specification for cathodic protection design

Specification for Cathodic Protection Design Version 2.0

Petroleum Development Oman L.L.C.

Specification for

Cathodic Protection Design

UNRESTRICTED Document ID : SP-1128December 2005 Filing key : xxxx

Keywords:

Cathodic protectionDesign requirementsMonitoringInternal wetted surfacesImpressed currentSacrificial anodesGroundbedCurrent densityProtection potentialPipelinesBuried structuresAnodesTransformer rectifiersTest facilities/junction boxesIsolating couplings

This document is the property of Petroleum Development Oman, LLC. Neither the whole nor any part of this document may bedisclosed to others or reproduced, stored in a retrieval system, or transmitted in any form by any means (electronic, mechanical,reprographic recording or otherwise) without prior written consent of the owner.


Version No. Date Author Scope / RemarksERD-65-12 Aug.91 TTH/5 Original ERD Document

1.0 Sept.99 OTT/11 Updated and in new PDO format2.0 Dec.05 UEC/121 Updated with minor changes

INSTRUCTIONS TO USER

Make sure this is the latest issue of this specification. Refer to the EMDS for the last issue date.

Where this Specification refers to DEPs and International Standards, it refers to the issues that were in-usewhen the author wrote this Specification. Exceptions are references to specific issues. If you use DEPs orInternational Standards with this Specification, make sure you use the latest issues.

Do not change this Specification without approval. Only the Custodian, the Corporate FunctionalDiscipline Head (CFDH) who owns this Specification, can give approval for changes. If you think theSpecification is not correct, write your comments on a copy of the User Comment Form. The form is thelast page of this Specification.

Specification for Cathodic protection Design Version 2.0

SP-1128 December 20052

Contents

Authorised For Issue Error! Bookmark not defined.

1 PREFACE 51.1 Introduction 51.2 Applicability 51.3 Language and units of measurement 5

2 FACILITIES TO BE PROTECTED 62.1 Facilities Requiring Protection 62.2 Selection of Type of Cathodic Protection System 6

2.2.1 External Protection 62.2.2 Internal Protection 7

3 CATHODIC PROTECTION PERFORMANCE CRITERIA 83.1 General 83.2 Protection Criteria 8

3.2.1 Impressed Current Systems 83.2.2 Sacrificial Anode Systems 8

3.3 Current Requirements 93.3.1 General 93.3.2 Pipeline Current Requirements 93.3.3 Well Casings 10

3.4 Avoidance of Cathodic Protection Interaction 113.4.1 General Guidelines 113.4.2 Testing 11

4 SITE SURVEYS 124.1 Introduction 124.2 Description of Terrain 124.3 Soil Resistivity Measurements 124.4 Soil Investigation 124.5 Current Drainage Tests 124.6 Stray Currents 13

5 CATHODIC PROTECTION DESIGN DETAILS 145.1 Introduction 145.2 Design Requirements 14

5.2.1 Isolation and Earthing 145.2.2 Cable Sizing 145.2.3 Hazardous Areas 145.2.4 Electrical Isolation 15

5.2.4.1 Buried In-Station Pipework Tanks and Vessels 155.2.4.2 Buried In-Station Pipework 155.2.4.3 Interstation Pipelines and Main Transmission Pipelines 15

5.2.5 Flowlines and Short Buried Sections 165.2.5.1 Well Casings 16

5.2.6 Electrical Earthing 165.2.6.1 Tanks and Vessels 165.2.6.2 Buried In-Station Pipework, Interstation and Transmission Pipelines 165.2.6.3 Transmission Pipelines Paralleling Overhead High Voltage Power Lines 175.2.6.4 Well Casings 17

5.3 External Cathodic Protection 175.3.1 Current Source 17

5.3.1.1 Impressed Current 17



5.3.1.2 Current Capacity of DC Source 185.3.1.3 Sacrificial Anodes 18

5.3.2 Station Tanks, Vessels, In-Station Pipework and Interstation Pipelines 185.3.3 Transmission Pipelines 185.3.4 Buried Sections of Above ground Pipelines and Flowlines 195.3.5 Well Casings 195.3.6 Groundbeds 19

5.3.6.1 General 195.3.6.2 Groundbed Resistance and Soil Resistivity 205.3.6.3 Positioning 20

5.4 Internal Cathodic Protection 215.4.1 General 215.4.2 Sacrificial Systems 21

5.4.2.1 Anodes 215.4.2.2 Anode Quantity 215.4.2.3 Anode Distribution 225.4.2.4 Anode Fixing 225.4.2.5 Anode Monitoring 22

5.4.3 Impressed Current Systems 225.4.3.1 Anodes 225.4.3.2 Anode Quantity 225.4.3.3 Anode Fixing 225.4.3.4 Anode Monitoring 23

6 MONITORING AND TEST FACILITIES 246.1 Introduction 246.2 Tanks and Vessels 24

6.2.1 External CP Potential Measurement 246.2.1.1 Tanks 246.2.1.2 Vessels 24

6.2.2 Internal CP Potential Measurement 246.2.2.1 Tanks 246.2.2.2 Vessels 24

6.3 Buried In-Station Pipework 256.3.1 Potential Monitoring 25

6.4 Interstation and Main Transmission Pipelines 256.4.1 Potential Monitoring 256.4.2 Isolating Joint / Insulated Flange 256.4.3 Drain Point 256.4.4 Combined Drain Point and Isolation Joint / Insulated Flange 256.4.5 Buried Cathodic Protection Coupons 256.4.6 Foreign Service Bonding 256.4.7 Cased Crossing 256.4.8 Grouted Sleeve 256.4.9 Buried Sections Of Surface Laid Pipeline/High PressureGas Flowlines 26

7 Appendix A Glossary of Definitions, Terms and Abbreviations 277.1 Standard Definitions 277.2 Special Definitions 277.3 Abbreviations 287.4 Calculation of ICCP Station Spacing For Main Transmission Pipelines 297.5 Groundbed Resistance Calculations 31

7.5.1 General 317.5.2 Horizontal Groundbeds 317.5.3 Vertical/Borehole Groundbeds 32

7.6 Sacrificial Anode Example Calculation 32



8 References 35

9 USER COMMENT FORM 36



1 PREFACE

1.1 Introduction

This Specification gives the minimum requirements for the design of cathodic protection systems forinternal surfaces of tanks and vessels, the external surfaces of tank bottoms, buried vessels, buried in-station pipework, buried flowline sections, interstation pipelines, main transmission pipelines and wellcasings.

Marine facilities (e.g. jetties and sub-sea pipelines), internal surfaces of pumps, valves etc and internalsurfaces of pipelines are not dealt with in this Specification.

1.2 Applicability

If this Specification is applicable to the work that you do, you shall obey its instructions. You shall getapproval, in writing, from the Custodian, the CFDH Corrosion who owns this Specification, before you useprocedures other than those that this Specification specifies.

This Specification is not applicable retroactively.

1.3 Language and units of measurement

You shall use the English language and the International System (SI) units of measurement in alldocuments and drawings. Where the SI unit is a conversion of a manufactured dimension, you can put theoriginal dimension, in brackets, after the SI units. For example, 50mm (2in) pipe.



2 FACILITIES TO BE PROTECTED

This section defines the structures which shall be cathodically protected. It also gives guidance on the typeof CP system(s) which may be employed on specific structures.

