Cathodic protection (CP) is an effective technique used to prevent corrosion of buried pipelines. Buried pipelines are subject to various forms of corrosion due to the presence of moisture, soil, and other environmental factors. CP provides a means of preventing corrosion by applying a small electric current to the pipeline to offset the electrochemical reactions that lead to corrosion.
There are two primary methods of cathodic protection for buried pipelines: impressed current cathodic protection (ICCP) and sacrificial anode cathodic protection (SACP).
ICCP involves the use of an external power source to generate a current that flows from anodes to the buried pipeline, thereby protecting it from corrosion. The anodes are usually made of graphite, mixed metal oxides, or high-silicon cast iron, and they are placed in the soil near the pipeline. The power source is usually a rectifier, which converts alternating current (AC) to direct current (DC) and controls the current output to the pipeline.
This article is about Cathodic Protection of Buried Pipelines which is as standard of SAES-X-400 Saudi Aramco and can download pdf also.
Cathodic Protection of Buried Pipelines
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SAES-X-400General Design Requirements for Cathodic Protection of Buried Pipelines
The design requirements for cathodic protection (CP) of buried pipelines are outlined in various industry standards and codes. Some general design requirements for CP of buried pipelines include:
- Coating and corrosion control: The pipeline must have an appropriate coating and cathodic protection system to protect against corrosion. The coating must be in good condition, and the CP system must be designed to maintain a protective potential on the pipeline.
- Current density: The CP system must be designed to provide sufficient current density to the pipeline to achieve a protective potential, but not so much as to cause hydrogen embrittlement or other damage to the pipeline.
- Anode placement: The anodes must be placed at regular intervals along the pipeline to ensure adequate current distribution and to maintain the protective potential. The number and spacing of the anodes depend on factors such as soil resistivity, coating quality, and pipeline size.
- Anode type: The anodes used in the CP system must be selected based on factors such as soil resistivity, pipeline size, and expected current demand. Common types of anodes include graphite, mixed metal oxide, and high-silicon cast iron.
- Power supply: The power supply used in the CP system must be designed to provide a stable, direct current output to the anodes and must be able to adjust the current output to maintain the desired protective potential on the pipeline.
- Monitoring: The CP system must include a monitoring system to ensure that the protective potential is maintained and that the anodes are functioning properly. The monitoring system should include reference electrodes, test points, and other instrumentation to measure the potential difference between the pipeline and the surrounding soil.
- Maintenance: The CP system must be regularly inspected and maintained to ensure that it continues to function properly. This may include cleaning the anodes, replacing damaged coatings, and repairing or replacing damaged components of the system.
Fundamental Design Calculations for Cathodic Protection of Buried Pipelines
The fundamental design calculations for cathodic protection (CP) of buried pipelines are important to ensure the system provides adequate protection against corrosion. These calculations include:
- Current requirement calculation: This calculation determines the amount of current required to protect the pipeline from corrosion. The current requirement is based on the pipeline size, coating quality, soil resistivity, and the desired level of protection. The current requirement is typically expressed as amperes per unit length of pipeline.
- Anode sizing calculation: This calculation determines the size and number of anodes required to provide the necessary current to protect the pipeline. The anode sizing calculation is based on the current requirement and the expected current output of the anodes. The anodes must provide enough current to maintain the protective potential on the pipeline without exceeding the safe current density limit.
- Anode placement calculation: This calculation determines the spacing and placement of the anodes along the pipeline. The spacing of the anodes is based on the anode size, current output, and soil resistivity. The anodes must be spaced close enough to ensure adequate current distribution and to maintain the protective potential along the entire length of the pipeline.
- Rectifier sizing calculation: This calculation determines the size of the rectifier required to provide the necessary current to the anodes. The rectifier must be able to provide a stable direct current output and adjust the current output as needed to maintain the protective potential on the pipeline.
- Voltage drop calculation: This calculation determines the voltage drop along the pipeline and the anode bed. The voltage drop is based on the resistance of the soil, the size of the pipeline, and the current flowing through the system. The voltage drop must be considered to ensure that the protective potential is maintained along the entire length of the pipeline.
These fundamental design calculations are essential for the proper design and installation of the CP system and for ensuring that the pipeline is adequately protected against corrosion.
Production Pipelines – Special Considerations
Production pipelines have some special considerations that must be taken into account when designing and implementing a cathodic protection (CP) system. Some of these considerations include:
- Coating quality: The quality of the coating on production pipelines is critical for effective CP. The coating must be of high quality and free from defects, as any discontinuities or damage to the coating can compromise the CP system.
