NACE-CP L-I (1)

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CATHODIC PROTECTION TRAINING COURSE

LE VE L ONE

TH E ORY, DE SI GN, SURVE Y ME TH ODS & TE CH NI QUE S Reference Manual (I ssue 3 – May2014)

TRAINING PROGRAM Prepared ByR J WATERHOUSE, MICorr1 of 113

PAGE INDEX SECTION

SUBJECT

THEORY THEORY THEORY THEORY THEORY THEORY THEORY

The corrosion process Basis of cathodic protection The corrosion mechanism diagram Electromotive force series of metals Practical galvanic series Comparison of reference electrodes with a Cu/CuSO4 @ 25° c Types of corrosion cell on pipelines

THEORY THEORY THEORY THEORY THEORY THEORY THEORY THEORY THEORY THEORY THEORY THEORY THEORY DESIGN DESIGN DESIGN DESIGN DESIGN DESIGN

Pipeline corrosion mechanism Basic theory of cathodic protection Cathodic protection process flow chart Sacrificial anode systems Advantages and limitations of sacrificial anode system Impressed current system Advantages and limitations of impressed current system Pipeline (structure) cathodic protection criteria Corrosion interaction interference – testing Protective coatings & coating process flow chart Insulating devices DC blocking devices AC mitigation Survey methods & techniques (pipelines) Ohms law Calculate the resistance of a horizontal anode groundbed A horizontal anode groundbed layout Calculate the resistance of a horizontal anode groundbed Calculate the resistance of a vertical anode groundbed

DESIGN DESIGN DESIGN DESIGN DESIGN DESIGN DESIGN DESIGN DESIGN

Calculate the resistance of a vertical anode group Calculate the resistance of a deep vertical anode groundbed Selection of cable size to meet a given maximum voltage drop Cathodic protection xlpe/pvc cable characteristics Calculation of system voltage Graphite and silicon iron anode consumption rates Graphite and silicon iron rod anodes metric dimensions and weights Coke breeze backfill specifications Galvanic anode materials table of capacities, potentials and consumption rates Calculate the weight requirement of sacrificial anodes Calculate the life expectancy of sacrificial anodes Magnesium and zinc anode specification Sacrificial anode design Typical magnesium or zinc anode installation Pipeline attenuation calculations (coated pipelines) Soil resistivity measurements Testing of cathodic protection insulation flanges (Isolation joints) Determine current flow measurements on pipelines Determining remote earth High soil resistivity: voltmeter correction Pipe-to-soil (P/S) potential measurements Close interval (pipe to soil) potential surveys (CIPS) DC voltage gradient survey (DCVG) Well casing potential profile Current-potential profiles (E log I) Pipeline steel casing/sleeving evaluation

DESIGN DESIGN DESIGN DESIGN DESIGN DESIGN SURVEY METHODS SURVEY METHODS SURVEY METHODS SURVEY METHODS SURVEY METHODS SURVEY METHODS SURVEY METHODS SURVEY METHODS SURVEY METHODS SURVEY METHODS SURVEY METHODS


PAGE

4 5 6 7 8 9 10 11 – 14 15 16 17 18 19 20 21 – 22 23

24 – 25 26 27 28-29 30 – 31 32 – 33 34 35– 36 37 – 38 39 – 41 42 – 45 46 – 47 48 49 50 – 51 52 53 – 54 55 – 56 57 58 59 – 60 61 62 – 64 65 – 66 67 – 68 69 – 75 76 – 78 79 – 83 84 – 85 86 87 – 88 89 – 91 92 – 94 95 – 98 99 100 –101

PAGE INDEX SECTION

SURVEY METHODS CONSTRUCTION DEFINITIONS ACKNOWLEDGMENTS

SUBJECT

Structure to water potential measurements Pipeline cable identification colour coding system Cathodic protection definitions Standards & Publications


PAGE

102–103 104 –105 106 –111 112

CATHODIC PROTECTION TRAINING COURSE THEORY THE CORR OSI ON PRO CE SS Corrosion -

Corrosion is an electrochemical process between a metal and its environment, resulting in its progressively degradation or destruction.

Corrosion is a serious problem and requires control, corrosion of some degree is inevitable but should the corrosion be sufficient to result in loss of plant or production, then it is said that Corrosion Damage has occurred. Corrosion damage is both expensive and dangerous; it is far cheaper to apply an effective corrosion control system, than to allow the corrosion process to result in loss of company plant and equipment or cause dangerous conditions to allow injury or death to personnel. Corrosion can be prevented, reducing or arresting by a variety of applications, these can be summarised as follows:

     

Paint systems Coating systems Material selection Environmental changes Cathodic Protection Systems Combination systems

Cathodic Protection is a system that can be applied to structures which is a means of rendering a metal immune from corrosion attack by causing direct current to flow from its electrolyte environment into the entire metal surface.


CATHODIC PROTECTION TRAINING COURSE THEORY BASI S OF CATHO DI C PRO TECTI ON Corrosion requires the existence of anodic and cathodic areas. The corrosion process in aerated water is shown in the corrosive mechanism diagram. If electrons are caused to flow into the surface from an external source, then the positive ions leave the metal surface less readily, but the cathodic process is enhanced. There are certain conditions, which must be met before a corrosion cell can function, these are: 

There must be an anode and a cathode.



There must be an electrical potential between the anode and the cathode (Potential difference). This potential can result from variety of conditions on pipelines.



There must be a metallic path electrically connecting the anode and cathode. (Normally this will be the pipeline itself).



The anode and cathode must be immersed in an electrically conductive electrolyte, which is ionised – meaning that some of the water molecules (H 2O) are broken down into positively charged hydrogen ions (H ⁺) and negative charged hydroxyl ions (OH¯). (The usual soil moisture, or water, surrounding pipelines normally fulfils this condition).

Once these conditions are met, an electric current will flow and metals will be consumed at the anode. The corrosive mechanism diagram, illustrates diagrammatically the conditions outlined above as applied to iron anodes – the consumption of which is the basis for most pipeline corrosion problems. Anodic areas of pipelines made of metals other than iron (steel) generally can be consumed in a similar fashion. The amount of metal that will be removed is directly proportional to the amount of current flow. One ampere of direct current discharging into the usual soil electrolyte can remove approximately twenty pounds of steel in one year (20 Pounds/Ampere/Year). This is based on the electrochemical equivalent of the metal involved; other metals other than steel will be removed at other rates, some more and some less.


CATHODIC PROTECTION TRAINING COURSE THEORY THE CORROS I ON MECHANI SM DIAGRAM


CATHODIC PROTECTION TRAINING COURSE THEORY EL E CTROMOTIVE F ORCE SER I ES O F ME TALS

_______________________________________________ METAL VOLTS _______________________________________________ Magnesium Aluminium Zinc Iron Tin Lead Hydrogen Copper

-2.37 -1.66 -0.76 -0.44 -0.14 -0.13 -0.00 +0.34 to +0.52

Silver Platinum Gold

+0.80 +1.20 +1.50 to +1.68

_______________________________________________



THEORY PRA CTI CAL GALVA NIC SERI ES

____________________________________________________________ METAL VOLTS ____________________________________________________________ Commercially Pure Magnesium Magnesium Alloy (6% Al, 3% Zn, 0.15% Mn) Zinc Aluminium Alloy (5% Zn) Commercially Pure Aluminium Mild Steel (clean and Shiny) Mild Steel (rusted) Cast Iron (not graphitized) Lead Mild Steel in Concrete Copper, Brass, Bronze High Silicon Iron Mill Scale on Steel Carbon, Graphite, Coke

-1.75 -1.60 -1.1 -1.05 -0.80 -0.50 to - 0.80 -0.20 to -0.50 -0.50

-0.50 -0.20 -0.20 -0.20 -0.20 +0.30

____________________________________________________________



THEORY COMPARI SON OF R E F E RE NCE E LE CTRO DE S WI TH A CO PPE R/COPPER SULPHATE E LE CTRODE @ 2 5° C

Reference electrode.

Pipe to electrode reading equivalent to -0.85 volt with respect to Copper/Copper Sulphate Electrode.

To correct pipe to electrode readings to equivalent reading with respect to Copper/ Copper Sulphate Electrode.

Silver-Silver Chloride (0.1 NKCL solution)

-0.84 volts

Add -0.01 volts

+0.25 volts

Add -1.10 volts (1)

Pure Zinc (Special High Grade)

(1) Based on Zinc having an open circuit potential of -1.10 volts with respect to Copper Sulphate Electrode



THEORY TYPE S OF CORR OSI ON CEL L ON PIPE LI NE S There are a number of conditions that can establish anodes and cathodes on a pipeline. Knowing these conditions, which can be responsible for active corrosion, can be of immeasurable value when designing and installing a pipeline system. By taking steps to eliminate them, the subsequent task of maintaining the pipeline in a corrosion-free condition will be simplified. 

