Trailing Wire Survey

Trailing Wire Survey is a specialized technique used to assess the effectiveness of cathodic protection on subsea and submerged pipelines. The method enables continuous measurement of pipeline-to-electrolyte potential along the pipeline route and provides valuable information about cathodic protection performance, coating condition and areas of increased corrosion risk.

Italcorr provides Trailing Wire Survey services for subsea pipelines, offshore structures and other submerged metallic infrastructure, including measurement planning, field data acquisition, processing, interpretation and technical reporting.

What Is a Trailing Wire Survey?

The Trailing Wire method is used to measure the electrochemical potential of a submerged pipeline relative to a reference electrode.

During the survey, a reference electrode is moved along the pipeline route while maintaining an electrical connection to the pipeline through a long trailing wire.

The measured potential represents the voltage difference between the pipeline and the reference electrode at a specific location.

By continuously recording these measurements along the pipeline, it is possible to create a detailed pipeline potential profile and evaluate the effectiveness of the cathodic protection system.

Principle of Operation

The basic measurement circuit consists of:

• the submerged pipeline;
• electrical connection to the pipeline;
• trailing wire;
• reference electrode;
• high-impedance voltmeter or data acquisition system;
• positioning system.

One conductor is electrically connected to the pipeline, while the reference electrode is positioned in the surrounding seawater close to the pipeline.

The voltage between the pipeline and reference electrode is continuously measured and recorded.

The reference electrode may be deployed from a vessel, ROV or other subsea inspection platform.

Reference Electrodes

For offshore and marine cathodic protection surveys, silver/silver chloride (Ag/AgCl/seawater) reference electrodes are commonly used.

The stability and condition of the reference electrode are critical because measurement accuracy directly depends on the reliability of the reference potential.

Before and after the survey, reference electrodes should be checked and verified according to the applicable inspection procedure.

Where required, dual reference electrodes can be used to provide additional measurement confidence.

Survey Procedure

A typical Trailing Wire Survey includes several stages.

1. Survey Preparation

Before field measurements, available technical information is reviewed, including:

• pipeline route;
• pipeline diameter and length;
• coating system;
• installation depth;
• cathodic protection design;
• sacrificial anode locations;
• previous CP survey results;
• subsea crossings;
• pipeline connections;
• isolation arrangements;
• previous inspection data.

The measurement interval and survey configuration are then established.

2. Electrical Connection

A reliable electrical connection is established between the measurement system and the pipeline.

The connection may be made at an accessible pipeline termination, platform, landfall, test facility or other suitable location.

Electrical continuity must be verified before starting the survey.

3. Reference Electrode Deployment

The reference electrode is deployed into the seawater and positioned close to the pipeline.

Depending on the project, deployment can be performed using:

• ROV;
• inspection vessel;
• diver;
• subsea survey frame;
• towed measurement system.

The distance between the reference electrode and pipeline should be controlled as much as practical to obtain consistent measurements.

4. Continuous Potential Measurement

The inspection platform travels along the pipeline route while pipeline-to-electrolyte potentials are continuously recorded.

Measurements are correlated with position information obtained from the navigation system.

This produces a continuous potential profile along the inspected pipeline.

Parameters Recorded During the Survey

Depending on the project requirements, the following information can be recorded:

• pipeline-to-seawater potential;
• GPS or subsea position;
• chainage;
• water depth;
• reference electrode position;
• pipeline burial condition;
• distance from the pipeline;
• anode locations;
• crossings;
• subsea structures;
• measurement time;
• environmental conditions;
• detected anomalies.

The synchronization of electrical measurements with positioning data is particularly important for accurate interpretation.

Cathodic Protection Assessment

The principal purpose of Trailing Wire Survey is to determine whether the pipeline maintains adequate cathodic protection along its entire length.

The recorded potential profile can reveal:

• adequately protected sections;
• areas of insufficient protection;
• abnormal potential variations;
• possible anode deterioration;
• changes in CP current distribution;
• possible electrical discontinuities;
• influence of connected structures;
• possible interference effects.

Potential values are evaluated according to the applicable cathodic protection criteria and project specifications.

Detection of Cathodic Protection Anomalies

A continuous potential profile makes it possible to identify localized anomalies that may not be detected by measurements performed only at widely spaced test locations.

Typical indications may include:

• gradual reduction in protection level;
• localized potential shifts;
• abnormal potential peaks;
• insufficiently polarized pipeline sections;
• influence from nearby structures;
• abnormal conditions near pipeline crossings;
• possible problems with sacrificial anodes.

Such areas can be selected for more detailed ROV inspection or additional electrochemical measurements.

Assessment of Sacrificial Anode Performance

Many offshore pipelines are protected using galvanic sacrificial anodes, typically installed at predetermined intervals along the pipeline.

Trailing Wire Survey data can assist in evaluating the overall performance of the anode system.

Potential variations can be correlated with:

• anode locations;
• pipeline joints;
• field joints;
• subsea structures;
• pipeline crossings;
• exposed or buried sections.

Where abnormal CP performance is detected, anodes can subsequently be visually inspected using an ROV.

Pipeline Coating Condition

Trailing Wire Survey does not directly measure coating damage.

However, deterioration of pipeline coating can increase cathodic protection current demand and influence the measured potential distribution.

Changes in the potential profile may therefore indicate areas requiring further investigation.

Potential anomalies should be correlated with other available information before conclusions about coating condition are made.

ROV-Based Trailing Wire Survey

For deep-water or inaccessible pipelines, the survey can be performed using a Remotely Operated Vehicle (ROV).

