Specification for Cathodic Protection Design

Specification for Cathodic Protection Design

Version 2.0

Petroleum Development Oman L.L.C.

UNRESTRICTED December 2005

Document ID : SP-1128 Filing key : xxxx

Specification for Cathodic Protection Design Keywords: Cathodic protection Design requirements Monitoring Internal wetted surfaces Impressed current Sacrificial anodes Groundbed Current density Protection potential Pipelines Buried structures Anodes Transformer rectifiers Test facilities/junction boxes Isolating couplings

This document is the property of Petroleum Development Oman, LLC. Neither the whole nor any part of this document may be disclosed 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. ERD-65-12 1.0 2.0

Date Aug.91 Sept.99 Dec.05

Author TTH/5 OTT/11 UEC/121

Version 2.0

Scope / Remarks Original ERD Document Updated and in new PDO format 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-use when the author wrote this Specification. Exceptions are references to specific issues. If you use DEPs or International Standards with this Specification, make sure you use the latest issues.

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

Specification for Cathodic protection Design

Version 2.0

Contents Authorised For Issue

Error! Bookmark not defined.

1 PREFACE 1.1 Introduction 1.2 Applicability 1.3 Language and units of measurement

5 5 5 5

2 FACILITIES TO BE PROTECTED 2.1 Facilities Requiring Protection 2.2 Selection of Type of Cathodic Protection System 2.2.1 External Protection 2.2.2 Internal Protection

6 6 6 6 7

3 CATHODIC PROTECTION PERFORMANCE CRITERIA 3.1 General 3.2 Protection Criteria 3.2.1 Impressed Current Systems 3.2.2 Sacrificial Anode Systems 3.3 Current Requirements 3.3.1 General 3.3.2 Pipeline Current Requirements 3.3.3 Well Casings 3.4 Avoidance of Cathodic Protection Interaction 3.4.1 General Guidelines 3.4.2 Testing

8 8 8 8 8 9 9 9 10 11 11 11

4 SITE SURVEYS 4.1 Introduction 4.2 Description of Terrain 4.3 Soil Resistivity Measurements 4.4 Soil Investigation 4.5 Current Drainage Tests 4.6 Stray Currents

12 12 12 12 12 12 13

5 CATHODIC PROTECTION DESIGN DETAILS 5.1 Introduction 5.2 Design Requirements 5.2.1 Isolation and Earthing 5.2.2 Cable Sizing 5.2.3 Hazardous Areas 5.2.4 Electrical Isolation 5.2.4.1 Buried In-Station Pipework Tanks and Vessels 5.2.4.2 Buried In-Station Pipework 5.2.4.3 Interstation Pipelines and Main Transmission Pipelines 5.2.5 Flowlines and Short Buried Sections 5.2.5.1 Well Casings 5.2.6 Electrical Earthing 5.2.6.1 Tanks and Vessels 5.2.6.2 Buried In-Station Pipework, Interstation and Transmission Pipelines 5.2.6.3 Transmission Pipelines Paralleling Overhead High Voltage Power Lines 5.2.6.4 Well Casings 5.3 External Cathodic Protection 5.3.1 Current Source 5.3.1.1 Impressed Current

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5.3.1.2 Current Capacity of DC Source 5.3.1.3 Sacrificial Anodes 5.3.2 Station Tanks, Vessels, In-Station Pipework and Interstation Pipelines 5.3.3 Transmission Pipelines 5.3.4 Buried Sections of Above ground Pipelines and Flowlines 5.3.5 Well Casings 5.3.6 Groundbeds 5.3.6.1 General 5.3.6.2 Groundbed Resistance and Soil Resistivity 5.3.6.3 Positioning 5.4 Internal Cathodic Protection 5.4.1 General 5.4.2 Sacrificial Systems 5.4.2.1 Anodes 5.4.2.2 Anode Quantity 5.4.2.3 Anode Distribution 5.4.2.4 Anode Fixing 5.4.2.5 Anode Monitoring 5.4.3 Impressed Current Systems 5.4.3.1 Anodes 5.4.3.2 Anode Quantity 5.4.3.3 Anode Fixing 5.4.3.4 Anode Monitoring

18 18 18 18 19 19 19 19 20 20 21 21 21 21 21 22 22 22 22 22 22 22 23

6 MONITORING AND TEST FACILITIES 6.1 Introduction 6.2 Tanks and Vessels 6.2.1 External CP Potential Measurement 6.2.1.1 Tanks 6.2.1.2 Vessels 6.2.2 Internal CP Potential Measurement 6.2.2.1 Tanks 6.2.2.2 Vessels 6.3 Buried In-Station Pipework 6.3.1 Potential Monitoring 6.4 Interstation and Main Transmission Pipelines 6.4.1 Potential Monitoring 6.4.2 Isolating Joint / Insulated Flange 6.4.3 Drain Point 6.4.4 Combined Drain Point and Isolation Joint / Insulated Flange 6.4.5 Buried Cathodic Protection Coupons 6.4.6 Foreign Service Bonding 6.4.7 Cased Crossing 6.4.8 Grouted Sleeve 6.4.9 Buried Sections Of Surface Laid Pipeline/High PressureGas Flowlines

