- Close Interval Surveys (CIS) – also known as pipe-to-soil and potential gradient surveys – are one of several methods for assessing effectiveness of cathodic protection (CP) systems used on buried pipelines.
- CIS are often used in External Corrosion Direct Assessment (ECDA) pipeline inspections. Normally, at least two separate inspection tools, such as CIS, must be used to obtain desired confidence in inspection results.
- CIS are used to measure the potential (voltage) difference between buried pipes and their surrounding soil.
- Where a pipe is located under concrete or asphalt, precautions must be taken to ensure good electrical contact with surrounding soil. This is typically accomplished by boring holes through the concrete or asphalt to allow contact with the soil.
Applicability of Close Interval Surveys
- CIS are used to assess the performance of installed CP systems on underground pipelines
- CIS can also be used to detect some coating holidays (defects). CIS is not effective for detecting very small coating holidays.
- CIS is not effective if a pipeline is located under frozen ground; where the pipe coating is disbonded resulting in a non-continuous electrical path to the soil; where the pipe is enclosed in a casing; where the pipeline is located in rocky terrain; or where the pipe is located under a paved road. In these cases, another assessment tool should be considered.
Three basic types of Close Interval Surveys
“On/off” potential surveys measure the potential difference between the pipe and the ground surface, as the cathodic protection is switched on and off. On-off surveys are used to evaluate the performance of the CP system against system performance criteria, and can be used to detect some coating anomalies
“Depolarized” potential surveys measure the potential difference between the pipe and the ground surface after the cathodic protection current has been switched off long enough for the pipe-to-soil potential to stabilize. Depolarized potential is used to evaluate CP system effectiveness by measuring polarization decay.
“On” potential surveys measure the potential difference between the pipe and the ground surface at regular intervals, while the cathodic protection is operating in its normal mode. “On” potential surveys are used when CP current sources cannot be interrupted.
Date of Revision: 12012011