Dissimilar metal corrosion cells



Corrosion resulting from dissimilar soils



Different aeration corrosion cells



New pipe and old pipe corrosion cells Steel in concrete (acts as dissimilar soils)

 

Mill scale corrosion (acts as dissimilar metal corrosion



THEORY PIPE LI NE CO RROS I ON MECHANI SM



THEORY PIPE LI NE CO RROS I ON MECHANI SM DIF FE RENT S OI LS



THEORY PIPE LI NE CO RROS I ON MECHANI SM NEW S TEEL PIPELI NE RE PAI R



THEORY PIPE LI NE CO RROS I ON MECHANI SM STE E L I N CONCR E TE


CATHODIC PROTECTION TRAINING COURSE THEORY BASI C THE ORY OF CATHO DI C PRO TECTI ON There a various conditions that will result in pipeline corrosion. These conditions allow the formation of anodic and cathodic areas to be formed on the pipeline metal surface (Areas of coating damage i.e. ‘Holidays’), at the anodic areas the pipeline corrodes, current will be flowing from the pipeline into the surrounding electrolyte (soil or water). Likewise, where current was flowing from the electrolyte onto the pipe, the pipe surface was made cathodic and did not corrode.

Taking the above into consideration, it becomes obvious that if every bit of the exposed metal on the surface of the pipeline could be made to collect current, it would not corrode because the entire surface would be cathodic. This is what Cathodic Protection does. Direct current is forced to flow from a source external to the pipeline onto all surfaces of the pipeline. When the amount of current flowing is adjusted properly, it will overpower corrosion current discharging from all anodic areas of the pipeline and there will be a net current flow onto the pipeline surface at these points. The entire surface then will be cathodic and the protection complete (i.e. Cathodically Protected).

In order for the cathodic protection system to work, current is discharged from an earth connection (groundbed). In discharging current to earth, the groundbed materials are subject to corrosion. Because the sole purpose of this groundbed is to discharge current, it is desirable to use materials, which are consumed at much lower rates (Pounds (Kgs)/Ampere/Year).

The cathodic protection system does not stop corrosion, but eliminates the corrosion on the pipeline (structure) and concentrates the corrosion in a known controlled location, i.e. the groundbed. At this known controlled location the current discharge at the groundbed can be designed for a reasonable long life i.e. 20 – 30years. It also can be easily tested and replaced at the end of its useful life without endangering the pipeline system being protected.

There are two main systems adopted for pipeline cathodic protection design, these are:



Sacrificial anode systems (galvanic anodes)



Impressed current system (utilizing external power supply)


CATHODIC PROTECTION Process Flowchart

Start

Guidelines Philosophy Feasibility

Identify Items to be Protected

Determine Surface Area to be Protected

Guidelines ES-30.99.37.001 BS 7361

Develop CP Scheme

Select CP System

S a c r if ic ia lAnode

Guidelines

Impr e s se dC ur r ent

Select Type of Power Supply eg AC, Diesel, Solar

Select Anode Type eg Al, Zn, Mg

Guidelines Eng Specs NACE RPs Budget

Design CP System

Procure Materials

Tender for Installation

Install CP System

Commission System Initial Survey Final Adjustments

Handover Issue Commissioning Report

Routine Survey/ Monitoring

Survey Analysis and Reporting

Further Analysis Identify Fault

Yes

Design Modification Required?

No

No


CP System OK?

Yes

Replace Depleted Anodes Make Repairs

Guidelines


THEORY SE CR I F I CI AL AN ODE SYSTE MS Galvanic (or sacrificial) cathodic protection makes practical use of dissimilar metal corrosion. The galvanic anode is connected to the structure it is protection either directly or thru a test station so it can be monitored.

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85V,PIP E P rim a ry s t ru c t u re Cathode TRAINING PROGRAM Prepared ByR J WATERHOUSE, MICorr17 of 113

M a g ne s


THEORY ADVANTAGE S AND LI MI TA TI ONS OF SE CR I F I CI AL A NOD E SYSTE MS Advantages of sacrificial anode systems      

No External power source required Low maintenance requirements Small current output resulting in little or no stray current inter-ference Easy to install Easy to add anodes in most cases Provide uniform distribution of current

Limitations of sacrificial anode systems   

Low driving voltage/current output Many anodes may be needed for poorly coated structures May be ineffective in high-resistivity environments



THEORY I MPRESS E D CURRENT SYS TEM An impressed current cathodic protection system (ICCP) consists of an external power source and anodes. The power source forces current to flow from the anode to the structure through the electrolyte. The power source for an impressed current system produces direct current (DC) Types of power sources include:  Rectifiers  Solar Cells  Engine Generators  Wind-powered Generators


I

DC

CATHODIC AREA

STEEL

NEUTRALAREA

CATHODIC AREA


THEORY ADVANTAGE S AND LI MI TA TI ONS OF I MPR E SSE D CUR RE NT SYSTE M Advantages of impressed current system  

Flexible with capability to handle a wide range of voltage and current outputs Satisfy high current requirements with a single installation


  

Effective in protection uncoated and poorly coated structures Effective in high-resistivity environments Less anode consumption that with galvanic anodes

Limitations of impressed current system   

Higher inspection and maintenance cost that with galvanic anodes Requires external power Constant power supply cost



High risk of causing stray current interference

Diagram: Deep-well groundbed


THEORY PIPE LI NE (STRU CTURE) CAT HODIC PRO TECTION C RI TERI A E xtract from British Standard - BS 7361 : Part 1 : 1991 Cathodic Protection Code of practice for land and marine application


The potential difference between a metal (pipeline) surface and the electrolyte (soil or water) is normally used as the criteria that satisfactory cathodic protection has been achieved. The minimum values that need to be achieved (the protection potentials) are listed in table 1. Values more positive than these will allow corrosion to occur. The value depends on the reference electrode used. In practice the potential may vary considerably over the surface of the metal (pipeline). It is therefore important in assessing, for example, a cathodic protection installation on a buried pipeline, to ensure that the measurements should enable the least negative metal (pipeline)/soil potential to be located. This will necessitate measuring at a sufficient number of points along the route of the pipeline, taking into account the considerations to ensure the test equipment is in good working order, calibrated, good electrical contact is made with the structure (pipeline) and the placing of the reference cell (half cell) is correctly undertaken. Unless the reference electrode is placed very close to the metallic surface, (pipeline), measurement of the potential difference between the metallic surface (pipeline surface) and the electrolyte can be considerable affected by the potential drop produced by the protected current as it flows through the surrounding electrolyte to the structure (pipeline). This effect, which is often referred to as ‘IR drop’, has the effect of making the measured potential more negative than the ac tual potential at the metal (pipeline)/electrolyte interface. Unless a method of measurement is used that eliminates or sufficiently reduces the effect of the IR drop adjusting the Cathodic Protection current to the value that gives the relevant protection potential shown in table 1 may not provide full protection. IR drop is dependant on electrolyte resistivity and is particularly pertinent to buried structures (pipelines). With a coated structure (pipeline) the resistance of the coating will also have an effect. The instantaneous-off potential method as a means of minimizing IR drop error is rapidly gaining acceptance. However, alternative techniques are likely to be developed, and any technique that can be shown to reduce IR errors to acceptable levels may be employed, (i.e. Use of Cathodic Protection Analyser (CPA) instrumentation, test point coupons, etc.).

Subject to the above considerations, Cathodic Protection of a pipeline is achieved by bringing it to the potential shown in table 1 or to a more negative potential.

As shown in table 1, a more negative potential is recommended for iron and steel when they are installed in conditions that are likely to become anaerobic, for example clay soils. This is because of the effect of sulphate reducing bacteria.

TABLE 1 MINIMUM POTENTIALS FOR CATHODIC PROTECTION Metal or Alloy

Reference electrode (and condition of use)

Copper/copper sulphate (i n soils and fresh water)


Silver/silver chloride/sea-water (see note1)

Zinc/sea-water (seenote1)

V

V

V

- 0.85

- 0.8

+ 0.25

- 0.95

- 0.9

+ 0.15

Iron and Steel aerobic environment anaerobic environment

NOTE 1: For use in clean, undiluted and aerated sea-water. The sea-water is in direct contact with the metallic electrode. NOTE 2: All figures have been rounded to the nearest 0.05 V.

CRITE RI A FOR EF FE CTI VE CATHO DI C PRO TECT I ON E xtract fromBritish Gas E ngineering Standard B GC/PS/E CP1 All structure to soil potential referred to in this specification are with r espect to a saturated Cu/CuSO4 reference electrode unless otherwise stated. The cathodic protection system shall be designed to ensure that the pipe to soil potentials at the point of minimum protection shall be maintained at a more negative potential than -0.95 V, at the point of maximum protection not more negative than -1.2 V, over the entire structure as measured using a technique which eliminates errors due to volt drop in the soil i.e. polarised

potentials. (A CIPS polarised instantaneous ‘OFF’ potential survey shall be conducted over the entire buried sections of thepipeline).

