AC Assessment, Corrosion Prediction, Safety and Mitigation

Quick Guide

By Joe Pikas

Overview Introduction The NEW AC Mitigation PowerTool© now with unlimited pipeline and electric

transmission tower lines to truly understand, model and mitigate the underground

pipeline AC induced current integrity problem. The AC Mitigation PowerTool© has been

developed to assist the engineer or technician to model and mitigate or modify the

design of pipeline cathodic protection systems in order to reduce the AC current density

effects to meet the criteria specified either by a client or the National Association of

Corrosion Engineers (NACE) standard.

The Pipeline Research Council International (PRCI) AC Predictive & Mitigation software

developed by Electro Sciences, Inc. and Dr. John Dabkowski (PRCI Catalog #L51835)

has been the defacto industry safety & analysis standard since 1999. The PRCI AC

Predictive & Mitigation software was developed to handle multiple pipelines utilizing the

same right-of-way with overhead High Voltage (HV) Alternating Current (AC) power

lines. A NEW and enhanced version of the PRCI field proven and field tested

computational engine has been ported to work on current and future “cloud” and mobile

devices as well as the traditional desktop/laptop.

Features

Now able to address Complex Corridors with Unlimited pipelines and high voltage transmission lines can be assessed versus the limitation of the PRCI software

Integrated Modeling can follow complex pipelines and tower lines that are networked I.e. diverge or that intersect with each other.

Multiple scenarios can be run with Steady State, Faults or mitigation in minutes o Multiple or Individual graphs

Ease of manual or Excel data input o Can show data in total

Data can be visualized using graphs and Google Earth

Cloud or desktop version

Detailed reports show graphs with data

Ease of determining multiple mitigation cables and bonds

Functions

Integrate Google Earth with GIS capability for visualization of pipeline and tower locations

Faster computational modeling o Steady State o Faults with Arc Radius o Mitigation

Input and Output to/from Excel and Google Earth o Latitude/Longitude coordinates

Interim reports can be generated at any time in the process

Data validation and error checking

Change units of measurement at anytime

Operational defaults are built in for common tower settings

Mitigation design allows bonds and multiple mitigation cables Problem Electrical power lines carry an electrical current, where as a magnetic field is produced around the wire which induces into the buried pipe. This linking causes an alternating voltage and current to be induced onto a parallel collocated pipeline. When a pipeline is located in the vicinity of a power line, it is subject to several electrical effects depending upon the operational status of the line. In addition, lightning stroke or other cause, the power line will experience a short circuit condition known as a fault. The focus of the AC predictive guide is based on inductive coupling and fault conditions from co-located power lines using Technical Toolboxes AC Mitigation PowerTool©.

1. AC Pipeline Prediction

a. Electrostatic Interference (Capacitive when pipe is above ground)

i. Function of voltage not current

ii. Transfer of small amounts of power to pipeline

iii. Can result in high voltages on short sections

iv. Considered nuisance voltages

b. Induced Voltages (Electromagnetic) i. Function of tower current not voltage ii. Power transfer is proportional

iii. Line current

iv. Length of parallelism

v. Inverse to separation distance vi. Results in high voltages on long sections of pipeline

c. AC Faults i. Rare in US ii. Short duration iii. Weather conditions

1. High winds 2. Lightning

iv. Structural failure 1. Poor maintenance 2. Accidental damage

3. Vandalism/Terrorism 2. AC Mitigation

a. AC Decoupling Devices, Polarization Cells and Surge Protection

i. Dairyland Devices (See Appendix B) ii. Cu Cable for Parallel Grounding iii. Distributed Ground Beds iv. Discrete Ground Beds

b. Mitigation to NACE Standard <15VAC i. Personnel Hazard

c. Gradient Control Mats at Pipeline d. Appurtenances such as Test Stations & Valves

i. Grid with Zn, Cu ii. Decoupler

Practical Approach to Mitigating Corrosion

1. AC potentials induced into a pipeline are a function of:

a. Inverse distance between these parallel structures b. Pipe diameter c. Coating conductance/resistance d. AC tower loads

2. Induced AC into a pipeline or to earth is directly proportional to the strength of AC tower current load.

a. Longitudinal electrical fields (LEF) are induced into the earth. b. LEF can be field measured to estimate AC before a pipeline is constructed

3. Input Data

a. Tower Configurations i. Single Circuit Horizontal ii. Single Circuit Vertical iii. Double Circuit Horizontal iv. Double Circuit Vertical

b. Shield Wires i. Cross sectional height and horizontal displacement of the shield

(sky) wires from the tower center line are evident inputs. Program default accepts data for two wires with the assumption that the wires are periodically grounded to the tower grounds.

c. Phase Wires i. Physical placement of the wires, i.e., height and horizontal

displacement from the tower center line are obvious if and when data are available. Default values for typical circuits as a function of circuit voltage level are available from within the program data base.