24 24 24 24 24 24 24 24 24 25 25 25 25 25 25 25 25 25 25 25 26

7 Appendix A Glossary of Definitions, Terms and Abbreviations 7.1 Standard Definitions 7.2 Special Definitions 7.3 Abbreviations 7.4 Calculation of ICCP Station Spacing For Main Transmission Pipelines 7.5 Groundbed Resistance Calculations 7.5.1 General 7.5.2 Horizontal Groundbeds 7.5.3 Vertical/Borehole Groundbeds 7.6 Sacrificial Anode Example Calculation

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8 References

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9 USER COMMENT FORM

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1 PREFACE 1.1

Introduction

This Specification gives the minimum requirements for the design of cathodic protection systems for internal surfaces of tanks and vessels, the external surfaces of tank bottoms, buried vessels, buried instation pipework, buried flowline sections, interstation pipelines, main transmission pipelines and well casings. Marine facilities (e.g. jetties and sub-sea pipelines), internal surfaces of pumps, valves etc and internal surfaces 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 get approval, in writing, from the Custodian, the CFDH Corrosion who owns this Specification, before you use procedures 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 all documents and drawings. Where the SI unit is a conversion of a manufactured dimension, you can put the original dimension, in brackets, after the SI units. For example, 50mm (2in) pipe.

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2 FACILITIES TO BE PROTECTED This section defines the structures which shall be cathodically protected. It also gives guidance on the type of 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 or alloy 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 gas flowlines



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 shield that prevents the protective cathodic protection current from reaching its intended target of the tank’s bottom. The asphalt carpet does NOT offer protection to the tank’s bottom against corrosion in the absence of cathodic protection.



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



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

Note 2 

2.2 2.2.1

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

Selection of Type of Cathodic Protection System External Protection

Cathodic protection of buried external surfaces should, where technically and economically practical, be by Impressed Current Cathodic Protection (ICCP) systems. This covers all the external categories stated in section 2.2 above except for the part buried short lengths of above ground pipelines and flowlines which is explained 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 per DEP 31.40.30.31 & 31.40.30.32. SP-1128

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Where the use ICCP cannot be justified and application of sacrificial anode system is technically not feasible , the short buried section or road crossing of surface laid pipelines and high gas pressure flowlines shall be protected by the application of three layer factory applied PE or PP coating as applicable with an additional rock shielding coat to a minimum total thickness of 6mm. Coating field joints shall be kept to a minimum for such crossings. Field joints shall have a double seal arrangement which shall ensure that water cannot penetrate. Coating and field joints shall be inspected with high voltage holiday detector immediately prior to backfilling . For road crossings of LP flowlines where the use of cathodic protection can be justified, either ICCP or sacrificial anode systems may be used as applicable. In general LP flowlines road crossings may be protected 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 CP system 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 continuous unhibited 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) cathodic protection 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 .

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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 tool shall be used to confirm the effective application of cathodic protection.

3.2 3.2.1

Protection Criteria Impressed Current Systems

Impressed current CP systems shall be designed such that instantaneous “OFF” potentials can be measured for assessing the CP system performance. CP systems shall be designed to provide sufficient current to the structure, over its design life, to achieve an “OFF” potential over the entire structure, equal to or more negative than stated in Table 3.1. In particular, 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 induced stress cracking) due to over protection, “OFF” potentials for carbon steel shall not be more negative than the overprotection limit value as stated in Table 3.1. Some corrosion resistant steels and high strength steels (e.g. Duplex stainless steels) are more susceptible to hydrogen induced stress cracking than carbon steel. The protection criteria for structures made of such materials shall be determined on a case by case basis, but shall not under any circumstance be more negative than the over protection limit given in Table 3.1. When such materials are to be cathodically protected, the Company Materials and Corrosion department shall be consulted for specific recommendations and requirements. Anaerobic environments are not generally encountered on buried pipelines or other structures in the Sultanate of Oman. They may be encountered on internal CP systems however. The protection criteria potential shown in Table 3.1 shall be used for anaerobic conditions when medium (electrolyte) analysis confirms the presence of active sulphate reducing bacteria in anaerobic environments, or when consideration of the operating conditions allows that these may exist. 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

3.2.2

Protection potential for steel in aerobic soil environment.