2.1 Facilities Requiring Protection

The following steel structures shall be cathodically protected:

Internal tank and vessel surfaces where these contain an uninhibited water phase unless GRE lined oralloy steel cladExternal surfaces of tank bottoms ( See note 1 below )

Buried vessels

Buried in-station pipework

Buried flowlines (where specified- see note 2 below) Buried interstation pipelines

Buried transmission lines

Buried sections of above ground laid interstation or transmission pipelines and high pressure gasflowlines

Well casings (where specified- see note 2 below)

Note 1

The use of asphalt carpet beneath tank bottoms is NOT recommended as the carpet acts as a shieldthat prevents the protective cathodic protection current from reaching its intended target of the tanksbottom. The asphalt carpet does NOT offer protection to the tanks bottom against corrosion in theabsence of cathodic protection.

Experience in PDO has shown that the use of dry bitumen sand mixes and oiled sand under the tankbottom is appropriate.

Where tanks are to be placed on a concrete base and where cathodic protection is required, specificguidance shall be sought from the Materials and Corrosion Engineering Department (CFDH).

Note 2

The Materials and Corrosion Engineering Department will advise when CP is required for thesestructures.

2.2 Selection of Type of Cathodic Protection System

2.2.1 External Protection

Cathodic protection of buried external surfaces should, where technically and economically practical, beby Impressed Current Cathodic Protection (ICCP) systems. This covers all the external categories stated insection 2.2 above except for the part buried short lengths of above ground pipelines and flowlines which isexplained below.

For short buried sections e.g. road crossings, of surface laid pipelines and high pressure gas flowlines,either ICCP or sacrificial anode systems shall be used as applicable. These sections shall be coated as perDEP 31.40.30.31 & 31.40.30.32.



Where the use ICCP cannot be justified and application of sacrificial anode system is technically notfeasible , the short buried section or road crossing of surface laid pipelines and high gas pressure flowlinesshall be protected by the application of three layer factory applied PE or PP coating as applicable with anadditional rock shielding coat to a minimum total thickness of 6mm. Coating field joints shall be kept to aminimum for such crossings. Field joints shall have a double seal arrangement which shall ensure thatwater cannot penetrate. Coating and field joints shall be inspected with high voltage holiday detectorimmediately prior to backfilling .

For road crossings of LP flowlines where the use of cathodic protection can be justified, either ICCP orsacrificial anode systems may be used as applicable. In general LP flowlines road crossings may beprotected by application of a coating system in accordance with PCS-2 of SP-1246 and GU-368.

Pipelines installed in cased crossings shall be cathodically protected by either the principal pipeline CPsystem or, where required, by a dedicated sacrificial anode or ICCP system.

2.2.2 Internal Protection

Cathodic protection of the internal surfaces of hydrocarbon containing tanks or vessels, with a continuousunhibited water layer, shall be by sacrificial anodes. Unless the equipment is GRE lined or alloy clad.

For tanks which do not contain hydrocarbons (e.g. Fire Water Tanks, potable water tank) cathodicprotection of the internal surfaces shall be achieved using either impressed current or sacrificial anodes.

For potable water tanks that require protection ONLY sacrificial magnesium anodes shall be considered .



3 CATHODIC PROTECTION PERFORMANCE CRITERIA

This section specifies the criteria for the design and operation of CP systems.

3.1 General

The structure to soil potential is the criterion for effective cathodic protection. For well casings only,where potential measurements cannot be reliably made, a downhole casing current density profiling toolshall be used to confirm the effective application of cathodic protection.

3.2 Protection Criteria

3.2.1 Impressed Current Systems

Impressed current CP systems shall be designed such that instantaneous OFF potentials can be measuredfor assessing the CP system performance.

CP systems shall be designed to provide sufficient current to the structure, over its design life, to achievean OFF potential over the entire structure, equal to or more negative than stated in Table 3.1. Inparticular, on tank base plates, the OFF potential shall be achieved at the centre thereof.

To avoid detrimental effects on the applied coating (disbondment) or on the structure (hydrogen inducedstress cracking) due to over protection, OFF potentials for carbon steel shall not be more negative thanthe overprotection limit value as stated in Table 3.1.

Some corrosion resistant steels and high strength steels (e.g. Duplex stainless steels) are more susceptibleto hydrogen induced stress cracking than carbon steel. The protection criteria for structures made of suchmaterials shall be determined on a case by case basis, but shall not under any circumstance be morenegative than the over protection limit given in Table 3.1. When such materials are to be cathodicallyprotected, the Company Materials and Corrosion department shall be consulted for specificrecommendations and requirements.

Anaerobic environments are not generally encountered on buried pipelines or other structures inthe Sultanate of Oman. They may be encountered on internal CP systems however. Theprotection criteria potential shown in Table 3.1 shall be used for anaerobic conditions whenmedium (electrolyte) analysis confirms the presence of active sulphate reducing bacteria inanaerobic environments, or when consideration of the operating conditions allows that these mayexist. The CFDH Materials and Corrosion shall indicate if this requirement applies.

Table 3.1. Potential Limits for Cathodic Protection for ICCP Systems

ENVIRONMENT POTENTIAL

Instantaneous OFF Potential (mV)Cu/CuSO4 Reference Electrode

Protection potential for steel in aerobic soilenvironment.

-850

Protection potential for steel in anaerobic soilenvironment

-950

Over protection limit for corrosion resistant andhigh strength steels.

-1150

Over protection limit for carbon steel -1200

3.2.2 Sacrificial Anode Systems



Sacrificial anode systems are not normally designed to enable OFF potentials to be recorded. Cathodicprotection systems shall therefore be designed to provide sufficient current to achieve a minimum ONpotential on the structure, over the design life.

For tank and vessel internal sacrificial anode systems, using Company standard composition aluminiumanodes, the design shall provide sufficient current to achieve an ON potential equal to or more negativethan minus 800mV, with respect to a silver/silver chloride reference electrode.

If used for cathodically protecting buried sections of above ground pipeline and flowlines, wheremagnesium alloy is the sacrificial anode material, the design shall provide sufficient current to achieve anON potential equal to or more negative than minus 1000mV with respect to a Cu/CuSO4 referenceelectrode.

3.3 Current Requirements

3.3.1 General

The total minimum current requirements for all new structures requiring cathodic protection shall becalculated from the area of the structure, the current density requirements and the estimated coatingbreakdown. Data for current density requirements and coating defect estimates are given in the Tables 3.2and 3.3.

When applying cathodic protection to the external surfaces of structures, care shall be taken to ensure thatan allowance is made in the design current requirement calculations for all metallic surfaces in contact withthe environment and electrically continuous with the structure. The size of the allowance shall depend onthe relative proximity of the cathodic protection groundbeds to the main structures to be protected andancillary structures (e.g. earthing systems). If remote groundbed(s) are used then the allowance shall be tothe full current density requirement to achieve cathodic protection on the main and ancillary structures; ifclose groundbeds are used then a smaller provision for the ancillary structures shall be used.

Where a number of structures, such as tanks, vessels and interstation pipework, are to be protected and/or avariety of coating systems have been used, each item shall be considered individually. The total currentrequirement shall then be the summation of individual current requirements.

When designing retrofit CP systems, current drainage tests (see Specification-SP-1129) shall be performedwherever possible to determine the minimum current requirements. The results of these tests shall becompared to calculated current demands and the highest value used to identify CP system capacity.

3.3.2 Pipeline Current Requirements

The Contractor shall carry out pipeline current attenuation calculations to determine the spacing betweencathodic protection stations as required during the pipeline life. The current densities in Tables 3.2 and 3.3shall be used as minimum design values for new projects. This data is valid for pipelines with operatingtemperatures upto 30C.

The current density values in Table3.2 are to be related to the total pipeline surface area and take intoaccount coating deterioration during the design life of the pipeline.

It is assumed that pipeline construction is carried out in a manner to avoid coating damage duringconstruction and operation.