d. Tower Data

i. Additional data required are the average tower ground resistance to remote earth and the average separation between the faulted transmission line towers (structures). Default values for these parameters are shown below:

e. Pipeline Data i. Diameter ii. Sections as shown below

iii. Location iv. Coating Quality v. PRCI Guideline for Estimating Pipe Coating Resistance (Next

Page)

(1-ohm m = 100-ohm cm)

Coating resistance in Kohm -ft2

Be careful of unit conversion

AC Corrosion

1. Current Density Criteria a. Does not usually occur at AC current densities of less than 1.9 A/ft2 (20

A/m2) b. May occur at AC current densities of 1.9 – 9.3 A/ft2 (20 - 100 A/m2) c. Can occur at AC current densities of greater than 9.3 A/ft2 (100 A/m2)

2. AC corrosion rates are: a. Highest at holidays having a surface area of 0.155 – 0.465 in2 (1 - 3 cm2) b. High CP c. Chloride environments

Mitigation

1. Most common mitigation approach is the grounding of the pipeline by means of buried horizontal wires, galvanic anodes, etc. See Screen Selection on next page:

a. Discrete (Ground Resistance and Node) b. Distributed (Anode Resistance Times Spacing) c. Parallel (Copper Cable(s) with De-Couplers) d. Combinations of above

Note: Examples of Steady State and Fault Voltage and Current Drafts are shown on the next two (2) pages.

Steady State Voltage and Current (Combined Graph)

Modified Calculation has been added to validate assessments to field measurements

and coating resistance estimations. Coating resistance is a primary factor in these

complex calculations.

Steady State with Mitigation with Distributed Ground Bed on Each End

(Individual Graph)

Fault Voltage and Current – Unmitigated (Individual Graphs)

Fault Voltage and Current with Mitigation (Individual Graphs)

AC Corrosion Calculations

1. High AC current density effects has resulted in AC corrosion.

2. AC modeling and mitigation is used to estimate AC voltage and AC current

densities.

3. AC current density can be calculated using a known area on a coupon.

4. Where

a. iAC = AC Current Density (A/m2)

b. VAC = pipe AC voltage to remote earth (volt)

c. ρ = soil resistivity (ohm-m)

d. d = diameter of a circular holiday

5. AC voltages as low as 1 VAC can produce high current densities at small

holidays in lower resistivity areas.

6. AC voltage required to for a current density of 100 A/m2 in 100 ohm-cm soil at a

1 sq cm holiday is: (See Chart below)

VAC = iAC x p x 3.1416 x 1sq cm /8

VAC = 0.393

AC Corrosion Chart: AC Voltage versus Holiday/Resistivity

Conclusion

The format developed for this AC Mitigation program makes many of the computational functions and much of the data input very easy for the user, thus making these calculations in an understandable way for unlimited pipelines and transmission towers. This is a first in this area of complex mathematical modeling. For example, data input for only one computer screen is required to exercise the program to assess the following including reporting.

Steady state induction predictions

Fault induction predictions

Mitigation design

Export data through Excel

GPS mapping using Goggle Earth

Help Section

Reports – Most Completed with Additional in Future Release

Current Density Graphs - Future Release

Upload data through Excel – Future Release

This easy to use interface makes the use of this cloud based program viable for number of potential users in your company than other available computer programs. See Appendix A (Getting Started) for an example of an AC project. Should there be any questions, training or consulting, please feel free to call:

Joe Pikas VP P.L. Integrity

ENGINEERING SERVICES

Technical

ToolBoxes

3801 Kirby Dr. Suite 520, Houston, TX 77098

C 832 758-0009 Preferred

O 713 630-0505 X-216

joseph.pikas1

jpikas@technicaltoolboxes.com

www.technicaltoolboxes.com

Appendix A

Getting Started with skyBox Tools

Logging in: http://acmitigationpowertool.com/

Select Steady State: (No Data)

Load Case: Example 1 P & 2 T and Press Go

Example of 1 Pipeline and 2 Power Transmission Lines:

Menu Bar Selections:

Transmission Lines

o Add

o Edit

o Delete

345 KV Power Line

160 KV

Power Line

6” Dia. Pipeline

Example of Pipelines:

Menu Bar Selections:

Transmission Lines

o Add

o Edit

o Delete

o Show Full Section (See Below)

Show Full Section:

Calculate Steady State Graph and Download Report:

Modified Calculation has been added to validate assessments to field measurements

and coating resistance estimations. Coating resistance is a primary factor in these

complex calculations.

Fault Current – Load Fault Case:

Calculate Fault Current, Arc Distance and Download Report:

Modified Calculation has been added to validate assessments to field measurements

and coating resistance estimations. Coating resistance is a primary factor in these

complex calculations.

