Specification for Cathodic Protection DesignVersion 2.0

Specification for Cathodic protection DesignVersion 2.0

Specification for Cathodic Protection DesignVersion 2.0

Petroleum Development Oman L.L.C.

UNRESTRICTEDDocument ID : SP-1128

December 2005Filing 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.DateAuthorScope / Remarks

ERD-65-12Aug.91TTH/5Original ERD Document

1.0Sept.99OTT/11Updated and in new PDO format

2.0Dec.05UEC/121Updated 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.

Contents

Error! Bookmark not defined.Authorised For Issue

1PREFACE51.1Introduction51.2Applicability51.3Language and units of measurement52FACILITIES TO BE PROTECTED62.1Facilities Requiring Protection62.2Selection of Type of Cathodic Protection System62.2.1External Protection62.2.2Internal Protection73CATHODIC PROTECTION PERFORMANCE CRITERIA83.1General83.2Protection Criteria83.2.1Impressed Current Systems83.2.2Sacrificial Anode Systems83.3Current Requirements93.3.1General93.3.2Pipeline Current Requirements93.3.3Well Casings103.4Avoidance of Cathodic Protection Interaction113.4.1General Guidelines113.4.2Testing114SITE SURVEYS124.1Introduction124.2Description of Terrain124.3Soil Resistivity Measurements124.4Soil Investigation124.5Current Drainage Tests124.6Stray Currents135CATHODIC PROTECTION DESIGN DETAILS145.1Introduction145.2Design Requirements145.2.1Isolation and Earthing145.2.2Cable Sizing145.2.3Hazardous Areas145.2.4Electrical Isolation155.2.4.1Buried In-Station Pipework Tanks and Vessels155.2.4.2Buried In-Station Pipework155.2.4.3Interstation Pipelines and Main Transmission Pipelines155.2.5Flowlines and Short Buried Sections165.2.5.1Well Casings165.2.6Electrical Earthing165.2.6.1Tanks and Vessels165.2.6.2Buried In-Station Pipework, Interstation and Transmission Pipelines165.2.6.3Transmission Pipelines Paralleling Overhead High Voltage Power Lines175.2.6.4Well Casings175.3External Cathodic Protection175.3.1Current Source175.3.1.1Impressed Current175.3.1.2Current Capacity of DC Source185.3.1.3Sacrificial Anodes185.3.2Station Tanks, Vessels, In-Station Pipework and Interstation Pipelines185.3.3Transmission Pipelines185.3.4Buried Sections of Above ground Pipelines and Flowlines195.3.5Well Casings195.3.6Groundbeds195.3.6.1General195.3.6.2Groundbed Resistance and Soil Resistivity205.3.6.3Positioning205.4Internal Cathodic Protection215.4.1General215.4.2Sacrificial Systems215.4.2.1Anodes215.4.2.2Anode Quantity215.4.2.3Anode Distribution225.4.2.4Anode Fixing225.4.2.5Anode Monitoring225.4.3Impressed Current Systems225.4.3.1Anodes225.4.3.2Anode Quantity225.4.3.3Anode Fixing225.4.3.4Anode Monitoring236MONITORING AND TEST FACILITIES246.1Introduction246.2Tanks and Vessels246.2.1External CP Potential Measurement246.2.1.1Tanks246.2.1.2Vessels246.2.2Internal CP Potential Measurement246.2.2.1Tanks246.2.2.2Vessels246.3Buried In-Station Pipework256.3.1Potential Monitoring256.4Interstation and Main Transmission Pipelines256.4.1Potential Monitoring256.4.2Isolating Joint / Insulated Flange256.4.3Drain Point256.4.4Combined Drain Point and Isolation Joint / Insulated Flange256.4.5Buried Cathodic Protection Coupons256.4.6Foreign Service Bonding256.4.7Cased Crossing256.4.8Grouted Sleeve256.4.9Buried Sections Of Surface Laid Pipeline/High PressureGas Flowlines267Appendix A Glossary of Definitions, Terms and Abbreviations277.1Standard Definitions277.2Special Definitions277.3Abbreviations287.4Calculation of ICCP Station Spacing For Main Transmission Pipelines297.5Groundbed Resistance Calculations317.5.1General317.5.2Horizontal Groundbeds317.5.3Vertical/Borehole Groundbeds327.6Sacrificial Anode Example Calculation328References359USER COMMENT FORM36

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 in-station 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.