Where errors due to voltage drop in the soil cannot be eliminated, conventionally measured pipe to soil potentials at minimum protection shall be maintained at a more negative potential than 1.25 V, at the point of maximum protection no greater than -1.95 V. I t should be noted that evidence shall beproduced to guarantee full cathodic protection has been achieved. (A CI PS

‘ON’ potential survey shall be conducted over the entire buried sections of the pipeline).


CATHODIC PROTECTION TRAINING COURSE THEORY

CORROSION INTE RACTI ON – I NTERF ERE NC E TEST I NG The application of cathodic protection to a buried or immersed structure (pipeline) causes direct current to flow in the earth or water in its vicinity. Part of the protection current traverses nearby buried or immersed pipes, cables, jetties, tanks or similar structures, or ships alongside (termed secondary structures), which may be unprotected and the corrosion rate on these structures may, increase at points where the current leaves them to return to the primary structure. This effect is described as corrosion interaction. Corrosion interaction can be minimised or eliminated by exercising care during the design and construction stages. The magnitude of any positive changes of structure/electrolyte potential on neighbouring structures will depend mainly on the follows: 

The quality & standard of coating on primary structure



The quality and standard of coating on secondary structure



The spacing between the primary and secondary structure (crossing or parallel)



The distance between the groundbed and secondary structure



The soil or water resistivity



Keep to a minimum the structure/electrolyte potentials on the primary structure



Increase electrical path between primary structure and secondary structure, use of electrical shields

Corrosion interaction testing (Interference testing) shall be conducted as part of any applied Cathodic Protection systems commissioning program and where necessary remedial actions can be undertaken to eliminate any corrosion interaction caused by the installed scheme. The changes in structure/electrolyte potentials due to interaction will vary along the length of the secondary structure and a negative potential change at any point will often indicate the presence of positive changes at other parts of the structure. For most metals, only positive potential changes are liable to accelerate corrosion. The usual object of interaction testing is, therefore, to find the area where the potential change is positive, to locate, by testing a number of positions, points at which the potential changes locally reaches a maximum and to assess each maximum value with sufficient accuracy. The maximum positive potential change at any part of the secondary structure, resulting from interaction testing, shall not exceed +20 mV, this figure was adopted on the basis of information provided by the ‘Joint Committee for the Co-ordination of Cathodic Protection of Buried Structures’ when the previous British Standard BS Code was Cathodic Protection 1021 : 1973, was being drafted. Subsequent experience within the industry has provided no indication that corrosion damage occurs when this limit is respected.


CATHODIC PROTECTION TRAINING COURSE THEORY PROTECTI VE CO ATI NGS The function of a coating is to reduce the areas of metal (pipeline surface) exposed to the electrolyte (soil or water). By this means it is possible to reduce greatly the current density required for Cathodic Protection. The fact that the current is spread more uniformly along the structure (pipeline) may reduce the number of points, which Cathodic Protection need be applied. A coating should, ideally, have a high electrical resistance and be continuous, i.e., there should be few ‘holidays. It should be resistant to any chemical or bacterial action to which it might be exposed, and withstand all temperature variations to which it may be subjected; no blisters or laminations should exist and the coating should adhere strongly to the surface to be protected; it should have satisfactory ageing characteristics and adequate mechanical strength. Ability to resist abrasions, and have strong mechanical strength is also an important application requirement for pipelines. Coating may take the form of paints (above ground or in pits), epoxy or similar based resins (FBE), or of thermoplastic materials such as polyethylene (PE) or polyvinyl chloride (PVC) either sintered, extruded or in sheet form. Enamels of coal-tar or bitumen reinforced with inorganic fibres, and petroleum wax impregnated fabric tapes are also used. The most suitable of coating depends on the type of structure and its environment. In deciding the type of coating to be used the aim should be to achieve overall economy in the combined cost of the protective structure and of the initial and running costs of the protection scheme. Due regard should be paid to the life expected of the structure and to the economics of repairing the coating should this become necessary. In buried structures (pipelines), a secondary but important function of the coating is to reduce the current flowing thereby reducing the potential gradient in the surrounding soil and thus decreasing interaction with neighbouring buried structures (interference). The protected current, particularly if strongly negative potential are used, may allow hydrogen gas to be produced at the cathode which will have the effect of lifting the coating from the pipeline (structure), causing dis-bondment and laminations at the metallic surface, (a polarized potential more negative than –1200 mV should be avoided). As part of a cathodic protection scheme the coating is the prime protection devise against external corrosion of the structure. It should be noted however; that although coating a structure (pipeline) drastically reduces the external tendency for external corrosion to take place, at ‘Holidays’ the corrosion rate will increase significantly. Therefore the use of protective coatings in pipeline construction should always be associated with the application of Cathodic Protection.


Coating/Wrapping Process flowchart Start

Identify Location Below Ground

Above Ground

Subsea Select Surface

Select Coating System

Internal External

Surface Preparation

Select Coating System

New Product

Coating Application

Select Colour Codes

Fail

Quality Control Testing

Surface Preparation

Pass Record Details

Coating Application

Finish Inspection of existing Coatings

Fail

Pass

Pass Condition Assessment

Record Details

Fail Repair or Replace


Quality Control Testing

Finish

CATHODIC PROTECTION TRAINING COURSE THEORY I NSULAT I NG DE VI CES Insulating devices designed to electrical isolation of catholically protected pipelines and to seal the flanges and prevent electrical leakage of the transported gas within the piping system. Each kit shall include the following components: Insulating central gasket  Insulating bolt sleeves  Insulating bolt washers  Steel bolt washers Type “E” gaskets perfectly center onprecisely located bolt holes. Since their outside diameter are the same, foreign material is prevented from “shorting” the flange insulation.

Type “F” gaskets are made without boltholes, to fit tightly inside the overall bolt hold circle of the flange faces. The outside diameter of the gasket fits tightly in place assuring a well centered position.


CATHODIC PROTECTION TRAINING COURSE THEORY DC BLO CKING DEVI CES The DC block functions by using a blocking capacitorto capture the flow of low-level DC currents through a coaxial cable. It does this while allowing the higher frequency AC currents used in radio signal transmission to pass through in a process called coupling. The DC block can produce limitations on the transmission of all lower level frequencies, including AC frequencies, so modification of the system is sometimes required. This process of modification is called tuning the circuit. Examples of DC blocking devices as follow:

   

DC blocking cell ( alkaline nickel – nickel) Diodes Capacitor ( Usually large value electrolytic capacitors) Polarisation cell ( Kirk cell and PCR’s)


CATHODIC PROTECTION TRAINING COURSE THEORY AC MI TI GATI ON Pipelines sharing a right of way with power lines may be subject to electrical interference because of inductive and conductive effects. Magnetic induction (coupling) acts along the entire length of a pipeline which is approximately parallel to the power line and can cause significant pipeline potentials even at relatively large separation distances. Conductive interference due to currents in the soil is of particular concern where the pipeline is close to transmission line structures that may inject large currents into the soil during power fault conditions. Such structures include transmission line tower or pole foundations and substation grounding systems. The effects of power system interference on pipelines are due to the voltage difference between the pipeline metal and the soil. In term of safety for personnel and public, a potential shock hazard exists when someone touches an exposed part of the pipeline (such as a valve) while standing on soil that is at a significantly difference potential. This “touch voltage” is defined as the difference in potential between the pipeline metal and the ground above the pipeline. Similarly, the “step voltage” is the difference in potential between a person’s feet or the difference in ground potential between two points spaced 3 ft (less than 1 m) apart. Power line and pipeline interference can damage the pipeline and its coating. Excessive coating stress voltage (the difference between the pipe steel potential and the local soil potential) can degrade and puncture the coating. In the case of an extreme soil potential rise, the pipeline wall itself can be damaged or punctured.

POWER LINE AUTHORITY TOWER

A

A


A mitigation system designed to protect a pipeline subject to AC interference. An effective method of pipeline AC interference mitigation is the gradient control wire system. It protects against both inductive and conductive interference and provides Cathodic Protection (CP). Gradient control wires consist of one or more bare zinc conductors buried parallel to and near the pipeline and regularly connected to it. They are a highly effective means of mitigating excessive pipeline potentials due to both inductive and conductive interference, and they can provide CP. Gradient control wires accomplish their task by “evening out” pipeline and soil potential differences.


