Cathodic Protection Surveys: What ROV Pilots Do on CP Inspection Jobs

Cathodic protection (CP) surveys are one of the most common ROV inspection tasks in the offshore oil and gas industry. Subsea structures — jackets, pipelines, manifolds, and subsea trees — rely on cathodic protection systems to prevent corrosion. ROV-based CP surveys measure the effectiveness of these systems by recording the electrical potential of the steel relative to the surrounding seawater. Accurate CP data is critical for structural integrity management and regulatory compliance.

Understanding Cathodic Protection Basics

Cathodic protection works by making the steel structure the cathode in an electrochemical cell, preventing the oxidation reaction that causes corrosion. This is achieved either through sacrificial anodes (typically aluminum or zinc alloys) or impressed current systems. CP surveys measure the potential difference between the steel and a reference electrode (usually silver/silver chloride or zinc) to determine whether the protection level is adequate. The target potential range is typically between -800 mV and -1100 mV versus Ag/AgCl, though specific criteria vary by standard and operator.

CP Probe Types and Deployment

• Proximity CP probes — held close to the structure surface without contact, suitable for coated surfaces
• Contact CP probes — pressed directly against bare steel for more accurate readings
• Stab probes — inserted into dedicated measurement receptacles on subsea equipment
• Half-cell reference electrodes — silver/silver chloride or zinc, must be calibrated before use
• Multi-probe arrays — allow multiple readings per positioning, increasing survey efficiency

Survey Execution Checklist

• Verify probe calibration certificate is current (calibrated within the past 12 months)
• Function-test the CP measurement system on deck before deployment
• Confirm the survey grid or measurement point locations from the client scope of work
• Record water temperature and salinity as these affect readings
• Take readings at each specified point, holding the probe stable for the required settling time
• Photograph each measurement location with the ROV camera for verification
• Record anode dimensions (length, width, thickness) to estimate remaining life

Anode Measurement and Assessment

• Measure remaining anode dimensions using ROV-mounted calipers or laser scaling
• Calculate anode wastage percentage by comparing to the original installed dimensions
• Document anode condition — cracking, detachment, uneven consumption
• Note any anodes that are fully consumed or missing
• Record the anode location identifier for correlation with the CP management database

Data Recording Best Practices

CP survey data must be recorded systematically with each reading linked to a specific location identifier, timestamp, and set of environmental conditions. ThrusterLog supports structured data entry for CP surveys, allowing pilots to log readings alongside dive records and attach photographs to specific measurement points. This structured approach simplifies the generation of CP survey reports and ensures that the data trail is complete for engineering review.

Interpretation Fundamentals

• Potentials more positive than -800 mV (vs Ag/AgCl) indicate under-protection — corrosion risk
• Potentials between -800 mV and -1100 mV generally indicate adequate protection
• Potentials more negative than -1100 mV may indicate over-protection — risk of hydrogen embrittlement
• Trends over time are more informative than single readings — compare with historical data
• Anomalous readings should be flagged for engineering review rather than dismissed