NACE CP Interference January 2008

CP Interference

January 2008 ©NACE International, 2006

IMPORTANT NOTICE Neither the NACE International, its officers, directors, nor members thereof accept any responsibility for the use of the methods and materials discussed herein. No authorization is implied concerning the use of patented or copyrighted material. The information is advisory only and the use of the materials and methods is solely at the risk of the user. It is the responsibility of the each person to be aware of current local, state and federal regulations. This course is not intended to provide comprehensive coverage of regulations. Printed in the United States. All rights reserved. Reproduction of contents in whole or part or transfer into electronic or photographic storage without permission of copyright owner is expressly forbidden.

Acknowledgements The scope, desired learning outcomes and performance criteria of this course were developed by the CP Task Group under the auspices of the NACE Education Administrative Committee. The time and expertise of several members of NACE International have gone into the development of this course—and its task analysis, course outline, student manual, classroom lab manual, presentation slides, and examinations. Their dedication and efforts are greatly appreciated. On behalf of NACE, we would like to thank the task group for its work. Their efforts were extraordinary and their goal was in the best interest of public service—to develop and provide a much needed training program that would help improve corrosion control efforts industry-wide. We also wish to thank their employers for being generously supportive of the substantial work and personal time that the members dedicated to this program.

CP Interference Course Development Task Group Paul Nichols, Task Group Chairman Brian Holtsbaum Kevin Parker David A. Schramm Steven R. Zurbuchen Steven Nelson Donald R. Mayfield

Shell Global Solutions, Houston, Texas CC Technologies Canada, Ltd., Calgary, Alberta CC Technologies, Mt. Pleasant, Michigan EN Engineering, Woodridge, Illinois EN Engineering, Topeka, Kansas Columbia Gas Transmission, Charleston, West Virginia Dominion Transmission, Delmont, Pennsylvania

CP Interference Daily Course Outline DAY ONE Introduction, Welcome, Overview Chapter 1

Stray Current Interference

DAY TWO Chapter 2

DC Interference (Includes Experiment 2-1)

DAY THREE MORNING Chapter 2

DC Interference

AFTERNOON Chapter 3

AC Interference (Includes experiments 3-1, 3-2, and 3-3)

DAY FOUR Chapter 3

AC Interference

DAY FIVE MORNING Chapter 3

AC Interference

AFTERNOON Chapter 4

Telluric Current Interference

DAY SIX MORNING Exam

CP Interference Course Manual © NACE International, 2006 January 2007

Introduction

Introduction The Cathodic Protection (CP) Interference course is a six-day course focusing on alternating current (AC) and direct current (DC) interference. The course includes in-depth coverage of both the theoretical concepts and the practical application of identifying interference and interference mitigation techniques. Students will learn to identify the causes and effects of interference as well as conduct tests to determine if an interference condition exists and perform calculations required to predict AC interference. The course is presented in a format of lecture, discussion and hands-on, in-class experiments, case studies and group exercises. There is a written examination at the conclusion of the course.

Who Should Attend This course is designed for persons who have extensive CP field experience, a strong background in mathematics, and a strong technical background in CP.

Prerequisites • CP 3–Cathodic Protection Technologist certification recommended • Minimum of 3 years CP work experience

Length The course begins at 1 p.m. on Sunday and concludes Friday afternoon. Daily class hours: 8 a.m. to 6:30 p.m. Monday through Thursday and 8 a.m. to 3 p.m. Friday.

Reference Book Students will receive the CP Interference Course Manual prior to the start of the course. A course manual on CD-ROM will be provided to students on-site. CP Interference Course Manual

© NACE International, 2006 July 2007

1

Introduction

Quizzes and Examinations There will be four (4) quizzes distributed during the week and reviewed in class by the instructors. This course has a written final examination. The final examinations will be given on Friday. The written final examination is open-book and students may bring reference materials and notes into the examination room. Non-communicating, battery-operated, silent, non-printing calculators, including calculators with alphanumeric keypads, are permitted for use during the examination. Calculating and computing devices having a QWERTY keypad arrangement similar to a typewriter or keyboard are not permitted. Such devices include but are not limited to palmtop, laptop, handheld, and desktop computers, calculators, databanks, data collectors, and organizers. Also excluded for use during the examination are communication devices such as pagers and cell phones along with cameras and recorders. A score of 70% or greater on the examination is required for successful completion of the course. All questions are from the concepts discussed in this training manual. You will receive written notification of your exam results as quickly as possible. Your results will not be available on Friday.

Introductions We would like for each of you to stand, one at a time and introduce yourself to the class. Tell us: •

Your name

•

Your company’s name and location

•

Your job function

•

Your experience related to CP Interference.

CP Interference Course Manual

© NACE International, 2006 July 2007

2

CP Interference Course Manual Table of Contents General Course Information Daily Course Outline Introduction Chapter 1–Stray Current Interference 1.1 Historical Background ...........................................................

1:1

1.2 Typical Stray Current Circuit Arising from a Transit System Operation .................................................................

1:5

1.3 Stray Current Charge Transfer Reactions on a..................... Metallic Structure

1:6

1.4 Effects of Stray Current on Metallic Structures .....................

1:9

1.4.1 1.4.2 1.4.3

At the Current Discharge Location...................................... At Area of Current Pick-Up ................................................. Along the Structure .............................................................

1:9 1:15 1:19

1.5 Summary ..............................................................................

1:21

Summary of Equations..................................................................

1:22

Figures Fig. 1-1 Fig. 1-2 Fig. 1-3 Fig. 1-4 Fig. 1-5 Fig. 1-6

Early Electric Trolley.............................................................. Pipe-to-soil Potential Changes due to Transit System Stray Current Activity were Recorded on Smoked Charts.. Co-efficient of Corrosion at Different Frequencies for Iron Electrode Denoted as Average Electrode Loss........... Typical Stray Current Paths Around a DC Transit System .... Typical Stray Current Interference on a Metallic Underground Structure ....................................................... Simplified pH Pourbaix Diagram For Iron in Water at 25ºC Showing Potential Shift Direction for Current Pick-up and Discharge at Low pH ...................................................

CP Interference Course Manual © NACE International, 2006 June 2007

1:1 1:3 1:4 1:6 1:6 1:8

Fig. 1-7 Fig. 1-8 Fig. 1-9 Fig. 1-10 Fig. 1-11 Fig. 1-12 Fig. 1-13 Fig. 1-14 Fig. 1-15a Fig. 1-15b Fig. 1-16

Simplified pH Pourbaix Diagram For Iron in Water at 25ºC Showing Potential Shift Direction for Current Pick-up and Discharge at High pH................................................... Current Discharge from a Metal Structure to Earth via an Oxidation Reaction ........................................................ Superposition of a Stray Current and a Cathodic Protection Current at a Metal/Electrolyte Interface .............................. Randle’s Electrical Circuit Model of a Metal/Electrolyte Interface.............................................................................. Theoretical Conditions of Corrosion, Immunity and Passivation of (a) Aluminum at 25ºC and (b) Lead at 25ºC ................................................................. Comparison of Zn and Al Coatings for Corrosion Resistance as Functions of pH ........................................... Typical Section Through a Joint in Two Types of PCCP ....... Cathodic Blistering/Disbondment of Protective Coating ........ Stray Current Discharge and Pick-Up Around an Electrically Discontinuous Joint Though the Earth.............. Stray Current Discharge and Pick-Up Through the Internal Aqueous Medium Around an Electrically Discontinuous Bell and Spigot Joint on Cast Iron Piping.... Stray Current Circuit in an AC Electrical Distribution System................................................................................

1:9 1:10 1:10 1:14 1:16 1:17 1:18 1:19 1:19 1:20 1:20

Tables Table 1-1 Theoretical Consumption Rates of Various Metals and Substances ..................................................................

1:12

Table 1-2 Electrochemical and Current Density Equivalence with Corrosion Rate....................................................................

1:13

Chapter 2–DC Interference 2.1 Introduction ...........................................................................

2:1

2.2 Detecting Stray Current ........................................................ 2:23 2.2.1 Mitigation of Interference Effects from Impressed Current Cathodic Protection Systems ..................................... 2:24 a. Source Removal or Output Reduction .......................... 2:25 b. Installation of Isolating Fittings...................................... 2:26 c. Burying a Metallic Shield Next to the Interfered-with Structure .................................................................... 2:27 d. Installation of Galvanic Anodes on Interfered-with Structure at Point of Stray Current Discharge............ 2:28 e. Installation of an Impressed Current Distribution System on the Interfered-with Structure at Point of Stray Current Discharge................................................................... 2:33 f. i. Installing a Bond Between the Interfered-with and

CP Interference Course Manual © NACE International, 2006 June 2007

Interfering Structures................................................ ii. Calculation of Bond Resistance ............................... g. Use of Coatings in the Mitigation of Interference Effects 2.2.2

2:33 2:35 2:40

Other Sources of DC Stray Current .................................... 2:41 a. DC Transit Systems ...................................................... 2:42 i. Analysis of Transit System Stray Currents ............... 2:44 ii. Mitigation of Transit System Stray Currents ............. 2:51 b. High Voltage Direct Current (HVDC) Electrical Transmission Systems ..................................................................... 2:55 c. DC Welding Operations ................................................ 2:57

Experiment 2-1:

To Demonstrate DC Interference and Its Mitigation........................................................

2:59

………………………………………………………. …..

2.64

Summary of Equations ............................................................................

2:65

Case Study

Figures Fig. 2-1 Fig. 2-2 Fig. 2-3 Fig. 2-4 Fig. 2-5 Fig. 2-6 Fig. 2-7a Fig. 2-7b Fig. 2-8 Fig. 2-9 Fig. 2-10 Fig. 2-11 Fig. 2-12 Fig. 2-13 Fig. 2-14 Fig. 2-15 Fig. 2-16

Parallel Current Paths in the Earth .................................... Parallel Current Paths in a Pipeline Cathodic Protection Section................................................................................ Parallel Current Paths in Vertically Stratified Soil Conditions Parallel Current Paths in Horizontally Stratified Soil Conditions........................................................................... Polarization Test Results....................................................... Stray Current in a Metallic Structure Parallel to a Cathodically Protected Structure ........................................ Voltage vs. Distance from a Vertically Oriented Anode ......... Multiple Vertical Anodes Connected to a Common Header Cable ..................................................................... Multiple Horizontal Anodes Connected to a Common Header Cable ..................................................................... Hemispherical Electrode........................................................ Cathodic Protection Circuit Model with Foreign Structure Intercepting the Anode Gradient......................................... Potential Profile along the Interfered-with Structure .............. Electrical Model for Interfered-with Pipe ................................ Attenuation Model.................................................................. Voltage Gradient in the Earth Around a Cathodically Protected Bare Pipeline ...................................................... Cathodic Protection Circuit Model ......................................... Cathodic Protection Circuit Model with Foreign Structure Intercepting the Anode Gradient......................................... Stray Current in a Foreign Metallic Structure that Intercepts both the Anodic and Cathodic Voltage Gradient.................

CP Interference Course Manual © NACE International, 2006 June 2007

2:1 2:2 2:3 2:3 4:5 2:5 2:6 2:7 2:8 2:9 2:11 2:14 2:14 2:15 2:18 2:18 2:19 2:20

Fig. 2-17 Cathodic Protection Circuit Model with Foreign Structure Intercepting both Anodic and Cathodic Voltage Gradient... Fig. 2-18 Stray Current in a Foreign Metallic Structure that Intercepts the Cathodic Protection Gradient........................................ Fig. 2-19 Cathodic Protection Circuit Model for Foreign Structure Intercepting the Cathodic Voltage Gradient........................ Fig. 2:20 Typical Potential Profile on an Interfered-with Structure that Intersects both Anodic and Cathodic Voltage Gradient with the Current Source Interrupted..................... Fig. 2-21 Current Changes In and Near an Interfered-with Structure ... Fig. 2-22 Stray Current Arising from Installation of Isolating Fittings .... Fig. 2-23 Using a Buried Metallic Cable or Pipe as a Shield to Reduce Stray Current Interference..................................... Fig. 2-24 Cathodic Protection Current Model for a Buried Metallic Shield Connected to the Negative Terminal of the Transformer-Rectifier.......................................................... Fig. 2-25 Interference Mitigation using Galvanic Anodes at Stray Current Discharge Location ................................................ Fig. 2-26 Electrical Circuit Model for Mitigating Stray Current Interference at a Stray Current Discharge Site Using Galvanic Anodes................................................................. Fig. 2-27 Potential Profile Changes on a Pipeline where Stray Current is Discharging in an End-Wise Pattern .................. Fig. 2-28 Interference Mitigation Using a Resistance Bond.................. Fig. 2-29 Measurements Required to Determine Size of Resistance Bond Re .............................................................................. Fig. 2-30 Use of a Dielectric Coating to Mitigate Interference .............. Fig. 2-31 Typical Stray Current Paths Around a DC Transit System .... Fig. 2-32 Typical Structure-to-Soil Potential Recording with Time Caused by Interference from a DC Transit System ............ Fig. 2-33 Current Clamp Used to Measure Pipeline Currents .............. Fig. 2-34 Line Current Survey to Locate Source of Interference Using IR-Drop Test Stations ............................................... Fig. 2-35 Line Current Plots for Example in Figure 2-34 ...................... Fig. 2-36 Exposure Survey to Locate Point of Maximum Exposure...... Fig. 2-37 Exposure Survey Plots for Example in Figure 2-36 ............... Fig. 2-38 Mutual Survey to Confirm Source of Interference.................. Fig. 2-39 Pipe-to-Soil Potential Versus Pipe-to-Rail Potential for Example in Figure 2-38....................................................... Fig. 2-40 Exposure Survey Conducted Without the Measurement Of Pipeline Currents ........................................................... Fig. 2-41 Exposure Survey Plots for Example in Figure 2-40 ............... Fig. 2-42a Typical Embedded Track Installation..................................... Fig. 2-42b Typical Direct-Fixation Isolating Fastener ............................. Fig. 2-43 Typical Utilities Drainage System at a Transit Substation ..... Fig. 2-44 Schematic Showing Circulating Current between Transit Substations Through Direct Bonds to Utilities .................... Fig. 2-45 Forced Drainage Bonds Using a Potential Controlled Rectifier............................................................................... Fig. 2-46 Electrical Schematic for a HVDC System ..............................

CP Interference Course Manual © NACE International, 2006 June 2007

2:20 2:21 2:22 2:23 2:24 2:26 2:27 2:28 2:29 2:30 2:33 2:34 2:36 2:41 2:42 2:43 2:44 2:45 2:46 2:47 2:48 2:48 2:49 2:50 2:50 2:52 2:52 2:52 2:53 2:54 2:55

Fig. 2-47 Potential-Time Plot for a Metallic Structure being Interfered-with by a HVDC System..................................... Fig. 2-48 Stray Current Caused by DC Welding Operations ................

2:57 2:58

Experiment Schematic No. 1................................................................... Experiment Schematic No. 2................................................................... Experiment Schematic No. 3...................................................................

2:59 2:60 2:61

Table 2-1 Specific Leakage Resistances and Conductances in 1000 Ω-cm Soil or Water ....................................................... Table 2-2 Types of Reverse Current Switches ......................................

2:13 2:54

Tables

Chapter 3–AC Interference 3.1 Introduction ........................................................................... 3.1.1 3.1.2 3.1.3

Experiment 3-1:

3:1

Electrostatic (Capacitive) Coupling..................................... 3:2 Electromagnetic (Inductive) Coupling ................................. 3:11 Conductive Coupling (Resistive Coupling) During Powerline Fault Conditions........................................................................... 3:14 To Demonstrate the Effects of Electrostatic Induction .............................................................................

3:16

3.2 Basic Theory of Electromagnetically Induced Voltages ........

3:19

3.2.1 3.2.2 Experiment 3-2:

AC Circuit Theory ............................................................... The Nature of Induced AC Pipeline Voltages .....................

3:19 3:34

To Demonstrate the Effects of Electromagnetic Induction .............................................................................

3:42

3.3 Induced AC Voltages ............................................................

3:44

3.3.1 3.3.2 Experiment 3-3:

Factors that Affect the Longitudinal Electric Field............... Factors that Affect the Pipeline Voltages............................

3:44 3:48

To Further Investigate the Effects of Electromagnetic Induction .............................................................................

3:57

3.4 Deleterious Effects of AC Interference..................................

3:60

3.4.1 3.4.2

3.4.3

Electric Shock Hazards....................................................... AC Corrosion ...................................................................... .1 Theory........................................................................... .2 AC Corrosion Case Histories........................................ .3 AC Corrosion Field Test Procedures ............................ Fault Current Effects...........................................................

CP Interference Course Manual © NACE International, 2006 June 2007

3:60 3:67 3:67 3:75 3:90 3:93

3.5 Induced AC Voltage Prediction and Mitigation Calculations . 3.5.1 3.5.2 3.5.3

3:95

Data Gathering ................................................................... Field Estimation of LEF....................................................... Measurement and Interpretation of Soil Resistivity Data....

3:95 3:97 3:98

3.6 Prediction of Steady-State Induced AC Voltages..................

3:102

3.6.1 3.6.2 3.6.3 3.6.4 3.6.5

Introduction ......................................................................... Calculation of Pipeline Electrical Characteristics................ Sectionalization of Pipeline-Powerline Route ..................... Determination of Longitudinal Electric Field (LEF) ............. Calculation of Induced Pipeline Voltages ...........................

3:102 3:102 3:106 3:107 3:110

3.7 Prediction of Fault Voltages ..................................................

3:115

3.7.1 3.7.2 3.7.3 3.7.4

Introduction ......................................................................... Conductive Coupling Due to Fault Currents ....................... Inductive Coupling Due to Fault Currents........................... Other Related Calculations................................................. (a) Ground Electrode Resistance .................................... (b) Step and Touch Potential .......................................... (c) Conductor Size ..........................................................

3:115 3:115 3:122 3:123 3:123 3:125 3:126

3.8 Equipment for AC Mitigation .................................................

3:126

3.8.1 3.8.2 3.8.3

DC Decoupling Devices...................................................... Test Stations....................................................................... Sacrificial Anodes ...............................................................

3:126 3:138 3:139

Group Activity – AC Mitigation System Design .......................................

3:142

Summary of Equations ............................................................................

3:145

Figures Fig. 3-1a Single Horizontal 3φ Circuit with Shield Wires....................... Fig. 3-1b Distribution System (1φ 4kV Primary and 2φ 240V Secondary with Neutral)........................................................................ Fig. 3-2 AC Voltage Waveforms in a 3φ Circuit .................................. Fig. 3-3 Elements of a Capacitor ........................................................ Fig. 3-4 Electrostatic Coupling during Pipeline Construction .............. Fig. 3-5 Voltage Divider Circuits – Resistive (left) and Capacitive (right) .................................................................................. Fig. 3-6 Calculation of Typical Capacitance Values for a Pipe on Skids .............................................................................. Fig. 3-7 Calculation of Typical Electrostatically Induced Voltage for a Pipe on Skids.............................................................. Fig. 3-8 Calculation of Typical Shock Current Resulting from Electrostatic Coupling ......................................................... Fig. 3-9 Calculation of Typical Electrostatically Induced Voltage

CP Interference Course Manual © NACE International, 2006 June 2007

3:2 3:2 3:2 3:3 3:4 3:5 3:6 3:7 3:8

for an Automobile................................................................ Fig. 3-10 Calculation of Typical Electrostatically Induced Voltage for a Buried Pipe ................................................................. Fig. 3-11 Electromagnetic Field Created by Current Flow in a Wire..... Fig. 3-12 Electromagnetic Induction in a Multiple-Turn, Iron-Core Transformer ........................................................................ Fig. 3-13 Electromagnetic Induction in a Single-Turn, Air-Core Transformer ........................................................................ Fig. 3-14 Electromagnetic Coupling Between a Pipeline and an Overhead AC Powerline ..................................................... Fig. 3-15 Conductive Coupling During Line-to-Ground Fault Conditions........................................................................... Fig. 3-16 Determination of Voltage on a Transformer Secondary ........ Fig. 3-17 Effect of Interconnecting the Secondary Windings ................ Fig. 3-18 Effect on Polarity on a Series Combination of DC Voltage Sources .............................................................................. Fig. 3-19 Effect of “Polarity” on a Series Combination of AC Voltage Sources .............................................................................. Fig. 3-20 In-Phase 60 Hz AC Waveform .............................................. Fig. 3-21 Typical Electrical Distribution Transformer ............................ Fig. 3-22 Typical Residential Electrical Service .................................... Fig. 3-23 AC Waveforms on a Residential Electrical Service ............... Fig. 3-24 Plot of General Equation for Sinusoidal AC Waveforms........ Fig. 3-25 Typical Phasor Diagram ........................................................ Fig. 3-26 Series Combination of AC Voltage Sources .......................... Fig. 3-27 Phasor Diagram for Problem in Figure 3-26 .......................... Fig. 3-28 Determination of Current through a Capacitor....................... Fig. 3-29 Voltage and Current Waveforms for a Purely Capacitive Circuit.................................................................................. Fig. 3-30 Determination of Current through an Inductor ....................... Fig. 3-31 Voltage and Current Waveforms for a Purely Inductive Circuit.................................................................................. Fig. 3-32 Phasor Representation of a Three-Phase Circuit .................. Fig. 3-33 Electric Model of Single Pipe Section .................................... Fig. 3-34 Simplified Electrical Model of Single Pipe Section................. Fig. 3-35 Simplified Electrical Model of Single Pipe Section................. Fig. 3-36 Series Combination of Multiple Pipe Sections ....................... Fig. 3-37 Series Combination of Two Pipe Sections ............................ Fig. 3-38 Series Combination of Two Pipe Sections (Simplified).......... Fig. 3-39 Circuit Analysis Using Kirchhoff’s Law................................... Fig. 3-40 Circuit Analysis Using Kirchhoff’s Law................................... Fig. 3-41 Induced AC Voltage Profile Along Two-Section Pipe Method of Figure 3-39 ........................................................ Fig. 3-42 Profile of Induced AC Voltages and their Phase Angles along any Pipeline having Uniform Electrical Characteristics .................................................................... Fig. 3-43 Effect of Electrical Length of Pipeline on AC Voltage Profile. Fig. 3-44 Double Vertical Circuit ........................................................... Fig. 3-45 Quadruple Vertical Circuit...................................................... Fig. 3-46 Single Delta Circuit ................................................................