Mitigation – Sample Using Distributed Anode Ground Beds:

Load Saved Mitigation and Determine Best Strategy Using Cloud Base Interface:

Use a computational iterative process

Review principles and grounding techniques for mitigation

o Sunde Equations

o Dwight’s Curves

Use Technical Toolboxes Training and Consulting for complex circuits and

projects

Use modified calculation to validate field measurements

Fault Current Mitigation Using Distributed Anodes:

Steady State Mitigation Using Distributed Anodes:

Reports – Steady State Voltage and Current with Mitigation

Reports – Fault Voltage and Current with Mitigation

Mitigation Considerations:

Types of grounding are reviewed:

(1) Discrete grounds such as deep wells can be used to mitigate widely spaced voltage peaks or nodes.

(2) Distributed vertical or horizontal anode strings are used at nodes or isolated voltage peaks. Distributed grounding such as a parallel horizontal buried copper wire bonded to the pipeline can be used (See paragraph 3 below for additional details on de-couplers). This approach is used more often used when multiple closely spaced voltage peaks exist on the pipeline and to isolate grounding from the CP system. Both of these approaches are accommodated by Cloud Based computer program. This program requires an iterative procedure to determine an effective final design and requires successively reruns with new mitigation resistance values until a satisfactory reduction in the induced voltage levels is obtained.

(3) Wire or long line copper cable(s) using a bonded horizontal cable i.e. 1/0 or greater as the grounding element. This grounding system provides a grounding impedance to achieve the best or lowest bound to achieve satisfactory steady state voltage levels. Dairyland De-couplers and related devices are used to isolate the grounding copper cable, anodes, ground mats, etc. (a) Copper grounding cables with and without backfill connected to

Dairyland de-couplers have a history of good performance record with trouble free operations.

(b) Zinc ribbon grounding has been used in the past; however, limitations due to being part of the CP system have resulted in performance issues.

(4) Combinations of grounding can be used that are based on geometry of pipes and transmission lines and soil resistivity constraints.

Note: For additional AC Mitigation training and consulting, call Technical Toolboxes or visit our website www.techncialtoolboxes.com

References:

1) PR-200-9414 AC Predictive and Mitigation Techniques – Final Report

Appendix B

AC Mitigation System Checklist

Dairyland

Inductive voltage mitigation equipment:

Decoupler

Grounding material (bare copper, zinc, etc.)

Isolated conductors connecting Decoupler and grounding material

Conductor attachment method: thermite welding, pin brazing, plus

coating/sealing system

Enclosure or pedestal for Decoupler

Disconnection means for Decoupler testing or close interval surveys (Isolation

switch). Select switch with AC steady-state and AC fault ratings equal to the

Decoupler ratings.

Measurement equipment:

Test station

Coupon for AC current density measurement (or multiple function coupon)

Step and touch voltage protection near above-grade structures and connections:

Gradient control mat

Thermite welding molds/charges

Anode for mat protection

Decoupler for zinc grounding mat isolation from pipeline

Isolated conductor

Isolating joint protection – subject to AC induction:

Decoupler

Decoupler mounting brackets appropriate for insolated joint type

Note: Dairyland Solid-State Decoupler (SSD) and Polarization Cell Replacement

(PCR) data and information are on the next page for mitigation selection.

Dairyland Solid-State Decoupler (SSD)

AC RMS 60Hz Fault Current Ratings

Model 1 cycle 3 cycles 10 cycles 30 cycles Associated

Conductor Size

3.7KA 6,500 5,000 4,200 3,700 #6

5.0KA 8,800 6,800 5,700 5,000 #2

All SSD Models:

AC- RMS, 60Hz steady-state current rating: 45A

Environmental Rating: IP68

Hazardous Location Rating: Division 2 and Zone 2

Dairyland Polarization Cell Replacement (PCR) AC-RMS 60Hz Fault Current Ratings

Model 1 cycle 3 cycles 10 cycles 30 cycles Associated

Conductor Size

PCR-3.7KA

6,500 5,000 4,200 3,700 #6

PCR-5KA 8,800 6,800 5,700 5,000 #2

PCR-10KA

20,000 15,000 12,000 10,000 #2

PCR-15KA

35,000 27,000 21,000 15,000 #2/0

All PCR Models:

AC- RMS, 60Hz steady-state current rating: 45A (Optional 80A)

Environmental Rating: NEMA 4X (Optional NEMA 6P)

Hazardous Location Rating: Division 2 and Zone 2

Application notes for AC mitigation:

Select AC fault current rating that exceeds modeled or calculated values

expected on pipeline.

Select standard AC steady-state current rating unless modeled or measured

conditions require the higher 80A PCR rating.

Most AC mitigation sites are classified as Div. 2/Zone 2 or are "ordinary"

locations. A Div. 2/Zone 2 product will cover either. If Div. 1/Zone 1 ratings are

needed, use model PCRH.

Apply product environmental ratings suitable for the site location. Most are

typically above grade and not subject to flooding. If below grade, select IP68 or

NEMA 6P.

Select conductor size to match or exceed Decoupler AC fault current rating.

Note: For more information contact www.dairyland.com