Cathodic Protection Report

ZAMZAMA CONDENSATE PIPELINE CP SYSTEM FOR

BHP BILLITON BY

NURICON PETROSERVICES (PTE.) LIMITED 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi, Pakistan Phn : 92-21-273 5051 ( 4 Lines ) Fax : 92-21-273 4162 Email : [email protected]

Report for: Client: Doc. Title Project:

BHP BILLITON NPSL Reference: PETROCHEM ENGG. & IND. CONSULTANTS Contract No: CP SYSTEM OF 6” DIA 28 Km PIPELINE Zamzama Condensate Pipeline CP System Design for BHP BILLITON

PETROCHEM:07/002 PEC/NU-2006/002 Doc. No.: Main Report

CATHODIC PROTECTION SURVEY RESULTS & SYSTEM DESIGN FOR:

BHP BILLITON

FACILITIES:

BHP GAS FIELDS, SIND, PAKISTAN

PROJECT:

ZAMZAMA CONDENSATE PIPELINE CP SYSTEM DESIGN REVIEW

CLIENT:

PETROCHEM ENGG. & INDUSTRIAL CONSULTANTS

CONTRACT DOC NO.

PEC/NU-2006/002

CONTRACT DATE:

APRIL 10, 2006

FIELD SURVEY BY:

MAQSOOD-UZ-ZAMAN, NPSL, KARACHI ASIF SIDDIQUI, NPSL, KARACHI TARIQ JAMEEL, NPSL, KARACHI PERVAIZ ABBASI, NPSL, KARACHI BURHAN, NPSL, KARACHI

REPORT BY:

MAQSOOD-UZ-ZAMAN, NURICON PETROSERVICES, KARACHI, PAKISTAN

FINAL REPORT DATE:

DECEMBER 2006

NPSL REFERENCE NO.

PETROCHEM :07/002

NURICON PETROSERVICES (PTE.) LTD. 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi Phn : (92-21) 273 5051 ( 4 Lines ) Fax : (92-21) 273 4162

1 E-mail : [email protected]

Report for: Client: Doc. Title Project:

BHP BILLITON NPSL Reference: PETROCHEM ENGG. & IND. CONSULTANTS Contract No: CP SYSTEM OF 6” DIA 28 Km PIPELINE Zamzama Condensate Pipeline CP System Design for BHP BILLITON

PETROCHEM:07/002 PEC/NU-2006/002 Doc. No.: Main Report

TABLE OF CONTENTS

LIST OF ABBREVIATIONS............................................................................................. 3 INTRODUCTION & BACKGROUND ............................................................................. 5 SCOPE OF WORK............................................................................................................. 6 OBJECTIVE ....................................................................................................................... 6 SOIL RESISTIVITY SURVEY ......................................................................................... 6 DESIGN CONCEPTS ........................................................................................................ 7 BILL OF QUANTITIES ................................................................................................... 10 APPENDIX:

SOIL RESISTIVITY

APPENDIX:

SOIL CONDUCTIVITY / RESISTIVITY

APPENDIX:

DESIGN CALCULATIONS

APPENDIX:

DRAWINGS

1. Schematic Arrangement of CP System Along Pipeline ROW ..........................................................................Drawing No. CP-LAY-01 2. Typical Shallow Vertical Groundbed Details ....................Drawing No. SVGB-LAY-01 3. Typical Anode Junction Box (AJB) Details .........................Drawing No. TYP-AJB-01 4. Typical Burial Detail for Pair of Earthing Electrodes & their Cable Connections to Insulating Flanges ............................................................................ Drawing No. EE-01 5. Typical Cable Connections to the Pipeline ........................Drawing No. TYP-CCON-01 6. Test Post Details ............................................................Drawing No. TP-01sheets 1 & 2 7. Typical Cable Trench Details ...........................................................Drawing No. CT-01 8. Current Measuring Test Station.................................................. Drawing No. CMTS-01

NURICON PETROSERVICES (PTE.) LTD. 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi Phn : (92-21) 273 5051 ( 4 Lines ) Fax : (92-21) 273 4162

2 E-mail : [email protected]

Report for: Client: Doc. Title Project:

