AC Interference Mitigation Study

AC INTERFERENCE MITIGATION STUDY

GULF JYOTI INTERNATIONAL

SITE SURVEY REPORT

AC INTERFERENCE MITIGATION STUDY FOR NEW OVERHEAD HVAC POWERLINE CROSSING EXISTING GAS PIPELINES CLIENT:

GULF JYOTI INTERNATIONAL Order Ref No. GJI-001/PO/GP/110 GJI-001/PO/GP/110 (Contract #. CNE/005B/2006)

CP VENDOR: Corrpro Companies Middle east LLC

SITE SURVEY REPORT Corrpro Project No. IND- 206 CORRPRO DOC. NO. : IND- 206-250 Rev. A

A

10/12/07

Issued for Approval

T. Mehmood

R. Sandip

R. Fielding

REV

DATE DD/MM/YY

REMARKS

PREPARED BY

CHECKED BY

APPROVED BY

CORRPRO Document Name

CORRPRO Document No.

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TABLE OF CONTENTS 1.0

INTRODUCTION......................... ........................... ......................... ........................... .... 3

2.0

APPLICABLE CODES / STANDARDS AND SPECIFICATION ............................. ........ 3

3.0

DISCUSSION OF AC INTERFERENCE MITIGATION (ACIM) ............................ ......... 3

4.0

MITIGATION OBJECTIVES............................... .......................... ........................... ....... 4

5.0

CODES AND STANDARDS........................ ........................... ............................ ............ 5

6.0

PROCEDURES.................................. ......................... ........................... ........................ 5

7.0

RIGHT OF WAY (ROW) GEOMETRY ......................... ............................ ...................... 6

8.0

SOIL RESISTIVITY CONSIDERATION .......................... ............................ ................... 6

9.0

EXPOSURES & EFFECTS OF ALTERNATING CURRENT AND LIGHTING............... 7

9.1

RESISTIVE COUPLING (ELECTROLYTIC) ........................ ............................ .............. 7

9.2

CAPACITIVE COUPLING ......................... ......................... ........................... ................. 7

9.3

INDUCTIVE COUPLING ......................... ......................... ........................... ................... 7

9.4

POWER ARC ...................... ........................... ......................... ........................... ............ 8

9.5

LIGHTNING............................. ........................... ......................... ........................... ........ 8

9.6

SWITCHING SURGES OR OTHER TRANSIENTS................... ............................ ........ 8

10.0

PRE-DESIGN SURVEY ......................... ......................... ........................... .................... 8

5.1

SOIL RESISTIVITY .................................................................................................................................. 8

5.2

WENNER 4-PIN METHOD.................................................................................................................... 8

11.0

RESULTS AND OBSERVATIONS.......................... ......................... ........................... . 10

12.0

SOIL RESISTIVITY RESULTS .......................... .......................... ............................ .... 10

13.0

CONCLUSIONS AND RECOMMENDATIONS.................................. .......................... 11



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INTRODUCTION Corrpro Companies Middle East (CCME) has been retained by Gulf Jyoti International to carryout a detailed Engineering Study on the Impact of new 400KV OHL on existing DUSUP Gas Pipelines in Crossing & Parallelism and to design, supply materials, supervise the installations and test & Commission the mitigation measures (if required). A site survey was conducted by CCME to obtain sufficient information and record data as required for the AC Mitigation Study. The survey was performed generally in accordance with the relevant project documents, applicable international standards and normal industry practices. The intent of this document is to outline the tests and results of the survey and provide recommendation for the system design.