-850

Protection potential for steel in anaerobic soil environment

-950

Over protection limit for corrosion resistant and high strength steels.

-1150

Over protection limit for carbon steel

-1200

Sacrificial Anode Systems

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Sacrificial anode systems are not normally designed to enable “OFF” potentials to be recorded. Cathodic protection systems shall therefore be designed to provide sufficient current to achieve a minimum “ON” potential on the structure, over the design life. For tank and vessel internal sacrificial anode systems, using Company standard composition aluminium anodes, the design shall provide sufficient current to achieve an “ON” potential equal to or more negative than minus 800mV, with respect to a silver/silver chloride reference electrode. If used for cathodically protecting buried sections of above ground pipeline and flowlines, where magnesium alloy is the sacrificial anode material, the design shall provide sufficient current to achieve an “ON” potential equal to or more negative than minus 1000mV with respect to a Cu/CuSO 4 reference electrode.

3.3 3.3.1

Current Requirements General

The total minimum current requirements for all new structures requiring cathodic protection shall be calculated from the area of the structure, the current density requirements and the estimated coating breakdown. Data for current density requirements and coating defect estimates are given in the Tables 3.2 and 3.3. When applying cathodic protection to the external surfaces of structures, care shall be taken to ensure that an allowance is made in the design current requirement calculations for all metallic surfaces in contact with the environment and electrically continuous with the structure. The size of the allowance shall depend on the relative proximity of the cathodic protection groundbeds to the main structures to be protected and ancillary structures (e.g. earthing systems). If remote groundbed(s) are used then the allowance shall be to the full current density requirement to achieve cathodic protection on the main and ancillary structures; if close 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 a variety of coating systems have been used, each item shall be considered individually. The total current requirement shall then be the summation of individual current requirements. When designing retrofit CP systems, current drainage tests (see Specification-SP-1129) shall be performed wherever possible to determine the minimum current requirements. The results of these tests shall be compared 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 between cathodic protection stations as required during the pipeline life. The current densities in Tables 3.2 and 3.3 shall be used as minimum design values for new projects. This data is valid for pipelines with operating temperatures upto 30°C. The current density values in Table3.2 are to be related to the total pipeline surface area and take into account 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 during construction and operation. For protection of pipelines with elevated operating temperatures the minimum design current densities given in Table3.2 shall be increased by 25% per 10°C rise in temperature above 30°C. The increase shall be compounded per 10°C rise in temperature. For pipelines, or other structures, operating at temperatures above 60°C the Company Materials and 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 a time that current drainage tests may be conducted to establish the actual current requirement. SP-1128

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Table 3.2. Design Current Densities For Different Pipeline Coatings PIPELINE LIFE (Years) COATING TYPE

0-5

5 - 15

15 - 30

CURRENT DENSITY (mA/m²) Fusion bonded epoxy Liquid epoxy Coal tar epoxy Polyethylene Polypropylene

0.010

0.020

0.05

0.002

0.005

0.01

The current densities given in Table3.2 already include the current requirements due to the expected coating 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 as detailed in Tables 3.3.and 3.4. Table 3.3. Minimum Current Density Requirements for Non-coated Steel in Common Environments Environment Ohm.m Soil with resistivity of: >10 1-10 <1 Water with resistivity of: >1.5 0.5-1.5 <0.5

Current Density mA/m

2

10 20 35 50 75 110

Table 3.4. Assumed Coating Defects for the Purpose of CP Design Calculations

Surface Coating

Assumed Coating Defects (%)

None (Bare steel) and Galvanised Steel

100

PCS 2,7 & 8

10

When other coating systems are used the Company Materials and Corrosion department shall be consulted during CP design. 3.3.3

Well Casings

When specified by the Company, well casings shall be provided with cathodic protection.

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The application of cathodic protection on well casings should be undertaken retrospectively so that current requirements 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 one and 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 that cathodic protection is achieved with a given current. The data determined from such tests shall form the basis for the cathodic protection design for all well casings 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 well casings. Such design criteria shall be established by the PDO approved pulsed rectified CP vendors.