CATHODIC PROTECTION TRAINING COURSE DESIGN CONSIDERATIONS SUR VE Y ME TH ODS & TE CH NI QUES (PI PE LI NE S) The Cathodic Protection system shall be designed using a sound engineering foundation using true and factual data. Much of the required data for the design will be obtained from carrying out various on-site surveys and testing methods. The surveys and test methods adopted shall provide adequate information to ensure the design engineer is able to provide a Cathodic Protection system that will meet the design life (requirement of the owner) of the pipeline, be effective (meets the criteria for full protection) and is cost efficient. Various testing and survey techniques can be conducted on an underground pipeline, the choice of which would be subject to the specific requirements of the project being designed.

DATA REQUIRED TO EXECUTE DESIGN & COMMISSION SYSTEM

The data required for undertaking a Cathodic Protection design for a pipeline can be summarised as follows:

1) NEW PIPELINE CONSTRUCTION PROJECT Data obtained from the pipeline initial engineering mechanical design package and proposed construction method documentation: Pipeline material, weight & wall thickness Pipeline diameter Pipeline external coating system Pipeline proposed special backfill consideration Special crossing, i.e. river crossings, railway, thrust bores, steel casings, etc. Pipeline route, i.e. urban, open country, desert, utility corridor, etc. Foreign service pipelines and/or foreign structures crossing the pipeline Pipeline jointing method, welded, couplings, socket and joint, etc. Proposed insulation joints/flanges Pipeline off-take(s) and spur line(s), included in project or proposed for future development Data obtained from undertaking field surveys and testing: Resistivity measurements along pipeline route Resistivity measurements at possible Impressed Current groundbed site(s) Physical survey along pipeline route Locate possible source of any man made stray current, DC powered equipment, etc. Locate possible source of induced current, overhead power lines, etc. Take soil samples and obtain laboratory test results


2) PREVIOUSLY INSTALLED PIPELINE (NEW OR OLD) Data obtained from the pipeline as built engineering mechanical drawings & engineering design documentations. Pipeline material, weight & wall thickness Pipeline diameter Pipeline external coating system Pipeline proposed special backfill consideration Special crossing, i.e. river crossings, railway, thrust bores, steel casings, etc. Pipeline route, i.e. urban, open country, desert, utility corridor, etc. Foreign service pipelines and/or foreign structures crossing the pipeline Pipeline jointing method, welded, couplings, socket and joint, etc. Installed Insulation Joints (I/J or I/F) locations Pipeline off-take(s) and spur line(s), included in project or proposed for future development Data obtained from undertaking field surveys and testing: Resistivity measurements along pipeline route Resistivity measurements at possible Impressed Current groundbed site(s) Physical survey along pipeline route, inspecting for possible groundbed location, suitable AC supply, special crossing, pig trap stations, etc Inspect & test insulation joints/flanges (if installed) Current drain test & line current survey Take soil samples and obtain laboratory test results Natural pipe/soil potential survey Test for possible stray current conditions Test installed steel casings for shorted condition Coating resistance survey (if necessary)

3) COMMISSIONING SYSTEM The data required by the commissioning procedure will include the following field survey requirements. Pipe/soil potential survey (natural) T/R operating settings (ampere(s)/voltage) switch settings, etc. Test current output of all anodes installations Test all cables for continuity Inspection and testing of installed insulation joints/flanges Pipe/soil potential survey (energised) Initial close interval, polarised (instant off), potential survey (CIPS) Pearson survey (new pipelines) and/or if found necessary from (CIPS) survey Bell hole examination if necessary Interference testing Final adjustment if found necessary from interference testing results. 12-month, Close interval, polarised (instant off), potential survey (CIPS) 12-month, Pearson survey if found necessary from CIPS survey



DESIGN CONSIDERATIONS OHMS LAW Ohm’s Law states that a current in an electrical circuit is equal to the electromotive force divided by the resistance. Expressed in the symbols this becomes:

Ohms Law

I

V 

R

When: I = the current in amperes V = the electromotive force (potential) in volts R = the resistance in ohms Ohms law not only applies to electrical circuits, but also to the ground currents involved in the corrosion and corrosion mitigation by means of cathodic protection. When cathodic protection is used for underground structures, the circuit is in effect from the anode through the soil/electrolyte to the structure, then along the structure to the cable/wire leading back to the anode, or to the transformer rectifier and in turn to the anode. Whether the current is supplied by means of a sacrificial anode or transformer rectifier, it is necessary to calculate the anticipated current output per anode/groundbed for a given electromotive force. To calculate the anticipated current, it is necessary to determine the resistance of the anode/groundbed, whether the anode/groundbed is vertical or horizontal.



DESIGN CONSIDERATIONS OHMS LAW Example:

Ohms Law

I

V 

R

when: I = the current in amperes V = the electromotive force (potential) in volts R = the resistance in ohms Example: To calculate circuit resistance: Standard magnesium anode potential Min protective potential required Current required for potential change Driving voltage,

= - 1.550 volt = - 0.850 volt = 0.075 amp

-1.55 volt - 0.85 volt = 0.70 volt

Therefore by ohms law:

R

Circuit Resistance

R

V 

I 0. 7



0.075


= 9.33 ohms


DESIGN CONSIDERATIONS CALCULAT E TH E R E SI STANCE OF A HO RI ZONTAL ANO DE G ROUNDBE D A horizontal anode may consist of either (1) a rectangular (Normal Square) column of compacted coke breeze backfill, with a number of graphite or high-silicon iron anodes, imbedded therein, as in the diagram (A) or as diagram (B) a horizontal piece of scrap steel pipe. In either case, the basic dimensions are the length L, the diameter D (use width for a square column) and the depth S. When all three of these are expressed in feet, and the resistivity is given in ohms-cm, then the resistance R in ohms is given by the following equations: Expression using the common logarithm: 0.012. .

R

L

log

4. L

log

D

L

0.87

S

0.87. S L

Alternatively, the following expression may be used, in which the natural logarithm appears instead of the common. R

 . ln 4. L 192 . L D

ln

L

2

S

2. S L

or

R

0.00521. . L

2 4. L 2.3. log

2 4. L. S ( D.S )

2

L

S L

2

S

L

2

L

1

Note: The natural logarithm may be computed from the common logarithm by the expression: ln( x)


2.303 . log( x)


DESIGN CONSIDERATIONS A HORI ZONTA L ANODE GROUNDBE D LAYOUT

DIAGRAM A



DESIGN CONSIDERATIONS A H ORI ZONTA L ANODE GROUND BE D - LAYOUT SCR AP STE E L

DIAGRAM B



DESIGN CONSIDERATIONS CALCULAT E TH E R E SI STANCE OF A HO RI ZONTAL ANO DE G ROUNDBE D

EXAMPLE ONE Find the resistance to earth of a 200-foot length of 24” pipe buried at a depth of 4 feet in 12,000 ohm-cm soil. A) Computed using natural logarithms:



R

R

. ln

4. L

192 . L

12000

. ln

192 . 200

ln

D

4. 200

L

2. S

2

S

ln

L

200

2

2

4

2. 4 200

R = 2.482 ohms

B) Computed using common logarithms: R

R

0.012. . L

0.012. 12000 . 200

log

log

4. L D

4.200 2

log

L

0.87

S

log

200 4

0.87. S L

0.87

0.87. 4 200

R = 2.483 ohms

The agreement between the two examples is close enough.



DESIGN CONSIDERATIONS CALCULAT E TH E R E SI STANCE OF A HO RI ZONTAL ANO DE G ROUNDBE D

E XAMP LE TWO Find the resistance to earth of a 100 m length of 300 mm width buried at a depth of 1200 mm in 12,000 ohm-cm soil. A) Computed using natural logarithms: R

R

12000 192 . 100. 3.281

. ln



. ln

4. L

192. L

ln

D

4. 100. 3.281 0.3. 3.281

L S

ln

2

2. S L

100. 3.281 1.2. 3.281

2

2. 1.2. 3.281 100. 3.281

R = 1.837 ohms

B) Computed using common logarithms: R

R

0.012. . L

log

4. L D

0.012 . 12000 . 4. 100. 3.281 log 100. 3.281 0.3. 3.281

log

log

L S

0.87

100. 3.281 1.2. 3.281

0.87. S L

0.87

R = 1.837 ohms

There is agreement between the two examples.


0.87. 1.2. 3.281 100. 3.281


DESIGN CONSIDERATIONS CALCU LATE THE RE SISTAN CE OF A VER TICAL ANO DE GRO UNDBE D To calculate the anticipated current, it is necessary to determine the resistance of the anode, where a single anode is employed, or the resistance of a group of anodes. Considerations will first be given to the calculations for a single vertical anode, then to groups of vertical anodes. A formula commonly used to calculate the resistance of a single vertical anode is: R



2.  . L

. ln

4. L

1

r

When: R = resistance to earth of a single vertical anode in ohms  = soil resistivity in ohm-cm L = length of anode in cm r = radius of anode in cm This formula (expression) may be modified to a more readily usable form in which the length and diameter of the anode are given in feet. This arrangement of the formula is: R

0.00521. . L

8. L 2.3. log D

1

When: R = resistance to earth of a single vertical anode in ohms  = soil resistivity in ohm-cm L = length of anode in feet D = diameter of anode in feet

Note: all dimensions of anode used to take into account backfill.


