coating condition.
When DC current flows in the soil to or from bare steel/metal in contact with the soil, as in cathodic protection, a voltage gradient is established in the soil and a specific current flow pattern is established on the pipeline. This voltage gradients and current flow patterns on a specific pipeline may vary with defect size, pipeline coating and soil resistance.
A current flowing on a buried conductive structure produces a magnetic field directly proportional to the magnitude of the applied current. By resolving components of the magnetic field from above ground, the original current can be determined accurately.
GPS current interrupters generally cover a range of 10 amps to 200 amps. The current interrupters can be installed in either the AC supply to the rectifier or the DC output of the rectifier. A sufficient number must be installed such that all rectifiers that could influence the potential of the pipeline in the area to be surveyed must be interrupted synchronously.
Cathodic protection comprises the application of both electrical and chemical principles to prevent the corrosion of suitably submerged steel structures.
In simple terms the steel structure requiring corrosion protection is rendered cathodic (negative) and the circuit is completed with an anodic (positive) component. Current flows from the anode to the cathode thereby preventing the oxidation (corrosion) of the steel.
EXPERT CORROSION SOLUTIONS FOR BOTH OFFSHORE AND INLAND STRUCTURE
• DCVG (Direct Current Voltage Gradient) surveys,
• pipe-to-soil potential surveys,
• soil resistivity surveys,
• current demand surveys,
• stray DC current surveys, and
• AC mitigation surveys amongst others.
Our team of experts provide full service management from the conceptual phases of a project through to the final stages of implementation. This would typically include setting out the scope of work, budget estimates, field surveys, reporting, detailed design, tender, schedule of rates and quantities, tender process, adjudication, tender award and project management.