3.4 3.4.1

Avoidance of Cathodic Protection Interaction General Guidelines

Where a CP system is designed for installation on a structure which is in the vicinity of a foreign structure the design shall include features to minimise possible interaction. Design features to be considered shall include the provision of supplementary anodes, facilities to allow resistive bonds between the structures, selective positioning of material with the appropriate insulating properties and selective positioning of the groundbed. 3.4.2

Testing

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

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4 SITE SURVEYS 4.1

Introduction

Prior to designing a cathodic protection system for a pipeline a pre-design route survey shall be carried out. Information obtained during previous surveys for the proposed pipeline route may be used provided that the 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:

4.3



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, other pipelines, wadis)



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

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 the pipeline and known data (maximum 5 kms separation) but measurements should at least be carried out when there are visual changes in soil characteristics at a maximum separation of 2kms. 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 a minimum of 2 measurements shall be carried out. Soil resistivity measurements shall be made using four terminal (Wenner) resistivity method. When the soil resistivity measurements are used to locate suitable places for surface groundbeds, the four terminal 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 and bacterial 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 be performed to determine the current and optimal current distribution. This may necessitate temporary installation of one or more groundbeds and DC power sources (e.g. batteries or portable rectifiers), timerunits, and test facilities to the pipeline under investigation.

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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 measured at regular points along the pipeline are within the protection criteria as given in Table 3.1. A further allowance for future deterioration of the coating, during the remaining life of the pipeline, shall also be made. If the pipeline has been previously cathodically protected, historical data and data trends may be used to determine the current demand.

4.6

Stray Currents

The Contractor shall investigate possible sources of detrimental DC stray currents and include proposals in the 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 at the time of commissioning. Stray currents shall be eliminated at the source by suitable insulation or other means. If this is impossible other 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, the length and separation of parallelism(s), number of crossings, crossing angle(s) and AC rating shall be recorded. The requirements of SP-1102 and SP-1114A shall also be considered during the survey.

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5 CATHODIC PROTECTION DESIGN DETAILS 5.1

Introduction

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

5.2 5.2.1

Design Requirements Isolation and Earthing

Structures cathodically protected using close groundbeds shall not be isolated from adjacent plant and earthing systems but shall be isolated from reinforcing bars in concrete constructions associated with the structure. Structures which are cathodically protected using remote groundbeds shall be electrically isolated from common earthing schemes, foreign structures, pipelines and from reinforcing bars in concrete constructions. 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 drawings STD-7-5001, 5002 and 5003 respectively. The CP design shall be such as to minimise the number of isolating devices by placing them, wherever possible, in common pipework but always between a cathodically protected structure or pipeline and other earthed plant items. Structures isolated from common earthing schemes shall have an independent earthing system as detailed in section 5.2.4. Isolation from common earthing systems shall be used to stop current flow through the common earth bonds to structures which are required to be outside the CP system, and thus avoid excessive 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 and the groundbed shall not be more than 5V at the full rated output of the T/R. Preferred cable sizes have 10, 35 or 70mm² cross sectional areas. Groundbed to Solar Generator to CP structure/pipeline: 70mm² cable shall be used. In both the above cases the driving potential between the groundbed and structure / pipeline shall be sufficient 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.

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These may be located adjacent to the structure. Appropriate safety measures (e.g. Class A work permit, gas test, intrinsically safe meters) shall be taken during monitoring activities. Isolation shall be effected outside hazardous areas or be protected against inadvertent short circuiting across 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 by the use of close anodes or groundbeds which are located sufficiently close to ensure that current flows preferentially to the vessels or tank whilst achieving an even spread of protection. Under these circumstances 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-metallic materials. Buried in-station steel pipework shall be cathodically protected using a close anode system, designed to ensure that current flows preferentially to the pipework whilst achieving an even spread of protection. It is not, therefore, required to provide any isolation of this pipework from tanks, vessels or plant earthing systems. 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 of internally coated monobloc isolating joints (DEP 31.40.21.31 refers), installed above ground at both ends of the pipeline. If the product transported by the pipeline is an electrolyte (e.g. water) or it may be anticipated that it may contain an electrolyte at any time during the life of the pipeline, then isolating spools designed as per the following rules shall be installed. Where a pipeline has an HDPE liner, isolating spools may not be required. 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 less than 5% of the pipeline volume, the overall length of the isolating spool shall be four times the pipe diameter (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 following formula:

L  (400/  ) D 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

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(c) Internally coated pipeline spool and holiday free when tested by high voltage spark detection equipment. 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 into which they flow. This shall be done by the use of an isolating flange. For new stations that have close groundbeds this will not be required. Short buried sections of above ground pipelines shall be isolated from the above ground section by use of isolating flanges. For road crossings that have CP the buried protected section shall be isolated from the above ground section. 5.2.5.1