CATHODIC PROTECTION TRAINING COURSE DESIGN CONSIDERATIONS CALCU LATE THE RE SISTAN CE OF A VER TICAL ANO DE GRO UNDBE D

E XAMP LE ON E Find the resistance of a single vertical anode with a diameter of 6” (15 cm) and a length of 4 feet (120 cm), (including backfill), to be installed in 4,000 ohm-cm soil.

R

R

0.00521. .

8. L 2.3. log D

L 0.00521 . 4000. 4

1

8. ( 4 ) 2.3. log 0.5

1

R = 5.2(4.1589 - 1) R = 16.43 ohms

Alternatively calculated in cm using the following expression: R

R



2.  . L 4000 2.  . 120

. ln

4. L

1

r

. ln

4. 120

1

7.5

R = 5.305(4.159 - 1) R = 16.75 ohms




DESIGN CONSIDERATIONS CALCU LATE THE RE SISTAN CE OF A VER TICAL ANO DE GRO UNDBE D

E XAMPLE TWO To determine the current output for a single vertical magnesium anode with a backfill dimensions of 6” (15 cm) and 5 feet (150 cm) long, in a soil resistivity of 6,000 ohm-cms. The structure is to be protected to a minimum potential of 0.9 volts.

R

R

0.00521. . L

8. L 2.3. log D

0.00521 . 6000.

( 8. 5 ) 2.3. log 0.5

5

1

1

R = 6.24(4.382 - 1) R = 21.14 ohms

If the structure is to be protected at a structure-to-copper sulphate potential of 0.9 volts, substituting ohms law, using the difference in the anode-to-soil potential and the design potential, as the electromotive force (driving Potential), gives: I

I

V 

R

15 . 5 0 .9 



2110 .

I = 0.031 amperes

Thus it may be seen that the anode, under the conditions given, would have an output of 0.031 A, or 31 mA of current.



DESIGN CONSIDERATIONS CALCU LATE THE RE SISTAN CE OF A VE RTI CAL ANO DE GRO UP Anodes are frequently installed in groups. In these cases the effective resistance of the group is taken into consideration in calculating the current output of the group. If a group say of 6 anodes were to be installed, the resistance of the group would not be equal to the resistance of one anode divided by the number of anodes in the group. Dividing the resistance of a single anode in the group and multiplying by a correction factor would determine the resistance of the group. This factor takes into consideration the mutual interference effect of the anodes in the group. The formula used to calculate the correction factor for groups of vertical anodes is:

.2

R 1

log

cot 2

.

.3

cot 2 4. L log r

.n

.

cot 2 1

Where: n



AcrCot

L ns

n = number of anodes in parallel r = radius of anodes in cm L = length of anode in cm s = spacing between anodes in cm

See chart and graph for correction factors to be applied to a number of anode groups.



DESIGN CONSIDERATIONS CALCU LATE THE RE SISTAN CE OF A VE RTI CAL ANO DE GRO UP Correction factor chart for various anode groups No in group 5 anodes 8 anodes 10 anodes 12 anodes 14 anodes 16 anodes 18 anodes 20 anodes 24 anodes 27 anodes

5 feet spacing 1.62 1.86 1.97 2.05 2.14 2.21 2.29 2.31 2.41 2.48

10 feet spacing 1.33 1.44 1.51 1.56 1.60 1.63 1.65 1.68 1.73 1.76

15 feet spacing 1.22 1.30 1.35 1.38 1.40 1.42 1.44 1.46 1.49 1.50

20 feet spacing 1.16 1.23 1.25 1.28 1.29 1.32 1.34 1.35 1.37 1.38

25 feet spacing 1.13 1.18 1.20 1.22 1.24 1.25 1.26 1.27 1.29 1.30

30 feet spacing 1.11 1.14 1.16 1.18 1.19 1.21 1.22 1.23 1.24 1.25

33 anodes

2.60

1.81

1.54

1.41

1.33

1.27

Correction factor graph for various anode groups



DESIGN CONSIDERATIONS CALCU LATE THE RE SISTAN CE OF A VE RTI CAL ANO DE GRO UP E XAM PLE ONE To determine the resistance of a group of 8 anodes, with a diameter of 4” (10 cm), and a length of 5 feet (150 cm), in a soil resistance of 5,000 ohms-cm. The anodes are to be spaced at 10-foot intervals. The resistance of a single anode is:

0.00521. .

R

R

8. L 2.3. log D

L

0.00521 . 5000.

1

( 8. 5 ) 2.3. log 0.33

5

R = 5.2 (4.797-1)

1

R = 19.748 ohms

Correction factor for 8 anodes with 10 foot spacing, see attached chart, is: CF = 1.44 Therefore the resistance of the 8-anode group is: R

19.748 

x1. 44

8

R = 3.555 ohms

To determine the current output for a group of 8 magnesium anodes, the driving force would be the difference between the anode-to-soil potential (1.55 volts for magnesium), and 0.90 volts for the protected structure giving:

I

1. 55 0 .90 



3. 555




0.1969amperes


DESIGN CONSIDERATIONS CALCU LATE THE RE SISTAN CE OF A VE RTI CAL ANO DE GRO UP Alternative Design Formula for Vertical Anode Group To calculate several anodes in parallel (group), the alternative expression is: Rn

0.00521. 1 . 8. L 2.3. log . ( N L) D

L . 2. 2.3. log( ( 0.656 . N ) ) S

1

When: Rn = resistance to earth in ohms, of the vertical anodes in parallel 1= soil resistivity in ohm-cm N = number of anodes in parallel L = length of anode in feet D = diameter of anode in feet S = anode spacing in feet

EXAMPLE To calculate the resistance of a impressed current groundbed consisting of 10 silicon iron anodes, of 3” diameter and 60” in length, installed in 8” dia holes 10 foot deep, the active column length being 8 feet, installed every 10 foot in parallel in a soil resistivity of 1000 ohm-cm. Rn

0.00521 . 1000. 8. 8 2.3. log 10. 8 0.666

1

8 . 2. 2.3. log( ( 0.656 . 10) ) 10

R = 0.0651(4.565-1+(1.6 x 1.88)) R = 0.428 ohms

Note: The internal resistance of the anode to the carbonaceous backfill has been ignored due to the number of anodes in-group. The effect of the internal resistance becomes less as the number of anodes increase.



DESIGN CONSIDERATIONS CALCU LATE THE RE SISTAN CE OF A DEE P VERTI CAL AN ODE GR OUNDBE D Calculating the resistance of a Deep Vertical Anode Groundbed (DAGB) When: ρ = Soil resistivity in ohm-cm L = Length of groundbed in feet D = Diameter of anode/groundbed in feet Then the resistance R in ohms is given by:

2. ( L) 0.012. . log D R

2 L

Alternative DAGB Design Calculation When: ρ = Soil resistivity in ohm-cm t = depth of groundbed in m L = length of groundbed in m d = diameter of groundbed in m



R

100 L ( 4. t ) ( 3. L) . ln 2 . . d ( 4 .t ) L 2.  . L


EXAMPLE To determine the resistance of a deep vertical anode groundbed with a diameter of 9” (0.230 m) and a length of 365 feet (111 m), at a ground bed depth of 200 feet (61 m), in a soil resistivity of 1,000 ohms-cm. The ground resistance R can be calculated using feet as follows: 2. ( L) 0.012. . log D 2

R

L

0.012 .1000 . log

2. ( 365 ) 0.755 2

R

365 R = 0.108

Alternatively calculated in metres using the following expression: 

R

100 . L ( 4. t ) ( 3. L) ln 2. . . . d ( 4. t ) L 2 L 

R

100 . ln 2.  . L

2.

L . ( 4. t ) ( 3. L ) d ( 4. t ) L

R = 0.102




DESIGN CONSIDERATIONS SE LE CTI ON OF CAB LE SI ZE TO ME E T A GI VE N MA XI MUM VOLTAGE DROP ON A CATHO DI C PROT E CTION SYST E M Max. Allowable cable volt drop =

Given . Max . voltdrop .( ) volts x 103 Route . Length .( ) meters xCircuit . Amps

EXAMPLE Given max. Volt drop = 2 volts Route Length

= 150 meters

Circuit Amps

= 50 A

Max. Allowable cable volt drop

=

2 . x10

3

150 x50



0.266 mV / A m /

Hence, from the table characteristics of XLPE/PVC cable the minimum cable size, which could be used for this cathodic protection system would be 95 mm².