CP Interference Course Manual © NACE International, 2006 June 2007

3:9 3:10 3:11 3:12 3:13 3:13 3:14 3:20 3:20 3:21 3:21 3:22 3:23 3:23 3:24 3:25 3:26 3:27 3:27 3:30 3:31 3:32 3:33 3:33 3:35 3:36 3:36 3:36 3:37 3:37 3:37 3:38 3:39 3:39 3:40 3:44 3:44 3:45

Fig. 3-47 Effect of Phase Conductor Separation .................................. Fig. 3-48 Phase Arrangements for a Double Vertical Circuit ................ Fig. 3-49 Effect of Phase Arrangement on LEF Magnitude for Variation of d/s Ratios (and for the specific case Where ρ/s2 = 1Ω/m, s/h=0.3, and I=1000A) ....................... Fig. 3-50 Simple Pipeline-Powerline Corridor (Plan View).................... Fig. 3-51 AC Voltage Profile Along an Electrically Short Pipeline (Uniform Conditions – No Grounding) ................................ Fig. 3-52 Electrical Service Analogy for Pipeline-Powerline Corridor In Figure 3-50 ..................................................................... Fig. 3-53 AC Voltage Profile Along an Electrically Short Pipeline (Non-Uniform Conditions – No Grounding)......................... Fig. 3-54 Effect of Grounding One End of Electrical Service Secondary........................................................................... Fig. 3-55 Effect of Grounding One End of Pipeline in Figure 3-50 ....... Fig. 3-56 Effect of Grounding Both Ends of Pipeline or Adding Distributed Grounds............................................................ Fig. 3-57 Effect of an Insulator at the Midpoint of the Pipeline ............. Fig. 3-58 AC Voltage Profile Along an Electrically Long or Lossy Pipeline (Uniform Conditions – No Grounding)................... Fig. 3-59 AC Voltage Profile Along an Electrically Long or Lossy Pipeline (Zero Resistance Ground at Distance = 0) ........... Fig. 3-60 Effect of an Insulator at the Midpoint of an Electrically Long Pipeline ...................................................................... Fig. 3-61 Fibrillating Current vs. Body Weight (Various animals – 3 second shock duration)....................................................... Fig. 3-62 Possible Body Current Paths................................................. Fig. 3-63 Example of Typical Touch and Step Potentials at an Energized Structure ............................................................ Fig. 3-64a Coefficient of Corrosion at Different Frequencies for Iron Electrode Denoted as Average Electrode Loss........... Fig. 3-64b Maximum Penetration Depth as a Function of Test Duration at Constant Cathode DC Current Density (2A/m2) and Differing AC Current Density .......................... Fig. 3-65a Effect of CP Potential on AC Corrosion Rate ........................ Fig. 3-65b Effect of CP Potential on AC Current Density........................ Fig. 3-65c Pit Cluster and Pinhole Perforation (Case History No. 1) ...... Fig. 3-65d Hemispherical Shell of Hardened Soil Surrounding Anomaly (Case History No. 3) ............................................ Fig. 3-65e Hemisphere of Hardened Soil and Corrosion Pit (Case History No. 3) ........................................................... Fig. 3-65f Pinhole Corrosion Failure Following Removal of Repair Clamp (Case History No. 4)................................................ Fig. 3-65g Pipeline-Powerline Route (Case History No. 4)..................... Fig. 3-65h Nodule of Corrosion Products Protruding Through Coating (Case History No. 4).............................................. Fig. 3-65i Corrosion Pit After Removal of Coating and Corrosion Products (Case History No. 4) ............................................ Fig. 3-65j Effects of Installing Ground Electrodes at Sites A and B (Case History No. 4) ...........................................................

CP Interference Course Manual © NACE International, 2006 June 2007

3:46 3:46 3:47 3:49 3:49 3:50 3:50 3:51 3:52 3:52 3:53 3:54 3:55 3:56 3:61 3:63 3:64 3:68 3:71 3:73 3:74 3:76 3:80 3:81 3:82 3:83 3:85 3:85 3:88

Fig. 3-65k Effects of Installing Ground Electrodes on AC Current Densities (Case History No. 4) ........................................... Fig. 3-66 Fault Damage to CP Bond..................................................... Fig. 3-67 Field Estimation of LEF Magnitude Using Horizontal Wire Method ....................................................................... Fig. 3-68 Soil Resistivity Measurement Using the Wenner Four-Pin Method................................................................................ Fig. 3-69 Determination of Pipeline Coating Resistance ...................... Fig. 3-70 Determination of Pipeline Internal Impedance....................... Fig. 3-71 Sectionalization of Pipeline-Powerline Route ........................ Fig. 3-72 Pipeline-Powerline Geometry for Calculation of LEF............. Fig. 3-73 Typical Series of Curves for Determining LEF....................... Fig. 3-74 Simple Pipeline-Powerline Corridor (Plan View).................... Fig. 3-75 Simple Pipeline-Powerline Model .......................................... Fig. 3-76 Equivalent Circuit for Line-to-Ground Fault ........................... Fig. 3-77 Distribution of Fault Current Along Powerline........................ Fig. 3-78 Distribution of Fault Current Along Powerline........................ Fig. 3-79 Calculation of Earth Voltage at Pipe due to Faulted Tower ... Fig. 3-80 Approximate Length of Pipeline Affected by Faulted Tower.. Fig. 3-81 Resistance of Coating Holiday to Earth ................................. Fig. 3-82 Modified Resistance of Coating Holiday to Earth due to Localized Soil Ionization Effects ..................................... Fig. 3-83 AC Pipeline Voltages Induced by Overhead Faulted Powerline (Per 1000 A of Fault Current)............................. Fig. 3-84 Motor Operated Valve – Effects of Grounding on Induced AC and CP Currents ........................................................... Fig. 3-85 Electrical Isolation of Motor Operated Valve from Pipeline.... Fig. 3-86 Electrical Grounding Schematic of Motor Operated Valve Showing Two Alternative Locations for a DC Decoupling Device................................................................................. Fig. 3-87 Decoupling Device Installed by Electrical Utility Between Primary and Secondary Grounds ....................................... Fig. 3-88 Isolation-Surge Protector Installed across Isolating Flange... Fig. 3-89 Electrical Schematic of One Model of Solid-State DC Decoupling Device.............................................................. Fig. 3-90 DC Decoupling Device Installed Across Insulating Flange for Lightning Protection....................................................... Fig. 3-91 AC Current Being Measured Through a Polarization Cell ..... Fig. 3-92 Polarization Cell Construction ............................................... Fig. 3-93 Corrosion of Plates Within a Polarization Cell ....................... Fig. 3-94 Grounding Cell....................................................................... Fig. 3-95 Electrolytic Capacitor............................................................. Fig. 3-96 Failure of Electrolytic Capacitors in Stray Current Area ........ Fig. 3-97 Metal-Oxide Varistors (MOVs)............................................... Fig. 3-98 Explosion-Proof Surge Protection Device Installed Across Insulator .................................................................. Fig. 3-99 Test Station Varieties (left to right): a) Terminals Exposed To Public; b) Terminals Covered by a Plastic Cap (Locking or Non-Locking); c) Dead-Front Terminals; d) Aluminum Test Station with Padlocked Cover.....................................

CP Interference Course Manual © NACE International, 2006 June 2007

3:89 3:95 3:98 3:99 3:103 3:104 3:107 3:109 3:110 3:111 3:111 3:115 3:116 3:117 3:118 3:119 3:120 3:121 3:123 3:127 3:128 3:129 3:130 3:131 3:131 3:132 3:133 3:133 3:134 3:135 3:135 3:136 3:137 3:138

3:139

Fig. 3-100 a) Zinc Ribbon Anode of Various Sizes; b) Zinc Ribbon Being Installed in Pipe Trench ............................................ Fig. 3-101 Effect of Gypsum on Restoration of Zinc Potential in Bicarbonate-Rich Soil ......................................................... Fig. 3-102 Potential of Magnesium Versus AC Current Density in a Fe-Mg Cell ...................................................................

3:140 3:140 3:141

Tables Table 3-1 Table 3-2 Table 3-3 Table 3-4 Table 3-5

Effects of 60 Hz AC Body Currents on Humans .................... Let-Go Currents from Dalziel’s Experiments ......................... Let-Go Currents from Dalziel’s Experiments ......................... Voltage Puncture Levels for Various Holiday-Free Coatings. Specific Leakage Resistances and Conductances................

CP Interference Course Manual © NACE International, 2006 June 2007

3:60 3:62 3:63 3:94 3:103

Chapter 4–Telluric Current Interference 4.1 Background Theory .............................................................. 4.1.1 4.1.2

4:1

Distributed Source Transmission Line Equations ............... Factors that Affect the Induced Electric Field ..................... (a) Solar Cycle Variations................................................ (b) Sun’s Rotational Frequency....................................... (c) Earth’s Rotation ......................................................... (d) Plasma Magnetic Field Direction ............................... (e) Proximity of Pipeline to a Sea Coast.......................... (f) Pipeline Latitude ........................................................ Factors that Affect the Pipeline Lineal Impedance (Z) and Shunt Admittance (Y).......................................................... (a) Effect of Coating Quality ............................................ (b) Effect of Isolating Fittings........................................... (c) Effect of Pipeline Directional Change ........................

4:13 4:13 4:14 4:15

4.2 Measuring the Geomagnetic Intensity and Determining the Electric Field (E)..............................................................

4:16

4.3 Interference Effects of Telluric Current on Pipelines .............

4:18

4.1.3

4.3.1 4.3.2

4.3.3 4.3.4 4.3.5

General Considerations ...................................................... Corrosion ............................................................................ (a) Theoretical Considerations ........................................ (b) Calculating the Corrosion Rate .................................. (c) Telluric Corrosion Case Studies on Cathodically Protected Piping...................................................... Impact on Accuracy of Current and Potential Measurements ................................................................................... Impact of Telluric Current on Pipeline Coatings ................. Impact on Output of a CP Rectifier .....................................

4.4 Mitigating the Effects of Telluric Current ............................... 4.4.1

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4:18 4:18 4:18 4:22 4:27 4:29 4:31 4:32

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Mitigating Corrosion Impact ................................................ (a) Making the Pipeline Electrically Continuous and Grounded ................................................................ (b) Using CP.................................................................... (i) Sacrificial Anodes ................................................ (ii) Impressed Current Systems ................................ Compensating for Measurement Error Caused by ............ Telluric Current ..........................................................

4:33 4:34 4:35 4:39

4.5 Summary ..............................................................................

4:49

Summary of Equations..................................................................

4:51

4.4.2

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Figures Fig. 4-1 Interaction of Solar Particles on the Earth’s Magnetic Field .. Fig. 4-2a Plasma Charge Distribution around the Earth during Quiescent Period ................................................................ Fig. 4-2b Plasma Charge Distribution around the Earth during a Magnetic Storm................................................................ Fig. 4-3 This Plot Shows the Current Extent and Position of the Auroral Oval in the Northern Hemisphere, Extrapolated From Measurements Taken During the Most Recent Polar Pass of the NOAA POES Satellite for September 16, 2004 at 14:22 UT .......................................................... Fig. 4-4 Schematic of Geomagnetic Induction Directly into a Pipeline and the Resulting Change in Pipeline Potential that is Produced ............................................................................ Fig. 4-5 Quiet Day Variation in the Geomagnetic Field and the Associated Change in the Electric Field and the Pipe-toSoil Potential....................................................................... Fig. 4-6 P/S Potential and Telluric Current in a Long Pipeline Exposed to an Induced Electric Field of 1 V/km, Having an Impedance of 0.1 Ω /km and an Admittance of 0.15 Ω /km................................................... Fig. 4-7 Equivalent Circuit for a Short Section of Pipeline .................. Fig. 4-8 History of Geomagnetic Effects on Ground Technology........ Fig. 4-9 Pipe-to-soil Potential Variations with Time ............................ Fig. 4-10 Charge Accumulation at the Coast Resulting from Larger Induced Currents in the Sea Compared to in the Land. The Charge Accumulation Increases the Electrical Potential of the Earth’s Surface Near the Coast ................. Fig. 4-11 Electric Field, E, Generated by Seawater Moving with Velocity, v, Through the Earth’s Magnetic Field, B ............. Fig. 4-12 Geomagnetic Hazard Percentage of Probability of Occurrence ......................................................................... Fig. 4-13 Telluric Induced Voltage Profile vs Distance for a Pipeline with Different Attenuation Constants..................... Fig. 4-14 Calculated Telluric Induced Voltage at the End of a Long Pipeline as a Function of Coating Conductance for an East-West Electric Field of 0.1V/km .............................. Fig. 4-15 Effect of Isolating Fittings on the Telluric Induced Voltage Profile on an Electrically Short Pipeline .............................. Fig. 4-16 Effect of Pipeline Directional Change on the Telluric Induced Voltage.................................................................. Fig. 4-17 Average Occurrence of 3-Hour Intervals with the Magnetic Activity Index Kp Equal to or Greater than a Specified Value. Kp=9 Corresponds to a Severe Magnetic Storm .... Fig. 4-18 Peak Electric Field Magnitudes as a Function of Kp ............. Fig. 4-19 Oxidation Reaction at Pipe Surface During Telluric Current Discharge in the Absence of CP............................ Fig. 4-20 Reduction Reactions During Negative Cycle Telluric

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and CP Current Pick-up...................................................... 4:19 Fig. 4-21 Steel Surface pH versus Applied CP Current Density ........... 4:20 Fig. 4-22 Polarization Curves after Several Days of Potentiostatic Polarization ......................................................................... 4:20 Fig. 4-23 Experimental Anodic Polarization Curve of Steel in Hydroxide (pH 12.0)............................................................ 4:21 Fig. 4-24 Telluric Current Discharge from a Cathodically Protected Pipe .................................................................................... 4:22 Fig. 4-25 Coefficient of Corrosion at Different Frequencies for Iron Electrodes Denoted as Average Electrode Loss ......... 4:23 Fig. 4-26 Effect on Corrosion Rate of Reversing Direction of Current Compared to Steady State DC and Length of Time Between Reversals............................................................. 4:24 Fig. 4-27 Corrosion Current Density at a Coating Defect having an Applied Voltage of 1.0V in 1000 Ω-cm Soil for Various Coating Thicknesses ............................................. 4:25 Fig. 4-28 Chart Showing the Influence of Anodic Transient Time with Respect to Corrosion Experienced by Probe in Sandy and Clay Soil. Line (a) Represents the Corrosion Rate Expected from Faraday’s Law for the Clay Soil, and Line (B) for the Sandy Soil, Respectively ........................... 4:26 Fig. 4-29 Corrosion Pit at 112+307 (60 mils/497mils 07:30)................. 4:28 Fig. 4-30 Magnetic Field Intensity and Pipe-to-Soil Potential Superimposed..................................................................... 4:29 Fig. 4-31 Schematic of Potentially Controlled CP System Used to Mitigate Telluric Current Effects ....................................................... 4:30 Fig. 4-32 Current Flow and Calculated OFF Potentials during a GIC Incident........................................................................ 4:31 Fig. 4-33 Telluric Current Through a Bridge Rectifying Element During a Discharge Cycle ................................................... 4:32 Fig. 4-34 Schematic of a Telluric Bond Switch ..................................... 4:34 Fig. 4-35 Mitigation of Telluric Current Discharge Effects Using Galvanic Anodes................................................................. 4:35 Fig. 4-36 Effect of Connecting and Disconnecting Groups of Galvanic Anodes to a Pipeline Subjected to Telluric Current................................................................................ 4:36 Fig. 4-37 Maritimes DSTL Results Without Flanges ............................. 4:38 Fig. 4-38 Electrical Schematic at a Constant Voltage Transformer Rectifier During a Positive Telluric Voltage Fluctuation ...... 4:40 Fig. 4-39 Pipe Potential and Rectifier Current Output vs Time for An Impressed Current System Operating in Potential Control ................................................................................ 4:41 Fig. 4-40 Typical Pipe-to-Soil Potential Measurements at Test Station Having a Steel Coupon and Soil Tube ................... 4:42 Fig. 4-41 Typical Pipe-to-Soil Potential Recording at a Test Station Using a Coupon/Reference Probe.......................... 4:43 Fig. 4-42 Comparison Between Pipe/Coupon Potential with Time Recorded with Respect to a Copper-Copper Sulfate Reference on Grade and to a Coupon/Reference Probe Located at Pipe Depth ........................................................ 4:44

CP Interference Course Manual © NACE International, 2006 June 2007

Fig. 4-43 Pipe-to-Soil Potential Measurement Method to Compensate For Telluric Current Effects During a Close Interval CP Survey........................................................................... Fig. 4-44 CIPS Method Using One Moving and Two Stationary Data Loggers ...................................................................... Fig. 4-45 Pipe-to-Soil Potential Measurement Method to Compensate for Telluric Current Effects During a Close Interval CP Survey........................................................................... Fig. 4-46 Pipe Potential/Telluric Current Relationship at a Coupon Test Station......................................................................... Fig. 4-47 Four Wire Test Lead Arrangement for Measuring Pipe Current........................................................................

Appendices Appendix A – Curve Matching Appendix B – Pipe Data Table Appendix C – Anode Tables Appendix D – Wire Size Table Appendix E – Metric Conversion Table Appendix F – Dabkowski Paper NACE RP0177 NACE SP0169 NACE Glossary of Corrosion-related Terms Course Evaluation Instructor Evaluation

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CHAPTER 1 STRAY CURRENT INTERFERENCE

1.1

Historical Background

The term “interference” is understood in the pipeline industry as electrical interference and is defined as “any detectable electrical disturbance on a structure caused by a stray current where a ‘stray current’ is defined as a current in an unintended path”.1 This broad definition suggests that the structure, although often a pipeline, could be any metallic network such as electrical power grids and communication systems. Furthermore, although the interfering current is often a direct current (DC) from a cathodic protection (CP) impressed current source, the current can also originate from any electrical system that uses the earth either intentionally or inadvertently as a current path. Thus alternating current (AC) can also be included in the definition. Electrical interference concerns preceded the use of CP for corrosion control of pipelines. Telegraph systems were reported2 to interfere with the operation of the early telephone systems. Lighting systems, first introduced in about 1880, comprised arcs and incandescent lamps also interfered with the telephone systems, primarily because both the telephone system and the lighting systems used the earth as a current path. Then, in the late 1800s and early 1900s, street railways throughout North America were electrified.3 They ultimately led to the corrosion of cast iron watermains.

Figure 1-1: Early Electric Trolley (courtesy of East Bay Municipal Utility District, Oakland, CA)4 1

CP3 – Cathodic Protection Technologist Course, NACE International, June 1, 2004, p.3-1. Anderson, John M., The Fight Over the Highways, IEEE Power Engineering Review, December 1997, p.45. 3 Anderson, John M., First Electric Street Car, IEEE Power Engineering Review, Oct. 1999, p.32. 4 Lewis, Mark, Once Vagrant Current, Now Impressed Current Cathodic Protection, MP, Vol 36, July1997. 2

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Corrosion on watermains as a result of interference from a DC transit system was first reported by Stone & Forbes in 18945, just 6 years after a New England transit system began operation. In 1901, damage to water and gas mains in Toronto, Ontario, was reported6 as being due “to railway currents.” The currents reportedly affected the watermains for two reasons: deterioration of the rail joint bonds and the practice of bonding the watermains to the rails at certain locations. The U.S. Bureau of Standards began studying the stray current traction problem in 1910. The bureau would issue 15 reports by 1921. Many of the investigations involved field studies, during which temporary electrolysis committees were formed consisting of interested utility representatives. The corrosion resulting from stray current was initially referred to as “electrolysis,” a term defined as “the decomposition of a substance by the application of a current”.7 The widespread corrosion of iron watermains by stray transit system currents led to the formation in 1913 of the American Committee on Electrolysis.8 Stray current activity on underground structures arising from transit system operation is not steady-state but dynamic in terms of current and potential amplitude. It often reverses direction. Typical structure potential activity was recorded on smoked charts. These charts collect data as a stylus moving in response to a changing potential input removes the smoke from the chart, which is rotated by a clock drive. The dynamic nature of the stray current effect on pipe potential is shown in Figure 1-2.

5

Stone, C.A. and Forbes, H.C., Electrolysis of Water Pipes, New England Water Works Association, Vol. 9, pp.1894-5. 6 Knudson, A.A., Report on the Joint Investigation and Survey for Electrolysis on the Water and Gas Mains in the City of Toronto, Ontario, July 1, 1906. 7 The Oxford Encyclopedic English Dictionary, Oxford University Press, 1991. 8 Meany, J.J., A History of Stray Traction Current Corrosion in the United States, NACE, Corrosion’74, Paper 152, p.3.

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Figure 1-2: Pipe-to-Soil Potential Changes due to Transit System Stray Current Activity were Recorded on Smoked Charts

Because of the variable nature of the stray current activity, it is difficult to predict how much corrosion would occur. The Bureau of Standards conducted a study9 in which iron samples where subjected to AC discharge and current pick-up for different periods of time. The resulting corrosion was compared to corrosion produced by a steady-state DC of the same current density and discharge period. The results of this study, reported in 1916, are summarized in Figure 1-3.

9

McCollum, B. and Ahlborn, G.H., Influence of Frequency of Alternating and Infrequently Reversed Current on Electrolytic Corrosion, Technologic Papers of the Bureau of Standards, U.S. Dept. of Commerce, No. 72, 1916.

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100 90 80

LEGEND: Soil Soil + Na2CO3

70 60 50 40 30 20 10 0 -10

1/60S 1/15S

1S

5S

1M 5M 10M 1Hr.

2Days 2Weeks

D.C.

Logarithm of Length of Time of One Cycle

Figure 1-3: Coefficient of Corrosion at Different Frequencies for Iron Electrode Denoted as Average Electrode Loss

For short periods of reversals, the corrosion was only a small fraction of the corrosion at steady state. For equal periods of pick-up and discharge, the corrosion coefficient remained below 20% when the cycle remained below one hour. This meant that the corrosion occurring from dynamic stray currents was a function of the frequency. At 60hz the corrosion rate was less than approximately 2% of the steady state value. R.J. Kuhn, who investigated the effects of transit system stray current activity on iron water mains in New Orleans, Louisiana, is credited with the discovery of CP. It occurred to him in 1928 that “ordinary corrosion could be prevented by reversing these currents”.10 Sir Humphrey Davy11 was the first person on record to use CP by applying zinc castings to protect the copper sheathing on British warships in 1824. Although a technical success, Davy’s application was a 10

Kuhn, R.J., Cathodic Protection of Underground Pipe lines from Soil Corrosion, API Proceedings, Nov. 1933, Vol. 14, p.164. 11 Davy, H., Philosophical Transactions of the Royal Society, London, 1824-1825.

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practical failure because the copper biofouled when the corrosion was stopped— thus reducing the speed of these sailing ships. It appears that neither Kuhn nor any other corrosion practitioner had knowledge of this. Hence, Kuhn is considered by one source12 as the “father” of CP (certainly as it applies to pipelines). Against this backdrop, stray current interference and its corrosion consequences for underground metallic structures were first evaluated. Today electrolysis committees exist throughout North America, and methods of mitigation that have subsequently been developed are commonly utilized. Sources of stray current interference are not confined to DC transit systems. They now include any electrical source that uses the earth either intentionally or inadvertently as a current path. This course addresses these sources and the mitigation methods that have been developed to mitigate not only the corrosion effects, but other deleterious consequences of stray current activity.