For protection of pipelines with elevated operating temperatures the minimum design current densitiesgiven in Table3.2 shall be increased by 25% per 10C rise in temperature above 30C. The increase shallbe compounded per 10C rise in temperature.

For pipelines, or other structures, operating at temperatures above 60C the Company Materialsand Corrosion department shall be consulted for advice on appropriate design current densities.In such circumstances, it may be required to provide temporary CP to the structure, until such atime that current drainage tests may be conducted to establish the actual current requirement.



Table 3.2. Design Current Densities For Different Pipeline Coatings

PIPELINE LIFE (Years)

COATING TYPE 0 - 5 5 - 15 15 - 30

CURRENT DENSITY (mA/m)

Fusion bonded epoxyLiquid epoxyCoal tar epoxy

0.010 0.020 0.05

PolyethylenePolypropylene

0.002 0.005 0.01

The current densities given in Table3.2 already include the current requirements due to the expectedcoating breakdown during the design life of the pipeline.

3.3.3 Tanks, Vessels and Buried Pipework

The Contractor shall carry out calculations based on resistivity data and coating breakdown factors asdetailed in Tables 3.3.and 3.4.

Table 3.3. Minimum Current Density Requirements for Non-coated Steel in Common Environments

Environment Ohm.m Current Density mA/m2Soil with resistivity of:>10 101-10 201.5 500.5-1.5 75



The application of cathodic protection on well casings should be undertaken retrospectively so that currentrequirements can be determined on an existing well casing.

The minimum current requirement for design purposes shall be determined by E log i tests on at least oneand preferably more of each type of casing completion in any given field.

Where conditions allow, a casing current density profiling tool should also be used to confirm thatcathodic protection is achieved with a given current.

The data determined from such tests shall form the basis for the cathodic protection design for all wellcasings in the field, with a suitable allowance made for any expected variation through the field.

This specification shall not be used for obtaining design criteria of pulse rectified CP systems for wellcasings. Such design criteria shall be established by the PDO approved pulsed rectified CP vendors.

3.4 Avoidance of Cathodic Protection Interaction

3.4.1 General Guidelines

Where a CP system is designed for installation on a structure which is in the vicinity of a foreign structurethe design shall include features to minimise possible interaction.

Design features to be considered shall include the provision of supplementary anodes, facilities to allowresistive bonds between the structures, selective positioning of material with the appropriate insulatingproperties and selective positioning of the groundbed.

3.4.2 Testing

Where it has been necessary to incorporate measures to mitigate possible CP interaction, InteractionTesting as described in BS 7361 (Part 1) : 1991 shall be carried out prior to commissioning.



4 SITE SURVEYS

4.1 Introduction

Prior to designing a cathodic protection system for a pipeline a pre-design route survey shall be carriedout.

Information obtained during previous surveys for the proposed pipeline route may be used provided thatthe date, conditions and source of such surveys are included in the site survey report.

4.2 Description of Terrain

The survey shall include general information of the terrain along the pipeline route, including:

Type of terrain and vegetation e.g. urban areas, industrial areas, farm land, forests,desert, rocks.

Visible relevant features and crossings (main roads, overhead power lines, otherpipelines, wadis)

All other information that is considered relevant to the design of a cathodic protectionsystem.

4.3 Soil Resistivity Measurements

Soil resistivity measurements shall be carried out along the route of the pipeline at pipeline depth.The number of measurements should be determined locally depending on the length of thepipeline and known data (maximum 5 kms separation) but measurements should at least becarried out when there are visual changes in soil characteristics at a maximum separation of2kms. On completion of the Project results shall be handed over to the Company

For each type of soil, readings should be taken in at least two different locations. At each location aminimum of 2 measurements shall be carried out. Soil resistivity measurements shall be made using fourterminal (Wenner) resistivity method.

When the soil resistivity measurements are used to locate suitable places for surface groundbeds, the fourterminal Wenner method shall also be used to determine the resisitvity at greater depths (25 m maximum).

For measurement of resistivity at depths greater than 25m the Schlumberger technique may be used.

Other soil resistivity measuring methods require approval by the CFDH for Materials and Corrosion.

4.4 Soil Investigation

If it may be assumed that corrosive conditions are present due to bacterial activity, further chemical andbacterial soil analysis shall be carried out. For test methods, refer to BS 1377 parts 3 and 9.

4.5 Current Drainage Tests

When designing a cathodic protection system for existing pipelines a current drainage test should beperformed to determine the current and optimal current distribution. This may necessitate temporaryinstallation of one or more groundbeds and DC power sources (e.g. batteries or portable rectifiers), timer-units, and test facilities to the pipeline under investigation.



To obtain relevant results, pipeline isolation equipment (Section 5) and monitoring facilities (Section 6)should be installed before current drainage tests are carried out.

The required current is determined when, after full polarisation is achieved, the OFF potentials measuredat regular points along the pipeline are within the protection criteria as given in Table 3.1. A furtherallowance for future deterioration of the coating, during the remaining life of the pipeline, shall also bemade.

If the pipeline has been previously cathodically protected, historical data and data trends may be used todetermine the current demand.

4.6 Stray Currents

The Contractor shall investigate possible sources of detrimental DC stray currents and include proposals inthe design on how to mitigate the effect of such stray currents.

If the effect of stray currents cannot be predicted, the Contractor shall carry out a stray current survey atthe time of commissioning.

Stray currents shall be eliminated at the source by suitable insulation or other means. If this is impossibleother measures such as installation of a current drainage system shall be designed.

The presence of overhead AC power transmission systems shall also be identified. As a minimum, thelength and separation of parallelism(s), number of crossings, crossing angle(s) and AC rating shall berecorded. The requirements of SP-1102 and SP-1114A shall also be considered during the survey.



5 CATHODIC PROTECTION DESIGN DETAILS

5.1 Introduction

Design of a correctly sized and located cathodic protection system is vital to achieving the required designlife. Technical considerations shall be the principal factor for consideration but where more than onetechnical solution is acceptable economics also becomes important. This Section addresses allconsiderations to be made by the Contractor during design of a cathodic protection system.

5.2 Design Requirements

5.2.1 Isolation and Earthing

Structures cathodically protected using close groundbeds shall not be isolated from adjacent plant andearthing systems but shall be isolated from reinforcing bars in concrete constructions associated with thestructure.

Structures which are cathodically protected using remote groundbeds shall be electrically isolated fromcommon earthing schemes, foreign structures, pipelines and from reinforcing bars in concreteconstructions.

This shall be accomplished using independent earthing, insulated flanges, monobloc isolating joints,electrically insulating sleeves and / or isolating spool pieces.

Standard insulated flanges, monobloc isolating joints and isolating spool pieces are shown in drawingsSTD-7-5001, 5002 and 5003 respectively.

The CP design shall be such as to minimise the number of isolating devices by placing them, whereverpossible, in common pipework but always between a cathodically protected structure or pipeline and otherearthed plant items.

Structures isolated from common earthing schemes shall have an independent earthing system as detailedin section 5.2.4. Isolation from common earthing systems shall be used to stop current flow through thecommon earth bonds to structures which are required to be outside the CP system, and thus avoidexcessive current demand and the possibility of failure to adequately protect the targeted structure.

5.2.2 Cable Sizing

Cables shall be sized as follows:

Groundbed to T/R to protected structure/pipeline: The voltage drop between the structure/pipeline andthe groundbed shall not be more than 5V at the full rated output of the T/R. Preferred cable sizes have 10,35 or 70mm cross sectional areas.

Groundbed to Solar Generator to CP structure/pipeline: 70mm cable shall be used.

