CIPS & DCVG Survey Procedures

Close Interval Potential Survey (CIPS) & DC Voltage Gradient (DCVG) Survey

Mitcorr Technical Guide Series | Field Survey Reference

1. Introduction

The CIPS and DCVG surveys are the two primary field techniques used to assess the integrity of cathodic protection on buried pipelines. CIPS measures the pipe-to-soil potential at close intervals along the full pipeline route, allowing the CP engineer to identify under-protected sections, isolating joint performance issues, and atypical potential shifts that may indicate interference or coating defects. DCVG surveys detect and characterise individual coating holidays by detecting DC voltage gradients in the soil created by CP current flowing to bare metal at defect locations. Together, these surveys provide a complete picture of pipeline CP integrity and are required by NACE SP0169 and most regulatory frameworks for periodic pipeline monitoring.

2. Pipeline Corrosion and CP Monitoring Context

A well-designed CP system maintains all sections of a pipeline at the protection potential criterion. However, system performance degrades over time due to coating deterioration, changes in soil conditions, shielding from third-party buried services, and loss of rectifier output. Annual CIPS and periodic DCVG surveys are the tools used to detect these changes before corrosion initiates or advances significantly.

3. Applicable Standards

4. Engineering Principles

4.1 Pipe-to-Soil Potential

The pipe-to-soil potential (PSP) or pipe-to-electrolyte potential is measured by placing a reference electrode on the ground surface directly above the pipeline and connecting a high-impedance voltmeter between the pipeline (via a test station or valve stem) and the reference electrode. The copper/copper sulphate electrode (CSE) is the standard reference for onshore applications (potential offset: +0.316 V vs. Standard Hydrogen Electrode).

Protection criterion (NACE SP0169): A minimum PSP of −0.850 V (CSE) measured with CP current applied and IR drop errors eliminated where practicable (polarised potential criterion).

4.2 IR Drop Error

When CP current is flowing along the soil path between the buried pipe and the surface reference electrode, there is a resistive voltage drop (I×R) superimposed on the true pipe-to-soil potential. This IR drop makes the measured potential more negative than the true steel surface potential. The on-potential therefore over-reads protection; the depolarised (instant-off) potential eliminates this error.

5. Close Interval Potential Survey (CIPS)

5.1 Equipment

Required equipment: calibrated high-impedance digital voltmeter (≥10 MO input impedance), CSE reference electrode with long porous tip for soil contact, current interrupter installed on the CP rectifier(s), GPS data logger, and connecting wire to the pipeline at the nearest test station or above-ground fitting.

5.2 Field Procedure

1. Connect the voltmeter positive terminal to the pipeline via the nearest test station. Connect the negative terminal to the CSE reference electrode.
2. Install a current interrupter on the CP rectifier(s) and synchronise interruption cycle: typically 0.8 s ON, 0.2 s OFF (5-second full cycle).
3. Begin walking the route with the reference electrode placed on the soil surface directly above the pipe at 1–2 m intervals (closer in complex areas).
4. Record GPS co-ordinates, on-potential, and instant-off potential at every measurement point. Annotate anomalies (road crossings, casing pipes, test stations, third-party crossings).
5. On-potential (V_ON): measured during the current-on period. Off-potential (V_OFF): measured 0.3–0.5 s after current interruption before depolarisation begins.

5.3 Data Interpretation

Plot V_ON and V_OFF profiles along the pipeline chainage. Evaluate:

• Sections where V_OFF is more positive than −0.850 V (CSE): under-protected, requires CP system adjustment or investigation.
• Rapid attenuation of potential: may indicate isolation failure or high coating breakdown.
• Very large IR drop (V_ON − V_OFF > 200 mV): high localised current demand, possible coating defect or interference.
• Fluctuating potentials with no obvious pattern: probable stray current interference (see MTG-05).

6. DC Voltage Gradient Survey (DCVG)

6.1 Principle

When CP current exits the pipeline at a coating holiday, it travels through the soil to the anode groundbed. This current flow creates a voltage gradient in the soil around the defect location. By walking along the pipeline with two CSE electrodes separated by approximately 1 m (connected to a sensitive millivoltmeter), the surveyor detects these gradients as the electrodes straddle the current field. Divergence of potential away from the defect appears as a gradient, and the precise holiday location is identified by the null-point (zero gradient) directly above the defect.

6.2 DCVG Severity Classification: %IR Method

The severity of a coating defect detected by DCVG is quantified as the percentage of the IR drop across the total CP circuit represented by the local potential gradient over the defect:

7. System Components

Survey infrastructure enabling CIPS and DCVG includes: flush-to-grade test stations at maximum 1 km intervals (less in urban areas), permanent reference electrodes at crossing points, current interruption relays on rectifiers, and GPS-enabled data loggers for survey correlation.

8. Monitoring and Maintenance

NACE SP0169 requires annual pipe-to-soil potential surveys. DCVG surveys are conducted whenever CIPS data reveals anomalies or on a scheduled basis (typically every 5–10 years for older pipelines). All survey data should be stored in a GIS-linked database for trend analysis across multiple survey years.

9. Conclusion

CIPS and DCVG surveys are the cornerstone of pipeline CP monitoring. Combined, they identify under-protected sections, locate coating defects for targeted repair, and provide the data record required for regulatory compliance and pipeline integrity management. A rigorous survey programme, executed by experienced CP personnel, is the most cost-effective means of extending pipeline service life and preventing costly corrosion failures.