AC Current Attenuation (ACCA) surveys are typically conducted to obtain a coating assessment for a large section of pipeline. When conducting a survey, the current attenuation readings are generally recorded at intervals of around 100 feet or may be Operator specific. Extended intervals allow for faster surveys of larger sections of pipeline.
The purpose of conducting ACCA surveys is to locate possible coating deficiencies, or holidays. Unlike other survey methods which are used to pinpoint coating defects, ACCA surveys are typically used to identify more general areas which may potentially have coating deficiencies. When a fault is encountered, the current quickly drops. A fault may be the result of coating damage. Loss of current is typically proportional to the amount of CP current being used at the fault.
While the accuracy of ACCA surveys is considered reliable over longer distances, they may not be as accurate as other methods over shorter distances. Industry practices usually dictate if an area of concern is indicated through an ACCA survey, another external direct assessment survey method such as ACVG or DCVG are used to pinpoint and confirm the ACCA results.