Well Casings

All well casings shall be isolated from flowlines, gas lift, gas / water injection lines or electrical earthing systems associated with Beam Pump / Submersible Pump producers, irrespective of whether CP is proposed at time of completion. Flowlines and water injection lines shall be isolated by spool pieces designed as detailed in the preceding sub-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 the proximity of the CP anode groundbeds to the structure and the type of earthing material. If the anode groundbed is located close to the structure then the CP current will flow preferentially to the structure and the effect of the earthing system will be minimised. This is the mandatory method that has been adopted by PDO for applying cathodic protection to all new structures. The following paragraphs are to be used as a guideline 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 the CP system. The size of this effect will depend on the material used for the earthing system. If copper earthing is used then very high current requirements are expected and the effects of current straying to foreign structures shall be evaluated and recommendations for alleviating their effect on existing equipment submitted to the Company. Such recommendations shall consider the use and efficiency of isolation 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 copper conductors and earthing electrodes constructed of DN50 galvanised steel pipe in accordance with the guidelines of DEP 33.64.10.10 – Gen, Section 6.4 ‘Earthing and Bonding’ current requirements will be considerably less than required to protect a structure which is earthed using copper. 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 in STD-7-3007.

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Transmission Pipelines Paralleling Overhead High Voltage Power Lines

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

Induced AC voltages may be hazardous to personnel, see Procedure-11345 on Safe Working 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 or phase imbalance, which present a hazard to personnel and may damage the pipeline coating.

From a cathodic protection point of view the induction of AC voltages on the pipeline can cause AC current to flow to earth via the rectifying elements of the transformer rectifier and the groundbed. This current, which has been half wave rectified, can flow back into the pipeline as DC current, cause increased DC potentials on the line, make the control and monitoring of the CP systems difficult and may possibly damage T/R components and the pipeline coating. Additionally, high AC potentials on the line may be hazardous 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 buried or surface pipelines where the overhead power lines are rated at 132kV or above, and the pipeline is separated therefrom by less than 500m over a minimum 0.5 km parallelism (Refer to SP-1114A). Different considerations are required for parallelisms between overhead powerlines and surface laid flowlines/pipelines, when lower voltage systems may create a hazard. In such cases, where the overhead power lines are rated at 33kV and the pipeline/flowline is separated laterally by less than 15m over a 0.5km 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 which may occur. These include overhead powerline rating, minimum/maximum separation of parallelism, length of parallelism, number and angle of overhead powerline/pipeline crossings, soil resistivity, coating conductance, type of AC powerline support pole (e.g. wood/metal) and any other factors as may be applicable on a case by case basis. If the Contractor is not sufficiently experienced to undertake this assessment, 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. Alternatively a galvanised steel earth may be used such that isolation is not required.

5.3 5.3.1

External Cathodic Protection 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 rated output voltage no greater than 48V and shall comply with DEP 33.64.10.10 - Gen. and Specification-SP1130 When a suitable continuous AC power supply is not available solar generators should be used. Alternative power sources (e.g. TEG’s) shall be subject to Company approval. The use of multi-channel power supplies shall be considered in appropriate circumstances, e.g. multi-tank external base CP systems with close anodes.

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All CP power sources shall be located in compounds which shall consist of a concrete foundation and be individually 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 be large enough to accommodate all of the necessary equipment (T/R’s, solar panels, mains switch box, current distribution box etc.) allowing a 1 m working space all around. The foundation shall be provided with conduit(s) for all cabling. Cables shall not be run on the concrete surface and shall not be cast in the concrete. Where a T/R or other non-solar power supply is used a sunshade shall be fitted over the compound, 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 it shall be located adjacent to the facility perimeter fence and the single mangate located in the perimeter fence. The compound fencing shall be to the same standard as the facility fencing and fitted with two gates so 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 with STD-7-7001. A review of development plans in the vicinity of the projected CP system shall be carried out. Where it is found that additional structures which will require CP are to be built, the CP design shall allow for this expansion. 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 be capable of providing at least 120% of the design current where current drainage testing has been performed and at least 130% of the design current where this has been determined through calculation. In any event the current source rating shall be minimum 10 amps, but for higher ratings should not be capable of providing more than 150% of the design current requirement, unless it can be shown to be technically or economically appropriate. 5.3.1.3

Sacrificial Anodes

Sacrificial magnesium anode cathodic protection systems may be employed for short buried sections where their use can be both technically and commercially justified. The maximum design life for sacrificial anode systems 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. These connections 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 each structure to common, centrally located NDB(s). Cables from these shall run to an NJB and cables from this shall 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 provide current. Distances between neighbouring stations shall be based on current attenuation calculations with due consideration 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 that pipeline then the pipelines shall have the facility to be electrically bonded at intervals of 10km. The operating history of the existing CP system and the current demand of the new line shall be reviewed to determine the need to provide additional CP stations.