The ROV can carry:

• Ag/AgCl reference electrodes;
• CP measurement equipment;
• video cameras;
• positioning equipment;
• depth sensors;
• sonar;
• additional inspection sensors.

The ROV follows the pipeline while the CP potential is continuously recorded.

This configuration allows cathodic protection measurements to be correlated with visual information about the pipeline, anodes and surrounding seabed.

Vessel-Based Survey

For suitable projects, the measurement system can also be deployed from a survey vessel.

The vessel follows the pipeline route while the reference electrode system is maintained in the water.

Accurate navigation and synchronization between position and potential measurements are necessary to create a reliable pipeline CP profile.

Pipeline Positioning

Accurate positioning is essential for interpretation of Trailing Wire Survey data.

Depending on the survey configuration, positioning may use:

• vessel GPS/GNSS;
• ROV navigation systems;
• USBL acoustic positioning;
• DVL;
• INS;
• pipeline tracking systems;
• known pipeline coordinates.

Electrical measurements are correlated with pipeline chainage to allow anomalies to be relocated during subsequent inspections.

Factors Affecting Measurements

Trailing Wire Survey results can be influenced by:

• distance between reference electrode and pipeline;
• seawater conductivity;
• reference electrode condition;
• pipeline burial depth;
• CP current distribution;
• nearby metallic structures;
• subsea cables;
• crossings;
• electrical interference;
• vessel or ROV movement;
• electrical continuity;
• measurement system resistance.

These factors must be considered during data processing and interpretation.

Buried Subsea Pipelines

Trailing Wire Survey can also be applied to buried subsea pipelines.

However, burial depth and seabed characteristics may affect the measured potential distribution.

Survey results should therefore be interpreted together with available information concerning:

• pipeline burial depth;
• seabed conditions;
• pipeline route;
• coating system;
• anode locations;
• previous inspection results.

Data Processing

After completion of the field survey, recorded measurements are processed and correlated with pipeline position.

Processing may include:

• validation of raw measurements;
• removal of invalid data;
• correction of positioning information;
• synchronization of CP and navigation data;
• calculation of pipeline chainage;
• preparation of potential profiles;
• identification of anomalies;
• comparison with CP criteria;
• comparison with previous surveys.

The final result is a detailed representation of cathodic protection performance along the pipeline.

Potential Profile

One of the principal outputs of the Trailing Wire Survey is a graph showing:

Pipeline-to-Seawater Potential vs Pipeline Chainage

This profile allows rapid identification of sections where cathodic protection conditions change.

Important pipeline features can also be displayed on the graph, including:

• sacrificial anodes;
• crossings;
• subsea structures;
• pipeline joints;
• landfall sections;
• detected anomalies.

Comparison with Previous Surveys

Periodic Trailing Wire Surveys make it possible to evaluate changes in cathodic protection performance over time.

Comparison of historical data can help identify:

• gradual anode depletion;
• changes in coating condition;
• increasing CP current demand;
• deterioration of particular pipeline sections;
• development of abnormal potential zones.

This makes the method useful not only for inspection but also for long-term pipeline integrity management.

Applications

Trailing Wire Survey can be used for:

• offshore oil pipelines;
• offshore gas pipelines;
• subsea water pipelines;
• marine pipelines;
• offshore platform pipelines;
• pipeline landfalls;
• subsea production systems;
• submerged metallic structures;
• offshore terminals;
• other cathodically protected subsea infrastructure.

Advantages of Trailing Wire Survey

The method provides several important advantages:

• continuous CP potential measurement;
• inspection of long subsea pipelines;
• identification of localized CP anomalies;
• assessment of overall CP effectiveness;
• evaluation of changes over time;
• compatibility with ROV inspection;
• correlation with pipeline position and depth;
• support for maintenance planning;
• reduction of unnecessary subsea intervention;
• creation of detailed CP potential profiles.

Integrated Subsea Pipeline Inspection

Trailing Wire Survey becomes particularly valuable when combined with other inspection techniques.

Italcorr can integrate cathodic protection data with:

ROV Visual Inspection
Assessment of pipeline condition, anodes, supports and subsea structures.

Subsea CP Measurements
Local potential measurements at selected pipeline locations.

Sonar Survey
Assessment of pipeline position and seabed conditions.

Pipeline Burial Assessment
Identification of exposed and buried pipeline sections.

Coating and Anode Inspection
Visual assessment of coating damage and sacrificial anode condition.

Combining several datasets allows potential anomalies to be correlated with the actual physical condition of the pipeline.

Trailing Wire Survey Reporting

The final technical report may include:

• pipeline description;
• survey methodology;
• equipment configuration;
• reference electrode information;
• measurement procedure;
• navigation and positioning method;
• pipeline route;
• potential measurements;
• potential vs chainage graphs;
• water depth profile;
• anomaly locations;
• CP performance assessment;
• comparison with applicable criteria;
• comparison with previous surveys;
• recommendations for further inspection or maintenance.

A table of anomalies can also be prepared with coordinates and pipeline chainage for subsequent ROV investigation.

Trailing Wire Survey Services by Italcorr

Italcorr provides Trailing Wire Survey services for the assessment of cathodic protection systems on subsea and submerged pipelines.

The service can include survey engineering, measurement planning, field data acquisition, ROV or vessel-based inspection support, processing of CP measurements, preparation of pipeline potential profiles and technical interpretation.

The objective is to identify areas of inadequate or abnormal cathodic protection, evaluate the performance of the CP system and provide reliable information for subsea pipeline integrity management.