Close Interval Potential Surveys (CIPS)

Close Interval Potential Surveys (CIPS)

The Need for Continuous Assessment

Traditional cathodic protection monitoring involves measuring pipe-to-soil potentials at established test stations, which are typically spaced every half-mile to a mile. While these discrete measurements verify that CP is functioning at those specific points, they leave large gaps in knowledge. A pipeline could have adequate protection at test station A and test station B, but suffer from severe corrosion due to a massive coating defect midway between them. To ensure the integrity of the entire structure and comply with stringent regulatory requirements (such as those from PHMSA), a continuous profile is required. This is achieved through a Close Interval Potential Survey (CIPS).

CIPS Methodology and Equipment

The primary objective of a CIPS is to measure the pipe-to-soil potential at very short, regular intervals—typically every 3 to 5 feet—directly over the centerline of the buried pipeline.

The Setup

1. Trailing Wire: The survey operator connects a spool of fine, insulated copper wire (often #30 AWG or similar) to the pipeline at a starting test station. As the operator walks the pipeline route, this wire unwinds, maintaining a continuous electrical connection to the structure.
2. Reference Electrodes (Half-Cells): The operator carries two matched copper/copper sulfate (Cu/CuSO4) reference electrodes, often integrated into walking canes for ergonomic continuous contact with the soil.
3. Data Logger: A high-speed digital data logger is worn by the operator. It measures the potential difference between the trailing wire (pipeline) and the reference electrodes in contact with the soil, recording the data along with GPS coordinates and distance.

ON and OFF Potentials (IR Drop Elimination)

A critical aspect of a modern CIPS is measuring the "instant-off" potential to eliminate IR drop error. When CP current flows through the soil (resistance 'R'), it creates a voltage drop (Voltage = Current x Resistance, or IR). If a potential measurement is taken while the CP rectifiers are on, the reading includes this IR drop, making the pipeline appear more negative (more protected) than it actually is at the pipe-to-soil interface.

To capture the true polarized potential, all CP rectifiers influencing the pipeline segment must be equipped with GPS-synchronized current interrupters. These devices cycle the rectifiers ON and OFF simultaneously across the system (e.g., 4 seconds ON, 1 second OFF).

During the survey, the data logger rapidly samples the voltage. It records:

• ON Potential: The reading just before the rectifiers switch off (includes IR drop).
• OFF Potential (Instant-Off): The reading captured within milliseconds after the rectifiers switch off, before the pipeline begins to depolarize. This reading is virtually free of IR drop and is used to evaluate against the NACE/AMPP -850 mV polarized criterion.

Interpreting CIPS Data

The massive amount of data collected during a CIPS is typically plotted on a graph, with distance along the pipeline on the X-axis and potential (millivolts) on the Y-axis. Both the ON and OFF potentials are plotted as continuous lines.

Identifying Anomalies

By analyzing the graphed data, corrosion professionals can identify specific areas of concern:

1. Dips in the Profile: A sharp positive shift (towards zero) in both ON and OFF potentials usually indicates a coating defect or "holiday." At a holiday, bare steel is exposed to the soil, draining CP current and depressing the local potential.
2. IR Drop Variations: A large divergence between the ON and OFF lines indicates a large IR drop, which could be caused by high soil resistivity in that area or unusually high current density concentrating at a large holiday.
3. Interference: Sudden, erratic spikes or abnormal shifts in the profile can reveal stray current interference from foreign pipelines, DC transit systems, or telluric currents.

Operational Considerations

Executing a successful CIPS requires meticulous planning. The pipeline must be properly located and marked to ensure the operator walks directly over the centerline; deviating from the centerline will introduce IR drop errors. Environmental factors, such as dry soil or paved roads, can cause high contact resistance for the reference electrodes, requiring the use of electrode sponges, water, or specialized techniques to maintain good electrical contact. Ultimately, CIPS provides the most comprehensive picture of a pipeline's CP health, identifying localized vulnerabilities that standard test station surveys inevitably miss.