In both the above cases the driving potential between the groundbed and structure / pipeline shall besufficient to drain the minimum required current from the structure / pipeline.

Ancillary equipment, Sacrificial Anodes etc.: see standard drawings Specification-SP-1136.

5.2.3 Hazardous Areas

All CP power sources and junction boxes shall be located outside of hazardous areas.

Test posts (e.g. for tanks, vessels) should be located outside of hazardous areas. When this is not feasible,e.g. for monitored internal anodes, under tank monitoring duct test posts and tank circumference soil pots.



These may be located adjacent to the structure. Appropriate safety measures (e.g. Class A work permit, gastest, intrinsically safe meters) shall be taken during monitoring activities.

Isolation shall be effected outside hazardous areas or be protected against inadvertent short circuitingacross the insulation.

5.2.4 Electrical Isolation

5.2.4.1 Buried In-Station Pipework Tanks and Vessels

Cathodic protection of the external bottoms of tanks and the external surfaces of buried vessels shall be bythe use of close anodes or groundbeds which are located sufficiently close to ensure that current flowspreferentially to the vessels or tank whilst achieving an even spread of protection. Under thesecircumstances isolation of this vessel or tank is not required.

Likewise isolation is not required for internal protection of tanks and vessels.

5.2.4.2 Buried In-Station Pipework

Whenever possible buried in-station pipework should be manufactured from suitable non-metallicmaterials.

Buried in-station steel pipework shall be cathodically protected using a close anode system, designed toensure that current flows preferentially to the pipework whilst achieving an even spread of protection. It isnot, therefore, required to provide any isolation of this pipework from tanks, vessels or plant earthingsystems.

5.2.4.3 Interstation Pipelines and Main Transmission Pipelines

Pipelines which have low BS&W (>1.0%) and gas lines shall generally be electrically isolated by means ofinternally coated monobloc isolating joints (DEP 31.40.21.31 refers), installed above ground at both endsof the pipeline.

If the product transported by the pipeline is an electrolyte (e.g. water) or it may be anticipated that it maycontain an electrolyte at any time during the life of the pipeline, then isolating spools designed as per thefollowing rules shall be installed. Where a pipeline has an HDPE liner, isolating spools may not berequired. Isolation is still required, and this may be achieved by the use of an isolating flange kit.

If the resistivity of the electrolyte is higher than 1 Ohm.m, or the volume occupied by the electrolyte is lessthan 5% of the pipeline volume, the overall length of the isolating spool shall be four times the pipediameter (with a minimum of one metre).

If the resistivity of the electrolyte is below 1 Ohm.m, or the volume of electrolyte is more than 5%of the pipeline volume, the length of an isolating spool shall be determined by the followingformula:

D )(400/L L =

Where:

L = length of spool (cm), = electrolyte resistivity (Ohm.cm)D = nominal pipe diameter (cm).

Acceptable isolating spools are:

(a) Glass reinforced epoxy pipe designed and manufactured in accordance with DEP31.40.10.19;(b) Pipe spool fitted with PE liner or RTP



(c) Internally coated pipeline spool and holiday free when tested by high voltage spark detectionequipment. Seek Materials and Corrosion Discipline advice for specific coating recommendations.

5.2.5 Flowlines and Short Buried Sections

For new facilities where CP is applied all flowlines shall be isolated from the station inlet manifold intowhich they flow. This shall be done by the use of an isolating flange. For new stations that have closegroundbeds this will not be required.

Short buried sections of above ground pipelines shall be isolated from the above ground section by use ofisolating flanges. For road crossings that have CP the buried protected section shall be isolated from theabove ground section.

5.2.5.1 Well Casings

All well casings shall be isolated from flowlines, gas lift, gas / water injection lines or electricalearthing systems associated with Beam Pump / Submersible Pump producers, irrespective ofwhether CP is proposed at time of completion.

Flowlines and water injection lines shall be isolated by spool pieces designed as detailed in the precedingsub-subsection.

Gas lift and gas injection lines shall be isolated by insulated flanges.

Any of the above devices shall be installed at the edge of the respective well pad location.

5.2.6 Electrical Earthing

5.2.6.1 Tanks and Vessels

The effect that the earthing system has on the cathodic protection system will largely depend on theproximity of the CP anode groundbeds to the structure and the type of earthing material. If the anodegroundbed is located close to the structure then the CP current will flow preferentially to the structure andthe effect of the earthing system will be minimised. This is the mandatory method that has been adopted byPDO for applying cathodic protection to all new structures. The following paragraphs are to be used as aguideline and are included to give an understanding of how existing systems effect electrical earthing.

If the anode groundbed is remote from the structure then the earthing system will have a large effect on theCP system. The size of this effect will depend on the material used for the earthing system. If copperearthing is used then very high current requirements are expected and the effects of current straying toforeign structures shall be evaluated and recommendations for alleviating their effect on existingequipment submitted to the Company. Such recommendations shall consider the use and efficiency ofisolation of the tank or vessel and the use of an independent earthing system.

If earthing of tanks and vessels consists of a dedicated earthing grid of insulated copperconductors and earthing electrodes constructed of DN50 galvanised steel pipe in accordancewith the guidelines of DEP 33.64.10.10 Gen, Section 6.4 Earthing and Bonding currentrequirements will be considerably less than required to protect a structure which is earthed usingcopper. In this case straying currents, to foreign structures, should not be a problem.

5.2.6.2 Buried In-Station Pipework, Interstation and Transmission Pipelines

Surge diverters shall be installed across all isolating joints and insulated flanges as shown inSTD-7-3007.



5.2.6.3 Transmission Pipelines Paralleling Overhead High Voltage Power Lines

Three main problems can exist from the parallelism of high voltage overhead power lines and buried orabove ground pipelines and flowlines:

Induced AC voltages may be hazardous to personnel, see Procedure-11345 on SafeWorking Procedures on Cathodic Protection Systems.

Induced AC currents can adversely effect the cathodic protection system

Very high transient voltages can occur during fault conditions, e.g. lightning strikes orphase imbalance, which present a hazard to personnel and may damage the pipelinecoating.

From a cathodic protection point of view the induction of AC voltages on the pipeline can cause ACcurrent to flow to earth via the rectifying elements of the transformer rectifier and the groundbed. Thiscurrent, which has been half wave rectified, can flow back into the pipeline as DC current, cause increasedDC potentials on the line, make the control and monitoring of the CP systems difficult and may possiblydamage T/R components and the pipeline coating. Additionally, high AC potentials on the line may behazardous to personnel engaged in routine pipe to soil potential measurements.

The Contractor shall submit proposals to the Company for mitigating the effects of induced AC on buriedor surface pipelines where the overhead power lines are rated at 132kV or above, and the pipeline isseparated therefrom by less than 500m over a minimum 0.5 km parallelism (Refer to SP-1114A). Differentconsiderations are required for parallelisms between overhead powerlines and surface laidflowlines/pipelines, when lower voltage systems may create a hazard. In such cases, where the overheadpower lines are rated at 33kV and the pipeline/flowline is separated laterally by less than 15m over a0.5km parallelism (Refer to SP-1102), proposals shall be made for mitigating the effects of induced AC.

In each case the Contractor shall consider all factors relating to the extent of potential AC voltage whichmay occur. These include overhead powerline rating, minimum/maximum separation of parallelism, lengthof parallelism, number and angle of overhead powerline/pipeline crossings, soil resistivity, coatingconductance, type of AC powerline support pole (e.g. wood/metal) and any other factors as may beapplicable on a case by case basis. If the Contractor is not sufficiently experienced to undertake thisassessment, he shall appoint another suitably qualified authority for this.

