Introduction
The Pipeline Toolbox is home to many tools and calculators. The PLTB User’s Guide presents information, guidelines and procedures for use during design, operations and integrity tasks for field or office applications. Pipeline cathodic protection is achieved by the supply of sufficient direct current to the external pipe surface, so that the steel-to-electrolyte potential is lowered on all the surface to values at which external corrosion is reduced to an insignificant rate. Cathodic protection is normally used in combination with a suitable protective coating system to protect the external surfaces of steel pipelines from corrosion.
User Guides
The PLTB User’s Guide presents information, guidelines and procedures for use during design, operations and integrity tasks for field or office applications.
An in depth analysis depicting gas cathodic protection. Utilize this PLTB User’s Guide to properly protect the external surfaces of steel pipelines.
Module/Applications
- Cathodic Protection Module Attenuation Calculation
- DCVG – %IR Drop
- Estimated Weight of a Magnesium Anode
- Power Consumption of a Cathodic Protection Rectifier
- Resistance to Earth of a Single Vertical Anode
- Resistance to Earth of Multiple Vertical Anode
- Resistance to Earth of a Single Horizontal Anode
- Resistance to Earth of a Multiple Horizontal Anode
- Required Number of Anodes and Total Current Requirement
- Rudenberg’s Formula for the Placement of an Anode Ground Bed
References
- NACE Standard Practices 0169 – Latest Edition “Control of Corrosion on Underground or Submerged Metallic Piping Systems”
- Uhlig’s Corrosion Handbook (2nd Edition) Edited by: Revie, R. Winston © 2000 John Wiley & Sons
- ISO 15589-2 Petroleum and Natural gas Industries Cathodic Protection Pipeline Transportation Systems
- Pipeline Corrosion and Cathodic Protection, Third Edition, Gulf Publishing Company
FAQ
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ASME B31G Original/Modified and RSTRENG?
ASME B31.G Original and Modified (0.85) are Level 1 assessment along with most other metal loss calculations that are limited to pits and short lengths of pitting clusters. RSTRENG is a Level 2 metal loss calculation; however, it is not limited just to pits, but most lengths of pitting i.e., 2500 mm (100 inches) using interaction rules that is not uncommon on some pipelines. Check Out
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Preventative and Corrective Measures to Control Corrosion Pitting?
A corrosion rate needed to set the re-inspection interval, reassess the performance metrics and their current applicability, plus to ensure the assumptions made are correct. At each direct examination where corrosion pitting is found, the operator should measure and record each pitting cluster. Check Out
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Estimating Internal Corrosion Rates?
Internal corrosion is most likely to occur where water first accumulates. Predicting these locations of water accumulation serve as a method for prioritizing local examinations. Predicting where water first accumulates requires knowledge about the multiphase flow behavior in the pipe requiring certain data. Check Out
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Estimating External Corrosion Rates?
All corrosion defects found during each direct examination should be measured, documented, and remediated as needed. At each excavation, the pipeline operator should measure and record generic environmental characteristics (such as soil resistivity, hydrology, drainage etc.). Check Out
Appendix of Definitions
A
- AC Corrosion
- Corrosion initiated and propagating under the influence of alternating current.
- AC Current Density (JAC)
- Unit: A/m2. The AC current density in a coating defect or in a coupon or probe used to simulate a coating defect of a certain area.
- AC Exposure
- Alternating voltages and currents induced on a structure because of the AC power system.
- AC Power Structures
- The structures associated with AC power systems.
- AC Power System
- The components associated with the generation, transmission, and distribution of AC.
- AC-Voltage (UAC)
- Unit: V. Voltage measured. The difference in AC potential between the pipeline and remote earth. The AC voltage is the ultimate driving force for the AC current density at a coating defect – which may cause corrosion – or the AC current density at grounding devices (including galvanic anodes) installed for mitigation purposes. The AC voltage is not a constant value since:
It will change over time primarily due to intermittent conditions in the AC power system, for instance, because household power consumption is different during daytime and nighttime.
It will change along the length of the pipeline since the induced voltage depends on the characteristics of the pipeline, characteristics of the interfering AC power system, as well as geometrical and geographical alignment.
- Affected Structure
- Pipes, cables, conduits, or other metallic structures exposed to the effects of AC or lightning.
- Anode
- Where negative electrode deteriorates or corrodes to protect the positive i.e., cathode or structure
- Anode Potential
- Anode-to-electrolyte potential
- Anode Sled
- Anodes installed on a structure and connected to the pipeline by a cable
- ArcGIS
- A geographic information system for working with maps and geographic information maintained by Esri.