1.2

Typical Stray Current Circuit Arising from a Transit System Operation

Figure 1-4 depicts stray current paths originating from the operation of an electric transit system. Although it is the intent that the DC operating current returns to the substation via the running rails (IR), some of the load current (IL) will pass through the earth (Ie) if the rail is in electrolytic contact with the earth. If there is a metallic structure in the earth, it, too, will carry some of the load current (IS). Therefore, the load current (IL)—after passing through the locomotive—divides into parallel paths. The amount of current in each path is inversely proportional to the resistance of each path relative to the total circuit resistance, as Equation 1-1 indicates. I path = where:

12

Ipath RT Rpath IL

= = = =

R T • IL R path

current in a path total resistance of parallel paths resistance of current path load current

von Baeckmann, W., Schwenk, W., and Prinz, W., Handbook of Cathodic Corrosion Protection, 3rd edition, Gulf Publishing Co., Houston, TX, 1997, p.16.

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[1-1]

Stray Current Interference

1:6

DC substation O/H power conductor

IL

IR ground

Is

running rails

Is

Is

p ic k - u p metallic structure (e.g.,watermain)

d is c h a r g e

Ie

Ie

Figure 1-4: Typical Stray Current Paths Around a DC Transit System

Hence, as the resistance of the rail path increases or the resistance of the alternative stray current path(s) decreases, a greater percentage of the load current will appear in the stray current path(s).

1.3

Stray Current Charge Transfer Reactions on a Metallic Structure

Figure 1-5 illustrates the typical stray current situation on an underground metallic structure that is not electrically connected to the source of stray current. The stray current pattern consists of a pick-up of stray current from the earth at one or more locations and the subsequent discharge of stray current to the earth at one or more locations.

Is Is

stray current pick-up

Is stray current discharge

Is

Figure 1-5: Typical Stray Current Interference on a Metallic Underground Structure

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The principal charge carriers in the earth are ions. They are electrons in the metallic structure. For these reasons, electrochemical reactions must transfer the charge between the structure and earth at both the pick-up and discharge locations. At the pick-up location(s), it is through reduction reactions that the electrical charges are transferred. Depending on the nature of the electrolytic environment, the reduction reactions can be one or more of the following: H3O+ + e–

Æ

HO + H2O

[a]

O2 + 2H2O + 4 e–

Æ

4OH–

[b]

2H2O + 2e–

Æ

H2↑ + 2OH–

[c]

Reaction [b] is favored in well-aerated soils and waters; reduction reaction [a] is favored in acidic soils or waters. Reduction reaction [c], which involves the breakdown of water molecules to hydrogen gas and hydroxyl ions, can occur under all conditions if there is sufficient over-voltage applied. At the discharge location, one or more of the following oxidation reactions transfers the electrical charge. M0

Æ

Mn+ + ne–

[d]

4OH–

Æ

O2 + 2H2O + 4e–

[e]

2H2O

Æ

O2 + 4H+ +

[f]

4e–

Reaction [d] tends to occur on most basic metals such as iron, copper, zinc, and aluminum when the electrolyte has an acid or neutral pH. Reaction [e] is more likely in electrolytes with a high pH. Reaction [f] is more likely to occur when the over-voltage reaches the oxygen line. The oxygen line is line “b” on the Pourbaix diagram for iron (Figure 1-6).

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-1.6 -2

16

0

2

4

6

8

10

12

14

16

pH (assuming passivation by a film of Fe2O3)

Figure 1-6: Simplified pH Pourbaix Diagram for Iron in Water at 25ºC Showing Potential Shift Direction for Current Pick-up and Discharge at Low pH

The Pourbaix diagram for iron in pure water represents three zones of thermodynamic stability: corrosion, immunity, and passivity based on a potential (SHE) vs pH relationship. Line (a) is the hydrogen line and line (b) is the oxygen line. Water is stable between these two lines. If the potential of iron is shifted to either of these lines, then oxygen is generated at line (b) and hydrogen gas at line (a). For an iron structure without CP that is exposed to a neutral or low-pH water, a current pick-up will cause the potential to shift in the negative direction toward the immunity zone and afford the structure some CP. Conversely, at the discharge location, the potential is shifted in the electropositive direction into the passive region if not at a low pH—where it would otherwise remain in the corrosion zone. On a cathodically protected structure as illustrated in Figure 1-7, where the electrolyte at the iron surface normally has a high pH, a current discharge resulting in a positive shift can produce a passive film given by the following reaction: Fe + 2H2O Æ Fe(OH)2 + 2H+ + 2e– CP Interference Course Manual © NACE International, 2006 January 2008

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-0.4 -0.8

2

-1.2 -1.6

-1.6 -2

16

10

12

14

16

pH (assuming passivation by a film of Fe2O3)

Figure 1-7: Simplified pH Pourbaix Diagram for Iron in Water at 25ºC Showing Potential Shift Direction for Current Pick-up and Discharge at High pH

The ferrous hydroxide formed is relatively stable at high pH. Because this reaction also produces hydrogen ions, the pH will decrease with time.

1.4

Effects of Stray Current on Metallic Structures

It is apparent that the effect of a stray current pick-up and a stray current discharge from an iron structure from a thermodynamic perspective can cause corrosion, passivation, or immunity, depending upon the direction of current and the pH of the aqueous electrolyte at the charge transfer location.

1.4.1 At the Current Discharge Location Identification of the current discharge site receives considerable attention in stray current investigations because it is the location where corrosion damage is most likely to occur on all metallic structures. When a current transfers from a metallic structure to earth (Figure 1-8), it must do so via an oxidation reaction that converts electronic current to ionic current.

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metal structure (electrons)

Is

O X I D A T I O N

Is

earth (ions)

Is

Figure 1-8: Current Discharge from a Metal Structure to Earth via an Oxidation Reaction

The generic oxidation reaction is the corrosion of the metal as in Equation 1-2. Mo Æ Mn+ + ne–

[1-2]

For steel, the oxidation reaction is: Feo Æ Fe++ + 2e–

[1-3]

A stray current discharge from a metallic structure may not cause corrosion attack if the structure is receiving CP (Figure 1-9). Whether the superposition of a stray current discharge and a CP current pick-up at a metal/electrolyte interface causes corrosion will depend on time and the relative magnitudes of these two currents.

metal structure

O X I D A T I O N

R E D U C T I O N

Is Is Is

Icp Icp earth

Icp Icp

Figure 1-9: Superposition of a Stray Current and a Cathodic Protection Current at a Metal/Electrolyte Interface

CP current transfers across the metal/earth interface via a reduction reaction, which produces hydroxyl ions in either of the three following reactions: H3O+ + e– Æ HO + H2O [1-4] O2 + 2H2O + 4e– Æ 4OH–

[1-5]

2H2O + 2e– Æ H2Ç + 2OH–

[1-6]

In the presence of a high concentration of hydroxyl ions, a possible oxidation reaction is given in Equation 1-7. The reaction involves the oxidation of hydroxyl ions to oxygen and water. 4OH– Æ O2 + 2H2O + 4e– [1-7]

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This latter reaction does not consume metal atoms; therefore, there is no corrosion damage. Hence, as long as the polarized potential at the structure electrolyte interface is not driven more electropositive than the CP criterion (e.g., –850mVcse for iron or steel), significant corrosion would not be expected. If the metal has a surface passive film or is a relatively inert material (such as some of the materials used for impressed current anodes), then not all of the stray current need transfer through a corrosion reaction. If the stray current polarizes the metal surface electropositively to the oxygen line on the Pourbaix diagram, then the hydrolysis[13] of water molecules by the following reaction 1-8 is likely. 2H2O Æ 4H+ + O2Ç + 4e–

[1-8]

This oxidation reaction does not result in the consumption of the metal surface, but it does produce an acidic pH from the generation of hydrogen ions. On an iron or steel structure without CP, the oxidation reaction is usually the dissolution of the metal according to Equation 1-9 Feo

Æ

Fe++ + 2e–

[1-9]

The severity of corrosion depends on the magnitude of the stray current and time as related by Faraday’s Law: Wt =

M t I corr nF

[1-10]

where: Wt = total weight loss at anode or weight of material produced at the cathode (g) n = number of charges transferred in the oxidation or reduction reaction Icorr = the corrosion current (A) F = Faraday’s constant of approximately 96,500 coulombs per equivalent weight of material (where equivalent weight = M ) n M = the atomic weight of the metal that is corroding or the substance being produced at the cathode (g) t = the total time in which the corrosion cell has operated (s) 13

Hydrolysis is defined as a double decomposition reaction involving the splitting of water into its ions and the formation of a weak acid or base or both. CRC Handbook of Chemistry and Physics, CRC Press, 53rd Edition, 1972-1973, PF-83.

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Given the atomic weight of pure iron as 55.85 g and assuming 100% efficiency and pure DC, the consumption rate of iron as illustrated in Table 1-1 is 9.13 kg/A-y. Table 1-1: Theoretical Consumption Rates of Various Metals and Substances Reduced Species

Oxidized Species

Al Cd Be Ca Cr Cu H2 Fe Pb Mg Ni OHZn

Al+++ Cd++ Be++ Ca++ Cr+++ Cu++ H+ Fe++ Pb++ Mg++ Ni++ O2 Zn++

Molecular Weight, M (g) 26.98 112.4 9.01 40.08 52.00 63.54 2.00 55.85 207.19 24.31 58.71 32.00 65.37

Electrons Transferred (n) 3 2 2 2 3 2 2 2 2 2 2 4 2

Equivalent Weight, M/n (g) 8.99 56.2 4.51 20.04 17.3 31.77 1.00 27.93 103.6 12.16 29.36 8.00 32.69

Theoretical Consumption Rate (Kg/A-y) 2.94 18.4 1.47 6.55 5.65 10.38 0.33 9.13 33.9 3.97 9.59 2.61 10.7

On pipelines, the total weight loss is usually less important than the penetration rate. By re-arranging Faraday’s Law, the weight loss per unit time per unit area is shown to be directly proportional to current density (i = I/A) as in Equation 1-11. Wt A tt

=

M i nF

[1-11]

Dividing this equation by the density (d) of the metal or alloy produces the corrosion rate (rcorr), which can be expressed in mm/y (Equation 1-12). rcorr

=

k M is nF d

where: M n i k d rcorr

= = = = = =

CP Interference Course Manual © NACE International, 2006 January 2008

atomic weight (g) number of charges transferred in corrosion reaction current density (μA/cm2) unit correction term ≈ 3.156 x 108 mm s/cm yr density (g/cm3) penetration rate in (mm/yr)

[1-12]

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1:13

Example: Using Equation 1-12 to calculate the penetration rate based on a current density of 1 A/m2 (10-4 A/cm2): where:

i = 10-4 A/cm2 d = 7.87 g/cm3

M = 55.85 g n = 2 F = 96,500 coulombs then:

rcorr =

3.156 × 10 8 mm s/ cm yr × 55.85g × 10 -4 A/cm 2 2 × 96,500 coulombs × 7.87 g/cm 3

= 1.16 mm/y

Table 1-2 gives the penetration rate, in mpy and 10-3 mm/y, equivalent to a current density of 1μA/cm2 for a number of common pure metals. Table 1-2: Electrochemical and Current Density Equivalence with Corrosion Rate for Some Common Pure Metals

Metal/Alloy Pure Metals Iron Nickel Copper Aluminum Lead Zinc Tin Titanium Zirconium

Element/ Oxidation State

Density (g/cm3)

Equivalent Weight (g)

Fe/2 Ni/2 Cu/2 Al/3 Pb/2 Zn/2 Sn/2 Ti/2 Zr/4

7.87 8.90 8.96 2.70 11.4 7.13 7.3 4.51 6.5

27.93 29.36 31.77 8.99 103.6 32.69 59.34 23.95 22.80

Penetration Rate Equivalent to 1 μA/cm2[1] (mpy)

10-3 mm/y[2]

0.46 0.43 0.46 0.43 1.17 0.59 1.05 0.69 0.45

11.6 10.8 11.6 10.9 29.7 15.0 26.6 17.4 11.5

Note: [1] A current density of 1 μA/cm2 is approximately = 1 mA/ft2 [2] 10-3 mm/y = 1 μm/y and 1 mpy = 25.4 μm/y

The foregoing corrosion rates apply to stray current situations involving a continuous DC discharge. Corrosion rates decrease for periodic reversals of DC and are substantially less for 60Hz AC (Figure 1-3).

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The low corrosion rate for a 60Hz current is attributed to the relatively low impedance of the interfacial capacitance. The structure/electrolyte interface can be modeled electrically by a Randle’s Circuit shown in Figure 1-10. where: Cdl = double layer capacitance (1-200 μF/cm2) Rp = polarization resistance (1-104 Ω-cm2)

Eac Rp

Iac,rp

Cdl

steel

Re

Re = resistance of steel surface to remote earth potential difference (volts) Eoc =

Iac

Iac = total AC crossing the interface

soil (electrolyte)

Ia,rp = total AC through polarization resistance Ia,dl = total AC through double-layer capacitance

Iac,dl

Figure 1-10: Randle’s Electrical Circuit Model of a Metal/Electrolyte Interface

This circuit model illustrates that the interface is not simply a resistance but a parallel combination of the polarization resistance (Rp) and a capacitor (Cdl) called the double-layer capacitance. Unlike DC, AC can pass through the doublelayer capacitance. There is no mass transfer in this current path and hence no corrosion polarization results from current transfer in this path. The proportion of AC (Iac,dl) through the double-layer capacitor is a function of the relative impedance of this path compared to the polarization resistance. The reactance (Xcdl) of the double-layer path is given by the following equation: Xc dl

=

1 2π f C dl

[1-13]

where: f = frequency (Hz) Cdl = capacitance (farads) Xcdl = reactance (ohms)

Assuming a 1cm2 surface area and mid-range values of both the polarization resistance and the double-layer capacitance as follows, then the proportion of AC through the capacitor can be calculated.

CP Interference Course Manual © NACE International, 2006 January 2008

Stray Current Interference

Assume:

1:15

Cdl = 100 μf/cm2 Rp = 103 Ω−cm2

Using Equation 1-13: Xc dl

=

1 2π 60 × 100 × 10 -6

=

10 4 376.8

=

10 4 120π

= 26.54 Ω

The total impedance Zt to 60Hz AC of the parallel combination of the polarization resistance (Rp) and the double-layer capacitance is therefore:

therefore:

1 Zt

=

1 + Rp

1 Zt

= 10 -3 + 37.7 × 10 −3 = 38.7 × 10 −3

Zt

=

10 3 38.7

1 Xc dl

=

1 + 10 3

1 26.54

= 25.8 Ω

Then the proportion of AC current through the double-layer capacitance is: I ac,dl

I ac,dl

=

=

Z t I ac, t Xc dl

25.8Ω × I ac, t = 0.974 I ac, t or 97.4% 26.5

Accordingly, only approximately 2.6% of the AC would pass through the polarization resistance and only the positive half-cycle of the current would be involved in the corrosion reaction.

1.4.2 At Area of Current Pick-Up At the area of current pick-up, a negative shift will result in cathodic polarization. If the foreign structure is mild steel, then there is a beneficial effect because the structure is receiving some measure of CP. If the structure is coated and has its CP Interference Course Manual © NACE International, 2006 January 2008

Stray Current Interference

1:16

own CP system, the additional polarization from the stray current pick-up may result in cathodic blistering of the coating. If the foreign structure is not mild steel but is made of an amphoteric metal such as aluminum, lead, or zinc, then the high pH developed at the structure/earth interface caused by the reduction reaction can effect “cathodic” corrosion. Amphoteric metals such as aluminum are susceptible to corrosion at both high and low pH. Figure 1-11 shows this phenomenon for aluminum.

(a) Aluminum

(b) Lead

Figure 1-11: Theoretical Conditions of Corrosion, Immunity, and Passivation of (a) Aluminum at 25ºC and (b) Lead at 25º C Source: Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, Houston, TX, 1974, p.172

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Stray Current Interference

1:17

70

Al

60

Zn

50 40 30 20 10 14

13

12

Alkaline

11

10

9

8

7

pH

6

5

4

3

2

1

Acid

Figure 1-12: Comparison of Zn and Al Coatings for Corrosion Resistance as Functions of pH

One can see that aluminum is particularly sensitive to high pH attack. Aluminum is often used underground for water irrigation systems, gas distribution piping in rural areas, AC secondary distribution conductors, and the sheathing on communication cables. Zinc and lead are also amphoteric metals. The corrosion rate of zinc, as indicated in Figure 1-12, is not as high as aluminum in alkaline conditions but is much greater in acid conditions. Lead sheathing was commonly used on belowground AC power cables. Not only are these amphoteric materials susceptible to corrosion according to Faraday’s Law at rates indicated in Table 1-2 at stray current discharge locations, but also at stray current pick-up locations. Prestressed concrete cylinder pipe (PCCP) used for both water and sewage transmission is composed of a mild steel inner cylinder, over which a highly stressed steel wire is wound to give the concrete/steel cylinder strength. Typical cross-sections of the two types of PCCP are shown in Figures 1-13a and 1-13b.

CP Interference Course Manual © NACE International, 2006 January 2008

Stray Current Interference

1:18

Prestressing Wire and Wire Fabric Around Bell or Thicker Bell Ring and Wire Fabric

Cement - Mortar Coating

Grout Joint After Installation

Prestressed Wire Steel Cylinder

Concrete Core

Rubber Gasket

Steel Bell Ring Cement Mortar Placed in Field or Other Protection

Steel Spigot Ring

a. Lined Cylinder Pipe Cement - Mortar Coating

Grout Joint After Installation

Concrete Core Steel Spigot Ring

Prestressed Wire

Rubber Gasket Cement Mortar Placed in Field or Other Protection

Steel Cylinder Steel Bell Ring

b. Embedded Cylinder Pipe Figure 1-13: Typical Section Through a Joint in Two Types of PCCP Source: Prestressed Concrete Pressure Pipe-Steel Cylinder Type for Water and Other Liquids, AWWA Standard C301, American Water Works Association, Denver, CO

The prestressing wire in these pipes is normally cold drawn steel with a yield strength in the order of 200 ksi. The cold-worked hardened surface of the wire makes it susceptible to hydrogen embrittlement. It is recommended that the polarized potential be limited to –970 mVcse or less negative to minimize the production of atomic hydrogen. If a stray current causes excessive cathodic polarization, then a catastrophic failure could occur.

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Stray Current Interference

1:19

If the foreign structure is coated at the stray current pick-up site, then coating blistering or disbondment can occur. Coating blistering is caused by the pressure build-up beneath the coating due to the movement of water through the coating, due to electroendosmosis. Electroendosmosis is defined as “the inward flow of a fluid through a permeable membrane due to an electric field”. The high pH produced by the reduction reaction at the metal surface can attack the coating adhesion bonds or a surface oxide layer, resulting in coating disbondment. DC

H2O _ _OH OH _ OH _ _ OH OH _ _ _ _ OH OH OH OH

H2O

soil

metal substrate

Figure 1-14: Cathodic Blistering/Disbondment of Protective Coating

1.4.3 Along the Structure Stray current in a metallic structure does not usually cause damaging effects between the stray current pick-up and discharge locations unless the current is very large or the structure is not electrically continuous. If the structure is electrically discontinuous (as is often the case with cast iron water distribution piping or PCCP transmission piping), the structure resistance (Rs) is greater than if it were electrically continuous, which reduces the magnitude of Is, but creates a current discharge/current pick-up pattern at each electrical discontinuity (Figures 1-15a and 1-15b).

electrically discontinuous joints

Is

Is

Is

Figure 1-15a: Stray Current Discharge and Pick-Up Around an Electrically Discontinuous Joint Through the Earth

In many of these structures not every joint is discontinuous, but localized corrosion will occur on the discharge side of the discontinuous joints. Furthermore, on water and sewer piping, there is not only a soil path for the stray

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Stray Current Interference

1:20

current but also an internal path through the aqueous medium as illustrated in Figure 1-15b. rubber seal

aqueous medium

Figure 1-15b: Stray Current Discharge and Pick-Up Through the Internal Aqueous Medium Around an Electrically Discontinuous Bell and Spigot Joint on Cast Iron Piping

Current in an AC distribution system can also affect the transformation characteristics in distribution transformers. At the AC distribution transformer, which supplies the AC service for an impressed current transformer-rectifier, a ground cable is normally run from the AC neutral to a ground rod at the base of the service pole. The ground rod, being relatively close to the groundbed, will pick up stray current. The distribution neutral and the AC phase conductor will carry the stray current to ground at remote transformers because DC does not encounter a high resistance through the primary winding. This circuit is illustrated schematically in Figure 1-16. Remote Distribution Transformer

CP AC Distribution Transformer

L1

Is,2

Is,2

N

N T/R

N

L2

L

Is,1 Is

CP groundbed

Is

Figure 1-16: Stray Current Circuit in an AC Electrical Distribution System

A DC in the primary or secondary windings of a transformer will produce a magnetic flux in the transformer core that will tend to saturate the core and thus spoil its voltage transformation properties. This is a deleterious effect that is in CP Interference Course Manual © NACE International, 2006 January 2008

Stray Current Interference

1:21

addition to the corrosion damage that results from the stray current discharging off the ground rod at the remote distribution transformer.

1.5

Summary

Stray current is an irrevocable factor to which all metallic underground structures are exposed because so many electrical systems use the earth as a current path. The following list of possible stray current sources is extensive: • • • • • • • • •

CP systems High-voltage AC transmission systems Low-voltage AC distribution systems High-voltage DC transmission systems AC and DC transit systems Welding operations Geomagnetically induced currents Low-frequency communication systems Land-line telephone systems.

Pipeline corrosion control practitioners are often acutely aware of the various sources of stray current, yet impressed current CP systems remain among the most prevalent stray current sources. As public pressure mounts to force more stray current sources into joint-use corridors, stray current control becomes increasingly important and decidedly more complex.