BHP BILLITON NPSL Reference: PETROCHEM ENGG. & IND. CONSULTANTS Contract No: CP SYSTEM OF 6” DIA 28 Km PIPELINE Zamzama Condensate Pipeline CP System Design for BHP BILLITON

PETROCHEM:07/002 PEC/NU-2006/002 Doc. No.: Main Report

LIST OF ABBREVIATIONS π

3.14.... (a constant)

ρb

Backfill resistivity (ohm-m)

ρs

Soil resistivity (ohm-m)

A AC or ac AP BHPB BHPBP

Ampere Alternating Current Anodic protection. BHP Billiton BHP Billiton Pakistan

CP CPP CPS

da

Cathodic Protection Casing potential profile Cathodic Protection System. Casing to Soil Potential (with respect to Saturated Cu-CuSO 4 Reference Electrode) Diameter of anode.

db DC or dc Dia ft, in, ICCP

Diameter of anode column/canister inclusive of backfil. Direct Current Diameter Foot, inch Impressed Current Cathodic Protection

ICCPS KM or km La Lb m

Impressed Current Cathodic Protection System Kilometer Length of anode. Length of anode column/canister including backfil Meter

mA MV or mv cm mm Na

Miliampere Milivolt Centimeter Millimeter No of anodes

NPSL PE Petrochem

Nuricon Petroservices (Pte.) Limited Polyethylene Petrochem Engineering (Pvt.) Limited Polarization Potential (with respect to Saturated Cu-CuSO 4 Reference Electrode)

CSP

Potential

NURICON PETROSERVICES (PTE.) LTD. 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi Phn : (92-21) 273 5051 ( 4 Lines ) Fax : (92-21) 273 4162

3 E-mail : [email protected]

Report for: Client: Doc. Title Project:

BHP BILLITON NPSL Reference: PETROCHEM ENGG. & IND. CONSULTANTS Contract No: CP SYSTEM OF 6” DIA 28 Km PIPELINE Zamzama Condensate Pipeline CP System Design for BHP BILLITON

PETROCHEM:07/002 PEC/NU-2006/002 Doc. No.: Main Report

PVC Rc Rg

Pipe to Soil Potential (with respect to Saturated Cu-CuSO 4 Reference Electrode) Polyvinyl Chloride Compound Cable resistance Resistance of single anode

Rgt ROW Rs Rt S

Resistance of multiple anodes to remote earth Right of way Resistance of structure to remote earth Total circuit resistance Spacing between the anode columns (wherever applicable)

Sa Sav SCP SCPS sqm

Spacing between bottom and top of consecutive anodes Center to center spacing between vertical anode/backfill columns Sacrificial Cathodic Protection Sacrificial Anode Cathodic Protection System Square meter

SSP

Structure to Soil Potential (with respect to Saturated Cu-CuSO4 Reference Electrode)

TEG

Thermo-electric generator

PSP

NURICON PETROSERVICES (PTE.) LTD. 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi Phn : (92-21) 273 5051 ( 4 Lines ) Fax : (92-21) 273 4162

4 E-mail : [email protected]

Report for: Client: Doc. Title Project:

BHP BILLITON NPSL Reference: PETROCHEM ENGG. & IND. CONSULTANTS Contract No: CP SYSTEM OF 6” DIA 28 Km PIPELINE Zamzama Condensate Pipeline CP System Design for BHP BILLITON