2.0

APPLICABLE CODES / STANDARDS AND SPECIFICATION The site survey was conducted in accordance with the following applicable documents:

3.0

•

BS 7361 Part 1: 1991

Code of Practice for Land and Marine Application

•

NACE RP 0169-83

Control of External Corrosion on Underground or Submerged Metallic Piping System

•

NACE RP 0177-2000

Mitigations of AC and Lightning Effects on Metallic Structure & Corrosion Control System

•

ASTM G 57

Field Measurement of Soil Resistivity using Wenner’s Four Electrodes Method

DISCUSSION OF AC INTERFERENCE MITIGATION (ACIM) Pipelines that share common Rights-of-Way (ROW) or cross the High Voltage AC (HVAC) transmission lines may become electrically energized by the electromagnetic fields generated by the surrounding power system. Currents flowing in HVAC system conductors produce very strong electromagnetic fields around the conductors. Electromagnetic fields of sufficient magnitude can produce undesirable AC voltages on the pipeline as well as on above ground appurtenances such as main line valves, remote valves, meter stations, test station leads and casing vents located along the corridor. The pipeline may exhibit electrical interference due to inductive



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SITE SURVEY REPORT and conductive coupling along the length of pipeline depending on the geometric relationship to the HVAC system. Geometry that is parallel or approximately parallel to the power line typically yields elevated potentials mostly dependent on the length of the parallelism. AC interference can result in an electrical shock hazard for persons touching above ground appurtenances or by step potentials in areas of elevated AC voltages. With voltages of 2500 to 5000 VAC, damage to pipe coating can occur. Elevated AC voltages greater than 5000 VAC can cause physical damage to the steel pipe structure itself resulting, although rarely, in pipe penetration. Inductive interference is the dominant interference mechanism under normal power line conditions. Induced potentials on an unmitigated pipeline can reach hundreds of volts at power line phase transpositions and locations where pipeline and power line geometry relationships change dramatically. An example of this is pipeline and/or HVAC points of inflection where acute or obtuse angles occur either entering or exiting the HVAC corridor, especially before or after a lengthy parallelism. During a phase to ground fault on the HVAC system, an arching fault, large currents can enter the ground near the fault location. Portions of these currents will flow along any metallic pipeline adjacent to the power line. Large fault currents injected via ground by HVAC structures increase the local soil potential. When pipelines are parallel to power line(s) they will be simultaneously subjected to both conductive and inductive interference. The induced pipeline potential can reach thousands of volts on an unmitigated pipe and can cause coating failure or even failure of the pipe itself. The magnitude of conductive interference is a function of several factors. In transmission line grounding systems, lower impedance values will present lower values of conductive interference voltage. The separation distance between a faulted structure and the pipeline impacts the voltage. Typically, the greater the separation distances the lower the voltage magnitude. Soil structure and soil resistivity also contribute to the levels of conducted AC and can inhibit mitigation efforts of elevated potentials as well. Bolted faults, or faults caused by direct contact between two phases or a single phase to a ground or static line can also cause major current flow. However, due to the immediacy of the fault, clearance times are typically much less than those in conditions during arching faults.

4.0

MITIGATION OBJECTIVES During worst case power line load conditions, touch voltages at exposed structures such as valves, metering stations, and pig launchers/receivers, and cathodic protection test stations must be sufficiently low to minimize shock hazards to operating personnel and the general public. NACE International standard RP0177-2001 recommends a 15 Volt limit (above ground and below ground) based on let go current thresholds for most people. Note that this is the potential difference between two parts of the pipeline that can be contacted simultaneously, or between the pipeline and the earth surface within reach of the above ground appurtenances.



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SITE SURVEY REPORT During fault conditions, as well as steady state conditions, step potentials must be sufficiently low to minimize or eliminate the possibility for shock hazards. ANSI/IEEE standard 80-1986 provides safety criteria based on the heart fibrillation current threshold. Safety limits are derived from empirical work performed by Dalziel and others. Based on a 0.5 second fault duration, the touch and step voltage safety limits are both approximately 164 volts for a 50 kg subject if the soil resistivity is very low and no protective layer such as crushed rock or asphalt is between the subject and earth. The safety limits increase as the fault duration decreases and the surface material resistivity increases. The pipe coating stress voltages must be maintained sufficiently low to prevent arcing through coating. This typically occurs for coating stress voltage on the order of 2.5 KV to 5 KV or higher for fusion bonded epoxy and poly coatings. For bitumen coating, glow arcing can occur at voltages as low as 1000 volts. This arcing can damage not only coating but also the steel wall, which in severe cases, could result in penetration of the pipeline.

5.0

6.0

CODES AND STANDARDS •

American Gas Association and Electric Power Research Institute (EPRI) – "Mutual Design Considerations for Overhead AC Transmission Lines and Gas Transmission Pipelines."