CP Interference Course Manual © NACE International, 2006 January 2008

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1:22

Summary of Equations I path =

[1-1]

where:

Ipath RT Rpath IL

= = = =

R T • IL R path

page 1:5

current in a path total resistance of parallel paths resistance of current path load current

[1-2]

Mo Æ Mn+ + ne–

page 1:10

[1-3]

Feo Æ Fe++ + 2e–

page 1:10

[1-4]

H3O+ + e– Æ HO + H2O

page 1:10

[1-5]

O2 + 2H2O + 4e– Æ 4OH–

page 1:10

[1-6]

2H2O + 2e– Æ H2Ç + 2OH–

page 1:10

[1-7]

4OH– Æ O2 + 2H2O + 4e–

page 1:10

[1-8]

2H2O Æ 4H+ + O2Ç + 4e–

page 1:11

Feo

[1-9]

Æ

Wt =

[1-10]

Fe++ + 2e– M t I corr nF

page 1:11

page 1:11

where: Wt = total weight loss at anode or weight of material produced at the cathode (g) n = number of charges transferred in the oxidation or reduction reaction Icorr = the corrosion current (A) F = Faraday’s constant of approximately 96,500 coulombs per equivalent weight of material (where equivalent weight = M ) n M = the atomic weight of the metal that is corroding or the substance being produced at the cathode (g) t = the total time in which the corrosion cell has operated (s)

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Wt A tt

[1-11]

M i nF

=

page 1:12

rcorr

[1-12]

=

kMi nF d

page 1:12

where: M n i k d rcorr

[1-13]

Z dl

=

= = = = = =

atomic weight (g) number of charges transferred in corrosion reaction corrosion current density (μA/cm2) unit correction factor ≈ 3.156 x 108 mm s/cm y density (g/cm3) penetration rate in (mm/yr)

1 2π f C dl

where: f = frequency (Hz) Cdl = capacitance (farads) Zdl = impedance (ohms)

CP Interference Course Manual © NACE International, 2006 January 2008

page 1:14

CHAPTER 2 DC INTERFERENCE

2.1

Introduction

The term “interference” in cathodic protection (CP) parlance means electrical interference as opposed to physical or chemical interference. Hence interference can be defined as any detectable electrical disturbance on a structure caused by a stray current. In turn, a stray current is defined as a current in an unintended path. Many electrical systems rely on the earth as a conducting medium either for the main transmission of electrical energy (as with CP systems) or as an electrical ground. Still, other systems—such as electrified transit systems—may not be adequately isolated from ground. Regardless, any electrical system that is in contact with the earth is a possible source of stray currents. As illustrated in Figure 2-1, a current entering the earth at point “A” has many parallel paths available at point “B.” In I4

A

I3 I2 I1

Rn R4 R3

B

R2 R1

Figure 2-1: Parallel Current Paths in the Earth

The amount of current in each path is inversely proportional to the resistance of each path. It can therefore be argued that current will take all available paths. If point “A” is considered an impressed current groundbed connected to the positive terminal of a transformer-rectifier and point “B” is a pipeline connected to the negative terminal, then the parallel current paths may all have similar resistances—in which case all the currents are the same. This is only possible in homogeneous soil where points “A” and “B” are a long distance apart and where the pipe has no lineal resistance. However, if the soil resistivity varies or the pipe has lineal resistance, the current paths will have unequal resistances as illustrated in Figure 2-2.

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DC Interference

2:2

R4,e

R1,e

R3,e

R2,e T/R

I2

I1

I3

I4

R3,p

R2,p

R4,p

R1,p drain point

Figure 2-2: Parallel Current Paths in a Pipeline Cathodic Protection System

It is apparent that each current path is composed of resistance through the earth (Re) plus a resistance through the pipe (Rp) from the point of current pick-up back to the drain point. Therefore, the total resistance (Rt,i) of each parallel path is different and given by Equation 2-1. Rt,i = Ri,e + Ri,p

[2-1]

Because the length of each current path is different both in the earth and in the pipe in any direction away from the drain point, the total resistance of each current path will increase with distance from the drain point. The amount of current in each path is given by Equation 2-2 Ii = where:

R t ,n R t ,i

It

[2-2]

Rt,n = the total resistance of n parallel paths 1 1 1 1 + + + R4 R3 R2 R1

1 = R t ,n

⋅⋅⋅

1 Rn

and:

It = I1 + I2 + I3 + I4 … In In stratified soil conditions where the soil resistivity or cross-sectional area of each stratum is different, even current paths of equal length will not have equal resistances as illustrated in Figures 2-3 and 2-4.

CP Interference © NACE International, 2006 January 2008

DC Interference

2:3

T/R

ρmod

till

R1,e

ρlow

clay

R2,e

ρhigh

R3,e

rock

Figure 2-3: Parallel Current Paths in Vertically Stratified Soil Conditions

ρhigh

ρlow

R4,e

R1,e

R3,e

R2,e T/R

I2

I1

I3 R2,p

I4

R3,p R4,p

R1,p drain point

Figure 2-4: Parallel Current Paths in Horizontally Stratified Soil Conditions

It is more common than not for soil geology to be stratified both vertically and horizontally and for the current in the low-resistivity soils to be proportionately greater than the high- or moderate-resistivity soils. Furthermore, the stratification need not be caused by different soils but can be due to similar soils with different moisture content. In the vertically stratified soils, the resistance of the current paths is not only a function of soil resistivity but also dependent upon the cross-sectional area of the current path (Equation 2-3). L [2-3] R i,e = ρ s A x,s where:

Ri,e = resistance of the current path (ohm [Ω]) ρs = resistivity of the soil CP Interference © NACE International, 2006 January 2008

DC Interference

2:4

L = length of current path Ax,s = cross-sectional area of soil path From a point source electrode like a CP groundbed, the cross-sectional area of the soil increases exponentially with distance from the electrode. Therefore, the resistance of each current path is not linear with distance from the source. Soil resistivities (ρs) are typically in the range of 103 to 106 Ω-cm whereas metal resistivities (ρm) are in the range of 10-5 to 10-6 Ω-cm. Hence the ratio of metal/soil resistivity can range from: ρm 10 -6 10 -5 = to ρs 10 6 10 3 ρm = 10 -8 to 10 -12 ρs

Put in perspective, for high soil resistivity (e.g., 106 Ω-cm) a metal object in the earth having a cross-sectional area of 100 cm2 or 10-2 m2 is equivalent in resistance to a cross-section of soil that is given by Equation 2-4. ρm = ρs substituting:

ρm = 10 -12 ρs

then:

A x,s =

A x,s =

A x,m A x,s

[2-4]

A x,m 10 -12 10 -2 10 -12

= 1010 m 2

That is, a metal conductor having a 0.01-m2 cross-sectional area is equal to a soil cross-sectional area of 1010 m2 if the soil resistivity is 106 Ω-cm. This means that when a metallic structure is present in the earth, it can be a very attractive current path—thus resulting in a stray current (Is) in the metallic structure as illustrated in Figure 2-5.

CP Interference © NACE International, 2006 January 2008

DC Interference

2:5

metallic structure

Is Is,4

Is,5

Is,3

Is,2

Is

Is,1

R4,e

R1,e

R3,e

R2,e T/R

I2

I1

I3 R2,p

I4

R3,p R4,p

R1,p drain point

Figure 2-5: Stray Current in a Metallic Structure Parallel to a Cathodically Protected Structure

The stray current is picked up on the foreign metallic structure where it is being impacted by the groundbed anodic voltage gradient. If there is no direct electronic path between the foreign structure and the pipeline, then the current will discharge from the metallic structure remote from the pick-up area. The amount of stray current in the metallic structure is a function of the resistance of the stray current paths and the driving voltage left at the location where the foreign metallic structure intersects the anodic voltage gradient. Currents from a single electrode, placed vertically in the earth, produce a voltage drop in the soil near the electrode that forms equipotential surfaces perpendicular to the current paths (Figure 2-6).

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DC Interference

2:6

Va,re

9 8

Vx,re

7 6

Va,x

5 4 3 2 1 0

x

CL

distance

Va,x

Figure 2-6: Voltage vs. Distance from a Vertically Oriented Anode

An equipotential surface has the same voltage difference between the anode and any place on its surface. Projection of each equipotential surface at grade and denoting its voltage and distance produces the voltage drop (Va,x) profile in the earth with distance from the anode, as illustrated. The voltage rise (Vx,re) in the earth with respect to remote earth can be calculated using Equation 2-5. Vx,re =

I ρ s ⎡ ⎛⎜ L + ⎢ln 2πL ⎢ ⎜⎝ ⎣

L2 + x 2 x

⎞⎤ ⎟⎥ ⎟⎥ ⎠⎦

where:

Vx,re = voltage rise in earth with respect to remote earth at a distance “x” from the anode CP Interference © NACE International, 2006 January 2008

[2-5]

DC Interference

2:7

I = anode current output ρs = soil resistivity L = length of anode For example, for a 10-m-long anode in 3000- Ω-cm soil having an output of 10A, the voltage rise at 100 m is: Vx,re =

10A × 30 Ω - m ⎡⎢ ⎛⎜ 10m + ln 2π 10m ⎢ ⎜ ⎣ ⎝

(10m )2 + (100m )2 ⎞⎟⎤⎥ 100m

⎟⎥ ⎠⎦

⎡ ⎛ 10m + 100.5m ⎞⎤ = 4.77 ⎢ln ⎜ ⎟⎥ 100m ⎠⎦ ⎣ ⎝

Vx,re

= 4.77 [ln 1.105] = 4.77 [0.1] = 0.48V

If a metallic structure was present 100m from this anode, it would be subjected to approximately 0.5V between that point and remote earth. This is the driving voltage that would produce a stray current in the structure. Most impressed current groundbeds, however, do not simply comprise a single electrode placed vertically in the earth. Rather, they typically consist of a number of electrodes placed either vertically or horizontally and interconnected by a common header cable ( Figures 2-7a and 2-7b).

Rgb,v s L

Ra,1

d

Ra,2

Ra,3

Ra,n

Figure 2-7a: Multiple Vertical Anodes Connected to a Common Header Cable

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DC Interference

2:8

Rgb,h

t d

CL s

L

Figure 2-7b: Multiple Horizontal Anodes Connected to a Common Header Cable

Calculation of the voltage rise to remote earth becomes more complicated for multiple anode groundbeds. The following procedure, which equates the resistance of the multiple anode groundbed to the resistance of hemispherical electrode, is a method of estimating the resistance remaining to remote earth. The estimate is then multiplied by the current output to obtain the voltage rise to remote earth. The first step is to calculate the resistance to remote earth of the multiple anode array using Sunde’s equation. Rv

=

⎧⎛ 8 L ⎞ ⎫ ρ 2L ln (0.656N) ⎬ ⎟ −1+ ⎨⎜ ln 2 π NL ⎩⎝ d ⎠ s ⎭

[2-6]

where: Rv ρ L d s

= = = = = N =

resistance of multiple vertical anodes to remote earth (Ω) soil resistivity (Ω-cm) length of anode (cm) diameter of anode (cm) anode spacing (cm) number of anodes

Note that this equation is simply Dwight’s equation divided by “N” with a “crowding” correction factor added. Example Calculation: As an example, calculate the resistance of 10 vertical anodes, each anode being a 1.5-m-long high silicon iron anode in a 30-cm. diameter by 2-m-long column of metallurgical coke. The anode spacing is 5 m and the soil resistivity is 6,000 Ω-cm. d = 0.3 m L = 2m S = 5m

CP Interference © NACE International, 2006 January 2008

ρ = 60 Ω-m N = 10

DC Interference

2:9

Therefore: Rn =

60 Ω - m 2 π 2m × 10 anodes

⎧⎛ 8 × 2 m ⎞ ⎫ 2× 2 m ln (0.656 × 10 anodes)⎬ ⎟ −1+ ⎨⎜ ln 5m ⎩⎝ 0.3 m ⎠ ⎭

= 0.478 {(3.98) – 1) + 0.8 (1.88)} = 0.478 {4.484} = 2.14 Ω

This resistance is then equated to an equivalent hemisphere in order to determine the hemisphere radius (r) where the equation for the resistance to remote earth of a hemispherical electrode is given in Equation 2-7 as follows: Rh where:

=

ρ 2πr

[2-7]

ρ = resistivity (Ω-m) r = radius of hemispherical electrode (m) Rh = resistance to remote earth (Ω)

Icp r1 r

Figure 2-8: Hemispherical Electrode

The radius of a hemispherical electrode having an equivalent resistance as the multiple anode groundbed is calculated by rearranging the previous equation.

CP Interference © NACE International, 2006 January 2008

r =

ρ 2π Rh

r =

60 Ω - m 6.28 × 2.14 Ω

[2-8]

= 4.46 m

DC Interference

2:10

Then the equivalent hemisphere has a radius of 4.46 m and the resistance included in the earth to a distance r1 is given by the following equation: =

R

ρ 2π

⎛1 1⎞ ⎜⎜ − ⎟⎟ r1 ⎠ ⎝r

[2-9]

Therefore 100m from the impressed current groundbed will incorporate a resistance of: R 100

=

60 Ω - m ⎛ 1 1 ⎞ − ⎜ ⎟ 6.28 ⎝ 4.46 m 100 m ⎠

= 9.55 (0.2242 – 0.0100) = 9.55 (0.2142) = 2.04 Ω

Then the resistance between a point in the earth 100 m from the center of the hemispherical electrode and remote earth is: R100 →∞ =

2.14 – 2.04 = 0.10 Ω

and the voltage rise per ampere of current put out by the groundbed will be 100 mV/A. Hence, a pipeline located 100 m from the 10 anode groundbed operating at 10A output would be subjected to 1 V with respect to remote earth. How much interference current (Is) is picked up by the pipeline would be a function of the pipeline resistance to earth in the pick-up area (Rs,e), the longitudinal resistance (Rs) of the pipeline between the current pick-up and discharge locations, and the resistance to remote earth (Rs,re) at the discharge location (Figure 2-9).

CP Interference © NACE International, 2006 January 2008

DC Interference

2:11

Ra,re

Icp

Rc,a

Icp '

A Rs,e

Is B Rs

Rc,p

Rs,re

remote earth

Icp Rp,re

where:

Icp

=

Icp '

+ Is

Rc,a & Rc,p = cable resistances Ra,re = anode resistance to remote earth Rp,re = pipe resistance to remote earth

Rs,e = resistance of foreign pipe to earth in a stray current pick-up area

Rs,re = foreign structure resistance to remote earth Rs = longitudinal resistance of foreign structure between pick-up and discharge sites

Figure 2-9: Cathodic Protection Circuit Model with Foreign Structure Intercepting the Anode Gradient

Calculation of the pipe-to-earth resistance can be carried out by a number of methods. For electrically short lengths of bare pipe (i.e., where attenuation is not significant) Equation 2-10 can be used. R s,e ρ L d t

where:

= = = =

=

⎧ (L )2 ⎫ ρ ln ⎨ ⎬ 2πL ⎩ td ⎭

[2-10]

soil resistivity length of pipe diameter of pipe depth below grade

Example Calculation: Assuming a 100 m long, 0.25 m diameter pipeline at 1 m depth in 60 Ω-m soil, the pipe resistance would be: R s,re

=

⎧ (100 )2 ⎫ 60 Ω - m ln ⎨ ⎬ 6.28 × 100 m ⎩1 × 0.25 ⎭

= 0.096 Ω × 10.6 = 1.02 Ω

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DC Interference

2:12

If the pipe is coated, then the voltage rise in the earth at the pipe will appear across the coating. In this case, the resistance of the pipe is the series combination of Equation 2-10 plus the resistance across the coating (Rs,c). To obtain Rs,c the specific coating resistance (r′e) is needed. Accordingly, given a good-quality coating having a specific coating resistance of 5 × 103 Ω-m 2 from Table 2-1 in 1000 Ω-cm soil, the specific coating resistance in 6000 Ω-cm soil (r′c @ 6000 Ω-cm) is then obtained by multiplying the specific coating resistance at 1000 Ω-cm (r′c @ 1000 Ω-cm) by the ratio of the actual soil resistivity divided by 1000 Ω-cm. rc′@ 6,000 Ω-cm = rc′@ 1,000 Ω-cm ×

6,000 Ω - cm 1,000 Ω - cm

= 5 × 103 Ω-m 2 × 6 = 3 × 104 Ω-m 2

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DC Interference

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Table 2-1: Specific Leakage Resistances and Conductances in 1000 Ω-cm Soil or Water Long Pipelines with Few Fittings

Average Specific Coating Conductance g′c

Average Specific Coating Resistance r′c

Quality of Work

Siemens/ft2

Siemens/m2

Ω-ft2

Ω-m2

Excellent

104

Good

1 x 10-5 to 5 x 10-5

1 x 10-4 to 5 x 10-4

2 x 104 to 105

2 x 103 to 104

Fair

5 x 10-5 to 1 x 10-4

5 x 10-4 to 1 x 10-3

104 to 2 x 104

103 to 2 x 103

Poor Bare Pipe (2” to 12”) (5cm to 30cm)

>1 x 10-4

>1 x 10-3

5 x 10-3

104

Excellent

1 x 10-4

>1 x 10-3

1000 Ω - m)

[3-71]

If arcing does not occur, then the fault current flows radially away from the tower footing. This produces a voltage gradient in the soil (Figure 3-78).

IFT VT

Vr r

Figure 3-78: Distribution of Fault Current along Powerline

In this case, the voltage of the earth at the pipeline (Vr) can be estimated by approximating the tower footing as a hemisphere having a radius req and calculating the voltage at a distance r away from the tower (Figure 3-79).

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AC Interference

3:119

req Vr

r

RC’ VP ZG

Remote Earth Figure 3-79: Calculation of Earth Voltage at Pipe Due to Faulted Tower

The equivalent radius of the tower footing is a function of the soil resistivity ρ and the tower-to-earth resistance RT. req =

ρ 2πRT

[3-72]

The voltage of the tower footing is: VT = I FT ⋅ RT = I FT ⋅

ρ 2πreq

[3-73]

Therefore, the voltage at a distance r from the tower footing is: Vr =

req ρ ⋅ I FT = VT 2π r r

[3-74]

Now, due to the resistance of the pipeline coating, the voltage of the pipeline Vp, will be less than the voltage of the earth immediately outside the coating. It will be determined by the following voltage divider (Figure 3-38).

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AC Interference

3:120

V P = Vr

ZG Z G + RC′

[3-75]

where: Vr = voltage of the earth in the vicinity of the pipe ZG = grounding impedance of pipeline Rc′ = modified coating resistance The voltage across the coating is therefore: VC = Vr − V P = Vr − Vr

⎛ ⎞ ZG ZG ⎟⎟ = Vr ⎜⎜1 − + Z G + RC ' Z R G C' ⎠ ⎝

[3-76]

The grounding impedances of a pipeline having a length L for the cases where the fault current is injected into the end of the pipeline, or into the middle of the pipeline, are as follows: ZG =

Z0 ⎛ ΓL ⎞ coth⎜ ⎟ 2 ⎝ 2 ⎠

Z G = Z 0 coth (ΓL)

(center injection)

[3-77]

(end injection)

[3-78]

The resistance presented to the fault current by the coating may be linear, depending on whether localized soil ionization occurs in the immediate vicinity of the coating holidays. The length of pipe affected by the fault current is typically equal to 2r, where r is the distance from the tower to the pipe (Figure 3-80).

r = distance from tower to pipeline

Affected Length of Pipe ≅ 2r

Figure 3-80: Approximate Length of Pipeline Affected by Faulted Tower

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For the case where soil ionization does not occur, the resistance offered by the coating along the affected length of pipe is: RC′ =

rC′ 2π ⋅ r ⋅ D

[3-79]

The resistance of a single holiday (Figure 3-81) having a diameter d would be: RH =

ρ 2d

Coating Holiday

[3-80]

Soil Resistivity: ρ

d

Pipe Wall

Figure 3-81: Resistance of Coating Holiday to Earth

The number of these holidays that might exist along the affected length of pipe would therefore be estimated based on how many holidays would be required in parallel to account for the coating resistance R′C. N=

RH RC′

[3-81]

The high current densities that can arise at the holidays may cause large voltage gradients may occur at the holidays. These gradients may be of sufficiently high magnitude to result in localized soil ionization effects, thereby effectively increasing the diameter of the holiday and decreasing the holiday’s resistance to earth (Figure 3-82).

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Ionized soil in vicinity of holiday as a result of AC fault current pick-up

Breakdown Voltage of Soil: VBD ≅ 1 MV/m to 2 MV/m Soil Resistivity: ρ

Voltage across coating: VC

Pipe Wall

d

Figure 3-82: Modified Resistance of Coating Holiday to Earth Due to Localized Soil Ionization Effects

The modified resistance of a coating holiday is given by: R H′ =

ρ ⋅ V BD 2π ⋅ Vr

[3-82]

where: VBD is the breakdown voltage of soil, which typically may range from 1 MV/m to 2MV/m The resistance of the coating when considering soil ionization therefore becomes: RC′′ =

R H′ N

[3-83]

and the pipeline voltage and the voltage across the coating therefore become: V P = Vr

ZG Z G + RC′′

⎛ ZG VC = Vr ⎜⎜1 − ⎝ Z G + RC′′

CP Interference Course Manual © NACE International, 2006 January 2008

[3-84]

⎞ ⎟⎟ ⎠

[3-85]

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3.7.3 Inductive Coupling Due to Fault Currents During fault conditions, the voltages induced on the pipeline will increase due to the increased current flow in the powerline—as well as to the current imbalance between the three phases. The induced voltages along the pipeline during fault conditions can be determined using the method in Section 3.6; however, the LEF occurring during fault conditions must be recalculated. This requires that the mutual impedance ZM between the faulted powerline and the pipe be determined.

Z M = j ⋅ f ⋅ μ 0 ⋅ 1n

(h − h′ + 2

ρ / j 2πfμ 0

)

2

+d2

(h + h′)2 + d 2

[3-86]

where: f μ0 ρ h h′ d

= = = = = =

frequency (Hz) permeability of free space average soil resistivity (Ω-m) average height of conductor average depth of pipeline mean distance from powerline to pipeline

The mutual impedance is then substituted into the following equation, along with the powerline fault current If, to determine the average electric field strength during fault conditions: E 0 = I fZM

[3-87]

This field strength is then used to determine the peak voltages occurring at the major discontinuities under fault conditions. As an alternative to the method described above, the graph in Figure 3-83 can be used to estimate pipeline voltage rise caused by inductive coupling with a fault current.

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600

10

20

5.6

8

10

30

40

50

70

100 200 300 500

10 11.2 12.5 14.8 18

25

31

40

14

17

20 22.5 26.5 32

45

55

71

18

25

31

36

40

90 100 12.5

32

45

55

64

71

56

80

100 112 125 148 180 250 310 400

100

140 170 200 225 265 320 450 550 710

190

250 310 350 400 475 560 800 1000 1250

500

400

300

47.5 56

85 100 143 175 225

200

100

0

5

10

Pipeline-Powerline Separation (m)

15

Length of Parallelism (km) Figure 3-83: AC Pipeline Voltages Induced by Overhead Faulted Powerline (Per 1000 A of Fault Current)10

3.7.4 Other Related Calculations In addition to calculating the effects of AC interference on a pipeline, other calculations are required. They include the following:

3.7.4(a)

Ground Electrode Resistance

Resistance of a hemispherical electrode having a radius r installed at grade: R=

ρ 2πr

[3-88]

Resistance of a circular plate electrode having a diameter D installed at grade:

10

Electricite de France (EDF) and Gaz De France (GDF). Recommendations for Protection of Steel Pipelines against Electrical Interference, 1967.