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If the pipeline is to be constructed inside steel casings or culverts at major crossing features, the requirement for cathodic protection of the carrier pipe inside the casing or culvert shall be considered. The provision 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 in section 2.4 and cathodically protected. The preferred method for achieving this, where economically justified, is via an impressed current source. The selection between use of existing current sources or installation 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 both technically and commercially justified. The maximum design life for sacrificial anode systems shall be 5 years. When considering a single flowline or pipeline buried section at the design stage, the Contractor shall take into account nearby or parallel buried pipework and any possible interference that may occur. In such cases 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 linked together 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 generators depending 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 most suitable 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 the structure(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-7-6004 STD-7-6005 STD-7-6006 or STD-7-6007 as appropriate.

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

The number and size of impressed current anodes shall be sufficient to operate at the calculated current output for 20 years. For horizontal and vertical shallow groundbeds PDO standard size Silicon-IronChrome 1525 mm x 75 mm anodes or cannistered anodes shall be used. For borehole groundbeds either Mixed 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 in horizontal, 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 flexible or 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 impressed current 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 remote earth 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 pin method for depths of 25, 20, 15, 10, 5, 2, 1 and 0.5 metres in accordance with DEP 30.10.73.10-Gen. For deeper resistivity data alternative methods (e.g. Schlumberger method) may be proposed for Company approval. For very deep borehole groundbeds (>100m) data on nearby groundbeds and water table depth may be used to design new groundbeds. In such cases the data and design shall be subject to Company confirmation 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 as piping, flowlines, tanks, vessels and other groundbeds should be 50 metres or so as to minimise the spread of current to other structures. Where groups of structures exist, shielding may occur. In these instances it is sometimes desirable to distribute borehole groundbeds such that the minimum separation is less than that given above. Where doubt 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 be minimised and shall not exceed 1000 metres. Close anode systems shall be positioned so as to minimise the spread of current to other structures whilst providing an even level of protection over the surface of the structure under protection.

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5.4 5.4.1

Version 2.0

Internal Cathodic Protection General

The internal surfaces of tanks and vessels, as defined in section 2.1, which contain an uninhibited continuous phase of water in normal operation shall be protected using a CP system separate to the external CP system. Only sacrificial anode systems shall be used for tanks which contain hydrocarbons. For tanks not containing 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. For tanks containing hydrocarbons anodes shall be either placed on the floor or on the walls depending on the coating 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 and achieve 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 20 year period. The number of anodes, N, has to satisfy two criteria:



Total Theoretical wt Individual anode wt



Total current required Individual anode current output

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

E  2000 - 27 (T - 20) -1

Where: E = capacity of anode material (Ah.kg ) T = operating temperature of electrolyte in degrees centigrade (°C) Current requirements for tank walls and floors and vessel surfaces shall be calculated as described in Section 3.5. For these calculations the area of the wall/surface shall be taken as the average area which under 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.

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Anode Distribution

Anode distribution in tanks and vessels is heavily influenced by the presence of internal baffles and localised flow regimes. These features will vary considerably depending on the nature of the vessels to be protected. As a general rule anodes should be spaced such that they “see” all of the areas which require protection; 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 the water phase to within 0.5 metres of the water height expected under normal operating conditions. (The minimum fixing height shall be 0.5 metres from the floor.) For tanks containing hydrocarbons the anodes shall 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 a staggered 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 steel supports welded to the steel surface. In onplot equipment the anodes shall be mounted by bolting and tack welding on steel supports welded to the steel surface. Welding of the supports shall take place during construction of the vessel and be in accordance with the appropriate construction code. After mounting, the steel surface around the support and the entire anode support and anode core shall be coated 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 shall be achieved by isolating the anode from the stand off’s and connecting it via a shunt. The cables shall exit the 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, silicon iron, platinised titanium or platinised niobium. The latter may be preferred in higher resistivity water environments. 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 anode must 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 removed without emptying the tank. SP-1128

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Care must be taken to ensure that the anodes will be submerged regardless of water level during the normal operation 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 an externally located junction box.

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6 MONITORING AND TEST FACILITIES 6.1

Introduction

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

6.2 6.2.1

Tanks and Vessels 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 base plates, extending from the tank centre to beyond the rim, to allow measurement of tank base plate potential by 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 be installed, adjacent to the duct, at tank centre and half radius. Tanks of diameter 10m or less shall only have one coupon installed at the tank centre. Coupons shall be positioned such that the cable tails terminate in a common 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 around the tank. Tanks of diameter upto 50m shall have 4 No. soil pots and tanks of greater 50m diameter shall have 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 in accordance 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 bore valves shall be provided to allow for insertion of reference electrodes. Fittings shall be clear of both internal 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, a second 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 current flow. 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.