B
- Bond
- A low-impedance connection (usually metallic) provided for electrical continuity.
- Breakdown Voltage
- A voltage in excess of the rated voltage that causes the destruction of a barrier film, coating, or other electrically isolating material.
C
- Capacitive Coupling
- The influence of two or more circuits upon one another, through a dielectric medium such as air, by means of the electric field acting between them
- Cathode
- The positive electrode that acquires electrons from the external circuit
- Closed-Circuit Anode Potential
- Anode potential while electrically linked to the pipeline to be protected
- Coating Breakdown Factor
- Ratio of current density required to polarize a coated steel surface as compared to a bare steel surface
- Coating Stress Voltage (AC Voltage With Respect To Close Earth)
- Unit: V. Difference in AC potential between the metallic surface of a coated structure and the earth in contact with the outer surface of the coating. Circuit theory and computer simulations suggest that the coating stress voltage is technically more accurately, the driving force for the AC current density at coating defects when AC mitigation in the form of ribbon anodes installed in the proximity of the coating defect.
- Cold Shut
- Horizontal surface discontinuity caused by solidification of the meniscus of the partially cast anodes as a result of interrupted flow of the casting stream
D
- DC Current Density
- Unit: A/m2. Refers to the DC current density a coating defect or a coupon or probe used to simulate a coating defect of a certain area.
- Direct Current (DC) Decoupling Device
- A device used in electrical circuits that allows the flow of AC in both directions and stops or substantially reduces the flow of DC.
- Driving Voltage
- Difference between the pipeline/electrolyte potential and the anode/electrolyte potential when the cathodic protection is operating
E
- Earth Current
- Electric current flowing in the earth.
- Electric Field
- One of the elementary energy fields in nature. It occurs in the vicinity of an electrically charged body.
- Electric Field Gradient
- Change in electrical potential per unit distance through a conductive medium, arising from the flow of electric current
- Electric Potential
- The voltage between a given point and a remote reference point.
- Electrochemical Capacity
- Total amount of electric charge that is produced when a fixed mass (usually 1 kg) of anode material is consumed electrochemically
NOTE: Electrochemical capacity is expressed in ampere hours
- Electrolytic Grounding Cell
- A device consisting of two or more buried electrodes installed at a fixed spacing, commonly made of zinc, and resistively coupled through a prepared backfill mixture. The electrical characteristics of a grounding cell include a small degree of resistance and a subsequent reduced voltage drop across the cell during a fault condition.
- Excessive Cathodic Protection (CP)
- Cathodic protection levels that lead to cathodic protection DC current densities exceeding 1 A/m2 or cathodic protection levels that lower the spread resistance due to cathodic reactions.
F
- Fault Current
- A current that flows from one conductor to ground or to another conductor due to an abnormal connection (including an arc) between the two. A fault current flowing to ground may be called a ground fault current.
- Final Current Density
- Estimated current density at the end of the lifetime of the pipeline
NOTE: Final current density is expressed in amperes per square meter
G
- Gradient Control Mat
- A system of bare conductors connected to the affected structure and placed on or below the surface of the earth, usually at above grade or exposed appurtenances, arranged and interconnected to provide localized touch-and-step voltage protection. Metallic plates and gratings of suitable area are common forms of ground mats, as well as conventional bare conductors closely spaced.
- Gradient Control Wire
- A continuous and long grounding conductor or conductors installed horizontally and parallel to the affected structure at strategic lengths and connected at regular intervals to provide protection to the structure and coating during steady-state and fault AC conditions from nearby electric transmission power systems.
- Ground
- An electrical connection to earth.
- Ground Current
- Current flowing to or from earth in a grounding circuit.
- Ground Electrode Resistance
- The ohmic resistance between a grounding electrode and remote earth.
- Grounded
- Connected to earth or to some extensive conducting body that serves instead of the earth, whether the connection is intentional or accidental.
- Grounding Grid
- A system of grounding electrodes consisting of interconnected bare conductors buried in the earth to provide a common electrical ground.
- Guarded
- Covered, fenced, enclosed, or otherwise protected by means of suitable covers or casings, barrier rails or screens, mats, or platforms; designed to limit the likelihood, under normal conditions, of dangerous approach or accidental contact by persons or objects.