PETROCHEM:07/002 PEC/NU-2006/002 Doc. No.: Main Report

DESIGN REPORT FOR 6” X 28 KM CONDENSATE PIPELINE FROM BHP’S ZAMZAMA GAS FIELD TO PARCO’S BUBAK STATION This document outlines the Cathodic Protection System Design and Engineering for the Ø6”x 28 km 3LPE coated Condensate Pipeline from BHP Billiton’s Zamzama Gas Field to Parco Booster Pumping Station at Bhubak. Petrochem Engineering is the consultant for design and engineering of the pipeline for BHP and they acquired the services of Nuricon Petroservices (Pte.) Limited (NPSL) to Design and Engineer the CP system for this pipeline and to carry out pre design surveys for this purpose. Zamzama is located near Dadu, Sind and is about 330 km northwest of Karachi, Pakistan. The pipeline would carry gas condensate from condensate tanks at Zamzama to condensate storage tanks at Bhubak for onward sale and distribution. INTRODUCTION & BACKGROUND Before carrying out the actual Soil Resistivity Survey by NPSL, the BHPB provided some soil resistivity data of the surrounding areas and sought NPSL advice whether a sacrificial anode system based on zinc ribbon can be designed for the pipeline or not. In response NPSL recommended an Impressed Current CP system as it was most suitable for that environment though they did not rule out the possibility of a Sacrificial CP system based on the zinc anode ribbon. BHPB in response advised to design a zinc ribbon based system and on NPSL’s recommendation they agreed on the discrete anode system as in NPSL’s opinion ribbon system did not prove successful in that area for other BHPB lines. After agreement for the type of CP system based on the BHPB provided data, NPSL team mobilized to the site and collected soil resistivity measurement data. The data showed that the soil resistivity in the upper layer is generally high averaging 52.87 ohm- m with few locations suitable for making the groundbeds. Preliminary design and the groundbed calculations including proposed groundbed locations were reported to BHPB that were in principal, agreed by them with few changes. Subsequently NPSL team was again mobilized to site to mark the precise groundbed locations on the drawings and also to confirm the resistivities. It was found that the resistivities of the upper layer have been changed that necessitated to take the resistivities of the deeper soil layers which were taken by using Geonics Model EM34 Soil Conductivity Meter. It is noted that the upper soil stratum is susceptible to the seasonal changes due to varying degrees of rain fall and cultivation cycles. Fertilizer, irrigation, seasonal water ponds are responsible for changes in the resistivities.

NURICON PETROSERVICES (PTE.) LTD. 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi Phn : (92-21) 273 5051 ( 4 Lines ) Fax : (92-21) 273 4162

5 E-mail : [email protected]

Report for: Client: Doc. Title Project:

BHP BILLITON NPSL Reference: PETROCHEM ENGG. & IND. CONSULTANTS Contract No: CP SYSTEM OF 6” DIA 28 Km PIPELINE Zamzama Condensate Pipeline CP System Design for BHP BILLITON

PETROCHEM:07/002 PEC/NU-2006/002 Doc. No.: Main Report

As a result of the resistivity changes the finally proposed groundbeds are kept deeper. Since higher resistivity reduces the groundbed current output therefore higher number of anodes is required but higher number of anodes would increase the installation cost therefore two anodes per anode column are proposed to limit the installation cost by reducing the boreholes to half. Total four groundbeds are proposed, two at the ends and two in between. Test Posts are designed to reduce the risk of vandalism which is a common problem in that area. NPSL proposed the average distance between test stations as 1.0 km to facilitate coating and Close Interval Potential Surveys (CIPS) in future however BHPB opted for a distance of 2.5 km between the subsequent test stations. To our understanding BHPB would install some Radio Transmitter test stations on experimental basis along the pipeline. Grounding or Earthing zinc electrodes are also proposed along with the surge diverters, to be installed across the insulating flanges. SCOPE OF WORK The detailed design report shall comprise the minimum requirement for the design, material, installation and commissioning for the external cathodic protection system to be installed for 6“x28 km Condensate pipeline from ZamZama to Parco’s Bubak Station. The Cathodic Protection System shall be designed as a stand alone system capable for operating independently of any other existing cathodic protection systems and shall be a sacrificial anode system. OBJECTIVE The cathodic protection system objective is to prevent pipeline from corrosion by making it sufficiently cathodic by supplying current to the pipeline from anodes so that the polarization potential along the whole pipeline is between -850 to -1200 mv with respect to the Cu-CuSO4 half cell. SOIL RESISTIVITY SURVEY Soil Resistivity survey was done by Wenner 4 Pin Method at 1.25 m and 2.5 m pin spacing at every 500 m along the entire pipeline right of way (ROW). The average resistivity is estimated to be 52.87 ohm- m and the data is appended at Appendix-Soil Resistivity. There were very few low resistivity locations where groundbed could be suitably placed. After acquiring this resistivity data and the approval of preliminary calculations, 4 Pin method was repeated at the precise groundbed locations however it was revealed that the resistivities have been changed and resistivities of deeper soil layers would need to be surveyed that may be less susceptible to the environmental changes. For this purpose Geonics Model EM34 was used to measure the soil conductivities that were converted to resistivities. NURICON PETROSERVICES (PTE.) LTD. 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi Phn : (92-21) 273 5051 ( 4 Lines ) Fax : (92-21) 273 4162