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R=

ρ 2D

[3-89]

Resistance of a vertical electrode, having a diameter D and length L, installed at grade: R=

ρ ⎡ ⎛ 8L ⎞ ⎤ ⎢ln⎜ ⎟ − 1⎥ 2πL ⎣ ⎝ D ⎠ ⎦

[3-90]

ρ 4L ln 2πL D

[3-91]

or alternatively: R=

Resistance of a vertical electrode, installed at a depth T: R=

⎛ 2 L 4T + 3L ⎞ ρ ⎟ ln⎜⎜ 2πL ⎝ D 4T + L ⎟⎠

T, L >> D

[3-92]

Resistance of a groundbed consisting of N electrodes separated by a uniform spacing S, each having a resistance R installed in a collinear array. RN =

1 N

⎡ ρ ⎛1 1 1 1 ⎞⎤ ⎢ R + πS ⎜ 2 + 3 + 4 + ... + N ⎟⎥ ⎝ ⎠⎦ ⎣

S≥L

[3-93]

or, when N is large, this simplifies to: RN =

ρ 1⎛ ⎞ ln(0.66 N ) ⎟ ⎜R + N⎝ πS ⎠

S≥L

[3-94]

Resistance of a horizontal electrode, installed at a depth T: R=

or alternatively:

CP Interference Course Manual © NACE International, 2006 January 2008

⎛ L2 ⎞ ρ ⎟ ln⎜⎜ 2πL ⎝ TD ⎟⎠

T, D D

[3-99]

Step and Touch Potential

The voltage at a distance x from the outside of a loop of wire located at grade, having a radius r, discharging a current I into the earth: V ( x) =

CP Interference Course Manual © NACE International, 2006 January 2008

Iρ ⎡ −1 r ⎤ sin x + r ⎥⎦ 2πr ⎢⎣

[3-100]

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Similarly, in a two-layer soil, this becomes: ∞ Iρ1 ⎡ −1 r ⎢sin V ( x) = + 2∑ k n sin −1 x+r 2πr ⎢ n =1 ⎣

3.7.4(c)

2r

(2nh )2 + x 2

+

(2nh )2 + (x + 2r )2

⎤ ⎥ ⎥⎦

[3-101]

Conductor Size

The minimum required conductor size (in circular mils) to prevent fusing during a fault: A = 197 ⋅ I

tc α r ρr ⎛ ⎜ Tm − Ta T CAP 1n ⎜1 + ⎜ 1 − Tr + Ta ⎜ αr ⎝

⎞ ⎟ ⎟ ⎟ ⎟ ⎠

[3-102]

where: tc = fault duration (s)

αr = thermal coefficient of resistivity ρr = conductor resistivity (μΩ-cm)

Tm = maximum allowable temperature (ºC)

Ta = ambient temperature (ºC) Tr = reference temperature (ºC) TCAP = thermal capacity (joules/cc/ºC) I = fault current (A) Note that for copper conductors at 20ºC, this formula simplifies to: A = 6.83I t c

3.8

[3-103]

Equipment for AC Mitigation

3.8.1 DC Decoupling Devices An important component of most AC mitigation systems is the DC decoupling device, which permits the flow of AC but blocks the flow of DC. Consider the case of a motor-operated valve on a pipeline, which must be electrically grounded for operational reasons and to satisfy the local electrical codes. As Figure 3-84 CP Interference Course Manual © NACE International, 2006 January 2008

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shows, electrical ground provides a low-resistance path to drain induced AC currents to earth—thereby lowering the induced AC pipeline voltages; however, it also needlessly picks up CP current intended for the pipeline itself. Therefore, CP current requirements increase, CP potentials decrease (both locally at the valve site and possibly elsewhere along the pipeline), and CP attenuation increases.

M

Induced AC Current

Induced AC Current CP Current

Figure 3-84: Motor Operated Valve – Effects of Grounding on Induced AC and CP Currents

One solution to this problem is to electrically isolate the valve from the pipeline (Figure 3-85). When this is done, a bond must be installed across the valve to maintain electrical continuity along the pipeline for CP purposes and to prevent the generation of an induced AC voltage peak. This approach solves the CP problems, but the pipeline has perhaps lost an important AC mitigation facility and pipeline voltages may consequently increase. The solution also requires that the insulators be above-grade because buried insulators may not be as effective and would be subject to stray current interference. If the valve is below-grade, it would now require its own separate CP system because it is isolated from the pipeline’s CP system. Finally, whenever insulators are installed on a pipeline that is exposed to induced AC interference there exists a risk that the insulators may be damaged as a result of fault currents.

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M

CP Current

Figure 3-85: Electrical Isolation of Motor-operated Valve from Pipeline

Rather than isolate the valve from the pipeline, a preferred solution would be to install a DC decoupling device between the valve and electrical ground. This would provide AC continuity but break the DC current path. Consider the electrical grounding schematic of a motor-operated valve (Figure 386). Installing a DC decoupling device in the ground connection between the electrical service entrance and the valve allows the valve to become isolated from the service entrance ground and secondary grounding system (perhaps a copper loop and some ground rods) and the extensive primary grounding system owned by the power company (consisting of pole grounds, substation grids, connections to watermains, etc).

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Service Entrance

Distribution Transformer Line Line

Fuse

Neutral

Arrestor

DC Decoupler

Motor Operated Valve

Line Primary

Neutral

Secondary

DC Decoupler Primary Ground

Secondary Ground

Service Entrance Ground

Figure 3-86: Electrical Grounding Schematic of Motor Operated Valve Showing Two Alternative Locations for a DC Decoupling Device

As an alternative, the DC decoupling device may also be installed between the primary and secondary grounding systems (Figure 3-87). This has the advantage of being able to isolate several grounded pipeline components in a station with a single DC decoupling device, although the local secondary grounding system would still be connected to the piping and would pick up some CP current.

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Figure 3-87: Decoupling Device Installed by Electrical Utility Between Primary and Secondary Grounds

The most reliable DC decoupling devices, which are also the most commonly used today, are solid-state devices such as the one shown in Figure 3-88. These devices have a very high DC impedance, a very low AC impedance, and can pass steadystate AC currents as well as lightning and fault currents. These devices are selfpowered. Although the internal construction of these devices may vary depending upon the particular manufacturer and the device requirements, the device may include the components shown in Figure 3-89. Steady-state AC current passes through the electrolytic capacitor. AC fault currents pass through the thyristors. The surge protector passes lightning currents. The inductor prevents the lightning currents from damaging the capacitor and thyristors.

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Figure 3-88: Isolation-Surge Protector Installed across Isolating Flange

Electrolytic Capacitor

– + Gate Thyristor

Thyristor

Inductor

Gate

Surge Protector

–

+

Figure 3-89: Electrical Schematic of One Model of Solid-State DC Decoupling Device

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DC decouplers may also be installed directly across insulating flanges to provide protection against lightning and fault current damage. Note, however, that the connections must be kept as short as possible (Figure 3-90) to provide protection against lightning. Otherwise, the voltage drop created across the inductance reactance of the lead wires alone may be enough to break down the insulator.

Figure 3-90: DC Decoupling Device Installed across Insulating Flange for Lightning Protection

Solid-state DC decouplers are reported to have a very low failure rate; should one fail, however, it will fail in the short-circuit mode. This is considered to be the fail-safe mode from an AC safety viewpoint, but it will be detrimental to the CP system. Prior to the advent of solid-state DC decouplers, the polarization cell (Figure 3-91) was used to pass steady-state and fault AC currents—as well as lightning currents—while maintaining DC isolation. The polarization cell consists of a series of nickel or stainless steel plates immersed in an alkaline hydroxide solution, such as the one that appears in Figure 3-92.

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Figure 3-91: AC Current Being Measured Through a Polarization Cell

O

Ho Ho

O O

Ho

O

Ho

O

Ho O KOH

Ho

Figure 3-92: Polarization Cell Construction

Depending upon its construction (the size, number, and spacing of the plates), the polarization cell is capable of carrying fault currents of tens of thousands of amperes. It has a very low AC impedance, which is typically in the 0.1-mΩ range. Initially, the cell also has a low DC resistance; but, as CP current passes through the cell, anodic and cathodic polarization of the plates occurs and a DC backvoltage develops.

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Although the polarization cell and the solid-state DC decoupler perform similar functions, the polarization cell has several disadvantages. Under steady-state AC load the plates tend to depolarize, allowing a significant amount of DC to pass through the cell. This reduces the effectiveness of the DC isolation that the cell is attempting to maintain, thereby reducing the effectiveness of the CP system. Moreover, it results in corrosion of the anodic plates inside the cell. As the plates corrode, the cell’s AC impedance increases. In severe cases, the cell may fail entirely.

Figure 3-93: Corrosion of Plates within a Polarization Cell

Simpler and less costly alternatives exist to the solid-state DC decouplers and polarization cells discussed above; however. these also tend to be less effective. The zinc grounding cell is similar in appearance to a packaged sacrificial zinc anode, except that two zinc electrodes are installed side-by-side inside the anode package and are separated by insulating blocks (Figure 3-94). The lead wires from the zinc electrodes are installed on opposite sides of a pipeline insulator. The lowresistivity anode backfill, when saturated, provides a low-resistance path between the two zinc electrodes—on the order of 0.03 Ω, which provides a reasonably lowimpedance path for AC. As CP current flows from the unprotected side of the flange, through the grounding cell, and back to the protected piping, the zinc electrode polarizes cathodically and thereby limits the amount of DC that can flow through the cell. However, in order to develop a significant back voltage across the insulator of 0.5 V, a substantial amount of DC is required—on the order of 500 mA—that compromises the effectiveness of the insulator. Furthermore, a steadyCP Interference Course Manual © NACE International, 2006 January 2008

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state AC load allows a significant amount of DC to pass through the cell, leading to the premature consumption of the anodic electrode and the eventual open-circuit failure of the cell; this results in an AC safety hazard. Cathodically protected side of insulator

This electrode polarizes cathodically and resists the flow of direct current

Insulating flange

Grounding cell consisting of two - 5 ft. long zinc anodes separated by insulating blocks and surrounded with low resistivity backfill

Figure 3-94: Grounding Cell

Another alternative for passing steady-state AC is the use of an electrolytic capacitor (a component of the solid-state DC decoupler). The capacitor can be connected between the pipeline and a ground electrode, such as a pipeline casing (Figure 3-95).

Figure 3-95: Electrolytic Capacitor

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Because the electrolytic capacitors are polarity-sensitive, the negative terminal must be connected to the more electro-negative structure (i.e., the pipeline). In cases where this polarity may reverse (such as in DC stray current areas) and when the capacitor is carrying AC, capacitors have been known to explode and/or catch fire (Figure 3-96).

Figure 3-96: Failure of Electrolytic Capacitors in Stray Current Area

Capacitors generally fail in short-circuit mode, which is the fail-safe condition from an AC safety viewpoint but which can be detrimental to the CP system. Capacitors are also susceptible to damage from electrical transients and benefit when they are paralleled with surge protectors, such as metal-oxide varistors (Figure 3-97).

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Figure 3-97: Metal-Oxide Varistors (MOVs)

Note that in cases where the pipeline casing is used as an AC ground electrode, the vent pipes (if applicable) will rise to the same AC voltage as the pipeline. Hence precautions must be taken to ensure that the public is protected from exposure to these voltages (e.g., cutting the vent pipes off below-grade). Solid-state devices are also available for installation across insulators to protect the insulator from transients. Such devices may be explosion-proof (Figure 3-98) and will conduct both AC and DC when a predetermined voltage limit is exceeded (e.g., +1V/-2V, +4V/-4V). Note, however, that such devices may not be appropriate for areas where steady-state induced AC voltages are present because any voltage higher than the voltage limit will cause the device to conduct— thereby compromising the effectiveness of the insulator.

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Figure 3-98: Explosion-Proof Surge Protection Device Installed across Insulator

3.8.2 Test Stations When a pipeline is exposed to induced AC voltages, the test leads at CP test stations can potentially expose pipeline personnel—as well as the public—to hazardous voltages. Test station selection is therefore an important consideration when designing an AC mitigation system. Figure 3-99 shows a number of different test station types. A test station in which the test lead terminals are exposed is obviously the poorest choice where induced AC voltages are present. Such a test station would also be a poor choice for CP purposes because the test leads could contact foreign metallic structures such as fences. The vast majority of commercially available test stations include a cover, thereby limiting the chance of contacting the test lead terminals. In some cases these covers can be easily removed, whereas a better choice in an area subject to AC interference would be a cover that incorporates a locking device. Such locking devices are not tamper-proof, however, and should not be relied upon to prevent public access to hazardous voltages. The safest test station choice, from a public safety point of view, is one in which the cover can be padlocked. The terminals inside such a test station should be of dead-front design to prevent accidental contact with AC voltages on the test leads by authorized personnel.

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Figure 3-99: Test Station Varieties (left to right): a) Terminals Exposed to Public; b) Terminals Covered by a Plastic Cap (Locking or Non-Locking); c) Dead-Front Terminals; d) Aluminum Test Station with Padlocked Cover

3.8.3 Sacrificial Anodes An important consideration when selecting anodes for CP and/or AC mitigation on a pipeline subject to steady-state induced AC interference is the effect that the AC current may have on the anode consumption rate. Consumption rates of both zinc and magnesium anodes increase with increasing current density. Sacrificial anodes are generally installed in wettable packages containing special backfill. In some cases, the anode may be installed as a continuous ribbon (Figure 3-100). This ribbon is often installed directly in the pipe trench without special backfill, but in some cases (such as where bicarbonates are present in the soil) the anode’s surface may passivate—causing its resistance to increase and rendering it ineffective as both a sacrificial anode and a ground electrode. Figure 3-101 illustrates the effect of a bicarbonate-rich soil on the potential of zinc and the subsequent reactivation of the zinc’s surface with the addition of calcium sulfate (CaSO4).

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Figure 3-100: a) Zinc Ribbon Anodes of Various Sizes; b) Zinc Ribbon Being Installed in Pipe Trench

-1.10

-1.00

-0.90

-0.80 Original Environment -0.70

600 ppm HCO2 73 ppm NO3 20 ppm CO3-2

-0.50

Room Temperature Saturated CaSO4 Added As Gypsum

-0.50

-0.40 0

20

40

60

80

TIME - DAYS

Figure 3-101: Effect of Gypsum on Restoration of Zinc Potential in Bicarbonate-Rich Soil

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In the case of magnesium anodes, AC causes the potential of magnesium to shift in the electropositive direction (Figure 3-102) and could even become electropositive with respect to the pipeline. This can be prevented by ensuring that the AC current density at the anode’s surface is maintained below 10 A/m2 (1 A/ft2). -1400 1 day 5 days 9 days

-1200

-1000

-800

-600

-400

-200

0

200

0 0

100 155

200 310

300 465

400 620

500 mA/in2 775 A/m2

AC Current Density

Figure 3-102: Potential of Magnesium Versus AC Current Density in a Fe-Mg Cell

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Group Activity – AC Mitigation System Design Introduction: The students shall break into groups, with each group preparing a design for a pipeline AC mitigation system. The students shall be given 60 minutes to prepare their designs, and 30 minutes will be available to present their designs to the class. Required Materials: 2-layer resistivity master curves Transparent log-log graph paper AC mitigation spreadsheet Laptop computer Course text Problem: A pipeline parallels a pipeline for a distance of L = 20 km at a constant separation distance of 15 m, as shown in Figure 1. The powerline has the geometry shown in Figure 2, where the spacing s between conductors is 5 m and the average height h of the conductors is 15 m. The powerline carries a maximum steady-state current of 1000A per phase and has a line-to-ground fault level of 20,000A. The pipeline has a diameter of 500 mm and is buried at a depth of 1.5 m. The pipe is coated with extruded polyethylene, which is considered to be an excellent coating, requiring a CP current density of 0.1 mA/m2. Assume that only 20-pound high-potential magnesium anodes are available for this project and that these are 1.5 m in length and 125 mm in diameter. A 50-m-long insulated wire was laid out along the pipeline route and was grounded at one end. The AC voltage to ground measured at the opposite end was found to be 0.6V. Soil resistivities along the pipeline route are typically 5000 Ω-cm and are generally uniform with depth—except at Location “C” (Figure 1), where a Wenner 4-Pin survey produced the following data:

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Soil Resistivity Data Pin Spacing Resistivity (Ω-cm) 2m 88,000 4m 55,000 6m 29,000 8m 16,000 10 m 10,000

Pipeline A

B

C

L

d

Powerline Figure 1 - Pipeline/Powerline Route

s

s

h

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Figure 2 - Powerlne Geometry

Tasks: 1) Determine the steady-state induced voltages on the pipeline at locations A, B, and C. 2) Design an AC mitigation system to satisfy the objectives as discussed in this course. 3) Determine the results of this mitigation system on induced voltages. 4) Present the findings to the class. Note: In order to produce a variety of solutions, one group should be assigned the case where both insulators are intentionally shorted and one group should be assigned the case where only the insulator at “A” is shorted.

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Summary of Equations C=

[3-1]

Q coulombs/volt V A d

page 3-3

1 2πfC

page 3-4

C=ε

[3-2]

XC =

[3-3]

Amean =

[3-4]

A1 × A2

V pipe

page 3-6

AC1 =

0.2m 2 × 5m 2 = 1m 2

AC 2 =

5m 2 × 20m 2 = 10m 2

V pipe =

[3-5]

page 3-3

C1 V powerline C1 +C s

page 3-7

0.9 × 10 −12 = 100 × 10 3 V = 1000V −12 −12 + 90 × 10 0.9 × 10

[3-6a]

[3-6b]

[3-7]

CP Interference Course Manual © NACE International, 2006 January 2008

VS N = S Vp Np

Vp Np

=

VS NS

v(t ) = Vm cos (ωt + φ)

page 3-19

page 3-19

page 3-24

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[3-8]

ω = 2πf

[3-9]

V = V∠φ

page 3-25

[3-10]

Α∠φ × Β∠θ = Α ⋅ Β∠( φ + θ)

page 3-28

[3-11]

Α∠φ ÷ Β∠θ = Α ÷ Β∠( φ − θ)

page 3-28

[3-12]

A = x + jy

page 3-29

[3-13]

x = ⏐A⏐cosφ

page 3-29

[3-14]

y = ⏐A⏐sinφ

page 3-29

[3-15]

j = −1

page 3-29

[3-16]

j = 1/90º

page 3-29

[3-17]

A/φ × j = A/φ × 1/90º = A/φ + 90º

page 3-29

[3-18]

A/φ × (-j) = A/φ × -1/90º = -A/φ + 90º = A/φ – 90º

page 3-29

[3-19]

A/φ ÷ j = A/φ ÷ 1/90º = A/φ – 90º

page 3-30

XC =

[3-20]

[3-21]

[3-24]

1 j 2πfC

page 3-30

V V = j 2πfCV = 2πfCV∠90° = XC ⎛ 1 ⎞ ⎟⎟ ⎜⎜ ⎝ j 2πfC ⎠

IC =

X L = j 2πfL

[3-22]

[3-23]

page 3-24

IL =

Vφ −φ

=

CP Interference Course Manual © NACE International, 2006 January 2008

page 3-32

V V V = = ∠ − 90° XL j 2πfL 2πfL 1.5 2 + (−.866) ⋅ Vφ −G 2

page 3-30

=

3 Vφ −G

page 3-32

page 3-34

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VO, L = ±

[3-25]

[3-27]

IB =

[3-28]

IB =

0.157 ts 0.116 ts

R =

page 3-49

2

VO, L = ±

[3-26]

[3-29]

E ⋅L

E

page 3-54

Γ

(70 kg body)

page 3-62

(50 kg body)

page 3-62

ρ 2D

page 3-65

[3-30]

R2Fp = 1.5ρ

page 3-65

[3-31]

R2Fss = 6ρ

page 3-65

[3-32]

V = R×I

page 3-65

[3-33]

V = ( Rbody + Rfeet) × Ibody

page 3-65

0.116

[3-34]

Vstep50 = (1000 + 6ρ)

[3-35]

Vtouch50 = (1000 + 1.5ρ)

[3-36]

Vstep70 = (1000 + 6ρ)

[3-37]

Vtouch70 = (1000 + 1.5ρ)

[3-38a]

CP Interference Course Manual © NACE International, 2006 January 2008

tS 0.116 tS

0.157 tS

Vss = Iss × Rbody

0.157 tS

page 3-65

page 3-65

page 3-66

page 3-66

page 3-67

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iAC =

[3-38b]

8VAC ρπd

page 3-74

[3-39a]

r = 0.08 I f ⋅ ρ

( ρ < 100 Ω - m)

page 3-93

[3-39b]

r = 0.047 I f ⋅ ρ

( ρ > 1000 Ω - m)

page 3-93

V AC L

page 3-98

V I

page 3-99

[3-42]

r′c= R ⋅ A

page 3-102

[3-43]

r′cc = ρ ⋅ L / A

page 3-103

[3-40]

LEF =

[3-41]

ρ a = 2πa

Rc ( Ω ) =

[3-44]

rc′ (Ω ⋅ m 2 ) rc′ = 2 πDL A pipe (m )

page 3-104

[3-45]

Gc =

πDL 1 = Rc rc′

page 3-104

[3-46]

gc =

Gc πD = L rc′

page 3-104

[3-47]

Zi =

⎧ [sinh (t n ) + sin (t n )] + j [sinh (t n ) − sin (t n )]⎫ ⎬ (2π)(0.0127 D ) ⎨⎩ cosh (t n ) − cos(t n ) ⎭ 0.5ωμ s ρ s

[3-48]

[3-49]

CP Interference Course Manual © NACE International, 2006 January 2008

tn =

0.036t ωμ s ρs

page 3-105

page 3-105

page 3-105

AC Interference

3:150

⎡ 1.12 ⎤ ⎡ ⎤ 1n ⎢ ⎥ j 1.85 ωμ 2 1 ′ ⎥ ⎢ 0 Γa Γ⎢ + •1n ⎥ = Zi + −1 2 − 1 ⎢ a′ Γ + jωμ ρ + jωε ⎥ 2π )⎦ ⎢Yi π (ρ + jωε)⎥ 0( ⎣ ⎣ ⎦ a′ =

[3-50]

[3-52]

Z0

[3-53]

α = ⏐Γ⏐cos(∠Γ) ⎡ 1.12 ⎤ 1n ⎢1 Γ a′ ⎥ = Γ⎢ + ⎥ −1 ⎢Yi π (ρ + jωε)⎥ ⎣ ⎦

page 3-106 page 3-106

page 3-108

E3φ = IA ⋅ ZMA + IB ⋅ ZMB + IC ⋅ ZMC + IS1 ⋅ ZMS1 + IS2 ⋅ ZMS2

Z M = j ⋅ f ⋅ μ 0 ⋅ 1n

[3-56]

(h − h′ + 2

ρ / j 2πfμ 0

) +d 2

page 3-108

2

page 3-108

(h + h′)2 + d 2

E = E1 + E2 + E3 + … + EN

[3-54]

Vx =

page 3-106

E = I φ ⋅ ZM

[3-54]

[3-58]

page 3-106

α = Re[Γ]

[3-51]

[3-55]

0.25 D 2 + 4h 2

{[

]

[

page 3-109

]

page 3-111

E0 Z 2 (Z1 − Z 0 ) − Z1 (Z 2 + Z 0 )e ΓL e −Γx − Z1 (Z 2 − Z 0 ) − Z 2 (Z1 + Z 0 )eΓL e Γ( x−L ) Γ (Z1 + Z 0 )(Z 2 + Z 0 )eΓL − (Z1 − Z 0 )(Z 2 − Z 0 )e −ΓL