I mportant:

Current carrying capacity of the cable must also be taken into consideration in final selection of cable size.


CATHODIC PROTECTION TRAINING COURSE DESIGN CONSIDERATIONS CATHO DI C PRO TECTI ON XLPE/ PVC C ABLE CHAR ACTERI STI CS TABLE of XLPE/PVC Cable to IEC.502 - 1983 (600/1000V) Nominal Area mm²

Number of Wires

Insulation Thickness mm

Sheath Thickness Mm

Overall Dia mm

Weight Kg/Km

Ohms per Km

1.5 2.5 4 6 10 16 25 35 50 70 95 120 150 185 240

1 1 1 7 7 7 7 7 7 19 19 19 19 37 37

0.7 0.7 0.7 0.7 0.7 0.7 0.9 0.9 1.0 1.1 1.1 1.2 1.4 1.6 1.7

1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.5 1.5 1.6 1.6 1.7

6.0 6.5 6.5 7.5 8.5 9.0 11.0 12.0 13.5 15.0 17.0 18.5 21.0 23.0 25.5

45 60 80 100 150 210 300 400 530 730 1000 1230 1520 1890 2440

3.08 1.83 1.15 0.727 0.524 0.387 0.268 0.193 0.153 0.124 0.0991 0.0754

Volt Drop at Conductor Operating Temperature of 70° C


mV Drop per A/m 14.000 8.500 5.500 3.550 2.100 1.350 0.850 0.650 0.455 0.315 0.225 0.180 0.125 0.115 0.090


DESIGN CONSIDERATIONS CALCULAT I ON OF SYST E M VOLTA GE General Considerations

1.

All cable losses.

2.

Ohmic resistance of anode or groundbed to remote earth or water.

3.

Potential change at cathode that will depend upon the following: 3.1

Attenuation - particularly relevant to pipelines.

3.2

Anode/cathode proximity.

3.3

Any maximum acceptable potential imposed by specification.

Calculating Allowances

1.

Cable

For calculation of all cable losses, refer to the following references found in this section: 1.1 Selection of cable size to meet a given maximum voltage drop on a cathodic protection system. 1.2 2.

Cathodic protection XLPE/PVC cable characteristics.

Ohmic Resistance

For calculation of ohmic resistance of anode or groundbed please refer to the relevant references found in this section: 2.1

Calculate the resistance of a horizontal anode groundbed.

2.2

Calculate the resistance of a vertical anode groundbed.

2.3

Calculate the resistance of a vertical anode group.


3.

Anode & Groundbed Related Items 3.1

Galvanic effect of coke/carbon backfill when used (“back e.m.f.”). Allow 1.70 volts

3.2

Titanium and Niobium Anodes: 3.2.1 Anode decomposition voltage

-1.35 volts

3.2.2 Anode over-potential

-0.50 volts

3.2.3 Cathodic decomposition voltage

-0.45 volts

3.2.4 Cathode over-potential (steel)

-0.40 volts _________

Therefore: Allow

-2.70 volts (“back e.m.f.”)



DESIGN CONSIDERATIONS GRAPH I TE AND SI LI CON I RON ANO DE CONS UMP TI ON RATES

Graphite Anodes Environment

Fresh Water Salt Water Carbonaceous Backfill

Current Density A/m² 3 10 10

Consumption Rate Kg/Ampere Year 0.10 - 0.20 0.10 - 0.20 0.05 - 0.10

Silicon Iron Anodes Environment

Fresh Water Salt Water Wet Carbonaceous Backfill Dry Carbonaceous backfill

Current Density A/m² 10 - 30 10 - 50 10 - 50

Consumption Rate Kg/Ampere Year 0.15 0.50 0.10

10 - 50

Negligible



DESIGN CONSIDERATIONS GRAPHI TE AND SI LI CON I RON RO D ANO DES ME TR I C DI ME NSI ONS AN D WE I GH TS

Anode Dia mm

Anode Length mm

Anode Surface Area m²

Silicon Iron Approx. Weight Kg 8.10 19.00 22.70 34.50 49.10 -

Anode Dia Inches

Anode Length Inches

0.109 0.194 0.243 0.304 0.365 0.486

Graphite Approx. Weight Kg 6.70 9.20 18.80

38 50 50 63 76 102

915 1220 1525 1525 1525 1525

1.5 2 2 2.5 3 4

36 48 60 60 60 60

114 102 152

1525 1830 1830

0.547 0.584 0.876

23.20 53.60

99.10 -

4.5 4 6

60 72 72



DESIGN CONSIDERATIONS GRAPHI TE AND SI LI CON I RON RO D ANO DES

Table of Amps per Anode at Varying Current Densities

Anode Dia mm

Anode Length mm

Anode Surface Area m² 0.109 0.194 0.243 0.304 0.365

Current Density Amps/m² 10 1.09 1.94 2.43 3.04 3.65




38 50 50 63 76

915 1220 1525 1525 1525

102 114 102 152

1525 1525 1830 1830

0.486 0.547 0.584 0.876

4.86 5.47 5.84 8.76

9.72 10.94 11.68 17.52

14.58 16.41 17.52 26.28

24.30 27.35 29.20 43.80



DESIGN CONSIDERATIONS COKE BREE ZE BACKF I LL SP ECI FI CA TIO NS Particle Size (Typical only)

Greater than 12.5 mm 12.5 mm to 6.3 mm 6.3 mm to 3.15 mm Less than 3.15 mm

1.3% 16.9% 25.8% 56.0%

50 ohms-cm max.

Resistivity

Chemical Analysis (Typical only)

Fixed Carbon

82.7% min.

Moisture content Ash Sulphur

5% max. 8.6% 1.04%

90% max.

Bulk Density

43/44 lb. per Cubic Foot 51 Cubic Feet per Ton 50 Cubic Feet per Tonne 19.77 Kg per Cubic Foot 698 Kg per Cubic Metre

Note: Density will vary with water content, above figures based upon approx. 5% by weight content.



DESIGN CONSIDERATIONS TYP I CAL CARBO NAC E OUS I MPRESS E D CURRENT ANO DE BACKF I LL SPE CI F I CA TI ON Particle Analysis (Typical)

Resistivity

Mesh

Retained

+ 35 35 x 65 65 x 100 - 100

0 15% 15% 70%

15

ohms-cm @ Atmospheric Pressure 0.19 ohms-cm @ 150 psig

Chemical Analysis (Typical only)

Carbon Moisture content Ash Sulphur Nitrogen Volatiles

96.3% 0.5% 0.1% 1.7% 0.8% 0.6%

Bulk Density

48 lb. per Cubic Foot 68 lb. per Cubic Foot @ 150 psig



DESIGN CONSIDERATIONS GALVANI C AN ODE MATER I ALS TABLE OF CAPAC I TI E S, POTENTI ALS AND CO NSU MPTION RATES Materials

Capacity in Sea Water

Capacity in Mud

Potential (volts)

Consumption Rate Sea Mud

Ah/lb

Ah/Kg

Ah/lb

Ah/Kg

Cu/CuSO4

Zinc

355

780

332

740

1.1

1.05

1.282

1.370

Galvalum I (aluminium)

1285

2830

not suitable

not suitable

1.1

1.05

0.353

-

Galvalum II (aluminium)

1230

2700

1139

2500

1.1

1.05

0.370

0.400

Galvalum III (aluminium)

1156 -

2550 -

998 567

2200 1250

1.14 1.11

1.09 1.06

0.392 -

0.455 0.800

Magnesium (High Purity)

-

-

in soil 560

in soil 1230

1.55

1.5

-

0.615

Galvomag (or equal)

-

-

560

1230

1.75

1.7

-

0.615


Notes

Ag/AgCl gm/Ah gm/Ah US Mil Spec. 18001 Unsuitable to anaerobic conditions e.g. seabed contains mercury Developed for use below mudline contains mercury @ 30°C @ 60°C suitable for free water & mud conditions no Mercury “normal” resistivity soils suitable for high resistivity soils

CATHODIC PROTECTION TRAINING COURSE DESIGN CONSIDERATIONS CALCU LATE THE W EI GHT REQU I RE MENT O F SACRIF I CI AL A NODES

Weight of anode material required (kg)

Conversion:



Current( Amps ) xDesignlife ( ) years x 8760 Capacity( AmperesHours / Kg ) xUtilisationFactor

1 Kg = 2.205 lb.

Ah/lb x 2.197 = Ah/Kg.

Typical Utilisation Factors

1.

For long slender anodes use

0.90 - 0.95

2.

For bracelet anodes use

0.75 - 0.80

3.