5.2.6.4 Well Casings

Electrical earths shall be isolated from thewell head by use of solid state polarisation devices. Alternativelya galvanised steel earth may be used such that isolation is not required.

5.3 External Cathodic Protection

5.3.1 Current Source

5.3.1.1 Impressed Current

Where a continuous AC power supply is available, CP current shall be supplied using a T/R with a ratedoutput voltage no greater than 48V and shall comply with DEP 33.64.10.10 - Gen. and Specification-SP-1130

When a suitable continuous AC power supply is not available solar generators should be used. Alternativepower sources (e.g. TEGs) shall be subject to Company approval.

The use of multi-channel power supplies shall be considered in appropriate circumstances, e.g. multi-tankexternal base CP systems with close anodes.



All CP power sources shall be located in compounds which shall consist of a concrete foundation and beindividually fenced and provided with an access gate. The access gate shall be a single personnel gate,with a lockable latch type closure, suitable for locking with a padlock. The concrete foundations shall belarge enough to accommodate all of the necessary equipment (T/Rs, solar panels, mains switch box,current distribution box etc.) allowing a 1 m working space all around. The foundation shall be providedwith conduit(s) for all cabling. Cables shall not be run on the concrete surface and shall not be cast in theconcrete. Where a T/R or other non-solar power supply is used a sunshade shall be fitted over thecompound, in accordance with SP-1283.

Compound fencing shall be either of the leaf gate type or the chain link type. If the compound is on-plot itshall be located adjacent to the facility perimeter fence and the single mangate located in the perimeterfence. The compound fencing shall be to the same standard as the facility fencing and fitted with two gatesso as to allow access to the compound from the outside and the inside of the facility.

All buried positive and negative cable runs shall be marked using cable route markers to comply withSTD-7-7001.

A review of development plans in the vicinity of the projected CP system shall be carried out. Where it isfound that additional structures which will require CP are to be built, the CP design shall allow for thisexpansion.

5.3.1.2 Current Capacity of DC Source

Design current requirements shall be determined as described in Section 3. The DC current source shall becapable of providing at least 120% of the design current where current drainage testing has beenperformed and at least 130% of the design current where this has been determined through calculation. Inany event the current source rating shall be minimum 10 amps, but for higher ratings should not be capableof providing more than 150% of the design current requirement, unless it can be shown to be technically oreconomically appropriate.

5.3.1.3 Sacrificial Anodes

Sacrificial magnesium anode cathodic protection systems may be employed for short buried sections wheretheir use can be both technically and commercially justified. The maximum design life for sacrificial anodesystems shall be 5 years.

5.3.2 Station Tanks, Vessels, In-Station Pipework and Interstation Pipelines

Each structure shall have a discrete drainpoint connection and a separate negative return cable. Theseconnections shall be made using welded pads to comply with STD-7-2001 or STD-7-2003.

Where groups of structures are to be protected using the same current source, cables shall run from eachstructure to common, centrally located NDB(s). Cables from these shall run to an NJB and cables from thisshall terminate at the current source.

5.3.3 Transmission Pipelines

A series of dedicated CP stations distributed along the length of the pipeline shall be used to providecurrent.

Distances between neighbouring stations shall be based on current attenuation calculations with dueconsideration for local variations in terrain and geology. Example calculations are given in Appendix 1.

Where a pipeline is being constructed parallel to an existing pipeline and is within 50 metres of thatpipeline then the pipelines shall have the facility to be electrically bonded at intervals of 10km. Theoperating history of the existing CP system and the current demand of the new line shall be reviewed todetermine the need to provide additional CP stations.



If the pipeline is to be constructed inside steel casings or culverts at major crossing features, therequirement for cathodic protection of the carrier pipe inside the casing or culvert shall be considered. Theprovision of supplementary cathodic protection shall then be designed on a case-by-case basis.

5.3.4 Buried Sections of Above ground Pipelines and Flowlines

All new buried sections of essentially above ground pipelines and flowlines shall be coated as stated insection 2.4 and cathodically protected. The preferred method for achieving this, where economicallyjustified, is via an impressed current source. The selection between use of existing current sources orinstallation of dedicated current sources shall be made by the specialist design engineer (Contractor).

Sacrificial magnesium anode cathodic protection systems may be employed where their use can be bothtechnically and commercially justified. The maximum design life for sacrificial anode systems shall be 5years.

When considering a single flowline or pipeline buried section at the design stage, the Contractor shall takeinto account nearby or parallel buried pipework and any possible interference that may occur. In suchcases necessary mitigation shall be considered during the design period.

5.3.5 Well Casings

Well casings may have dedicated groundbed and impressed current power supplies or may be linkedtogether in clusters such that one power supply / groundbed protects more than one well.

Power supplies shall be either conventional DC transformer-rectifiers, pulsed rectifiers or solar generatorsdepending on the field layout and application.

Borehole type groundbeds installed below the water table shall be used for well casing cathodic protection.

Principal factors influencing choice of power supply include :

Casing depth

Casing current demand

Separation between adjacent wells

Availability of AC power supply

The Contractor shall consider each well casing and field on a case-by-case basis and propose the mostsuitable option for approval.

5.3.6 Groundbeds

5.3.6.1 General

The selection of type and design of groundbeds shall take into account the following:

Soil resistivity at the location

The location shall be capable of providing satisfactory current distribution to thestructure(s) intended

Minimising the risk of harmful interference and installation costs.

Groundbeds shall be designed to comply with STD-7-6001,STD-7-6003, STD-7-6004STD-7-6005 STD-7-6006 or STD-7-6007 as appropriate.



The number of groundbeds shall be equal to or exceed the number of power supplysources for each location.

The number and size of impressed current anodes shall be sufficient to operate at the calculated currentoutput for 20 years. For horizontal and vertical shallow groundbeds PDO standard size Silicon-Iron-Chrome 1525 mm x 75 mm anodes or cannistered anodes shall be used. For borehole groundbeds eitherMixed Metal Oxide, Platinised Titanium / Niobium or Silicon-Iron-Chrome (1220 mm x 50mm size)anodes shall be used, based on a techno-economic review of required groundbed depth, active length,number of anodes and current requirement. Refer to section 5.1of SP-1130 for anode details. All anodes inhorizontal, vertical and borehole groundbeds shall be individually monitored via an anode junction box.

On new tanks or on tanks which have been re-bottomed the external anode system shall consist of flexibleor wire type ribbon anodes placed below, and in close proximity to, the tank base. The system design,anode type, sizing and its location and method of installation shall be subject to Company approval.

On pipelines installed in steel casings or concrete culverts, the anode system shall consist of impressedcurrent flexible or wire type anodes or sacrificial magnesium or zinc ribbon anode, as appropriate,designed on a case-by-case basis.

5.3.6.2 Groundbed Resistance and Soil Resistivity

Groundbeds (apart from close anode type groundbeds) shall be designed to have a resistance to remoteearth of less than 0.5 Ohm and to fulfil anode current output characteristics under normal soil conditions.Example design calculations are given in Appendix 2.

Soil resistivities, used in shallow groundbed calculations, shall be measured using the Wenner four pinmethod for depths of 25, 20, 15, 10, 5, 2, 1 and 0.5 metres in accordance with DEP 30.10.73.10-Gen. Fordeeper resistivity data alternative methods (e.g. Schlumberger method) may be proposed for Companyapproval.

For very deep borehole groundbeds (>100m) data on nearby groundbeds and water table depth may beused to design new groundbeds. In such cases the data and design shall be subject to Companyconfirmation and approval.