[

[3-59]

[3-60]

CP Interference Course Manual © NACE International, 2006 January 2008

ZG = Z0 coth (ΓL) Y AT =

N RA

]

}

page 3-112

page 3-112

AC Interference

3:151

Y AT N 1 = = L L ⋅ RA S ⋅ RA

[3-61]

Y M = Yi +

[3-62]

page 3-113

1 S ⋅ RA

page 3-113

N

V = P0 ∑ E Si ni e − jαλ i

[3-63]

page 3-113

i =1

P0 =

[3-64]

1 2Γ

page 3-113

[3-65]

ESi = ELi – ERi

page 3-113

[3-66]

ni = 10 kαλ i

page 3-114

[3-67]

e − jαλi = cos(−αλi ) + j sin( −αλi )

page 3-114

[3-68]

I FT =

V L −G ⎛ Z ⋅Z Z p + ⎜⎜ S T ⎝ Z S + ZT ZP =

[3-69]

[3-70]

r = 0.08 I f ρ

[3-71]

r = 0.047 I f ρ

⎛ ZS ⋅ ⎜⎜ ⎞ ⎝ Z S + ZT ⎟⎟ ⎠

V L −G IF

page 3-116

page 3-116

(for ρ < 100 Ω - m)

page 3-117

(for ρ > 1000 Ω - m)

page 3-117

ρ 2πRT

[3-72]

req =

[3-73]

VT = I FT ⋅ RT = I FT ⋅

CP Interference Course Manual © NACE International, 2006 January 2008

⎞ ⎟⎟ ⎠

page 3-118

ρ 2πreq

page 3-118

AC Interference

3:152

Vr =

[3-74]

V P = Vr

[3-75]

[3-76]

[3-77]

[3-78]

req ρ ⋅ I FT = VT 2πr r

VC = Vr − V P = Vr − Vr

ZG =

ZG Z G + RC′

[3-80]

[3-81]

[3-82]

[3-83]

[3-84]

[3-85]

CP Interference Course Manual © NACE International, 2006 January 2008

page 3-119

⎛ ⎞ ZG ZG ⎟⎟ = Vr ⎜⎜1 − + Z G + RC ' Z R G C' ⎠ ⎝

Z0 ⎛ ΓL ⎞ coth⎜ ⎟ 2 ⎝ 2 ⎠

page 3-119

(end injection)

page 3-119

rC′ 2π ⋅ r ⋅ D ρ RH = 2d

RC′ =

page 3-120

page 3-120

ρ ⋅ V BD 2π ⋅ Vr

page 3-121

R H′ N

page 3-121

RC′′ =

V P = Vr

page 3-120

RH RC′

N=

R H′ =

page 3-119

(centre injection)

Z G = Z 0 coth (ΓL)

[3-79]

page 3-118

ZG Z G + RC′′

⎛ ZG VC = Vr ⎜⎜1 − ⎝ Z G + RC′′

page 3-121

⎞ ⎟⎟ ⎠

page 3-121

AC Interference

[3-86]

3:153

Z M = j ⋅ f ⋅ μ 0 ⋅ 1n

(h − h′ + 2

ρ / j 2πfμ 0

)

2

+d2

(h + h′)2 + d 2 E 0 = I f ZM

[3-87]

page 3-122

page 3-122

[3-88]

R=

ρ 2πr

page 3-123

[3-89]

R=

ρ 2D

page 3-124

ρ ⎡ ⎛ 8L ⎞ ⎤ ⎢ln⎜ ⎟ −1⎥ 2πL ⎣ ⎝ D ⎠ ⎦

page 3-124

ρ 4L ln 2πL D

page 3-124

R=

[3-90]

R=

[3-91]

[3-92]

[3-93]

[3-94]

R= RN =

⎛ 2 L 4T + 3L ⎞ ρ ⎟ ln⎜⎜ 2πL ⎝ D 4T + L ⎟⎠

1 ⎞⎤ ρ ⎛1 1 1 1 ⎡ R+ ⎜ + + + ... + ⎟⎥ ⎢ N⎣ πS ⎝ 2 3 4 N ⎠⎦

RN =

[3-97]

ρ 1⎛ ⎞ ln(0.66 N ) ⎟ ⎜R + N⎝ πS ⎠

⎛ L2 ⎞ ρ ⎟ ln⎜ R= 2πL ⎜⎝ TD ⎟⎠

[3-95]

[3-96]

T, L >> D

R=

S≥L

T, D D

∞ Iρ1 ⎡ −1 r ⎢sin + 2∑ k n sin −1 2πr ⎢ x+r n =1 ⎣

A = 197 ⋅ I

[3-103]

CP Interference Course Manual © NACE International, 2006 January 2008

2r

(2nh )2 + x 2

+

(2nh )2 + (x + 2r )2

tc α r ρr ⎛ ⎜ Tm − Ta T CAP 1n ⎜1 + ⎜ 1 − Tr + Ta ⎜ αr ⎝

A = 6.83I t c

⎞ ⎟ ⎟ ⎟ ⎟ ⎠

⎤ ⎥ ⎥⎦

page 3-126

page 3-126

CHAPTER 4 TELLURIC CURRENT INTERFERENCE

4.1 Background Theory Telluric currents are currents that are geomagnetically induced in the earth and in metallic structures on the earth—such as powerlines and pipelines—as a result of the interaction of solar particles on the earth’s magnetic field (Figure 4-1). Here the earth’s magnetic filed is compressed on the sun side of the earth and stretched on the dusk side. The solar plasma arises from two solar phenomena: sun spot activity and corona mass ejections (CME), which are commonly referred to as solar flares. The geomagnetic storms that result from the interaction of the solar plasma with the earth’s magnetic field cause currents to be induced in the earth and metallic structures on the earth.

Figure 4-1: Interaction of Solar Particles on the Earth’s Magnetic Field Source: Place, Trevor and Sneath, T. Owen, Practical Telluric Compensation for Pipeline Close-Interval Surveys, NACE Corrosion 2000, Paper No. 741, Orlando, Florida, March 2001 (Powerpoint Presentation) (MP, Vol. 40(9), 2001 p.22

Charged solar particles, composed mostly of electrons and protons that enter the earth’s atmosphere, are deflected by the earth’s magnetic field. This creates current rings in the ionosphere centered around the north and south poles as well as at the equator (figures 4-2a and 4-2b). Electrons are deflected in one direction and protons are deflected in the opposite direction around the earth. This creates a current as the earth’s magnetic field narrows on the dark side of the earth. CP Interference Course Manual © NACE International, 2006 January 2008

Telluric Current Interference

Figure 4-2a: Plasma Charge Distribution around the Earth during Quiescent Period Source: Lerner, Eric J., Storms and Hurricanes Don’t Leave Off Where the Atmosphere Ends. Space, Discover, August 1995, p.60

Figure 4-2b: Plasma Charge Distribution around the Earth during a Magnetic Storm Source: Lerner, Eric J., Storms and Hurricanes Don’t Leave Off Where the Atmosphere Ends. Space, Discover, August 1995, p.60

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4:2

Telluric Current Interference

4:3

The current ring in the auroral regions forms an oval as shown in Figure 4-3. This “electrojet,” as it is sometimes called, typically contains more electrical charges than are generated by man on earth.

Figure 4-3: This plot shows the extent and position of the auroral oval in the northern hemisphere, extrapolated from measurements taken during the most recent polar pass of the NOAA POES satellite for September 16, 2004 at 14:22 UT. Source: http://www.sel.noaa.gov/pmap/pmapN.html - 9/16/2004

Because of the amplitude variation and directional changes in this electrojet current, a changing magnetic field is produced that induces an electric field in the earth and in any metallic conductor on or in the earth’s surface (Figure 4-4). Varying Magnetic Field

Figure 4-4: Schematic of Geomagnetic Induction Directly into a Pipeline and the Resulting Change in Pipeline Potential that is Produced Source: Boteler, D.H., Gummow, R.A. and Rix, B.C., Evaluation of Telluric Current Effects on the Maritimes and Northeast Pipeline, NACE Northern Area Eastern Conference, Ottawa, October 1999, Paper No. 8A.3, p. 8

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:4

Measured Pipe-to-Soil Potential

Calculated Electric Field

Measured Magnetic Field

The effect of the changing magnetic field caused by the electrojet, which creates both a changing electric field and a corresponding change in the pipe-to-soil potential on a pipeline, is shown in Figure 4-5.

Figure 4-5: Quiet Day Variation in the Geomagnetic Field and the Associated Change in the Electric Field and the Pipe-to-Soil Potential Source: Trichtchenko, L. et al, The Production of Telluric Current Effects in Norway, NACE Corrosion 2001, Houston, TX, March 2001, Paper No. 314

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Telluric Current Interference

4:5

The longer variations in these three parameters are a result of the night/day effect of the earth’s rotation. The short variations (i.e., minutes to hours) are a result of the variation of solar particles interacting with the earth’s magnetic field. It should be noted that this data was obtained during a quiet geomagnetic period on a pipeline in Norway located at approximately 60 degrees geogmagnetic latitude. For a long pipeline subjected to an induced electric field, the induced voltage and current profile is typically as shown in Figure 4-6. 10

15 E = 1 V/km

5 10 0 5 -5 -10

0

20

40

60

80

0 100

Distance (km) Figure 4-6: P/S Potential and Telluric Current in a Long Pipeline Exposed to an Induced Electric Field of 1 V/km, having an Impedance of 0.1 Ω /km and an Admittance of 0.15 Ω /km Source: Boteler, D.H. and Seager, W.H., Telluric Currents: A Meeting of Theory and Observation, NACE Canadian Region, Western Conference, Edmonton, Alberta, Feb. 1997.

This figure shows that for a long coated pipeline ungrounded at the ends, and subjected to an electric field of 1 V/km, the induced voltage reaches a peak at the end points and decreases with distance from either end toward the center and the voltage reaches zero in the middle of the pipeline. Note that the voltage to earth (pipe-to-soil potential) at each end is out of phase (i.e., when one end is positive, the opposite end is negative). Conversely, the current induced into the pipe is near zero at each end of the ungrounded pipe but reaches a maximum in the middle. This produces the counter-intuitive result that where the voltage peaks are greatest the current in the pipe is the least.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:6

4.1.1 Distributed Source Transmission Line Equations The induced voltage profile can be calculated using distributed source transmission line (DSTL) equations, as has been shown by Boteler and Cookson[1] using an electrical model of the pipeline shown in Figure 4-7. E

R

L

Vind

G

C

Figure 4-7: Equivalent Circuit for a Short Section of Pipeline

Each short section of pipeline is represented by a series impedance Z where Z = R + jwL, and a parallel admittance Y where Y = G + jwC and an induced electric field E represented by a voltage source. The response of the pipeline depends on the propagation constant γ and characteristic impedance Z0 given by: γ = Z0 =

ZY

[4-1a]

Z Y

[4-1b]

The voltage and current along the pipeline are then given by: dV dx

dI dx

1

= E − 1Z

[4-2]

= − VY

[4-3]

Boteler, D.H. and Cookson, M.J., Telluric Currents and Their Effects on Pipelines in the Cook Strait Region of New Zealand, Materials Performance, Vol. 25(3), March. 1986, p.27-32.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:7

Differentiation and substitution leads to the equations: d2V − γ 2V = dx 2 d 2I dx 2

dE dx

[4-4]

− γ 2 I = − YE

[4-5]

If the disturbance is uniform along the pipe, then the electric field does not vary with distance (i.e.,

dE = 0 ) and these equations then have solutions of the form: dx I =

E γE 0

V =

( 1 + Ae E γ

( Ae

- γ ( x − x1 )

-γ ( x − x1 )

+ Be - γ ( x2 − x1 )

− Be -γ ( x2 − x1 )

)

)

[4-6]

[4-7]

where A and B are constants determined by the conditions at the ends of the pipeline. For a long pipeline, of length L, terminated at ends 1 and 2 by impedances to ground Z1 and Z2 respectively, this becomes I =

V V E − 1 e - γx − 2 e - γ ( L − x ) Z0 Z0 Z

V(x) = − V1e - γx + V2 e - γ ( L − x )

[4-8]

[4-9]

where: V1

=

Z1 E × γ Z 0 + Z1

and

V2

=

Z2 E × γ Z0 + Z2

[4-10]

Reviewing these calculations and Equation 4-10, it is apparent that the magnitude of the induced voltage that appears at each end of the pipeline (i.e., V1 and V2) is directly proportional to the induced electric field (E), inversely proportional to the propagation constant (γ) and the characteristic impedance (Z0), and dependent on the relative impedances to ground, Z1 and Z2 at each end of the pipe.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:8

In practical terms the variables can be listed as follows into two categories—those factors that affect the induced electric field (E) and those that affect the longitudinal impedance (Z) and the shunt admittance (Y).

4.1.2 Factors that Affect the Induced Electric Field (E) The value of the induced electric field (E) is a function of the following factors and events: 4.1.2(a)

Solar Cycle Variations

The solar cycle produces peaks of solar activity at approximately 11-year intervals. These periods correspond to a change in the location of the north and south magnetic poles of the sun. This periodicity of solar activity is illustrated in Figure 4-8, which is a history of geomagnetic effects over the last 150 years. The intensity of sunspot activity on average appears to be increasing with time. The next peak should be expected from approximately 2011 to 2013, whereas a general quiescent period should be expected from 2005 to 2007. Geomagnetic Effects 100

Sunspot Number

80 150 60

100 40

50

Magnetic Disturbances

200

20

0

0 1860

1880

1900

1920 Year

1940

1960

1980

2000

Figure 4-8: History of Geomagnetic Effects on Ground Technology Courtesy of D.H. Boteler, Geological Survey of Canada, Geomagnetic Laboratory, Ottawa, Ontario, Canada

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4.1.2(b)

4:9

Sun’s Rotational Frequency

The sun’s rotational frequency of approximately 27 days will produce a variation in the solar plasma because sunspots and solar flares are not uniformly distributed over the sun’s surface. 4.1.2(c)

Earth’s Rotation

The earth’s rotation means that a metallic structure will experience the geomagnetic difference between the sun and night side of the earth. Voltage changes that have a repeating 24-hour variation are often called “diurnal” fluctuations. Diurnal fluctuations in pipe-to-soil potential are evident in Figure 49. Note that in this case the most electropositive potentials occur at midday.

-2000

-1500

-1000

-500

12:00

0:00

12:00

0:00

12:00

0:00

12:00

0:00

0 12:00

Pipe Potential wrt CSE (mV)

-2500

Time (Atlantic)

Figure 4-9: Pipe-to-Soil Potential Variations with Time

4.1.2(d)

Plasma Magnetic Field Direction

The direction of the plasma magnetic field has a significant impact on the magnitude of the induced electric field. When the solar particles leave the sun, the sun’s magnetic field at the point of emission is frozen in the plasma blob. When the plasma magnetic field is directed southward (against the earth’s magnetic field), then geomagnetic substorms produce a larger electric field. However, when the plasma magnetic field is northward, there are no significant changes in the induced electric field. Hence the impact of a corona mass ejection

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:10

on the induced electric field is very much a function of the alignment of its magnetic field relative to the earth’s magnetic field. 4.1.2(e)

Proximity of Pipeline to a Sea Coast

The proximity of the pipeline to a sea coast also introduces a potential change on a pipeline. As illustrated in Figure 4-10, a voltage gradient exists between the low-resistivity seawater and the higher-resistivity land. This is due to charge accumulation because of the larger induced currents in the sea compared to the land, which increases the electrical potential of the earth near the coast. This effect is also true on land at sudden transitions between high- and low-resistivity soils. Land

Sea

Earth Surface Potential

Figure 4-10: Charge accumulation at sea coast resulting from larger induced currents in the sea compared to in the land. The charge accumulation increases the electrical potential of the earth’s surface near the coast. Source: Boteler, D.H., Gummow, R.A. and Rix, B., Evaluation of Telluric Current Effects on the Maritimes and Northeast Pipeline, NACE Northern Area Eastern Conference and Exhibition, Ottawa, ON, October 1999, p.11.

Tidal activity can also generate ocean currents due to the Hall effect as illustrated in Figure 4-11.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:11

Magnetic Field

v

Sea

Land

E

Earth Surface Potential

Figure 4-11: Electric Field, E, generated by seawater moving with velocity, v, through the earth’s magnetic field, B Source: Boteler, D.H., Gummow, R.A. and Rix, B., Evaluation of Telluric Current Effects on the Maritimes and Northeast Pipeline, NACE Northern Area Eastern Conference and Exhibition, Ottawa, ON, October 1999, p.12.

As the water moves with a velocity (v) in a perpendicular magnetic field, positive and negative charges are forced in opposite directions perpendicular to the tidal direction. The potential difference created by this tidal dynamo can be approximated by the following equation. E = VBZW where:

Ε v BZ W

= = = =

[4-11]

the potential difference the water velocity the vertical component of the magnetic field the width of the water channel

Using this equation and assuming that the vertical component of the magnetic field was approximately 50×10-6 Tesla, a potential difference (E) of 52V was calculated for the Bay of Fundy—where some of the largest tides in the world occur.[2]

2

Boteler, D.H., Gummow, R.A. and Rix, B., Evaluation of Telluric Current Effects on the Maritimes and Northeast Pipeline, NACE Northern Area Eastern Conference and Exhibition, Ottawa, Ontario, October 1999, p.11.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4.1.2(f)

4:12

Pipeline Latitude

The location of the pipeline relative to the earth’s magnetic poles has a major impact on the magnitude of the induced electric field. Figure 4-12 illustrates the probability of a geomagnetic peak at about 0.2% at mid-latitude and decreases toward the north pole and the equator.

Figure 4-12: Geomagnetic Hazard Percentage of Probability of Occurrence Source: Molinski, Tom, Geomagnetically Induced Currents: - Causes, Effect, and Mitigation, IEEE Canadian Review – Fall 1996, p.13

The peak probability coincides with the general location of the auroral electrojet shown in Figure 4-3. Furthermore Figure 4-2b illustrates an electrojet located at the equator. There have been reports of telluric activity on pipelines located near the equator in Panama.[3]

3

Soto, Gonzalo, Control de Corrosion en El Oleoducto de Panama, El VIII Seminario Latinamericano de Corrosion y Electroquimica, 1985, City of Panama.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:13

4.1.3 Factors that Affect the Pipeline Lineal Impedance (Z) and Shunt Admittance (Y) Besides the geophysical factors that can affect the electric field magnitude, pipeline factors such as the lineal impedance (Z) and the shunt admittance (Y) also affect the induced voltage. Both the propagation constant (γ) and the characteristic impedance (Z0) of a pipeline, as indicated in Equations 4-1a and 41b, are dependent on these parameters. Small values of lineal impedance or small shunt admittance result in a small propagation constant that produces a more linear relationship between induced voltage and distance (Figure 4-13).

large γ (electrically long) 0

0 small

γ (electrically short)

Figure 4-13: Telluric Induced Voltage Profile vs Distance for a Pipeline with Different Attenuation Constants

A pipeline with a large propagation constant is considered electrically lossy or long and a pipeline with a small propagation constant is considered to be electrically “short.” 4.1.3(a)

Effect of Coating Quality

The shunt admittance of a coated pipeline is primarily a function of the coating quality and, to a lesser extent, the soil resistivity. Figure 4-14 shows that as the coating conductance increases, the voltage induced on a long pipeline decreases.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:14

25

E = 0.1 V/km 20

Coating = 1 microS/sq.m.

15

10 Coating = 10 microS/sq.m. 5 Coating = 100 microS/sq.m. 0

200

400

600

800

1000

Pipeline Length (km)

Figure 4-14: Calculated Telluric Induced Voltage at the End of a Long Pipeline as a Function of Coating Conductance for an East-West Electric Field of 0.1V/km Source: Boteler, D.H., Evaluation of Telluric Current Effects on the Maritimes and Northeast Pipeline, Geomagnetic Laboratory, Geological Survey of Canada, Ottawa, Aug. 1998, p.8.

A high-quality coating, although important for cathodic protection (CP) effectiveness, results in higher induced voltages because the induced current cannot easily leak to earth. 4.1.3(b)

Effect of Isolating Fittings

Isolating fittings in a pipeline produce a voltage peak on each side of the electrical isolation that are 180 degrees out of phase. Multiple isolating fittings therefore create multiple peaks, albeit with lesser voltage differences across the isolation (Figure 4-15).

0

0

1 isolating fitting in middle 3 isolating fittings no isolation

Figure 4-15: Effect of Isolating Fittings on the Telluric Induced Voltage Profile on an Electrically Short Pipeline

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:15

Note that telluric currents are alternating. Therefore, the polarity will change periodically and the voltage appearing across the isolator will be greater than the voltage-to-earth on either side of the isolator. 4.1.3(c)

Effect of Pipeline Directional Change

A change in pipe direction has the same impact on the telluric induced voltage (Vt) as it does with the induced AC voltage profile where the pipe crosses or leaves the powerline right-of-way. Because of the electromagnetic discontinuity created by the direction change, a voltage peak is created as illustrated in Figure 4-16 for an electrically long pipeline. Vt

pipeline bend

Figure 4-16: Effect of Pipeline Directional Change on the Telluric Induced Voltage Profile

The same effect as illustrated in Figure 4-16 will also occur at a sudden change in earth conductivity (e.g., clay/rock). But these earth conductivity changes are more difficult to predetermine than directional changes. In both cases, however, the induced voltage is also dependent on the direction of the induced electric field.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4.2

4:16

Measuring the Geomagnetic Intensity and Determining the Electric Field (E)

Geomagnetic activity is continuously monitored at observation posts around the world that record the geomagnetic disturbances caused by the solar wind. In North America, geomagnetic activity information can be obtained at www.geolab.nrcan.gc.ca in Canada and www.sel.noaa.gov in the U.S. The magnetic variations are recorded by magnetometers in units of nanoteslas (nT). There are a number of indexes that have been created to express the geomagnetic activity. For pipelines the Kp index is the most useful. The Kp index is an arithmetic average based on three-hour intervals. This index is logarithmic and spans from 0 (quiet) to 9 (severe), where activity greater than Kp 4 is considered a geomagnetic storm. The probability of a geomagnetic storm decreases logarithmically as the Kp index increases (Figure 4-17). 10- 0

2

10-1

2

10- 2

2 -3

10

2 -4

10

0

1

2

3

5

4

6

7

8

9

Kp

Figure 4-17: Average Occurrence of 3-Hour Intervals with the Magnetic Activity Index Kp Equal to or Greater than a Specified Value. Kp=9 Corresponds to a Severe Magnetic Storm Source: Boteler, D.H. and Rix, B., Telluric Current Considerations in the CP Design for the Maritimes and Northeast Pipeline, NACE Corrosion 2001, Houston, TX, March 2001, Paper No. 317.