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 tanks bottom. The asphalt carpet does NOT offer protection to the tanks 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

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

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 .

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 Protection Criteria

3.2.1 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

ENVIRONMENTPOTENTIAL

Instantaneous OFF Potential (mV)Cu/CuSO4 Reference Electrode

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

3.2.2 Sacrificial Anode Systems

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/CuSO4 reference electrode.

3.3 Current Requirements

3.3.1 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.

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 30C.

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 10C rise in temperature above 30C. The increase shall be compounded per 10C rise in temperature.

For pipelines, or other structures, operating at temperatures above 60C 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.

Table 3.2. Design Current Densities For Different Pipeline Coatings

PIPELINE LIFE (Years)

COATING TYPE0 - 55 - 1515 - 30

CURRENT DENSITY (mA/m)

Fusion bonded epoxy Liquid epoxy Coal tar epoxy0.0100.0200.05

PolyethylenePolypropylene0.0020.0050.01

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.

3.3.3Tanks, 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.mCurrent Density mA/m2

Soil with resistivity of:

>1010

1-1020

1.550

0.5-1.575

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 =

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 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.

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 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 External Cathodic Protection

5.3.1 Current Source

5.3.1.1 Impressed CurrentWhere 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-SP-1130

When a suitable continuous AC power supply is not available solar generators should be used. Alternative power 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-tank external base CP systems with close anodes.

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/Rs, 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.

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.

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-Iron-Chrome 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.

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 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:

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

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 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.

5.4.2.3 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 offs 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.

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.

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 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 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.

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 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 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 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-7-3005 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.

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 AGlossary 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.

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

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

Where:

L = length of pipeline (m)Ea = change in pipeline potential (V) at the drain point due to the application of impressed currentEm = Change in pipeline potential (V) at a point L due to the application 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 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 50C.

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

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

= 102,140mCurrent 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,

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 = 0.7VEm = 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 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 / mCalculate the value of a from:

( a =4.503 x 10-5 m-1

From equation (1),

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

Notes

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

sinh aL,

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

Design values for other coatings are as follows:

Coating TypeDesign Coating Resistance(15 - 30 year life)Ohm / m

Fusion bonded epoxyLiquid epoxyCoal Tar epoxy9,000

PolyethylenePolypropylene30,000*

* 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 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:

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

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;

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 or borehole.

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

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

The paralleling factor, Fn, is calculated using:

Fn =

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. The internal 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

= 55ACalculate 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.

Where,

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

For non-cylindrical anodes,

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 =

r = 11.3 cm

Therefore,

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

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

= 1.481 Amps

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

Step 2Calculate anode weight requirement, W, where,

From Section 5,

E =ET - 27 (T-20)

E =2000 - 27 (45 - 20)

E = 1325 Ah/kgTherefore,

W = 4,043 kg

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

8 References

PDO Standards

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

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

SP-1102Design of 33kV Overhead Lines

SP-1114ADesign of 132kV Overhead Lines

SP-1283Standard Drawings and Requirements for Sunshades

SP-1136Specification for Cathodic Protection Standard Drawings

SP-1099Electrical Installation Practice.

SP-1130Specification for Cathodic Protection Materials and Equipment

PR-1234Procedures for Safe Working on Cathodically Protected Structures.

DEP-31.40.30.31External Polyethylene and Polypropylene Coating for Line Pipe

DEP-31.40.30.32External Fusion-Bonded Epoxy Powder Coating for Line Pipe

DEP 33.64.10.10-Gen Electrical Engineering Guidelines

DEP 31.40.21.31Pipeline Isolating Joints (Amendments to MSS SP75)

DEP 31.10.73.10Cathodic Protection

International Standards

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

BS 1377Methods of Test For Soils for Civil Engineering Purposes

MSS SP75 Specification for High Test Wrought Butt Welding Fittings.

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.

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8SP-1128December 2005

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