6 E-mail : [email protected]

Report for: Client: Doc. Title Project:

BHP BILLITON NPSL Reference: PETROCHEM ENGG. & IND. CONSULTANTS Contract No: CP SYSTEM OF 6” DIA 28 Km PIPELINE Zamzama Condensate Pipeline CP System Design for BHP BILLITON

PETROCHEM:07/002 PEC/NU-2006/002 Doc. No.: Main Report

This method induces electromagnetic current into the soil and measures the induced current, knowing that the induction is proportional to the conductivities. This data is given at Appendix-Soil Conductivity/Resistivity. The analyses of all the acquired data lead to the conclusion that deeper groundbeds would be required to get the sufficient amount of protection current. It also necessitated the use of additional anodes than the required by anode weight calculations. DESIGN CONCEPTS The system design considerations are: Pipeline Information Diameter: Length: Wall Thickness: Coating Type: Coating Conductance: Isolation:

The pipeline has a nominal diameter of 6 inches and outer diameter of 6.625” (0.168 meter). The length of pipeline is about 28 km. The minimum wall thickness of the pipeline is 6 mm. 3 Layer Polyethylene Has an average coating conductance of 1.89x10-4 S/m2 at the end of its design life of 25 years. Electrically isolated from metallic structures along the ROW as well as at both the ends.

CP System Information Type: Anodes Material: Anode Size: Anode Weight: Package:

Sacrificial Groundbeds Zinc 2”x2”x60” long (50x50x1500 mm) 60 lbs (27.27 kg) The anodes are to be packaged in 50% hydrated gypsum (CaSO4) and 50% bentonite clay mixture. Package dimensions are given in the relevant drawing. No. of Groundbeds: Four: A, B, C and D Anode Columns: Each groundbed has 6 anode columns and 2 anodes per column. This is to reduce the installation complexities and costs since the holes are deeper than the normal for sacrificial anodes. Location of Groundbeds: See following table.

NURICON PETROSERVICES (PTE.) LTD. 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi Phn : (92-21) 273 5051 ( 4 Lines ) Fax : (92-21) 273 4162

7 E-mail : [email protected]

Report for: Client: Doc. Title Project:

BHP BILLITON NPSL Reference: PETROCHEM ENGG. & IND. CONSULTANTS Contract No: CP SYSTEM OF 6” DIA 28 Km PIPELINE Zamzama Condensate Pipeline CP System Design for BHP BILLITON

PETROCHEM:07/002 PEC/NU-2006/002 Doc. No.: Main Report

Serial

Groundbed Identification

1.

A

0+556

Zamzama Plant, near grave yard. Anodes and junction box to be installed within the plant fencing. Depth of inactive column: 26 meters.

2.

B

8+375

This location is near Goath (village) Gullan Punhwar. Depth of inactive column: 26 meters.

3.

C

19+318

This location is near the back of grid station. Depth of inactive column: 15 meters.

4.

D

27+720

Preferably within the Parco boundary wall where the pipeline enters the pumping station boundary. BHPB to decide with PARCO. Depth of inactive column: 10 meters.

Test Stations:

Serial

Chainage

Location/Remarks

See the following table. Test Stations with only two wires for only potential monitoring purpose are identified as TS whereas test stations having four or more wires, intended for potential monitoring as well as Line Current measuring, are identified as TS/CMP. Test stations are designed to reduce the risk of vandalism.

Type

Chainage

Identification

1.

Potential Monitoring 0+000 & Current Measuring

2.

Potential Monitoring

2+500

TS

3.

Potential Monitoring

5+000

TS/CMP

4.

Potential Monitoring

7+500

TS

5.

Potential Monitoring

10+000

TS

6.

Potential Monitoring

12+500

TS/CMP

7.