•

NACE International Standard RP0177-2000 - “Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems."

•

Canadian Standards Association C22.3 No. 6-M91 from the Canadian Electrical Code, Part III, entitled “Principles and Practices of Electrical Coordination between Pipelines and Electric Supply Lines." Particular attention shall be given to Section 5 – "Mitigation of Induced Voltage Effects during Pipeline Construction."

•

ASTM G57-95a Standard Test Method for Field Measurement of Soil Resistivity using the Wenner Four-Electrode Method.

•

National Electric Code NFPA 70, 2001

•

National Electric Safety Code / IEEE C2-1997

PROCEDURES The general outline of the procedure followed for AC interference mitigation design is as follows: The pipeline alignment sheets were carefully studied to note the existence and survey locations of high voltage power lines parallel to or crossing the pipeline ROW. All locations where the pipeline is approximately parallel to HVAC were surveyed. Transmission tower data, distances separating pipeline and power line, and soil resistivity data was collected for computer modeling in future; whereby, critical sections of the



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SITE SURVEY REPORT pipeline for elevated AC potentials near parallel sections could be modeled and Mitigation measures advised. Principle and Methodolo gy of Soil Resistiv ity Survey The Precision Soil Resistivity Instrument used for the survey was the MEGGER-DET5/4R. The ASTM G-57-95a (Wenner four pin method) was used for measuring soil resistance values along the ROW. Using this method, four probes are placed at equal distances from each other and along a straight line symmetrically over the measuring point. A known current is injected into the ground through a pair of current electrodes (the outermost two) and the resulting potential is measured through a second pair of electrodes known as the potential electrodes (the innermost two). The ratio between the measured potential and the current passed is the resistance in ohms. Using the indicated resistance value (R), the soil resistivity is calculated according to following equation:

ρ = 2 × π × A × R Where: ρ

A π

R

7.0

= mean value of soil resistivity in ohm-meters = distance between the probes (m) = constant = measured resistance (ohm)

RIGHT OF WAY (ROW) GEOMETRY The study reported here is based upon the evaluation of a ROW where the proposed / newly constructed 400 KV overhead HVAC Power Line crosses the existing Gas Pipelines. The crossing locations have been identified at three (3) different locations as follows: • • •

Near DEWA – L Station At approximately Km 30+700 along the DUSUP Sharjah Pipeline Corridor At approximately Km 62+000 along the DUSUP Sharjah Pipeline Corridor

It is noted that the project does not involve any section where the existing pipelines are parallel to the new / proposed 400KV power lines.

8.0

SOIL RESISTIVITY CONSIDERATION The soil resistivity measurements were made along the HVAC / Pipeline corridor using 4pin Wenner method at all three (3) proposed crossing locations. The soil resistivity measurements were carried out at depths of 1, 2.5, 5, 10 and 25 meters. Soil resistivity values at depths greater than 2.5 meters will be considered during computer modeling and calculations.



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SITE SURVEY REPORT From the soil resistivity values measured at 1 and 2.5 meter depths along the HVAC parallelism it is understood that the soil resistivity value of 0-1 meter layer is lower than the value obtained for 0-2.5 meter layer. Based on the results obtained, the value of soil resistivity considered is 100 ohm-m for calculating Zinc ribbon anode resistance to earth. Appendix A displays the soil resistivity results.

9.0

EXPOSURES & EFFECTS OF ALTERNATING CURRENT AND LIGHTING The following is a brief description of t he AC Mitigation is as under:

9.1

Resistive Coupling (Electrolytic) Grounded Structures of an AC power system share an electrolytic environment with other underground or submerged structures. Coupling effects may transfer AC energy to metallic structure in the earth in from of alternating current or the potential. Whenever a power system with a groundbed neutral has unbalanced conditions, current may flow in the earth. Substantial currents in the earth may result from phase-to-phase or phase to ground faults. A metallic structure in the earth may carry part of this current. Also a structure in the earth coated with an insulating material may develop a signification AC potential across the coating.