H
- Hydrogen-Induced Stress Cracking
- Cracking due to a combination of load and hydrogen embrittlement caused by the ingress of hydrogen formed at the steel surface due to the cathodic polarization
I
- IR Drop
- Voltage due to any current, measured between two points of the metal of the pipe or two points of the electrolyte, such as seawater or seabed, in accordance with Ohm’s law
NOTE: IR drop and electric field gradient are related terms
- Inductive Coupling
- The influence of two or more circuits upon one another by means of changing magnetic flux linking them together.
- Interference
- Any electrical disturbance on a structure because of stray current.
L
- Lightning
- An electric discharge that occurs in the atmosphere between clouds or between clouds and the earth.
- Load Current
- The current in an AC power system under normal operating conditions.
M
- Magnetic Field
- One of the elementary energy fields in nature. It occurs in the vicinity of a magnetic body or current-carrying medium.
- Master Reference Electrode
- Reference electrode, calibrated with the primary calibration reference electrode, used for verification of reference electrodes that are used for field or laboratory measurements
- Mean Current Density
- Estimated average cathodic current density for the entire lifetime of the pipeline
NOTE: Mean current density is expressed in amperes per square meter
O
- OFF Potential
- (Eoff ) Unit: V. DC potential measured between the pipeline and a reference electrode placed in the adjacent soil while the cathodic protection current is interrupted. This potential excludes voltage (IR) drops in the soil
- ON Potential
- (Eon) Unit: V. DC potential measured between the pipeline and a reference electrode placed in the adjacent soil while the cathodic protection current is flowing. This potential includes IR voltage drops in the soil. (IR drops).
- Over-Voltage Protector (Surge Arrester)
- A device that provides high resistance to DC and high impedance to AC under normal conditions within the specified DC and AC threshold rating and “closes” or has a very low resistance and impedance during upset conditions.
P
- Primary Calibration Reference Electrode
- Reference electrode used for calibration of master reference electrodes
- Protection Potential
- Structure-to-electrolyte potential for which the metal corrosion rate is considered as insignificant pitting resistance equivalent number
R
- Reclosing Procedure
- A procedure that normally takes place automatically whereby the circuit breaker system protecting a transmission line, generator, etc., recloses one or more times after it has tripped because of abnormal conditions such as surges, faults, lightning strikes, etc.
- Reference Electrode
- An electrode whose open-circuit potential is constant under similar conditions of measurement, which is used for measuring the relative potentials of other electrodes.
- Remote Earth
- A location on the earth far enough from the affected structure that the soil potential gradients associated with currents entering the earth from the affected structure is insignificant.
- Resistive Coupling
- The influence of two or more circuits on one another by means of conductive paths (metallic, semi-conductive, or electrolytic) between the circuits.
- Riser
- Part of an offshore pipeline, including any subsea spool pieces, which extends from the seabed to the pipeline termination point on an offshore installation
S
- Shock Hazard
- A condition considered to exist at an accessible part in a circuit between the part and ground or other accessible parts, if the steady-state open-circuit AC voltage is 15 V or more (root mean square [rms]). For capacitive build-up situations, a source capacity of 5 mA or more is recognized as a hazardous condition. For short circuit conditions, the permissible touch-and-step voltages should be determined in accordance with the methodology specified in accordance with IEEE(1) Standard 80.
- Solid-State DC Decoupler
- A dry type of DC decoupling device comprising solid-state electronics. The electrical characteristics of a solid-state decoupler are high resistance to low voltage DC and low impedance to AC.
- Spread Resistance (SR)
- Unit: Ω∙m2. Refers to the resistance from the pipe to earth through a specific coating defect with known area, or the resistance from a coupon or probe with a known exposed area, to earth. The spread resistance multiplied by the DC current density constitutes the voltage drop (IR) and the spread resistance is a factor proportionality between AC voltage and AC current density.
- Step Potential or Voltage (V)
- The potential difference between two points on the earth’s surface separated by a distance of one human step, which is defined as one meter, determined in the direction of maximum potential gradient.
- Stray Current
- Current flowing through paths other than the intended circuit.
- Surface Potential Gradient
- Change in the potential on the surface of the ground with respect to distance.
T
- Touch Potential or Voltage (V)
- The potential difference between a metallic structure and a point on the earth’s surface separated by a distance equal to the normal maximum horizontal reach of a human (approximately 1.0 m [3.3 ft]).
U
- Utilization Factor
- Fraction of the anodic material weight of a galvanic anode that can be consumed before the anode ceases to provide the minimum required current output.
V
- Voltage (V)
- The difference in electrical potential between two points.
- Voltage DROP
- Voltage (IR) drop due to any current, measured between two points of the metal of the pipe or two points of the electrolyte, such as seawater or seabed, in accordance with Ohm’s law