- Environmental factors: Production pipelines may be exposed to more extreme environmental conditions, such as higher temperatures and pressures, than pipelines used for transportation. These conditions can affect the performance of the CP system and must be considered in the design.
- Flow assurance: Production pipelines are often designed to transport fluids with high solids content, such as oil and gas. These fluids can cause issues with flow assurance, such as solids accumulation and flow restrictions, which can affect the performance of the CP system.
- Electrical isolation: Production pipelines may require electrical isolation from other structures and equipment in the area to prevent interference with the CP system. Electrical isolation can be achieved through the use of insulating flanges, coatings, or other methods.
- Monitoring: The CP system for production pipelines should include a comprehensive monitoring program to ensure that the pipeline remains adequately protected. This may include regular inspections, corrosion rate measurements, and other monitoring methods.
- Design life: The design life of the CP system for production pipelines must be considered. Production pipelines are often designed for long-term use, and the CP system should be designed to provide effective corrosion protection for the full design life of the pipeline.
- Maintenance: The CP system for production pipelines should be designed with maintenance in mind. Access to the pipeline and anode beds should be considered, and the system should be designed to allow for easy inspection and maintenance.
These special considerations are important for ensuring that the CP system for production pipelines is effective in protecting the pipeline from corrosion and that the pipeline remains safe and operational for its intended design life.
Impressed Current Cathodic Protection
Impressed current cathodic protection (ICCP) is a method of providing corrosion protection to buried or submerged metal structures, such as pipelines, tanks, and offshore platforms. ICCP uses an external power source, typically a rectifier, to apply a protective electrical current to the structure. This current is used to counteract the natural corrosion process that occurs when the metal structure comes into contact with the surrounding soil or water.
ICCP systems typically consist of anodes, reference electrodes, a rectifier, and a control panel. The anodes are typically made of a high-purity, low-consumption material, such as platinum-coated titanium or mixed metal oxide (MMO) anodes. The reference electrodes are used to measure the potential of the structure and ensure that the protective current is maintained at the correct level. The rectifier provides the electrical power needed to drive the ICCP system.
The design of an ICCP system involves a number of factors, including the type and size of the structure being protected, the soil or water environment, and the required level of protection. The design must also take into account any potential interference from other structures or electrical sources in the area.
ICCP systems have several advantages over other corrosion protection methods, such as sacrificial anodes. ICCP systems can provide a higher level of protection, even in high-resistivity soils or water environments. They also have a longer design life and can be more cost-effective over the long term.
However, ICCP systems also have some disadvantages. They require a power source, which can increase the initial cost of the system and require ongoing maintenance. The use of ICCP can also result in stray currents, which can cause corrosion problems in nearby structures if not properly managed.
Overall, ICCP is an effective method for providing corrosion protection to buried or submerged metal structures. Proper design and installation are critical for ensuring that the system is effective and reliable over its design life.
Temporary Cathodic Protection
Temporary cathodic protection (TCP) is a method of providing short-term corrosion protection to buried or submerged metal structures, such as pipelines or tanks, during construction or maintenance activities. TCP is typically used when the permanent cathodic protection system is not yet installed or is temporarily out of service.
TCP systems are usually based on galvanic corrosion protection. Sacrificial anodes made of a more reactive metal, such as zinc, are installed in a bed around the metal structure to be protected. The anodes are connected to the metal structure using a suitable cable or wire. When the metal structure comes into contact with the surrounding soil or water, an electrolytic cell is formed, and the anodes will corrode preferentially, sacrificially, to protect the metal structure from corrosion.
The design of a TCP system involves calculating the amount of anode material required to provide the required level of protection for the duration of the temporary protection period. The calculation takes into account factors such as the size and type of the metal structure, the expected corrosion rate, and the expected duration of the TCP system.
TCP systems have several advantages over other corrosion protection methods. They are relatively simple and inexpensive to install and require minimal maintenance. They can also provide a high level of protection for short-term corrosion protection needs.
However, TCP systems also have some disadvantages. They have a finite life and will eventually need to be removed or replaced. They may also be less effective in high-resistivity soils or water environments. In addition, TCP systems can cause stray currents, which can lead to corrosion problems in nearby structures if not properly managed.
Overall, TCP is an effective method for providing short-term corrosion protection to buried or submerged metal structures during construction or maintenance activities. Proper design and installation are critical for ensuring that the system is effective and reliable for the duration of the temporary protection period.
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