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6.3 6.3.1

Version 2.0

Buried In-Station Pipework 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 potential monitoring facilities shall be installed at each end of buried pipe sections. For longer buried pipe sections the maximum spacing between test facilities shall be 50m. For systems using discrete anodes (e.g. Si-Fe-Cr or MMO) distributed along the buried piping, monitoring facilities shall be located at the most remote points from the anode(s). If required other test facilities shall be installed in accordance with the relevant drawing shown in SP-1136

6.4 6.4.1

Interstation and Main Transmission Pipelines Potential Monitoring

Combined potential monitoring test posts/distance markers shall be installed at 2 km intervals along the pipeline route, unless the position of this test post coincides or is in close proximity (± 100m) to another type 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 test facility 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 all Drain 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-73005 and STD-7-3010 . Where one or more foreign pipelines parallel the protected line, but are not included in the protection scheme, 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 be installed in accordance with STD-7-3018. Where a casing is less than 10m in length a single test facility at one end of the casing is required. For casings of length 10m or greater test facilities shall be installed at each end of the casing. 6.4.8

Grouted Sleeve

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

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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.

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7 Appendix A Glossary of Definitions, Terms and Abbreviations 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 the company with services.

Vendor:

The person or organisation that supplies the company with materials and/or equipment.

Discipline:

A specific set of technical knowledge and skills

Corporate Functional Discipline Head:

The person within the Company responsible for the discipline to which the specification belongs. The CFDH approves the Specifications that apply to his discipline

User:

The person or organisation that reads, and uses the 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 electrolyte by maintaining the flow of electrical current through the electrolyte into the surface of the structure being protected. The flow of current into the surface of the structure results in a negative change in the surface to electrolyte potential of the structure. When a critical surface to electrolyte potential is achieved the structure surface is fully protected from corrosion. Cathodic protection can be achieved using impressed current 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 is protected by another separate system. Potential: Refers to the surface to electrolyte potential of a structure measured in Volts, with respect to a reference cell, unless specifically stated otherwise. Electrolyte: A liquid or the liquid component in a composite material in which electric current flows by the movement of ions. For the purposes of this Specification electrolyte shall indicate either soil and/or water.

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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 100 milliseconds after the cathodic protection system has ceased operation and with no current flowing to or from the structure. Impressed Current: Method of providing cathodic protection by connecting the structure to a DC power supply. Sacrificial Anode: Metals and alloys with a more negative electrochemical potential than steel which are connected 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

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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 be protected from a single cathodic protection station (in each direction from the drain point) can be estimated from the following equations.

Ea  Em cosh aL(V) ..(1) Where: L = length of pipeline (m) Ea = change in pipeline potential (V) at the drain point due to the application of impressed current Em = Change in pipeline potential (V) at a point L due to the application of impressed current g r a

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

These equations assume a number of conditions such as the use of remote groundbeds, uniform coating conductance, uniform pipe resistance and zero soil resistivity, although the latter is only really important on bare or poorly coated pipelines where it is significant compared with the coating resistivity. Any deviations from these conditions shall be taken into account and if necessary sections of the pipeline shall be 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 to API 5L X 42 pipe. The external coating is fusion bonded epoxy powder of nominal DFT 500 microns. Design temperature is 50°C. What is the end of life (30 year) current demand and how many cathodic protection stations are required due to attenuation of current along the pipeline ? a)

Pipeline design current demand is calculated using Table 3.2

Pipeline surface area  dL m² =  x (16 x 0.0254) x 80,000 m² = 102,140m² Current density at 30°C = 0.05 mA/m² Current density at 50°C = 0.05 x 1.25²

(Table 3.2)

= 0.078mA/m² Therefore design current demand, = 102,140 x 0.078 = 7967 mA = 7.967 Amps b) SP-1128

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Ea  EmCosh aL Assuming a ‘natural’ pipeline potential of minus 0.5V (wrt Cu/CuSO4 reference), then when the protection potential criteria limits are minus 1.2V at the drain point and -0.95V at the furthest point. Ea = 0.7V Em = 0.45V Calculate the value of ‘g’ g = surface area per metre length / resistance of 1m² of coating (in the absence of other information a value of 9,000 Ohm/m² is a reasonable design value for fusion bonded 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:

a  g.r  a = 4.503 x 10-5 m-1 From equation (1),

Ea  cosh aL Em 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 the value of Em, since the required potential shift at the mid-point between CP stations will be influenced by each station.

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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 CP stations. Notes 1.

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

I

a E m sinh aL, r

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 / m²

Fusion bonded epoxy Liquid epoxy Coal Tar epoxy

9,000

Polyethylene Polypropylene

30,000*

* Value based on 50% of calculated figure.

