Over-the-line Potential Survey

March 8, 2011

Cathodic protection systems have the need to be tested for effectiveness and to ensure quality protection on the material. This can be done by conducting a series of over-the-line potential survey or the close interval surveys. As defined, close interval surveys is a direct current (DC) measurement on pipelines done at a regular gaps to asses the level of protection on pipelines and immersed structures. The survey does not refer to cell-to-cell survey techniques. Using the coatings effectiveness or RTU remote we can evaluate the direction of the current that is passed on to the material that is to be protected.

                This survey is one of the most useful tools to ensure the quality of the protection provided by the cathodic protection systems. Cathodic protection systems have terminal units where this survey is made. Using close interval survey, the presence of copper sulfate can be detected which is an indication of corrosion in the metal.

                Close interval surveys provide measurement of possible corrosion area to a reference electrode directly above and at frequent intervals along the structure to be protected. There are three methods that may be used for taking the closely spaced results of close interval survey:

METHOD 1

Ø  Involves readings taken on a continuous basis at 3 to 5 ft. intervals along the structure

METHOD 2

Ø  Involves the use of a pair of CSE in a leapfrog fashion

METHOD 3

Ø  Measures the potential drops between two copper sulfate electrodes

The results by the cathodic protection systems are affected by corrosion current flowing to and from the structure. Such currents cause voltage drops in the immediate vicinity of the structure.