This figure shows that small disturbances occur frequently but the most severe storms are very infrequent. A Kp 6 storm, which is likely to occur 2 percent of the time, is considered significant because it relates to an average electric field magnitude of 100mV/km as shown in Figure 4-18.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:17

1000

100

10

1 0

1

2

3

5

4

6

7

8

9

Kp

Figure 4-18: Peak Electric Field Magnitudes as a Function of Kp Source: Boteler, D.H. and Rix, B., Telluric Current Considerations in the CP Design for the Maritimes and Northeast Pipeline, NACE Corrosion 2001, Houston, TX, March 2001, Paper No. 317.

Note that within a three-hour period for a Kp 6 storm, the electric field ranged from approximately 30 to 300 mV/km. The foregoing data were used in estimating the effect on a 762 mm diameter 1000-km pipeline running from Goldboro, Nova Scotia, through to New England at subauroral latitudes. For pipelines in other geographical locations, a similar plot could be obtained from the appropriate geomagnetic laboratory. This is necessary to produce an accurate prediction of tellurically induced voltages using the DSTL model.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4.3

4:18

Interference Effects of Telluric Current on Pipelines

4.3.1 General Considerations The impact of these geomagnetically induced currents has historically been considered more of a nuisance when measuring CP parameters than a serious corrosion concern. However, there are three main areas of concern regarding the effects of geomagnetically induced currents on coated pipelines: 1.

Corrosion during the positive half-cycles of the telluric waveform.

2.

Accuracy of pipeline current and potential measurements when determining the level of CP for comparison with industry criteria.

3.

Coating damaged caused by excessively negative potentials during the negative half-cycles of the telluric waveform.

4.3.2 4.3.2(a)

Corrosion Theoretical Considerations

During the time when telluric current transfers from the pipe to earth (positive portion of the telluric cycle), the charges must transfer through an oxidation reaction. For a steel pipe without CP, the primary oxidation reaction is corrosion of the steel (Figure 4-19) and as expressed by the following reaction: Fe° ⇒ Fe++ + 2e- (corrosion) Grade

it

Feo = Fe++ + 2eFigure 4-19: Oxidation Reaction at Pipe Surface During Telluric Current Discharge in the Absence of CP

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Theoretically, approximately 10kg of steel will be lost in 1 year for every ampere of continuous direct current (DC) that discharges. When a pipeline is being cathodically protected or is receiving telluric current (Figure 4-20), the charge transfer reactions can be one or both of the following depending on the soil conditions; H 3 O + + e-

or

2H2O + O2 + 4e-

⇒ H0 + H20 (in deaerated or acidic soils)

[4-13]

⇒ 4OH- (in alkaline or neutral aerated soils)

[4-14]

Figure 4:20: Reduction Reactions During Negative Cycle Telluric and CP Current Pick-up

Both these reduction reactions produce a high-pH environment, typically in the range of 10 to13, at coating flaws (holidays). The magnitude of the pH has been shown to be proportional to the logarithm of the current density[4] as shown in Figure 4-21.

4

Thompson, N.G., Barlo, T. J., Fundamental Process of Cathodically Protecting Steel Pipelines, 1983 International Gas Research Conference, p.279.

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14

13

12

11

10

9

8 10-9

10-7

10-8

10- 6

10- 5

10-4

C.P. Current Density, A/cm2

Figure 4-21: Steel Surface pH versus Applied CP Current Density

When positive charges transfer from a surface that has been cathodically protected, the initial oxidation reaction is therefore likely to result in the formation of a passive film (Figure 4-22).[5] Here it can be seen that, as the steel becomes progressively more cathodically polarized, the anodic polarization curve exhibits progressively more passive behavior. -0.5 -0.6

-0.7

Before polarization anodic polarization

-0.8 -200 mV mV -200 -0.9 -1

-400 mV

-1.1

-1.2 -1.3 0.0001

0.001

0.01

0.1

1

Current Density (mA/cm2 )

Figure 4-22: Polarization Curves after Several Days of Potentiostatic Polarization Source: Hesjevik, S.M. and Birketveit, O., Telluric Current on Short Gas Pipelines in Norway – Risk of Corrosion on Buried Gas Pipelines, NACE Corrosion 2001, Houston, TX, Paper No. 313, p16. (Figure redrawn) 5

Hesjevik, S.M. and Birketveit, O., Telluric Current on Short Gas Pipelines in Norway – Risk of Corrosion on Buried Gas Pipelines, NACE Corrosion 2001, Houston, TX, Paper No. 313, p16.

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Where the pipeline has been cathodically protected for a long period of time and the pH at the pipe/soil interface is high, the initial potential/current density relationship should be similar to that shown in Figure 4-23. Here, for a pH of 12, one can see that, as the potential moves from approximately –950mVsce, the current increases up to the primary passive potential of approximately –850mVsce, after which it diminishes as the potential moves through the passive range to the start of the transpassive region at approximately +600mVsce. Because the corrosion reaction is one that produces a passive film, then the initial corrosion rate (i.e., the current density) resulting from this anodic excursion would be low. +700

+500

-200

-400

-600 pH 12.0 -800

-1000

-1200 1

10

100

1,000

10,000

Current Density, Microamps/cm2

Figure 4-23: Experimental Anodic Polarization Curve of Steel in Hydroxide (pH 12.0) Source: Thompson, N.G., Lawson, K.M., and Beavers, J.A., “Exploring the Complexity of the Mechanism of Cathodic Protection”, Corrosion ’94, Paper No. 580, NACE International, 1994, p.11. (Figure redrawn)

If the telluric current discharge is sustained but the residual pH remains high, then the oxidation reaction could be expressed by Equation 4-15, the oxidation of hydroxyl ions, or by Equation 4-16, the hydrolysis of water (Figure 4-24); neither of these equations results in metal loss. 4OH- ⇒ 2H2O + O2↑ + 4e+

2 H2O ⇒ O2↑ + 4H + 4e

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-

[4-15] [4-16]

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Grade icp it 4OH2H2O

H2O + O2 + 4eO2 + 4H+ + 4e-

Figure 4-24: Telluric Current Discharge from a Cathodically Protected Pipe

Accordingly, the total corrosion that occurs at a coating defect as a result of current discharge is not strictly proportional to the charge transferred as would be predicted by Faraday’s Law for a steady-state DC. 4.3.2(b)

Calculating the Corrosion Rate

Cyclic variations in telluric current of equal amplitude and period will corrode steel less than a steady state DC of the same magnitude applied for the same time period, as previously discovered in a National Bureau of Standards (NBS) investigation[6] and as illustrated in Figure 4-25.

6

McCollum, B., Ahlborn, G.H., Influence of Frequency of Alternating or Infrequently Reversed Current on Electrolytic Corrosion, National Bureau of Standards Tech Paper No. 72, 1916.

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100 90 LEGEND: Soil Soil + Na2CO3

80 70 60 50 40 30 20 10 0 -10

1/60S 1/15S

1S

5S

1M 5M 10M 1Hr.

2Days 2Weeks

D.C.

Logarithm of Length of Time of One Cycle

Figure 4-25: Coefficient of Corrosion at Different Frequencies for Iron Electrodes Denoted as Average Electrode Loss

This study, which was commissioned to determine the relative corrosivity of stray currents arising from DC transit systems, has some merit with respect to telluric stray currents because the periods of activity are somewhat similar. In fact Campbell[7] produced the following mathematical relationship using the NBS findings to estimate the corrosion as a function of the telluric current cycle for a fixed amplitude: C = ( 4.7 ± 1.3) T+0.186

where:

[4-17]

C is percent of DC corrosion that would occur at the same amplitude T is the period of the current cycle in seconds

Peabody[8], as shown in Figure 4-26, also summarized the NBS findings in a different graphical representation that demonstrates a relationship between the logarithm of the period and the logarithm of the percentage of corrosion compared to an equal amount of DC.

7

Campbell, W.H., Induction of Auroral Zone Electric Currents Within the Alaska Pipeline, Pure and Applied Geophysics, Vol. 116, No.6, 1978, p.1167. 8 Peabody, A.W., Corrosion Aspects of Arctic Pipelines, Materials Performance, Vol. 18, No.5, May 1979, p.30.

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Time Interval in Hours Between Current Reversals

100

10

2 1 0.5

0.1

0.01

0.001 5

10

22

29

50

100

Percentage of Direct Current Corrosion Rate

Figure 4-26: Effect on Corrosion Rate of Reversing Direction of Current Compared To Steady State DC and Length of Time Between Reversals Source: Peabody, A.W., Corrosion Aspects of Arctic Pipelines, Materials Performance, Vol. 18, No.5, May 1979, p.30. (Figure redrawn)

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Although telluric frequencies cover a wide spectrum, the induced electric field peaks typically at periods between 30 minutes to 2 hours.[9] This corresponds to corrosion activity that would be approximately 22 to 29% of an equivalent DC. It should be noted, however, that diurnal telluric activity—which is typically less intense than the shorter telluric fluctuations—would produce a corrosion rate of approximately 50% of an equivalent DC because it would have a 12-hour period. The amount of stray telluric current produced during the positive period depends on the intensity of the telluric disturbances. On very well-coated modern pipelines, current transfer between the pipe and soil occurs primarily at small coating defects. Relatively small potential fluctuations in the order of 0.5 to 1.0V can produce a large current density as shown in Figure 4-27.[10] Here, for a 1-cmdiameter circular holiday in a 0.3-mm thick coating, which is a typical thickness for fusion bonded epoxy coatings, the current density for a soil resistivity of 1000 Ω-cm and a telluric voltage change of 1.0V, would be approximately 2500µA/cm2 and produce a corrosion rate of approximately 31.3mm/y.

Corrosion Current Density (µA/cm2)

100,000

t

t=0 t = 0.3mm 10,000

t = 1mm

Pipe Wall

t = 3mm

1µA/cm2 = 0.0125mm/a (Fe)

d

Soil

2,500 t = 10mm

1,000

100 0.1

1

10

100

1000

Defect diameter (mm) Figure 4-27: Corrosion Current Density at a Coating Defect having an Applied Voltage of 1.0V in 1000 ohm-cm Soil for Various Coating Thicknesses Source: Von Baeckmann, W., Schwenk, W., Handbook of Cathodic Protection, Portcullis Press, England, 1975, p.365. (Figure redrawn) 9

Campbell, W.H. and Zimmerman, J.E., Induced Electric Currents in the Alaska Oil Pipeline Resulting from Auroral Electrojet Current Sources, Geophysical Journal of the Royal Astronomical Society, Vol. 61, No.2, p.1164. 10 Von Baeckmann, W., Schwenk, W., Handbook of Cathodic Protection, Portcullis Press, England, 1975, p.365.

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The corrosion rate arising from Figure 4-26 for a 1-cm-diameter defect with a steady state voltage of +1V applied in 1,000 Ω-cm soil can be expressed as follows: CR = Ki y P

where:

Ki = corrosion current density factor (2.5 x 10-3 A/cm2 per volt) P = corrosion penetration factor (12.5 x 10-3 mm/y per 10-6 A/cm2) CR = corrosion rate (mm/yr)

To calculate the theoretical corrosion rate caused by telluric voltage fluctuations, modify the corrosion rate formula to account for the cyclic variations in the telluric wave form (Fp), the duration of time that the activity is present (Ft), and the magnitude of the telluric voltage (ΔVt ) as follows: CRtelluric = Ki y P y ΔVt y F(p) y F(t) ΔVt = change in potential of the pipe caused by telluric activity F(p) = fraction of steady state corrosion due to alternating period of the telluric current F(t) = fraction of time that telluric activity is present

As would be expected, the corrosion rate for a given potential change (ΔVt) varies with the soil resistivity and the anodic transient time as illustrated in Figure 4-28. (A) (B)

100

10 Clay soil 1

0.1 Sandy soil 0.1

1 10 Anodic Transient Time (min)

100

Figure 4-28: Chart Showing the Influence of Anodic Transient Time with Respect to Corrosion Experienced by Probe in Sandy and Clay Soil. Line (A) Represents the Corrosion Rate Expected from Faraday’s Law for the Clay Soil, and Line (B) for the Sandy Soil, Respectively. Source: Birbilis, N., Holloway, L.J. and Forsyth, M., Technical Note: Simulated Transient Loss of Cathodic Protection for Buried Pipelines, Corrosion, Vol. 61, No.5, May 2005, p.500. (Figure redrawn)

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This figure[11] shows the results of laboratory tests conducted on resistance probes placed in clay (4,000 Ω-cm) and sand (50,000 Ω-cm) soil. The probes were cathodically protected to –1000mVssc and subjected to an anodic transient to 0mVssc for 20% of the time for 1-minute, 10-minute, and 60-minute periods. Up to a 10-minute anodic period, the corrosion rate is less than 1% of the theoretical value based on the anodic current. It must be expected, however, that if the CP potential was less negative than –1000mVssc (IR drop free), then the corrosion rate would be greater than 1% of the theoretical value. 4.3.2(c)

Telluric Corrosion Case Studies on Cathodically Protected Piping

Although corrosion of steel pipelines due to telluric current activity is theoretically probable, it has not been considered by the pipeline industry as a serious threat to the integrity of cathodically protected pipelines. This view was probably a result of the findings of a study conducted by the American Gas Association on four pipelines in the U.S. between 1966 and 1970. Their investigation concluded that the effects are insignificant, both for coated, protected lines and for bare lines.[12] This study, however, focused on pipelines that were relatively short, located at latitudes lower than 46 degrees N, on relatively poorly coated pipelines, and during a period of relatively quiet telluric activity. Subsequent findings by other investigators[13,14] on existing, cathodically protected pipelines located in auroral zones using coupons showed that corrosion was mild but not insignificant. From a study on Norwegian pipelines, Henricksen, et al concluded that telluric current corrosion in auroral zones is approximately the same magnitude as normal soil corrosion where telluric corrosion is lacking.

11

Birbilis, N., Holloway, L.J. and Forsyth, M., Technical Note: Simulated Transient Loss of Cathodic Protection for Buried Pipelines, Corrosion, Vol. 61, No.5, May 2005, p.500. 12 Gideon, D.N. et al., Earth Current Effects on Buried Pipelines – Analysis of Observations of Telluric Gradients and their Effects, AGA Project PR-3-41, April 1970, p.71. 13 Henriksen, J.F. et al., Telluric Current Corrosion on Buried Pipelines, Proceedings of the 8th Scandinavian Corrosion Congress, NKM8, Helsinki, Vol. II, 1978, p.167-176. 14 Martin, B.A., Telluric Effects on a Buried Pipeline, Corrosion, Vol. 49(4), 1993, p.349.

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Martin[15], following a telluric corrosion study on a 515-km gas pipeline in northeastern Australia, reported corrosion rates in excess of 0.01 mm/y (4 mpy). A more serious case of tellurically caused corrosion was discovered in 2001 on a 24-in Ø natural gas pipeline east of Montreal, Quebec.[16] This fusion-bonded epoxy (FBE)-coated pipeline installed in 1998 was found to have a 60-mil pit at a subcriterion location identified during a close-interval potential survey; the results of the survey appear in Figure 4-29. The pipe-to-soil potential fluctuations at this location were later correlated with the magnetic field variation (Figure 4-30). 1800 1600 1400

Potential (-mVcse)

1200 1000

OFF ON crit

800 600 400 200 0 107000 -200

108000

109000

110000

111000

112000

113000

114000

115000

Kilometers

Figure 4-29: Potentials Measured with Rectifiers ‘ON’ and ‘OFF’ Source: Brochu, B., Telluric Current Effects on Underground Steel Pipelines, NACE Northern Area – Eastern Conference, Quebec City, August 2001

15 16

Ibid [14] p.349. Brochu, B., Telluric Current Effects on Underground Steel Pipelines, NACE Northern Area – Eastern Conference, Quebec City, August 2001.

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44000

Pipe-to-soil potentials 43900

43800

43700

43600

Ey Geomagnetic Field 43500 1 2 :0 0 :0 0

1 2 :2 8 :4 8

1 2 :5 7 :3 6

1 3 :2 6 :2 4

1 3 :5 5 :1 2

1 4 :2 4 :0 0

1 4 :5 2 :4 8

1 5 :2 1 :3 6

Figure 4-30: Magnetic Field Intensity and Pipe-to-Soil Potential Superimposed Source: Brochu, B., Telluric Current Effects on Underground Steel Pipelines, NACE Northern Area Eastern Conference, Quebec City, August 2001.

The high corrosion rate of approximately 15 mils/y was unlikely because of the high soil resistivity. Further investigation indicated that the pipeline at the corrosion location was situated above a low-resistivity (100 Ω-cm) graphite schist that extended northeastward for more than 100 km toward the Gulf of St. Lawrence. This soil anomaly was thought to provide a relatively low-resistance path between the pipeline and the Atlantic Ocean.

4.3.3

Impact on Accuracy of Current and Potential Measurements

In the absence of a stray current on a cathodically protected structure, a pipe-tosoil potential measured using a high-input resistance voltmeter will be the sum of the polarized potential (Ep) that appears across the pipe-to-earth interface and the voltage drop (Ve) in the earth due to the CP current (Icp) through the earth path resistance (Re) Equation 4-17. Vm = Ep + Ve where:

[4-18]

Ve = Icp • Re

The polarized potential (Ep) must be equal to or more electronegative than –850 mVcse in order to satisfy the NACE –850 mV criterion. It is usual on impressed

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current systems to momentarily interrupt the CP current (Icp = 0) so that an instant-off potential (Ei-off), measured the moment of interruption, is a reasonably accurate representation of the polarized potential (Ep). If a telluric current is present, the pipe-to-soil potential difference measurement will incorporate an additional voltage drop (Vt) owing to the telluric current in the soil path between the reference electrode and the pipe as represented by equation 4-19 and as illustrated in Figure 4-31. Vm = Ecp + Ve ± Vt

Test Station

[4-19]

Voltmeter

V Portable Reference Electrode

Grade

Icp Ve Pipe Test Lead

Vt

It

Ep

Figure 4-31: Schematic of Potentially Controlled CP System Used to Mitigate Telluric Current Effects Source: Gummow, R.A., Telluric Current Effects on Corrosion and Corrosion Control Systems on Pipelines in Cold Climates, NACE Northern Area Western Region Conference, Alaska, Feb. 2001, Paper CldCli01, p.12.

Because telluric current is alternating, the error can make the pipe appear either better protected or more poorly protected depending on its direction and change the polarized potential if the telluric current is sustained with time. Unlike an impressed current, the telluric current cannot be arbitrarily interrupted, which then compromises the accuracy of a pipe-to-soil potential measurement.

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4.3.4 Impact of Telluric Current on Pipeline Coatings NACE SP0169[17] cautions that “the use of excessive polarized potentials on externally coated pipelines should be avoided to minimize cathodic disbondment of the coating.” This precaution is typically being interpreted as a maximum polarized potential equal to –1200mVcse. During geomagnetic storms as illustrated by the calculated instant-off potential in Figure 4-32, –1200 mVcse potentials can easily result on well-coated pipelines during periods of telluric current pick-up. 5.5

2.0

5.0

1.5 Eon

1.0

4.5

Calculated Eoff 0.5

Current Density

4.0 3.5

0.0 -0.5

3.0

-1.0

2.5

-1.5

2.0

-2.0

1.5

-2.5

1.0

-3.0

0.5

-3.5

0.0

-4.0

-0.5

-4.5

-1.0

-5.0

-1.5

-5.5 03:50

03:55

04:00

04:05

04:10

04:15

04:20

04:25

04:30

04:35

04:40

-2.0 04:45

Time

Figure 4-32: Current Flow and Calculated OFF Potentials during a GIC Incident Source: Hesjevik, S.M. and Birketveit, O., Telluric Current on Short Gas Pipelines in Norway – Risk of Corrosion on Buried Gas Pipelines, NACE Corrosion 2001, Paper #01313. (Figure redrawn)

Cathodic disbondment and cathodic blistering both result from water migration through the coating due to electro-osmosis. Generally, the thicker the coating is and the better its moisture transmission resistance, then the less susceptible it is to 17

NACE Standard SP0169 – Control of External Corrosion on Underground or Submerged Metallic Piping Systems, NACE International. NACE International publishes three classes of standards: standard practices, standard material requirements, and standard test methods. Until June 23, 2006, NACE published standard recommended practices, but the designation of this type of standard was changed to simply standard practice. New standards published after that date will carry the new designation (SP), and existing standards will be changed as they are revised or reaffirmed.

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these effects. Accordingly, FBE, being a thin film coating, is particularly vulnerable to moisture transmission and consequential cathodic blistering at locations of poor coating adhesion and cathodic disbondment as a result of the high pH developed in the blister.

4.3.5 Impact on Output of a CP Transformer/Rectifier An impressed current transformer/rectifier will pass a telluric current through the rectifier element to its groundbed if the telluric current is in a discharge cycle, as illustrated in Figure 4-33.

It

Icp It Icp

groundbed

pipeline

Figure 4-33: Telluric Current Through a Bridge Rectifying Element During a Discharge Cycle

This will also be true for a center-tapped transformer/rectifier. When operating in constant voltage mode, the total output current (Io) of the transformer/rectifier will increase: that is:

Io = Icp + It

[4-20]

This is because the voltage difference (Vo) between the pipe and the groundbed is the sum of the superimposed telluric voltage and the transformer/rectifier output voltage (VTR): that is: Vo = VTR + Vt [4-21]

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But if the transformer/rectifier is operating on constant current, the CP current (Icp) will drop when the telluric current (It) is present: Icp = Io – It

that is:

[4-22]

This is undesirable because an increase rather than a decrease in the transformer/rectifier output would be preferred at the time of a telluric current discharge. Hence a transformer/rectifier should not be operated in constant current when the pipeline is subjected to telluric current activity.

4.4

Mitigating the Effects of Telluric Current

4.4.1 Mitigating Corrosion Impact 4.4.1(a)

Making the Pipeline Electrically Continuous and Grounded

Telluric voltages on pipelines arise from electromagnetic induction and are therefore analogous to induced alternating current (AC) voltages. Similarly, grounding the pipeline can be an effective method of mitigating telluric voltages just as it is with AC voltages. Telluric voltages, which appear across an insulated flange, can be reduced by electrically bonding around the isolating joint. As with AC mitigation, however, the bond must be designed to maintain the performance of the CP system. A telluric bond switch (Figure 4-34) has been used[18] to pass telluric current across an insulator separating onshore and offshore portions of a cathodically protected pipeline.

18

Boteler, D.H., Gummow, R.A., and Rix, B.C., Evaluation of Telluric Current Effects on the Maritimes and Northeast Pipeline, NACE International Northern Area Eastern Conference, Ottawa, Paper No. 8A, 3, October 24, 1999.