For flush mounted anodes use0.75 - 0.85



DESIGN CONSIDERATIONS CALCU LATE THE LI FE E XPECTANC Y OF SACRIF I CI AL AN ODES Designing a cathodic protection system for buried structures, using the sacrificial anodes principle, invariably involves three relationships the engineer must face. The engineer must decide the type of anode to be adopted and correlate the anode life in years and weight of anode for a particular design requirement. An understanding of some of the differences in performance, between zinc and magnesium anode installations, will be helpful in deciding which type of anode installation to use. Although the general rule-of-thumb would say that zinc anodes are better used in low resistivity soils (i.e. below 1500 ohm-cm) and magnesium anodes are better in higher resistivity soils. This rule is not universal but will depend on the application requirement. If the current output of a sacrificial anode of any given weight is known, its approximate useful life can be calculated. This calculation is based on the theoretical ampere hour per pound of the anode material, its efficiency and utilisation factor. The utilisation factor may be taken at 85 percent, (meaning that when the anode is 85% consumed, it will require replacement because there is insufficient anode material remaining to maintain a reasonable percentage of its srcinal current output.

1. To calculate the life expectancy of zinc the following expression can be used: Zinc Life in Years

= 42.5 x Pounds x Efficiency x Utilisation Milliamperes

2. To calculate the life expectancy of magnesium the following expression can be used: Magnesium Life in Years = 116 x Pounds x Efficiency x Utilisation Milliamperes

The indicated lives are at the efficiency factors shown in the example expressions, (90 percent for zinc, 50 percent for magnesium, (refer to specification data tables),. It should be noted that at very low current densities, efficiency of the anodes will suffer and actual lives would be less than indicated by the expression. This is particularly true of magnesium.


The resulting expressions being: Zinc Life in Years

= 32.51 x Pounds Milliamperes

Magnesium Life in Years = 49.30 x Pounds Milliamperes

EXAMPLE Calculate the life expectancy of a 10-kilogram magnesium anode having a design current delivery requirement of 75 milliamperes. From the data table of magnesium the efficiency factor used is 50 percent with utilisation factor at 85 percent. Therefore: Magnesium Life in Years = 49.30 x (10 x 2.2) = 14.46 years 75 Comparison using zinc utilising same weight for anode: Zinc Life in Years

= 32.51 x (10 x 2.2) = 9.54 years 75



DESIGN CONSIDERATIONS MA GNE SI UM AND ZI NC ANOD E SPE CI F I CA TI ON TABLE Approximate Data for Magnesium Anodes in Earth Specific Gravity Kilograms per Cubic Meter Theoretical Amp Hour per Kilogram Theoretical Kilograms per Amp per Year Percent Current Efficiency – Utilisation Percent Factor – Actual Amp Hour per Kilogram Actual Kilograms per Amp per Year Solution Potential - Volts to CuSO4 Standard Alloy High Potential Alloy Driving Potential to Pipeline/Structure Polarised to - .090 Volts to CuSO4 Standard Volts Alloy – High Potential Alloy - Volts

1.94 550 2200 4 50 85 - 90 1100 8 - 1.55 - 1.80

.055 0.80

TABLE Approximate Data for Zinc Anodes in Earth Specific Gravity Kilograms per Cubic Meter Theoretical Amp Hour per Kilogram Theoretical Kilograms per Amp per Year Percent Current Efficiency – Utilisation Percent Factor – Actual Amp Hour per Kilogram Actual Kilograms per Amp per Year Solution Potential - Volts to CuSO4 Driving Potential to Pipeline/Structure Polarised to - .090 Volts to CuSO4


7 2000 818 10.68 90 85 - 90 737 12 - 1.10 0.20


DESIGN CONSIDERATIONS SACR I F I CI AL AN ODE DE SI GN Example of onshore pipeline Basic Data. Pipeline Diameter (d) = 8” = 0.203m Pipeline Length (l) =10km =10,000m Pipeline Coating = 3 layer polyethylene = 3LPE Pipeline Native Potential = -0.55V (CSE) Average Soil Resistivity along right of way = 5,000 cm Required System Life = 20 yrs Required Protective Criteria =-0.85V (CSE) Isolation – Full Isolation by means of end line insulating monobloc joints = yes Choice of sacrificial anode materials i.

ii.

Magnesium alloy Magnesium alloy consumption rate Magnesium alloy open circuit potential Efficiency Zinc alloy Zinc alloy consumption rate Zinc alloy open circuit potential Efficiency

= 7.97kg/A-y = -1.7V (CSE) = 50%

= 11.5kg/A-y = -1.1V (CSE) = 90%

Magnesium Anodes Selected The most common size of magnesium anode is the D7 or 7.7kg Anode Packaged Diameter Anode Packaged Length

= 0.19m = 0.61m

Anode Magnesium Weight

= 7.7kg

General rules of thumb 1. If soil resistivity exceeds 1, 000 cm use magnesium anodes and never use magnesium anodesunder 1,000 cm only Zinc anodes. 2.

NEVER USE ALUMINIUM ANODES FOR ONSHORE BURIED APPLICATIONS



DESIGN CONSIDERATIONS SACR I F I CI AL AN ODE DE SI GN Anodes Required by Consumption Anode Weight required to meet a desired life is calculated using the following formula

Wt

=

IT x Lf x Ca Ua

=

0.189A x 20y x 7.97kg/A-y 0.9

= 33.5kg

where = IT Required current = 0.189A Ca = Magnesium alloy consumption rate = 7.97kg/A-y Ua = Efficiency = 50% Lf = Required life = 20 y Minimum number of anodes required to achieve life =33.5kg/7.7kg =4.3 =5 anodes Magnesium Anodes Required by Output Applying Ohm’s law

V = IA x RT

and

IA = V/ RT

Where V is the driving potential of the circuit and is the difference between the open circuit potential of the sacrificial anode and the protected potential of the pipeline. V = -1.7V (CSE) - -0.85V (CSE)

= 0.85V

IA is the anode current output RT is the total circuit resistance and comprises the resistance of the individual or grouped anodes (Ra) and the resistance of the pipeline (Rp) RT = Ra + Rp Resistance of Single Anode (Ra) The resistance of a single vertically installed anode is calculated based on the modified HB Dwight’s formula


L

Ra = 0.00159ρ Ln 8L d

Where ρ is the soil resistivity in Ωcm = L is the anode length in m = d is the anode diameter in m = Ra =

-1

5,000 cm 0.61m 0.19m 29.27Ω

Pipeline Resistance (Rp) Pipe Potential Difference (ΔE) = Protected potential– natural potential of the pipeline ΔE

=

-0.85V (CSE) -0.55V(CSE) –

= 0.30V

Pipeline Coating Resistance (CR) = Pipe Potential Difference / Applied Current Density CR

=

ΔE / Cd = 0.35V/0.03mA/m²

= 10,000

m²

Pipeline Resistance to Remote Earth (Rp) = Coating Resistance/Surface Area Rp

=

CR / SA =

10,000

m²/ 6280m² = 1.59

Total Circuit Resistance (Rt) RT

=

Ra + Rp

= 29.27

+ 1.59

= 30.86

Anode Current Output (IA) The output from one single anode is calculated as IA= V/RT = 0.85V/31Ω= 0.0275A Anodes Required by Output The number of individual anodes required is calculated by dividing the total required current by the anode current Anode quantity by current

= IT/IA =

This is acceptable√


0.189A/0.0275A

= 7 anodes

= 31


DESIGN CONSIDERATIONS TYPICAL MAGNE SI UM OR ZI NC ANO DE I NST ALLATI ON

Diagram A



DESIGN CONSIDERATIONS BLI ND MAGNESI UM OR ZI NC ANO DE I NSTALLAT I ON

Diagram B



DESIGN CONSIDERATIONS PI PELI NE ATT E NUA TI ON CALCU LATI ONS (COATED P I PELI NES) A theoretically model of the expected performance of a proposed cathodic protection system can be carried out using a pipeline attenuation calculation. This model will provide a theoretical analysis of the expected performance of the proposed cathodic protection designed for a pipeline, which is still in the planning stage, and/or is under construction, and/or when it is not possible to undertake a current drain test. It must be emphasised that this is a theoretical modelling only, and that variations in the construction practice, changes in environmental conditions, coating performance, external influences, and a verity of other unknown conditions can have radical influence on the outcome of the model result. Nevertheless, the model does provide some worthwhile information as to the expected performance of the proposed cathodic protection system, and can provide useful information as to suitable transformer rectifiers groundbed spacing, expected current distribution etc. The attenuation model calculation expression as follows: 8. Vv

L2 

. D. Id. R

Where: Vv

I. R. L 2

Where: R

1 C. D.  . d

When:

L2 = CP protective range, in km D = Pipe diameter, in m Id = Current density at drain point, in A/m² R = Pipeline resistance, in micro ohm/m C = Conductivity of pipe Coating, in mho's/mm² d = Pipe wall thickness, in m I = Current applied, A Vv = Voltage drop (i.e. drain point voltage - protective limit voltage), V


EXAMPLE Find the calculated theoretical maximum protected length of a proposed impressed current system for a 91.5 km, having a 0.406 m Dia, with a wall thickness of 0.009 m, coated with FBE having an expected coating conductivity value 6 mho's/mm², having a designed pipeline current requirement of 2.2 A, and a maximum volt drop from drain point to the pipeline protective limit (pipeline section end, isolation joint/flange) of 0.4 volts. Therefore; when: L2 = to be calculated, in km R = to be calculated, in ohm D = 0.406 m Id = 0.019 m C = 6 mho's/mm², d = 0.009 m I = 2.2 A Vv = 0.400 V R

1.1000 C. D.  . d

R = 14.519

L2

8. Vv .1000 

. D.Id. R

L2 = 95.366

Therefore; using the model provided, and assuming a pipeline coating conductivity of 6 mho's/mm² or better, the proposed cathodic protection system would provide adequate protection for 95.366 km. The groundbed should be located, if possible, approx. 45.75 km from one end of the pipeline, this being the middle of the pipeline section.