9.2

Capacitive Coupling The Electric field associated with potentials on power conductors can develop a potential on an inadequately grounded structure in the vicinity of the power system. The potential that the structure attains because of capacitive coupling varies with the power conductor potential and depend on many factors, including the geometric configurations of the structures involved. During construction, when the structure is aboveground or in an open trench, it may reach a dangerously high potential. When the structure is buried or submerged, the capacitive coupling effect usually is not significant unless the soil Resistivity is high, the structure is electrically isolated, or the structure is long.

9.3

Inductive Coupling AC current flow in power conductors produces an alternating magnetic field around these conductors. Thus, an AC potential can be induced in an adjacent structure within this magnetic field, and current may flow in that structure. The magnitude of the induced potential depends on many factors including the overall geometric configuration of the structure involved, the magnitude of the current in the power circuit, and any current imbalance. If the current in three power system are equal and the effected structure is equidistant from each of the conductors, the total induced voltage is zero. This, however, is seldom the, and induced the AC voltage is usually present on the affected structure. Greater electromagnetically induced potential may occur during a phase-to-ground or phase-to-phase fault in multiphase circuits because of the higher magnitude of the fault current in the systems. The leakage conductance to ground, caused by the resistive coupling of the effected structure, allows AC current flow between that structure and earth. This phenomenon, combined with other factors, results in the different values of



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SITE SURVEY REPORT AC structure-to-electrolyte potential along the affected structure. The higher the dielectric strength and resistance of the coating and the higher the soil resistivity, the greater the induced AC potential.

9.4

Power Arc During a fault to ground on an AC power system, the AC power structures and surrounding earth may develop a high potential with reference to remote earth. A long metallic structure, whether coated or bare, tends to remain at remote earth potential. If the resulting potential to which the structure is subjected exceeds breakdown potential of any circuit element, a power arc can occur and damage the circuit elements. Elements of specific concern include coatings, isolating fittings, bonds lightning arresters, and CP facilities.

9.5

Lightning Lighting strikes on the power system can initiate fault current conditions. Lightning strikes to a structure or the earth in the vicinity of a structure can produce electrical effects similar to those caused by AC fault current. Lighting may strike a metallic structure at some point remote from AC power system, also with deleterious effects.

9.6

Switching Surges or Other Transients A switching surge or other transient may generate abnormally high currents or potential on a power system, causing a momentary increase in inductive and capacitive coupling on the affected structures.

10.0

PRE-DESIGN SURVEY In order to ensure accurate AC Induction Mitigation (ACIM) modeling, a site survey was conducted to gather information required for the design. The work included general visual check of the project site, soil resistivity at representative locations 5.1

Soil Resisti vity

Resistivity is a common parameter for evaluating the corrosiveness of soil. It is the inverse of conductivity and is measured in the unit of ohm-centimeter (ohm-cm). In low resistivity soils, the corrosion rate is often greater than in high resistivity soils. Soil resistivity can be measured using different methods and for this scope the 4-pin Wenner method in accordance with ASTM G57 were used. 5.2

Wenner 4-pin Method

CCME conducted soil resistivity survey using 4-pin Wenner method along the route of the pipeline at every Overhead Power Line Crossing (“L” Station, Km 62+000 and near “H” Station). The measurements were recorded at 1m, 2.5m, 5m, 10m and 25m depth. The following describes the Wenner 4-Pin t echnique referenced above: CORRPRO Document Name


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Equipment required: 1. Nilsson Earth Resistance Meter Model 400 or equivalent 2. Cable harness 3. 4 stainless steel pins The Wenner 4-Pin method involves driving four steel pins in to the ground in a straight line. The pins were evenly spaced at an interval equal to the depth in which the average soil resistivity values are to be known. With the pins in this arrangement, a current was introduced into the earth through the outer two pins, which resulted in potential change between the inner two pins. By equating this current and potential change to Ohms Law (R = E/I) the resistance of the section of earth between the planes of the inner two pins was calculated. This resistance was typically measured in ohms. To determine the average soil resistivity value of the soil to the depth of the pin spacing, the following formula was used. p

=

2xΠxaxR

= = =

soil resistivity in ohm-m electrode spacing in meters measured resistance in ohms