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MOV

DUAL DIODE

To Offshore Pipeline

Auto Resetting Fuse

Variable Resistor

Shunt

Shunt To Onshore Pipeline

By-pass Switch

Figure 4-34: Schematic of a Telluric Bond Switch

Back-to-back diodes provide a fault path for the large telluric currents once the breakover voltage of the diodes (typically 0.8V) has been breached. These diodes are therefore rated to handle the largest expected telluric current typically arising from a once-per-year severe storm (i.e., Kp 9 on Figure 4-18). Adjustment of the variable resistor allows for a steady-state drain of current to balance the CP systems between the onshore and offshore sections of the pipeline. Lightning protection is provided by the metal oxide varistor. It is also possible to mitigate telluric effects by connecting the pipeline to electrical ground using AC coupling-DC isolating devices such as isolating surge protectors and polarization cells. Grouped galvanic anodes connected to each side of the isolating fitting can also be used but the anode capacity must be chosen to provide a reasonable life and with enough current output to compensate for any residual telluric current discharging from the pipeline. 4.4.1(b)

Using CP

CP systems can be designed and operated to mitigate telluric voltage fluctuations by a combination of two related mechanisms. Impressed current output can be increased to compensate for a telluric current discharge, or galvanic anodes can provide a grounding path for the telluric current to pass to earth. The capacity to perform these functions varies with the type and operating characteristics of the CP system relative to the operating characteristics of the pipeline system and the magnitude of the telluric activity.

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Sacrificial Anodes

Sacrificial CP systems have a limited voltage capacity to compensate for a telluric potential shift because they have a relatively small fixed output voltage. They do, however, offer an alternative path to earth for the telluric current (It) because of their low resistance to earth compared to a coated pipeline. Some proportion of the telluric current (Itl) will transfer to earth via the anode (Figure 4-35), depending primarily on the anode-to-earth resistance compared to the pipe-toearth resistance—both locally and looking down the pipe in the direction of the current. If the CP current (Icp) is equal to or greater than the residual telluric discharge current (Itll), then stray current corrosion will not occur on the pipe under the influence of the anode. I't''

It I't'

Icp

galvanic anode

Icp + I't I't where:

residual telluric current discharge

telluric current discharge from galvanic anode

It = I't + I''t + I''' t

Figure 4-35: Mitigation of Telluric Current Discharge Effects using Galvanic Anodes

This CP method, which makes the pipeline electrically lossy, has been used on the Trans-Alaska pipeline[19] in the form of a zinc ribbon anode that was placed at pipe invert elevation on each side of the pipe for the full extent of the underground portion of the pipeline. Grouping of zinc and magnesium sacrificial anodes at selected intervals has also been shown to be effective by Henriksen, et al.[20] when used on a pipeline in northern Norway, where the telluric potential fluctuations were reduced from ± 5 V to ± 0.1 V (Figure 4-36). Just as with induced AC mitigation, the more electrically lossy a pipeline is, the lesser the magnitude of the telluric voltage fluctuations.

19 20

Martin, B.A., Telluric Effects on a Buried Pipeline, Corrosion, Vol. 49 (4), 1993, p.343-350. Henriksen, J.F., Elvik, R. and Granasen, L., Telluric Current Corrosion on Buried Pipelines, Proceedings of 8th Scandinavian Corrosion Congress, Tehory andPraxis at Corroisons Prevention, Volume II, p.167-176, Helsinki, 1978.

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mV, Cu/CuSO4 Grounding

-3000 OUT

IN

OUT

IN

OUT

IN

IN

OUT

-2000 -1000 0 +1000 +2000 2100 5/10-74

2300

0100 6/10-74

0300

0500

0700

0900

Time

Figure 4-36: Effect of Connecting and Disconnecting Groups of Galvanic Anodes to a Pipeline Subjected to Telluric Current

For instance, if a 0.5-m-diameter coated pipeline has a conductance of 10-6 S/m2 in 10,000 Ω-cm soil (a reasonable expectation for modern coatings) then it has a conductance per 100m of 0.157 x 10-3 S. Example Calculation:

Consider a coating having a specific leakage conductance (G) of 10-6 S/m2 in 10,000 Ω-cm soil. For 100m of 0.5-m-diameter pipe, the leakage conductance (g) would be: g pipe = G 10,000 × A P = 10 S/m × 157 m -6

2

g pipe = 1.57 × 10 - 4 S

AP = πdl 2

= 3.14 × 0.5 m × 100 m = 157 m 2

Assume a packaged magnesium anode (9.1 kg × 1.52m lg) is attached to the piping for each 100m length. The conductance (g) of the anode to earth in 10,000 Ω-cm soil is given by the following equation, which is the reciprocal of the anode resistance as calculated by Dwight’s equation for a vertical electrode:

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g anode =

where: L = 1.52 m d = 0.12 m ρ = 100 Ω-m

1 R anode

=

2πL 1 × 8L ρ -1 ln d

=

6.28 × 1.52 m × 100 Ω - m

[4-23]

1 12.16 -1 ln 0.12

1 = 0.0263 S 3.61 = 26.3 × 10 -3 S

= 0.095 × g anode

The net conductance (gn) for a 100m of pipe with the anode attached is therefore: g n = g anode + g pipe = 26.3 × 10 -3 S + 0.157 × 10 -3 S = 26.5 × 10 -3 S

This is an increase in conductance of 167 times, which is well over two orders of magnitude. As Figure 4-37 shows, an increase in conductance of this order can significantly reduce the magnitude of telluric induced voltage (i.e., 90% reduction).

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Figure 4-37: Effect of Increased Coating Conductance on the Voltage on Each Side of Isolated Flanges

There is some belief that the telluric current seen on pipelines results primarily from current transfer (conductance) between the pipe and earth rather than from inductance directly. If this were the case, then magnesium anodes would be preferred over zinc anodes because they would not pick-up current until the pipeto-earth potential exceeded their open-circuit potential (approximately – 1.750Vcse). In contrast, zinc would accept telluric current when a potential of – 1.100Vcse was exceeded. Magnesium anodes would therefore lessen the amount of current pick-up and provide more CP current compared to zinc. There may also be net CP benefit with the use of sacrificial anodes in the presence of a telluric current. Results from an experiment[21] that applied a signal simulating a telluric wave form to a combination of a steel pipe and a zinc ribbon found that there was a net pick-up of the alternating signal on the pipe. Conversely, there was a net increase in the amount of current discharged from the 21

Unpublished results from research to determine the potential and current effects on a steel pipe/zinc ribbon couple, CORRENG Consulting Service Inc., Downsview, ON, Canada 1993.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:39

anode. This may be due to the fact that the anode does not pick-up the AC until its open-circuit potential is exceeded and the pipe does not discharge current until the anode potential is polarized electropositively to the pipe polarized potential. 4.4.1(b)(ii) Impressed Current Systems Impressed current CP (ICCP) systems can theoretically be designed with unlimited voltage capacity, although it is inefficient to continuously operate the system at higher voltages just to provide a buffer for the anticipated telluric positive voltage shift. Moreover, the very high negative potentials produced, as a result of operating ICCP systems at high current outputs, can cause cathodic disbondment of the coating. Martin[22] found that operating rectifiers in constant voltage or constant current mode had “little mitigative effect” because they caused “overprotection during local negative transients and underprotection during local positive transients”. There have been reports[23,24] that telluric voltage fluctuations are more pronounced near rectifier locations than between them. There is no doubt that anodic telluric currents will pass to earth through the rectifying element in the transformer-rectifier as discussed in Section 4.3.5. When operating in constant current mode (where Io is kept constant), CP current will be reduced by the amount of the telluric current through the rectifier— thereby diminishing the amount of CP available to counteract the residual telluric current discharge from the pipe. Hence, it would seem that—from a telluric current mitigation point of view—impressed current systems should not be operated in constant current mode.

22

Martin, B.A., Telluric Effects on a Buried Pipeline, Corrosion, Vol. 49 (4), 1993, p.345. Proctor, T.G., Experience with Telluric Current Interference in the Cathodic Protection of a Buried Pipeline in New Zealand, NACE, Corrosion /74, Paper No. 57, p11. 24 Private communication with Ian Munro, Corrosion Service Co. Ltd., Feb. 2001. 23

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:40

Martin[25] and other operators[26,27] have used the potential control mode to successfully ameliorate telluric currents even though Proctor[28] concluded that “the value of constant potential impressed current power sources in compensating for telluric current interference is questionable”. The voltage and current output of these units change automatically in response to the pipe potential as measured to a local reference electrode, as illustrated schematically in Figure 4-38. Potentially Controlled DC Power Supply -

S

R

+

Icp and It

Icp and It Permanent Reference Electrode/Coupon

Remote Groundbed

Figure 4-38: Schematic of Potentially Controlled CP System used to Mitigate Telluric Current Effects

Here the coupon potential is measured continuously with respect to the permanent reference electrode and compared to a pre-set potential in the controller of the DC power supply. When a telluric current attempts to discharge from the pipe/coupon, the reference senses the positive potential shift and the power supply immediately increases its output to maintain the set potential value. The impressed current system therefore presents a negative resistance path for the telluric current to earth and thus there is no residual discharge of telluric current from the pipe as long as the voltage or current output of the power supply is within its rating. A coupon is used to minimize IR drop between the reference electrode and the nearest holiday so that the rectifier can control to a potential that has minimal IR drop component. 25

Martin, B.A., Telluric Effects on a Buried Pipeline, Corrosion, Vol. 49 (4), 1993, p.345.. Peabody, A.W., Corrosion Aspects of Arctic Pipelines, Materials Performance, Vol. 18 (5), May 1979. 27 Degerstedt, R.M., Kennelley, K.J., Lara, P.F., and Moghissi, O.C., Acquiring “Telluric-nulled” Pipe-tosoil Potentials on the Trans Alaska Pipeline, Corrosion ’95, Paper No. 345, NACE International. 28 Proctor, T.G., Pipeline Telluric Current Difference as one Phase of a Wider Interdisciplinary Technological Problem, NACE, Corrosion /74, Paper No.60, p.16. 26

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:41

600

6

500

5

400

4

300

3

200

2

100

1

0

Pipe Potential wrt ZRE (mV)

-100

0 1

2

3

4

5

6

7

-1

-200

-2

-300

-3

-400

-4

-500

-5

-600

-6

-700

-7

-800

-8

-900

-9

-1000

-10 Day

Figure 4-39: Pipe Potential and Rectifier Current Output vs Time for an Impressed Current System Operating in Potential Control

Note that, in this example, the rectifier operates only when the pipe potential attempts to go more electropositive than the set potential of –100 mV/ZRE (approximately –1200 mV/cse). Telluric current is drained to earth during periods of telluric current discharge. During periods of telluric current pick-up, the current output goes to zero and thus limits the magnitude of the negative potential applied across the coating. This mode of operation effectively eliminates the positive telluric voltage fluctuations in the vicinity of the rectifier while minimizing excessively negative potentials and maximizing the life of the groundbed. This technique works most effectively when the transformer/rectifier and groundbed are located at peak locations of telluric current activity.

CP Interference © NACE International, 2006 January 2008

Rectifier Current (A)

The power supply voltage and current capacity must be sized to provide the needed CP current in addition to the amount of telluric current to be drained. This type of CP system functions as a telluric current “forced drainage” system. Its mitigating effect is illustrated in Figure 4-39, which compares typical rectifier current output and pipe potential over time.

Telluric Current Interference

4:42

4.4.2 Compensating for Measurement Error Caused by Telluric Current Because geomagnetically induced current cannot be arbitrarily interrupted, an alternative pipe-to-soil potential measurement method has been employed by some companies[29,30]. The method uses a small steel coupon installed next to the pipe, which is interconnected with the pipe inside a test station. The coupon simulates the pipe/soil surface at a defect in the coating. When the coupon is temporarily disconnected and the reference electrode is placed in the soil tube (Figure 4-40), both the telluric and CP voltage drops in the earth are removed from the measured potential difference and the “instant off” potential (Ep) of the coupon is measured for comparison to the –850 mVcse criterion. Test Station

Switch

Voltmeter V

Grade

Pipe Test Lead

Portable Reference Electrode

Non-metallic Tube filled with Sand/ Bentonite Mixture

Steel Coupon

Figure 4-40: Typical Pipe-to-Soil Potential Measurement at Test Station having a Steel Coupon and Soil Tube

29

Stears, C.D., Moghissi, O.C., Degerstedt, R.M., and Bone, L., Field Program on the Use of Coupons to Monitor Cathodic Protection of an Underground Pipeline, Corrosion ’97, Paper No. 564, NACE International, Houston, TX, 1997. 30 Greenwood, R., The Effects of Transient Stray Current on Cathodically Protected Pipelines, British Gas Engineering Research Report, July 1986, p4-6.

CP Interference © NACE International, 2006 January 2008

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This test arrangement is not suitable for recording the polarized potential with time, however, because the coupon has to be disconnected for each measurement. The use of a reference/coupon combination, as illustrated in Figure 4-41, has proved to be an excellent method of recording a polarized potential with time. The coupon in this device does not require disconnection because the reference is located inside the pipe coupon, where there is neither CP nor telluric voltage gradient.

Test Station

VR Recording Voltmeter

Grade

Pipe test lead

Coupon test lead Zinc Reference test lead

Coupon/Reference Probe

Figure 4-41: Typical Pipe-to-Soil Potential Recording at a Test Station Using a Coupon/Reference Probe

Figure 4-42 compares the pipe/coupon potential recorded to a CSE reference placed on grade and the reference located inside the coupon. The difference between the potential values is the soil voltage gradient caused by both the telluric and CP currents. Note that, despite the significant potential fluctuations in the potential measurement using a surface copper-copper sulfate electrode, the actual potential at the coupon/soil interface is relatively stable with time.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:44

-3000

-2500

Potential (mVCSE)

-2000 Potential wrt portable reference on Grade

-1500

-1000 Potential wrt Coupon Reference

-500 *For convenience, the readings were converted in mVCSE using a zinc potential of -1100 mVCSE 0 9:00

11:24

13:48

16:12

18:36

21:00

23:24

Time

Figure 4-42: Comparison between Pipe/Coupon Potential with Time recorded with respect to a Copper-Copper Sulfate Reference on Grade and to a Coupon/Reference Probe Located at Pipe Depth

Although the use of a coupon is a relatively simple solution at a test station, the measurement of telluric free potentials is more complex for close interval potential surveys (CIPS) where the reference is moved and placed over the pipe at intervals (typically < 3 m) along the route of the pipeline. Proctor[31] proposed a measurement method to compensate for the telluric induced voltage that involved the correction of the measured potential (Vm) with respect to the moving reference by the change in potential (ΔVf) measured with respect to a fixed reference located at a nearby test station such that where:

31

Vps = Vm ± ΔVf

[4-24]

ΔVf = Vfave ± Vf

[4-25]

Proctor, T.G., Experience with Telluric Current Interference in the Cathodic Protection of Buried Pipeline in New Zealand, Materials Performance, Vol. 13, No. 6, 1974, p29.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

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This measurement technique is illustrated in Figure 4-43 in which two separate data loggers are used to record the potentials with respect to the fixed and moving electrodes.

Roving Datalogger Synchronized Fixed or Moving Datalogger

Vm

Vf

Moving Reference

Fixed Reference

Survey Length

Figure 4-43: Pipe-to-Soil Potential Measurement Method to Compensate for Telluric Current Effects During a Close Interval CP Survey

This technique can be used with synchronous interruption of the rectifiers such that a telluric compensated “instant off” potential can be calculated in software from the recorded data. The accuracy of this technique depends on whether the average potential (Vfave) truly represents an average potential unaffected by telluric current and on the proximity of the fixed location to the moving electrode because long separation distances can introduce errors caused by potential differences in the earth parallel to the pipe route and to telluric current voltage drop in the pipe. Place and Sneath[32] have used a variation of the foregoing technique in combination with CP current interruption to produce close interval survey data that is telluric-compensated. Their test arrangement (Figure 4-44) uses two stationary data loggers, one at the start of the CIPS (Vrs) and one at the end of the survey span (Vrf). 32

Place, T., and Sneath, O., Practical Telluric Compensation for Pipelines, Proceedings, NACE Northern Area Western Conference, Saskatoon, Feb. 2000.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:46

Vrf

Vm

y

Vrs

x

Figure 4-44: CIPS Method using One Moving and Two Stationary Data Loggers

All data loggers are synchronized by referencing the global positioning system (GPS). The telluric compensation is a linear extrapolation of the telluric shift at each data logger relative to the moving reference’s proximity to each stationary reference. This correction routine, done in software, is expressed as follows; Vps = Vm ± ΔVrs • Where:

y x ± ΔVrf • x+y x+y

[4-26]

the ΔVrs and ΔVrf are the differences in potential compared to the average potential [Vrfave and Vrsave] recorded at each location over a period of time prior to the survey.

This technique tends to minimize the error inherent in the previous method when the distance between the moving reference and the single stationary data logger increases significantly. Both techniques assume that the telluric voltage amplitude is linear over the relatively short distances surveyed and that pipeline voltage drop error created by the telluric current in the pipe between the start and finish test stations is negligible. Also, each method is dependent on the validity of the prerecorded data that establish the average potential with time at the start and finish test stations. The shorter this period is prior to the survey, the greater will be the influence of short duration telluric activity and the less will be the effect of any diurnal telluric activity. Figure 4-45 compares the typical before and after correction pipe-to-soil potential data.

CP Interference © NACE International, 2006 January 2008

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Figure 4-x22: Comparison of Raw Pipe-to-Soil Potential Data to Compensated Data

Figure 4-45: Pipe-to-Soil Potential Measurement Method to Compensate for Telluric Current Effects During a Close Interval CP Survey

Note that this compensation technique did not remove all the telluric voltage fluctuations because of its limitations. Degerstedt, et al[33] have used a “telluric null” technique for surveys on the Trans Alaska Pipeline System, which overcomes some of the limitations of the foregoing survey methods. They recorded the potential and current parameters at a test station with time to produce a fundamental characteristic for each test location, as illustrated in Figure 4-46.

33

Degerstedt, Ross, M., Kennelley, K.J., Lara, P.F., Moghissi, O.C., Acquiring Telluric-Nulled Pipe-toSoil Potentials on the Trans Alaska Pipeline, Corrosion ’95, Paper No. 345, NACE International, Houston, TX, 1995.

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:48

+0.8 +0.6

I telluric (A) (downstream) Telluric Voltage Correction Factor

+0.4 +0.2

I telluric (A) (upstream)

-0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4

Telluric ‘Null’ Potential

-1.6

Pipe Potential (VCSE)

Figure 4-46: Pipe Potential/Telluric Current Relationship at a Coupon Test Station

The telluric current was measured using magnetometers placed on grade on each side of the pipeline. It can be seen that there is a linear relationship between the telluric current and the pipe potential and that, through regression analysis, the “telluric null” potential is identified as the intercept with the pipe potential axis. With a historical characteristic established at each test station, the CIS is conducted using GPS time stamping to record both pipe current magnitude and potential with respect to a moving reference. This potential is corrected relative to the voltage at the fixed electrodes at the adjacent test stations by an appropriate correction factor. In lieu of magnetometers, the pipe current can also be determined by measuring the voltage drop along the pipe as illustrated in Figure 4-47, although this arrangement would require installation of pipe test leads at each test station location. Where telluric current activity is anticipated, the four wire test arrangement should be installed at each test station location so that the telluric null method can be utilized. In addition, each test station should also incorporate

CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

4:49

a coupon/reference probe to facilitate the recording of pipe-to-soil polarized potentials with time. Ecal

V

Ip

Ical

Rp/l

L where: Ip = Icp ± It

Figure 4-47: Four Wire Test Lead Arrangement for Measuring Pipe Current

4.5

Summary

In order to maintain effective corrosion control on relatively long coated pipelines that have high leakage resistance and that are located in latitudes close to the magnetic poles and therefore subjected to telluric currents, the following measures should be taken: • Maintain good electrical continuity throughout the system. • Integrate mitigation facilities with the CP system to reduce the magnitude of the telluric voltage fluctuations in both the positive and negative directions. • Install test station facilities incorporating coupons that can be used to measure “telluric free” pipe-to-soil potentials. • Install four wire test station facilities so that the pipe current can be recorded with time. CP Interference © NACE International, 2006 January 2008

Telluric Current Interference

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• Use data loggers that are time synchronized and apply a correction factor to obtain accurate close interval pipe-to-soil data.

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Summary of Equations [4-1a]

γ =

[4-1b]

Z0 =

[4-2]

dV dx

page 4:6

Z Y

page 4:6

= E − 1Z

page 4:6

= − VY

page 4:6

dI dx

[4-3]

ZY

[4-4]

d2V − γ 2V = 2 dx

[4-5]

d 2I dx 2

I =

[4-6]

E γE 0

V =

[4-7]

I =

[4-8]

page 4:7

− γ 2 I = − YE

( 1 + Ae E γ

dE dx

( Ae

- γ ( x − x1 )

-γ ( x − x1 )

page 4:7

+ Be - γ ( x2 − x1 )

− Be -γ ( x2 − x1 )

)

)

V V E − 1 e - γx − 2 e - γ ( L − x ) Z Z0 Z0

V(x) = − V1e - γx + V2 e - γ ( L − x )

[4-9]

page 4:7

page 4:7

page 4:7

page 4:7

where: [4-10]

V1

=

Z1 E × γ Z 0 + Z1

CP Interference © NACE International, 2006 January 2008

and

V2

=

Z2 E × γ Z0 + Z2

page 4:7

Telluric Current Interference

4:52

E = VBZW

[4-11]

where:

Ε v BZ W

= = = =

the potential difference the water velocity the vertical component of the magnetic field the width of the water channel

Fe° ⇒ Fe++ + 2e- (corrosion)

[4-12]

H 3 O + + e-

[4-13]

page 4:11

page 4:18

⇒ H2 + OH- (in deaerated or acidic soils)

page 4:19

⇒ 4OH- (in alkaline or neutral aerated soils)

page 4:19

or [4-14]

2H2O + O2 + 4e-

[4-15]

4OH- ⇒ 2H2O + O2↑ + 4e-

page 4:21

[4-16]

2 H2O ⇒ O2↑ + 4H+ + 4e-

page 4:21

[4-17]

C = ( 4.7 ± 1.3) T+0.186

page 4:23

Vm = Ep + Ve

page 4:29

[4-18]

where:

Ve = Icp • Re Vm = Ecp + Ve ± Vt

[4-19]

Io = Icp + It

[4-20]

page 4:30 page 4:32

[4-21]

Vo = VTR + Vt

page 4:32

[4-22]

Icp = Io – It

page 4:33

g anode =

[4-23]

[4-24]

where: [4-25]

CP Interference © NACE International, 2006 January 2008

1 R anode

=

2πL 1 × 8L ρ -1 ln d

page 4:37

Vps = Vr ± ΔVf

page 4:44

ΔVf = Vfave ± Vf

page 4:44

Telluric Current Interference

[4-26]

Vps = Vm ± ΔVrs •

4:53

y x ± ΔVrf • x+y x+y

page 4:46

Where: the ΔVrs and ΔVrf are the differences in potential compared to the average potential [Vrfave and Vrsave] recorded at each location over a period of time prior to the survey.

CP Interference © NACE International, 2006 January 2008