Potential Monitoring

15+000

TS

TS/CMP Across insulating flange; can also measure current

NURICON PETROSERVICES (PTE.) LTD. 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi Phn : (92-21) 273 5051 ( 4 Lines ) Fax : (92-21) 273 4162

Remarks

8 E-mail : [email protected]

Report for: Client: Doc. Title Project:

BHP BILLITON NPSL Reference: PETROCHEM ENGG. & IND. CONSULTANTS Contract No: CP SYSTEM OF 6” DIA 28 Km PIPELINE Zamzama Condensate Pipeline CP System Design for BHP BILLITON

Chainage

Identification

Serial

Type

8.

Potential Monitoring

17+500

TS

9.

Potential Monitoring

20+000

TS

10.

Potential Monitoring

20+950

TS

11.

Potential Monitoring

21+100

TS

12.

Potential Monitoring

23+500

TS/CMP

13.

Potential Monitoring

26+000

TS

14.

Potential Monitoring

27+700

TS

15.

Potential Monitoring N/A & Current Measuring

PETROCHEM:07/002 PEC/NU-2006/002 Doc. No.: Main Report

Remarks

TS/CMP Across insulating flange; can also measure current

Cased Crossings:

There are no cased crossings along the pipeline ROW.

Earthing Electrodes:

Earthing electrodes are used at insulating flanges to shunt the electric surges as well as to bypass the lightening surge. This may also provide a little protection on the non-protected side of the insulating flange. Four electrodes, 1.4x1.4x60” each, are to be packaged in 50-50 bentonite/gypsum mixture. Electrodes are to be paired diagonally and joined diagonally.

Surge Diverters:

To diffuse the electric and lightening surges across the insulating flanges the surge diverters are proposed to supplement the action of earthing electrodes.

Cables:

All the proposed cables would have PVC insulation & sheath having stranded high purity annealed copper conductor and a voltage grade of 600/1000. Anode cable s would have 16 mm2 conductors, whereas negative cable would have a 50 mm2 conductor. All cables for test stations and earthing electrodes would have conductor size of 6 mm2 . Each anode of a particular groundbed is proposed to have equal anode-cable lengths to keep the cable resistance and hence the anode output, uniform.

NURICON PETROSERVICES (PTE.) LTD. 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi Phn : (92-21) 273 5051 ( 4 Lines ) Fax : (92-21) 273 4162

9 E-mail : [email protected]

Report for: Client: Doc. Title Project:

BHP BILLITON NPSL Reference: PETROCHEM ENGG. & IND. CONSULTANTS Contract No: CP SYSTEM OF 6” DIA 28 Km PIPELINE Zamzama Condensate Pipeline CP System Design for BHP BILLITON

PETROCHEM:07/002 PEC/NU-2006/002 Doc. No.: Main Report

BILL OF QUANTITIES Serial

Description

Unit

Quantity

1a.

Zinc anodes of 50x50x1500 mm having net metal weight of 27.27 kg (60 lb) with carbon steel insert and zinc conforming to American Military standard for pure zinc. Anode cable, 25 sqmm x 41 meter long, single core stranded copper with PVC insulation and PVC sheath of 600/1000 volt grade should be mechanically tightened to the steel insert and should also be brazed to it. This joint should be potted with the epoxy and top of the anode around this joint to be coated with epoxy to avoid preferential corrosion. Also, the joint should be strong enough to hold the weight of the anode and packaged backfill.

Each

24

1b.

Zinc anodes of 50x50x1500 mm having net metal weight of 27.27 kg (60 lb) with carbon steel insert and zinc conforming to American Military standard for pure zinc. Anode cable, 16 sqmm x 31 meter, single core stranded copper with PVC insulation and PVC sheath of 600/1000 volt grade should be mechanically tightened to the steel insert and should also be brazed to it. This joint should be potted with the epoxy and top of the anode around this joint to be coated with epoxy to avoid preferential corrosion. Also, the joint should be strong enough to hold the weight of the anode and packaged backfill.

Each

12

1c.