Where: p a R

To avoid erroneous test results the soil resistivity measurements were not conducted in the immediate vicinity of underground metallic structures. In addition, care was exercised to avoid taking `measurement’ near H.T power lines, railroads or plants where the reading could be affected by foreign earth currents. In dry soils it was necessary to moisten the soil immediately around the pins to obtain a measurement. The depth to which the pins are inserted into the soil was limited to 1/20 th of the pin spacing. CCME utilized a resistivity meter for all Wenner 4-pin soil resistivity measurements. Soil resistivity test was performed at representative location of the plant and pipe route. Data sheets of recorded soil resistivity measurements are included in the appendix section of this report. The purpose of the soil resistivity is to enable a satisfactory AC Mitigation system to be designed based on the resistivity of the electrolyte (soil). The resistivity is also a measurement of the aggressiveness of the environment. The resistivity and corrosiveness have an inverse relationship. The higher the resistivity the lower is the corrosiveness and vice versa. The following table (ASTM Special technical CORRPRO Document Name


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SITE SURVEY REPORT Publication741 page 69) provides general guidelines for evaluating corrosiveness of an environment with respect to resistivity values. Resistivity (in Ohm-cm)

11.0

the

Soil Classification

0 – 1,000

Very Low

Very Corrosive

1,000 – 5,000

Low

Corrosive

5,000 - 25,000

High

Mildly Corrosive

25,000 – 100,000

Very High

Relatively Corrosive

100,000 – above

Extremely High

Progressively Corrosive

RESULTS AND OBSERVATIONS The following are the results generated from the survey and are generally based on the available information and data collected at the site. Soil resistivity and other results are included in the appendix section of this report. Near the DEWA ‘L’ Station • •

The average soil resistivity recorded near the DEWA ‘L’ Station indicates a very low resistivity. There are existing Overhead Power Lines near the existing the Gas Pipelines and the AC potentials recorded were generally less than 15 V AC.

Near the KM 30+700 Along DUSUP Sharjah Gas Pipeline •

The average soil resistivity recorded at approximately KM 30+700 along the existing DUSUP Gas Pipeline corridor indicates a ‘very high’ resistivity.

•

There are existing Overhead Power Line crossing the existing Gas Pipeline Corridor and the AC potentials recorded at existing CP System test posts (in vicinity) were generally less than 15 V AC.

Near the KM 62 +000 Along DUSUP Sharjah Gas Pipeline

12.0

•

The average soil resistivity recorded at approximately KM 62+000 along the existing DUSUP Gas Pipeline corridor indicates a high resistivity.

•

There are existing Overhead Power Line crossing the existing Gas Pipeline Corridor and the AC potentials recorded at existing CP System test posts (in vicinity) were generally less than 15 V AC.

SOIL RESISTIVITY RESULTS The soil resistivity measurements recorded during the site survey were as f ollows:



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Sl

1

2

3

13.0

Survey Location

Near the DEWA "L" Station

62+000 (Approx. chainage of DUSUP Sharjah Pipeline Corridor) Km 30 + 700 (Approx. chainage of DUSUP Sharjah Pipeline Corridor)

Pin Spacing A (Meter)

Meter Reading R (ohm)

Soil Resistivity 2 x π x A x R x 100 (ohm- cm)

1

0.5

314

2.5

0.38

597

5

0.27

848

10

0.12

754

25

0.02

314

1

37.5

23550

2.5

10.3

16171

5

2.82

8855

10

0.27

1696

25

0

0.00

1

36.4

22859.2

2.5

31.1

48827

5

41.8

131252

10

10.7

67196

25

3.72

58404

Remarks

Near "L" Station Gate

Sand Dunes

Near "H" Station (Sand Dunes)

CONCLUSIONS AND RECOMMENDATIONS Based upon the survey results, AC Interference Mitigation Study shall be carried out to determine the mitigation measures as may be required to ensure that the induced AC voltage levels on the existing Pipeline due to the proposed new 400KV Power Lines are limited to less than 15 V AC. The modeling report with recommendations and mitigation requirements will be submitted separately.



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AC Interference Mitigation Study

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