Cathodic protection systems are crucial for maintaining the integrity of gas pipelines, and they perform a variety of functions for the pipeline itself.
Let’s discuss the different applications, types, and measurement of cathodic protection systems for pipelines and a few additional resources to help with pipeline maintenance.
What is a Cathodic Protection System?
A cathodic protection system is a method used to prevent corrosion of metal surfaces by converting them into cathodes of an electrochemical cell.
Although it’s a technique widely employed in the oil and gas industry to protect pipelines from corrosion, there are two primary types of CP systems: galvanic (sacrificial) anode systems and impressed current cathodic protection (ICCP) systems.
Galvanic (Sacrificial) Anode Systems
Galvanic anode systems rely on the natural potential difference between metals to drive the cathodic protection current.
Metals like magnesium, zinc, and aluminum serve as sacrificial anodes that corrode in place of the pipeline. These systems are advantageous because they do not require an external power source and need minimal maintenance.
- Advantages:
- No external power supply needed
- Minimal maintenance
- Cost-effective for smaller applications
- Limitations:
- Limited power output
- Shorter lifespan due to high consumption rates of anodes
- Limited control over the protection level
Impressed Current Cathodic Protection (ICCP) Systems
ICCP systems use an external DC power source to provide the necessary current for cathodic protection. This system can protect larger structures and offers greater control over the protection level. Anodes used in ICCP systems include graphite, high silicon cast iron, and mixed metal oxide.
Advantages:
- High current output: Suitable for large structures.
- Greater control and adjustability: Allows precise management of protection levels.
- Longer lifespan of anodes: Due to controlled current output.
Limitations:
- Requires an external power source: Dependence on a continuous power supply.
- More complex: Higher maintenance requirements.
- Higher initial installation cost: Due to the complexity and components involved.
Importance of Maintaining Correct CP Current Levels
Maintaining the correct level of CP current is crucial for the effectiveness of ICCP systems. Too little current can lead to insufficient protection, resulting in corrosion damage. Conversely, excessive current can cause over-protection, leading to problems such as coating disbondment and hydrogen embrittlement. Regular monitoring and adjustments are necessary to ensure the current levels are optimal, providing effective protection without causing additional issues.
Potential Risks of Incorrect Current Application
- Under-protection: Insufficient current fails to adequately protect the pipeline, allowing corrosion to occur.
- Over-protection: Excessive current can lead to:
- Coating Disbondment: The protective coating may detach from the pipeline, exposing it to corrosive elements.
- Hydrogen Embrittlement: Excessive hydrogen absorption can weaken the metal, making it brittle and prone to cracking.
By carefully managing and monitoring the CP current levels, ICCP systems can provide robust protection for gas pipelines, ensuring their longevity and reliability.
Key Measurements in Cathodic Protection
Accurate measurements are vital for the effectiveness of CP systems. Several critical measurements include pipe-to-soil potential, soil resistivity, instant OFF potential, corrosion coupons, Close Interval Potential Survey (CIPS), and Direct Current Voltage Gradient (DCVG).
Pipe-to-Soil Potential Measurements
These measurements help determine the effectiveness of the CP system by measuring the potential difference between the pipeline and the surrounding soil, which is typically measured using a reference electrode, such as a copper or copper sulfate electrode, placed in the soil above the pipeline.
Soil Resistivity Testing
Soil resistivity impacts the design and effectiveness of CP systems. Low resistivity soils generally require less protection current, whereas high resistivity soils need more.
To measure soil resistivity, the Wenner four-pin method is often used, which provides information about the corrosivity of the soil environment.
Instant OFF Potential
Instant OFF potential measurements are used to correct for IR drop errors in pipe-to-soil potential readings.
When the CP current is briefly interrupted, the potential measurement taken immediately after interruption (before any depolarization occurs) provides a true indication of the pipeline's potential without the influence of current flow.
This method is essential for obtaining accurate data on the level of protection provided by the CP system.
Corrosion Coupons
Corrosion coupons are small samples of pipeline material buried near the pipeline to simulate its exposure to the environment. These coupons help monitor the effectiveness of the CP system by mimicking how the pipeline would react if there were a defect in its coating. Measurements from corrosion coupons can include pipe-to-soil potentials and current densities, providing valuable data on the CP system’s performance.
Close Interval Potential Survey (CIPS)
CIPS involves taking pipe-to-soil potential measurements at regular intervals (typically every meter) along the pipeline.
This survey provides a detailed profile of the pipeline's protection level, identifying areas of inadequate protection that might not be detected with widely spaced test points.
During a CIPS, the CP current is interrupted synchronously with measurements taken at each interval to ensure accurate readings.
Direct Current Voltage Gradient (DCVG)
DCVG surveys are used to locate and size defects in the pipeline coating. When a pipeline with an impressed current CP system has a coating defect, a voltage gradient develops around the defect site.
By measuring the voltage gradient at the soil surface, technicians can pinpoint the location and severity of coating flaws. This method helps in accurately identifying areas that require repair to maintain effective protection.
Monitoring and Maintenance
Regular monitoring and maintenance are necessary to ensure that the CP system remains effective over time, which includes periodic checking of pipe-to-soil potentials, inspecting anodes, and ensuring the power supply functions correctly.
Automated remote monitoring systems can enhance this process by providing continuous data collection and alerts for any potential issues, thereby reducing the need for frequent manual inspections.
By incorporating these measurement techniques and maintaining diligent monitoring, cathodic protection systems can be optimized to ensure the long-term integrity and safety of gas pipelines.
Design Considerations for Cathodic Protection Systems
Several factors influence the design of CP systems. These include pipeline length, coating quality, soil characteristics, and environmental conditions. Proper design ensures that the system provides adequate protection throughout the pipeline’s lifespan.
- Pipeline Length and Coating Quality: Longer pipelines and those with lower quality coatings require more extensive CP systems. High-quality coatings can reduce the current demand, improving the efficiency of the CP system.
- Soil Characteristics: Soil resistivity and moisture content significantly affect the performance of CP systems. Designs must account for varying soil conditions along the pipeline route.
- Environmental Conditions: Factors like temperature, humidity, and presence of corrosive elements such as salts or chemicals influence the CP system design.
Polyguard’s Products and Their Role in Pipeline Protection
Polyguard offers a range of products that complement cathodic protection systems, enhancing their effectiveness and durability. Their solutions include protective coatings and wraps specifically designed for pipeline applications.
RD-6 Coating System
Polyguard’s RD-6 coating system is a non-shielding solution that remains effective even if disbondment occurs. This system allows cathodic protection currents to reach the pipeline surface, preventing corrosion even if the coating detaches.
IRO/IRO HD Outerwraps
The IRO (Impact Resistant Outerwrap) and IRO HD (Heavy Duty) products protect field joint coatings and mainline coatings from abrasion during directional drilling and other harsh installation methods. These outerwraps are compatible with cathodic protection systems, providing an additional layer of protection without hindering the CP current.
Engage with Polyguard for Comprehensive Pipeline Protection
Polyguard provides innovative solutions for pipeline corrosion prevention, integrating seamlessly with cathodic protection systems to enhance overall effectiveness.
By combining advanced coatings and wraps with proven CP techniques, Polyguard ensures your pipelines remain protected against the harshest conditions.
Contact us today for more information.