7.5 7.5.1

Groundbed Resistance Calculations General

In order to accurately predict the power requirements of a CP power source it is necessary to know the resistance of the output circuit. As the resistance to earth of the goundbed is a major part of the output circuit 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 anodes installed 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:

R

 4 L

 4L 4L  log e   log e 2 r S 

 

S S2  2 L 16 L2



      

Where,

2L r S ρ

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

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

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R

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 1000  4x2500 4x2500 240 2402  loge  loge 2    2 4 2500 15 240 2x2500 16x2500 

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 the following formula, which is based on the Modified Dwight Formula for a single vertical anode;

Rv 

  4L   1      (1)  log e 2L  r 

Where,

 L r

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

The above calculation directly yields the theoretical resistance to earth of a single vertical anode or borehole. For multiple vertical anodes, minimum parallel spacing 1m, the resistance to earth (Rn) of n anodes is given by.

Rn 

Fn n Rv

Fn R v    ( 2) n

= paralleling factor = number of anodes = resisance of a single anode to earth from equation ---(1)

The paralleling factor, Fn, is calculated using:

Fn = 1 

 log e(0.66n) SRv

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 m². The internal surfaces are coated in accordance with ERD-48-01 and the water has a resistivity of 0.25 Ohm.m.

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The design basis is as follows : Tank wetted area (average), A Water resistivity,  Operating Temperature, T Coating breakdown, Cb Design current density, Id Anode electrochemical efficiency, ET (@ 20°c) Anode Utilisation factor, U Design life, L (hours)

: : : : : : : :

5,000 m² 25 Ohm.cm 45°C 10% 110 x 10-3 A/m² (seeTable 3.3) 2000 Ah/kg 0.9 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.

Ra 

  4L   1  log e 2L  r 

Where,

L R

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

For non-cylindrical anodes,

r

C 2

Where,

C

= cross-section periphery of anode (m).

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

SP-1128

L W

= 61.0cm = 17.8cm

H

= 17.8cm

r

=

r

= 11.3 cm

2 x (17.8  17.8) 2

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Therefore,

25 4 x 61.0    1  log e 2 xx 61.0  11.3 

Ra 

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

Ia 

Ec  Ea Ra

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 Ia 

 0.80  {1.0} 0.135

= 1.481 Amps Therefore, 37 No. anodes will satisfy the current requirement. Step 2 Calculate anode weight requirement, W, where,

W

LxI r ExU

From Section 5,

E

=ET - 27 (T-20)

E

=2000 - 27 (45 - 20)

E

= 1325 Ah/kg

Therefore,

W

87660  5 kg 1325  0.9

W = 4,043 kg Therefore, 75 anodes (54.4 kg) would be required to satisfy both the weight and current requirements.

SP-1128

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8 References PDO Standards SP-1246

Specification for Painting and Coating of Oil & Gas Production 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 Sunshades

SP-1136

Specification for Cathodic Protection Standard Drawings

SP-1099

Electrical Installation Practice.

SP-1130

Specification for Cathodic Protection Materials and Equipment

PR-1234

Procedures for Safe Working on Cathodically Protected Structures.

DEP-31.40.30.31

External Polyethylene and Polypropylene Coating for Line Pipe

DEP-31.40.30.32

External Fusion-Bonded Epoxy Powder Coating for Line Pipe

DEP 33.64.10.10-Gen

Electrical Engineering Guidelines

DEP 31.40.21.31

Pipeline Isolating Joints (Amendments to MSS SP75)

DEP 31.10.73.10

Cathodic Protection

and

Requirements

for

International Standards BS 7361 Part 1: 1991

Cathodic Protection Code of Practice for Land and Marine Operations

BS 1377

Methods of Test For Soils for Civil Engineering Purposes

MSS SP75

Specification for High Test Wrought Butt Welding Fittings.

SP-1128

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9 User Comment Form User Comment Form If you find something that is incorrect, ambiguous or could be better in this Procedure, write your comments and suggestions on this form. Send the form to the Document Control Section (DCS). They make a record of your comment and send the form to the correct CFDH. The form has spaces for your personal details. This lets DCS or the CFDH ask you about your comments and tell you about the decision. Issue Date: Procedure Title: Details Number: Page Number:

Heading Number:

Figure Number:

Comments:

Suggestions:

User’s personal details Name:

Ref .

Signature:

Date:

Ind. : Phone: Document Control Section Actions

SP-1128

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December 2005


Comment

Version 2.0

Date:

CFDH

Number:

Ref. Ind.:

Recd.:

To CFDH:

CFDH Actions recd. Date:

Inits.:

Decision:

Ref.

  

Reject: Accept, revise at next issue:

Date:

Ind.

Accept, issue temporary amendment Comments:

Originator Advised:

Date:

Inits.:

Document

Date:

Inits.:

Control Advised.

SP-1128

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Specification for Cathodic Protection Design

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