5.3.6.3 Positioning

The minimum separation of horizontal and vertical groundbeds from any buried facilities such as pipelines,wells, flowlines, and other groundbeds shall be 200 metres.

The minimum horizontal separation of borehole groundbeds from any buried onplot facilities such aspiping, flowlines, tanks, vessels and other groundbeds should be 50 metres or so as to minimise the spreadof current to other structures.

Where groups of structures exist, shielding may occur. In these instances it is sometimes desirable todistribute borehole groundbeds such that the minimum separation is less than that given above. Wheredoubt over groundbed distribution arises the CFDH for Materials and Corrosion shall be consulted.

In any event the cable run between the CP power supply and its associated groundbed(s) should beminimised and shall not exceed 1000 metres.

Close anode systems shall be positioned so as to minimise the spread of current to other structures whilstproviding an even level of protection over the surface of the structure under protection.



5.4 Internal Cathodic Protection

5.4.1 General

The internal surfaces of tanks and vessels, as defined in section 2.1, which contain an uninhibitedcontinuous phase of water in normal operation shall be protected using a CP system separate to theexternal CP system.

Only sacrificial anode systems shall be used for tanks which contain hydrocarbons. For tanks notcontaining hydrocarbons either impressed current or sacrificial systems may be used.

Sacrificial anodes shall not be applied to parts of tanks or vessels lined with glass fibre reinforced epoxy(GRE) coating. If required anodes may be placed on the tank walls above the GRE lining.

5.4.2 Sacrificial Systems

5.4.2.1 Anodes

Aluminium anodes in accordance with Specification-SP-1130 shall be used for hydrocarbon service. Fortanks containing hydrocarbons anodes shall be either placed on the floor or on the walls depending on thecoating type used

For potable water service magnesium anodes shall be used.

For vessels anodes shall be mounted along the bottom of the vessel.

Anodes shall be of commercially available dimensions and weight to satisfy design requirements andachieve an even spread of current across the surface of the structure under protection.

5.4.2.2 Anode Quantity

The number of anodes required shall be determined such that the surfaces will be fully protected for a 20year period.

The number of anodes, N, has to satisfy two criteria:

wtanodeIndividualwtlTheoreticaTotal

outputcurrentanodeIndividualrequiredcurrentTotal

The electrochemical efficiency of the anode material shall be calculated using the equation:

20)-(T27-2000E

Where: E = capacity of anode material (Ah.kg-1)

T = operating temperature of electrolyte in degrees centigrade (C)

Current requirements for tank walls and floors and vessel surfaces shall be calculated as described inSection 3.5. For these calculations the area of the wall/surface shall be taken as the average area whichunder normal operating conditions is in contact with the water phase.

An example calculation to calculate the number of anodes required is given in Appendix B.



5.4.2.3 Anode Distribution

Anode distribution in tanks and vessels is heavily influenced by the presence of internal baffles andlocalised flow regimes. These features will vary considerably depending on the nature of the vessels to beprotected. As a general rule anodes should be spaced such that they see all of the areas which requireprotection; areas of low flow such as corners require a heavier concentration of anodes.

For tanks not containing hydrocarbons in which anodes are wall mounted they shall be positioned in thewater phase to within 0.5 metres of the water height expected under normal operating conditions. (Theminimum fixing height shall be 0.5 metres from the floor.) For tanks containing hydrocarbons the anodesshall be evenly distributed around the perimeter of the floor at a distance of 0.5 metres from the wall.Anodes which are required to protect the floor shall be evenly distributed over the entire area in astaggered fashion.

In drains vessels anodes shall be positioned in a line along the bottom of the vessel.

5.4.2.4 Anode Fixing

In tanks anodes shall be mounted with 0.3m of stand-off height by bolting and tack welding on steelsupports welded to the steel surface.

In onplot equipment the anodes shall be mounted by bolting and tack welding on steel supports welded tothe steel surface. Welding of the supports shall take place during construction of the vessel and be inaccordance with the appropriate construction code.

After mounting, the steel surface around the support and the entire anode support and anode core shall becoated to the same standard as the internal coating.

5.4.2.5 Anode Monitoring

At least one anode in each tank shall be mounted such that its current output can be monitored. This shallbe achieved by isolating the anode from the stand offs and connecting it via a shunt. The cables shall exitthe tank via a coffadam arrangement as per STD - 7-4003.

Monitoring of vessel anodes is not required.

5.4.3 Impressed Current Systems

5.4.3.1 Anodes

Impressed current systems shall only be used in tanks / vessels which do not contain hydrocarbons.

Impressed current anodes for internal cathodic protection shall be selected from mixed metal oxide, siliconiron, platinised titanium or platinised niobium. The latter may be preferred in higher resistivity waterenvironments.

5.4.3.2 Anode Quantity

Anodes shall be selected to provide a 20 year design life when operating at the design current.

The current requirements for the area to be protected shall be determined as indicated in section 3.5

5.4.3.3 Anode Fixing

Anodes shall be suspended from a suitable mounting fixed to the underside of the tank roof. The anodemust be suitably insulated from the mounting to prevent shorting.

A facility to remove the anode for inspection should be incorporated so that the anode may be removedwithout emptying the tank.



Care must be taken to ensure that the anodes will be submerged regardless of water level during the normaloperation of the structure.

5.4.3.4 Anode Monitoring

All impressed current anodes or anode strings used for internal CP shall be individually monitored via anexternally located junction box.



6 MONITORING AND TEST FACILITIES

6.1 Introduction

Regular monitoring of cathodic protection systems is vital to maintaining the design life integrity of thestructure. This Section specifies the minimum requirements for design of monitoring systems for all typesof cathodic protection system within the scope of this Specification.

6.2 Tanks and Vessels

6.2.1 External CP Potential Measurement

6.2.1.1 Tanks

All new or re-bottomed tanks shall have a slotted, non-metallic monitoring duct installed below the baseplates, extending from the tank centre to beyond the rim, to allow measurement of tank base plate potentialby insertion of a portable reference electrode as detailed in drawing STD-4001.

On all new and re-bottomed tanks of diameter greater than 10m potential measurement coupons shall beinstalled, adjacent to the duct, at tank centre and half radius. Tanks of diameter 10m or less shall only haveone coupon installed at the tank centre. Coupons shall be positioned such that the cable tails terminate in acommon test facility adjacent to the above ground access point to the duct.

Potential measurement soil pots shall be installed within 1m of the tank rim, located equidistantly aroundthe tank. Tanks of diameter upto 50m shall have 4 No. soil pots and tanks of greater 50m diameter shallhave 6 No.

All shall be in accordance with STD-7-4001

6.2.1.2 Vessels

All new buried vessels shall have two potential measurement coupons and associated soil pots inaccordance with STD-7-4002.

6.2.2 Internal CP Potential Measurement

6.2.2.1 Tanks

When cathodically protecting internal tank surfaces one or more 2 inch nozzles complete with full borevalves shall be provided to allow for insertion of reference electrodes. Fittings shall be clear of bothinternal and external obstructions or remote frame works and should be easily accessible from the outside.

In all cases a fitting shall be positioned as close as possible to the tank floor.

Where a water level equal to or greater than 4 metres from the base is expected during normal operation, asecond 2 inch fitting shall be installed. This shall be positioned 0.5 metres below the expected water level.

For sacrificial systems one anode in each tank shall be installed to allow external monitoring of currentflow. Monitoring cable(s) shall exit the tank via a cofferdam, all in accordance with STD-7-4003.

6.2.2.2 Vessels

The internal CP system of onplot equipment does not require the installation of monitoring facilities.