Zinc anodes of 50x50x1500 mm having net metal weight of 27.27 kg (60 lb) with carbon steel insert and zinc conforming to American Military standard for pure zinc. Anode cable, 16 sqmm x 26 meter, single core stranded copper with PVC insulatio n and PVC sheath of 600/1000 volt grade should be mechanically tightened to the steel insert and should also be brazed to it. This joint should be potted with the epoxy and top of the anode around this joint to be coated with epoxy to avoid preferential corrosion. Also, the joint should be strong enough to hold the weight of the anode and packaged backfill.

Each

12

2.

4-Electrode Zinc Earthing Cell (each electrode of size 1.4”x1.4”x60”) packaged in 50-50 bentonite/gypsum mixture. Electrodes to be diagonally paired and connected and each pair to have 5 meter cable to connect to the insulating flanges.

Package

2

NURICON PETROSERVICES (PTE.) LTD. 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi Phn : (92-21) 273 5051 ( 4 Lines ) Fax : (92-21) 273 4162

10 E-mail : [email protected]

Report for: Client: Doc. Title Project:

BHP BILLITON NPSL Reference: PETROCHEM ENGG. & IND. CONSULTANTS Contract No: CP SYSTEM OF 6” DIA 28 Km PIPELINE Zamzama Condensate Pipeline CP System Design for BHP BILLITON

Serial

Description

PETROCHEM:07/002 PEC/NU-2006/002 Doc. No.: Main Report

Unit

Quantity

3.

Cadweld Charges, 15 gram

Each

40

4.

Cadweld Mould

Each

1

5.

Test Stations (see drawing No. TYP-TP-01).

Each

10

6.

Test Station/Current Measuring Post having 6 each M6 Terminals. Terminals to be of stainless steel, copper or brass, complete with terminal box, fixtures and accessories.

Each

5

7.

Anode bond box, to be able to accommodate 13 shunts, 12 anode cables, one No. Negative Cable and one Test Cable. Junction box to be complete with all fixtures and accessories including bus bar.

Each

4

8.

Royston Handy Caps with primer or Raychem's Perp Repair system for coating repair, and covering of cadweld.

Each

40

9.

Stubs, 50x50x6 mm each (to be cut from ? 6” pipe piece), to be welded on the pipeline for cadwelding/cable connections to the pipe. Welding to be done by the pipeline contractor.

Each

40

10.

Negative cable, 50 mm2 Cu/PVC/PVC, single core stranded, 600/1000 volt grade.

Meter

40

11.

Test cable, 6 mm2 Cu/PVC/PVC, single core stranded, 600/1000 volt grade. Black Color.

Meter

550

12.

Test cable, 6 mm2 Cu/PVC/PVC, single core stranded, 600/1000 volt grade. Red Color.

Meter

500

13.

Test cable, 6 mm2 Cu/PVC/PVC, single core stranded, 600/1000 volt grade. Blue Color.

Meter

40

14.

Miscellaneous items e.g., cable lugs, nut-bolts, negative & test cables, coating repair material, sand, cement, crush for junction boxes’ grouting, etc.

Lot

1

15.

SERVICES - Installation, Test and Commissioning

Lump Sum

1

16.

Surge Diverters ( see attached data sheet )

Each

2

NURICON PETROSERVICES (PTE.) LTD. 4th Floor, Mubarak Manzil, 39 Garden Road, Karachi Phn : (92-21) 273 5051 ( 4 Lines ) Fax : (92-21) 273 4162

11 E-mail : [email protected]

POWER SUPPLY SYSTEMS Isolating spark gaps

Dimension drawing EXFS

EXFS L / EXFS KU EXFS L200

Installation of EXFS

For indirect connection/earthing of functionally separate parts of installations when being affected by lightning

EXFS L ...: Ex Isolating spark gap for aboveground installation

Unit for lightning equipotential bonding according to IEC 62305 for hazardous areas (Zone 2) Corresponds to "ATEX Directive" 94/9/EC Corrosion-resistant enclosure of zinc die casting with plastic cover and flexible conductor connection For bridging insulating pieces, insulating flanges, etc. in cathodic protected pipe sections

EXFS L200 Lightning impulse current (10/350 µs) current (10/350) Classfication of lightning current carrying capability acc. to prEN 50164-3 Nominal discharge current (8/20 µs) current (8/20)

50 kA N 100 kA

100% lightning impulse sparkover voltage Lightning impulse sparkover voltage