6.3 Buried In-Station Pipework

6.3.1 Potential Monitoring

Buried in-station pipework shall be protected by close anode systems. Monitoring facilities, therefore,shall consider the buried length of pipework and the anode type/number installed. As a minimum potentialmonitoring facilities shall be installed at each end of buried pipe sections. For longer buried pipe sectionsthe maximum spacing between test facilities shall be 50m. For systems using discrete anodes (e.g. Si-Fe-Cror MMO) distributed along the buried piping, monitoring facilities shall be located at the most remotepoints from the anode(s). If required other test facilities shall be installed in accordance with the relevantdrawing shown in SP-1136

6.4 Interstation and Main Transmission Pipelines

6.4.1 Potential Monitoring

Combined potential monitoring test posts/distance markers shall be installed at 2 km intervals along thepipeline route, unless the position of this test post coincides or is in close proximity ( 100m) to anothertype of test point. Installation shall comply with STD-7-3012.

6.4.2 Isolating Joint / Insulated Flange

An isolating joint / insulated flange (or spool) test facility shall be installed at all pipeline isolating joints /insulated flanges(spools). The installation shall be in accordance with STD-7-3007.

6.4.3 Drain Point

A test station in accordance with STD-7-3003 shall be installed at every drain point connection.

6.4.4 Combined Drain Point and Isolation Joint / Insulated Flange

Where drain point and isolating joint / insulated flange (spool) test facilities coincide, a combined testfacility in accordance with STD-7-3019.

6.4.5 Buried Cathodic Protection Coupons

Coupon test facilities in accordance with STD-7-3016 shall be installed at the mid-points between allDrain Point test facilities.

6.4.6 Foreign Service Bonding

Foreign service test facilities shall be installed at all foreign service crossings in accordance with STD-7-3005 and STD-7-3010 .

Where one or more foreign pipelines parallel the protected line, but are not included in the protectionscheme, test facilities complete with bond boxes shall also be installed at 5 km intervals.

6.4.7 Cased Crossing

Where a pipeline is cased, for example at road crossings, then cased crossing test facilities shall beinstalled in accordance with STD-7-3018. Where a casing is less than 10m in length a single test facility atone end of the casing is required. For casings of length 10m or greater test facilities shall be installed ateach end of the casing.

6.4.8 Grouted Sleeve

Where grouted sleeves are installed on pipelines these shall also have test facilities in accordance withSTD-7-3017 provided to allow for a bond between pipeline and sleeve.



6.4.9 Buried Sections Of Surface Laid Pipeline/High PressureGas Flowlines

Standard CP test post shall be installed per section buried in accordance with STD 7-3001.



7 Appendix A Glossary of Definitions, Terms andAbbreviations

The following terms and abbreviations used in this document, are defined below:

7.1 Standard Definitions

The list that follows tells you the meaning of some words in all Specifications:

Company: Petroleum Development Oman LLC

Contractor: The person or organisation that supplies thecompany with services.

Vendor: The person or organisation that supplies thecompany with materials and/or equipment.

Discipline: A specific set of technical knowledge and skills

Corporate Functional DisciplineHead:

The person within the Company responsible forthe discipline to which the specification belongs.The CFDH approves the Specifications thatapply to his discipline

User: The person or organisation that reads, and usesthe information, in this and other Specifications

Shall: Indicates a requirement

Should: Indicates a recommendation.

May: Indicates a possible course of action.

7.2 Special Definitions

Cathodic Protection: Process to reduce or prevent corrosion of structures in contact with an electrolyteby maintaining the flow of electrical current through the electrolyte into the surface of the structure beingprotected. The flow of current into the surface of the structure results in a negative change in the surface toelectrolyte potential of the structure. When a critical surface to electrolyte potential is achieved thestructure surface is fully protected from corrosion. Cathodic protection can be achieved using impressedcurrent or sacrificial anode systems.

Structure: The electrically continuous steel plant or equipment to be protected using cathodic protection.(Not inclusive of pipelines)

Foreign Structure (or Pipeline): A structure or pipeline which is either not Cathodically Protected or isprotected by another separate system.

Potential: Refers to the surface to electrolyte potential of a structure measured in Volts, with respect to areference cell, unless specifically stated otherwise.

Electrolyte: A liquid or the liquid component in a composite material in which electric current flows bythe movement of ions. For the purposes of this Specification electrolyte shall indicate either soil and/orwater.



Drain Point: The point on a structure or pipeline to which the current return (negative) cable is attached.

ON Potential: Electrical potential measured while cathodic protection system is operating.

OFF Potential Or Instantaneous OFF Potential: Electrical potential measured within 100milliseconds after the cathodic protection system has ceased operation and with no current flowing to orfrom the structure.

Impressed Current: Method of providing cathodic protection by connecting the structure to a DC powersupply.

Sacrificial Anode: Metals and alloys with a more negative electrochemical potential than steel which areconnected to structures to provide cathodic protection. They are consumed during cathodic protection,require periodic Specification and are typically alloys based on Aluminium, Zinc or Magnesium.

7.3 Abbreviations

AC: Alternating Current

CP: Cathodic Protection

DC: Direct Current

DP: Drain Point

FBE: Fusion Bonded Epoxy (Coating)

FRP: Fibre Reinforced Polymer

GRE: Glass Fibre Reinforced Epoxy (Coating)

ICCP: Impressed Current Cathodic Protection

NDB: Negative Distribution Box

NJB: Negative Junction Box

PCS: PDO Painting and Coating System (Refer to Specification-48-01)

PE: Polyethylene (Coating)

PP: Polypropylene (Coating)

T/R: Transformer/Rectifier



7.4 Calculation of ICCP Station Spacing For Main Transmission Pipelines

If impressed current cathodic protection is applied to a long pipeline, the length of pipeline that may beprotected from a single cathodic protection station (in each direction from the drain point) can beestimated from the following equations.

..(1)aL(V)cosh EmEa

Where:

L = length of pipeline (m)Ea = change in pipeline potential (V) at the drain point due to theapplication of impressed currentEm = Change in pipeline potential (V) at a point L due to theapplication of impressed current

g = coating conductance per unit length (mho/m)r = pipeline resistance per unit length (Ohm/m)a = square root of the product of g and r

These equations assume a number of conditions such as the use of remote groundbeds, uniform coatingconductance, uniform pipe resistance and zero soil resistivity, although the latter is only really importanton bare or poorly coated pipelines where it is significant compared with the coating resistivity. Anydeviations from these conditions shall be taken into account and if necessary sections of the pipeline shallbe treated separately i.e. coating system changes, pipeline diameter changes.

EXAMPLE

An 80 km pipeline of 16 inch nominal diameter and a wall thickness of 0.344 inches is manufactured toAPI 5L X 42 pipe. The external coating is fusion bonded epoxy powder of nominal DFT 500 microns.Design temperature is 50C.

What is the end of life (30 year) current demand and how many cathodic protection stations arerequired due to attenuation of current along the pipeline ?

a) Pipeline design current demand is calculated using Table 3.2

mdLareasurfacePipeline

= x (16 x 0.0254) x 80,000 m

= 102,140m

Current density at 30C = 0.05 mA/m (Table 3.2)Current density at 50C = 0.05 x 1.25

= 0.078mA/m

Therefore design current demand,

= 102,140 x 0.078= 7967 mA

= 7.967 Amps

b) Pipeline current attenuation (spread) is calculated from equation (1) above,



aL EmCoshEa

Assuming a natural pipeline potential of minus 0.5V (wrt Cu/CuSO4 reference), then when the protectionpotential criteria limits are minus 1.2V at the drain point and -0.95V at the furthest point.

Ea = 0.7VEm = 0.45V

Calculate the value of g

g = surface area per metre length / resistance of 1m of coating

(in the absence of other information a value of 9,000 Ohm/m is a reasonable design value for fusionbonded epoxy coating. See note 1 for basis and values for other coating materials).

g = .d.1 / 9000

= 1.28 / 9000

= 1.419 x 10-4 mho / m

Calculate the value of r

r = steel resistivity / cross sectional area of pipe= 0.16 x 10-6 / ( x 16 x 0.0254 x 0.344 x 0.0254)

= 0.16 x 10-6 / 0.0112

= 1.429 x 10-5 Ohm / m

Calculate the value of a from:

rg.a

a = 4.503 x 10-5 m-1

From equation (1),

m

a

EE

cosh aL

1.556 = cosh aL

1.011= aL

= 22.45 km

One CP station would be insufficient to protect the whole of the pipeline.

The introduction of additional CP stations on the same pipeline require, in theory, modification of thevalue of Em, since the required potential shift at the mid-point between CP stations will be influenced byeach station.



In such instances, the minimum allowable potential shift (Em) at the mid-point shall be taken as 0.25 V,this equating to a theoretical pipeline potential at that point of 1.00V, when under the influence of two CPstations.

Notes

1. Coating resistance figure based on 75% of the value obtained by back-calculations in the formula :

mEr

aI sinh aL,

and equation (1), based on current density figures given in Table 3.2.

Design values for other coatings are as follows:

Coating Type Design Coating Resistance(15 - 30 year life)

Ohm / m

Fusion bonded epoxyLiquid epoxyCoal Tar epoxy

9,000

PolyethylenePolypropylene

30,000*

* Value based on 50% of calculated figure.

7.5 Groundbed Resistance Calculations

7.5.1 General

In order to accurately predict the power requirements of a CP power source it is necessary to know theresistance of the output circuit. As the resistance to earth of the goundbed is a major part of the outputcircuit resistance its calculation is of obvious importance.

7.5.2 Horizontal Groundbeds

For a PDO standard horizontal groundbed consisting of 1525 mm x 75mm Silicon-Iron-Chrome anodesinstalled in a 300mm x 300mm trench of carbonaceous backfill at 1.2m depth in soil of 1000 Ohm.cm.Groundbed resistance is calculated using the Dwight Formula:

2

2

16224log4log

4R

LS

LS

SL

er

Le

L

Where,

2L = total length of groundbed (cm)r = radius of groundbed section (cm)S = depth to centreline of groundbed (cm) = soil resistivity (Ohm.cm)

Therefore, if L = 2500cm, r = 15cm, S = 240cm and = 1000 Ohm.cm



2

2

250016240

250022402

24025004log

1525004log

250041000R

xx

xe

xe

R = 0.0318 (loge 666.67 + loge 41.67 2 + 0.048 0.0006)

R = 0.0318 (6.502 + 3.73 2 + 0.048 0.0006)

R = 0.624Ohm

7.5.3 Vertical/Borehole Groundbeds

For vertical anode and borehole groundbeds, the resistance to earth (R) is best calculated from thefollowing formula, which is based on the Modified Dwight Formula for a single vertical anode;

)1(14log2

Rv

r

Le

L

Where,

= soil resistivity (Ohm.cm)L = length of groundbed (cm)r = radius of groundbed (cm)

The above calculation directly yields the theoretical resistance to earth of a single vertical anode orborehole.

For multiple vertical anodes, minimum parallel spacing 1m, the resistance to earth (Rn) of n anodes isgiven by.

)2(Rn n

RF vn

Fn = paralleling factorn = number of anodesRv = resisance of a single anode to earth from equation ---(1)

The paralleling factor, Fn, is calculated using:

Fn = )66.0(log1 neSRv

Where, S = spacing between electrodes.

7.6 Sacrificial Anode Example Calculation

A crude oil storage tank has an average surface area in contact with the water phase of 5,000 m. Theinternal surfaces are coated in accordance with ERD-48-01 and the water has a resistivity of 0.25 Ohm.m.



The design basis is as follows :

Tank wetted area (average), A : 5,000 mWater resistivity, : 25 Ohm.cmOperating Temperature, T : 45CCoating breakdown, Cb : 10%Design current density, Id : 110 x 10-3 A/m (seeTable 3.3)Anode electrochemical efficiency, ET (@ 20c) : 2000 Ah/kgAnode Utilisation factor, U : 0.9Design life, L (hours) : 87660 hours (10 years)

Step 1

Calculate total current requirement, Ir,

Ir = Id x A x Cb

= 110 x 10-3 x 5,000 x 0.1

= 55A

Calculate anode resistance, Ra,

For long slender stand off anodes a minimum of 300mm from the protected structure surface,anode resistance is given by the Dwight Formula.

14log

2Ra

r

Le

L

Where,

L = length of anode (cm)R = radius of anode (cm)

For non-cylindrical anodes,

2r

C

Where,

C = cross-section periphery of anode (m).

Therefore, utilising a commercially available anode size, weight 54.4kg, with dimensions.

L = 61.0cmW = 17.8cm

H = 17.8cm

r =2

)8.178.17(2 x

r = 11.3 cm



Therefore,

1

3.110.614log

0.61225Ra xexx

Anode current output (per anode), La, using Ohms Law:

RaEaEc

Ia

Where,

Ec = design protective potential = -0.80V (vs Ag/AgCl)Ea = design closed circuit potential of anode = -1.00V (vs Ag/AgCl)Ra = Anode Resistance

135.0}0.1{80.0Ia

= 1.481 Amps

Therefore, 37 No. anodes will satisfy the current requirement.

Step 2

Calculate anode weight requirement, W, where,

ExUrLxI

W

From Section 5,

E =ET - 27 (T-20)

E =2000 - 27 (45 - 20)

E = 1325 Ah/kg

Therefore,

kg9.01325587660W

W = 4,043 kg

Therefore, 75 anodes (54.4 kg) would be required to satisfy both the weight and currentrequirements.



8 References

PDO Standards

SP-1246 Specification for Painting and Coating of Oil & GasProduction Facilities.

SP-1129 Specification for Construction, Installation &Commissioning of Cathodic Protection Systems.

SP-1102 Design of 33kV Overhead Lines

SP-1114A Design of 132kV Overhead Lines

SP-1283 Standard Drawings and Requirements forSunshades

SP-1136 Specification for Cathodic Protection StandardDrawings

SP-1099 Electrical Installation Practice.

SP-1130 Specification for Cathodic Protection Materials andEquipment

PR-1234 Procedures for Safe Working on CathodicallyProtected Structures.

DEP-31.40.30.31 External Polyethylene and Polypropylene Coatingfor Line Pipe

DEP-31.40.30.32 External Fusion-Bonded Epoxy Powder Coating forLine Pipe

DEP 33.64.10.10-Gen Electrical Engineering Guidelines

DEP 31.40.21.31 Pipeline Isolating Joints (Amendments to MSSSP75)

DEP 31.10.73.10 Cathodic Protection

International Standards

BS 7361 Part 1: 1991 Cathodic Protection Code of Practice for Land andMarine Operations

BS 1377 Methods of Test For Soils for Civil EngineeringPurposes

MSS SP75 Specification for High Test Wrought Butt WeldingFittings.



9 User Comment Form

User Comment Form

If you find something that is incorrect, ambiguous or could be better in this Procedure, write your commentsand suggestions on this form. Send the form to the Document Control Section (DCS). They make a record ofyour comment and send the form to the correct CFDH. The form has spaces for your personal details. This letsDCS or the CFDH ask you about your comments and tell you about the decision.

ProcedureDetails

Title: Issue Date:

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specification for cathodic protection design

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