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES SOI L RE SI STI VI TY ME ASUR E ME NTS One of the major factors affecting corrosion rates is the electrical resistance offered to the flow of corrosion current. The principal elements of this resistance are associated with: a) any protective insulating coating on the structure and b) the resistance of the soil path. A protective coating applied to a structure will significantly increase this resistance to the point where a perfect high quality coating will prevent corrosion completely. Practical considerations of construction practice and effects of soil stress will produce inevitable flaws and imperfections in the coating during and after installation. Therefore account must be taken of the resistance of the soil path. Measurements of resistivity, by which the electrical characteristics of soil can be compared, are used to make an assessment of the corrosion hazard. The lower the resistivity, (measured in “ohm centimetres” or “ohm meters”), the more will be the tendency for significant corrosion current to flow. Variations in soil type will promote the development of differences in electrical potential at the structure surface. These will in turn constitute the voltage difference responsible for causing corrosion current to flow. Changes in soil resistivity will also afford an indication of changes in soil type, e.g. Heavy waterlogged clays exhibit a less resistivity, whilst dry sandy soil will exhibit a high resistivity. Two factors emerge which have an important bearing on the likelihood of corrosion occurring: 1) The order of magnitude of the soil resistivities, e.g. High, medium, low. 2) Changes in soil resistivity immediately surrounding the structure.


Soil resistivity is classified as follows in BS CP1021/ BS 7361 part 1; Soil Resistivity ohm-cm / (ohm-m)

Classification

0 -cm / (0 -m) to 1,000 -cm / (10 -m) Severely corrosive 1,000 -cm / (10 -m) to 5,000 -cm / (50 -m) Corrosive 5,000 -cm / (50 -m) to 10,000 -cm / (100 -m) Moderately corrosive 10,000 -cm / (100 and Slightly corrosive -m)over This corrosion classification of soils represents, in a very general way, a means of predicting areas in which corrosion of metals may become a problem. It is not valid to say that serious corrosion will occur because a specific soil resistivity is less than 1000 ohm-cm /10 ohm-m, or that corrosion will not occur because soil resistivity are above 10,000 ohm-cm /100 ohm-m. The classification shown does not, for example, recognise related dissimilar metal combinations. The classification of soils in this manner remains a useful tool to generally indicate corrosivity and if used within limits as a general guide to provide for better corrosion judgement.


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES SOI L RE SI STI VI TY ME ASUR E ME NTS THE WE NNER F OUR- PIN ME THO D The most common and reliable method of measuring the resistivity is by the 4 pin Wenner method that was developed by the American Natural Bureau of Standards. This involves the insertion of four pins into the ground at even spacing and in a straight line. The two outer pins are current electrodes, and the two inner pins, are potential electrodes that measure the potential drop between the two outer electrodes, all pins are equally spaced. The depth of the soil sample under test is equal to the pin spacing “a”. The resistance can be measured and the soil resistivity calculated according to the following formula:





2 aR

Where: soil resistivity in one ohm-cm a == distance between pair of pins in cm R = resistance in ohms Alternatively; for quick reference during field operations the measured value of resistance using the Wenner four pin method is taken and then multiplied by a factor to obtain the average soil resistivity at the location tested. This factor is 191.5 times the pin spacing in feet or 6.283 times the pin spacing in cm. For site convenience the following (rule of thumb) Table is can be used: TABLE - PRACTICAL SPACING FOR 4-PIN RESISTIVITY MEASUREMENTS Pin Spacing

feet

inch

2 5 7 10 13 15

7 3 10 5 1 8

Factor

Pin Spacing

Factor

cm 500 1000 1500 2000 2500 3000


80 160 240 320 400 480

500 1000 1500 2000 2500 3000

EXAMPLE Obtain the average soil resistivity for a proposed horizontal groundbed to be installed at a depth of 160 cm, using the rule of thumb method and using the formula expressed above. The pin spacing “a” is given as 5 ft 3” and the resultant resistance measured is 2.68 ohms. 1) calculating using rule of thumb method:  = Factor x measured resistance in ohms

Therefore;  = 1000 x 2.68 = 2680 ohm-cm

2) calculating using equation:  = 2 x  x spacing in cm x measured resistance in ohms

Therefore;  = 2 x 3.1415 x (63 x 2.54) x 2.68 = 2694 ohm-cm

Note: The results obtained from both methods are close enough and either could be used for calculating to determine groundbed design requirements. A number of instruments have been developed which give the results directly in ohms. The instruments, which measure resistance, use alternate current in order to avoid undesirable polarisation effects at the interface between the electrode and the ground.


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES SOI L RE SI STI VI TY ME ASUR E ME NTS LAYER RE SISTIVI TY CA LCU LATIONS

“ BARNES PROCEEDURE” It is very useful to know the average soil resistivity in a specified layer at a given depth. This information will aid design considerations when determining anode placement, quantity, spacing, groundbed length, system selection, material selection, etc. One mathematical procedure, known as the “Barnes Method”, this method is based on calculating the resistivity of the soil in each incremental layer of soil. This is done by using the data derived from a “Wenner “ four pin soil resistivity test data series but extending the calculations by determining the conductivity of each increment layer and converting this conductivity to resistivity. It should be understood that this method is not infallible because soil layers must be of uniform thickness and parallel to the surface to be accurate. Where this is true, each added layer of earth must increase the total conductivity from the surface or the bottom of the added layer no matter what the resistivity of the added layer may be. If the conductivity continues to increase with depth, it is an indication that conditions approach the ideal closely enough to make the method usable. Decreases in the conductivity at any point in a series are an indication that the soil layers are too distorted to permit use of the method for analysis of data at that level. The actual calculation should be evident from the typical figures given in the following tables 1a and 1b, but for clarity an example is given below.


LAYER RE SISTIVI TY CA LCU LATIONS

“ BARNES PROCEEDURE” 1.

EXAMPLE

The “Barnes Method” can be demonstrated by applying the procedure to the following set of soil resistivity data. TABLE 1A WENNER 4 PIN DATA Measured Resistance Correction Factor R1

Pin Spacing

0.80 1.60 2.40 3.20 4.00 4.80

2.200 1.000 0.833 0.750 0.645 0.570

500 1000 1500 2000 2500 3000

1.1.

Soil Resistivity to stated depth (ohm-cm) 1100 1000 1250 1500 1610 1710

TABLE 1B

Barnes Procedure

Soil conductance to stated depth 1

Mhos

Increase in conductance 

1

Increase in resistance 1

Mhos

R1



R1

1

Correction factor F

 R2 ohms

R1

Layer soil resistivity

Layer depth meters

ohm-cm F x R2

0.455

-

-

-

1100

0.00 - 0.80

1.000

0.545

1.84

500

920

0.80 - 1.60

1.200

0.20

5.00

500

2500

1.60 - 2.40

1.330

0.130

7.70

500

3850

2.40 - 3.20

1.550

0.220

4.55

500

2275

3.20 - 4.00

1.750

0.200

5.00

500

2500

4.00 - 4.80

When: (1)

Resistance obtained from site tests as detailed in TABLE 1A.



SURVEY METHODS & TECHNIQUES SOI L RE SI STI VI TY ME ASUR E ME NTS SOI L BOX ME ASUR E ME NTS The soil box is a useful and convenient means of taking soil resistivity and in obtaining accurate results. It is used to measure the soil resistivity from samples taken from excavations, trial holes, auger holes, etc.,. A soil box consists of a plastic container with metal end plates for passing current through the sample, which is placed and compacted into the box. Potential terminals permit measurement of volt drop across a section of the sample. The maximum sample resistivity that can be measured is limited to the maximum value of resistance, which the instrument being used is capable of recording. Commercially available units are available from most cathodic protection equipment suppliers. The soil box is also used to verify the soil resistivity measurements of specialised materials such as on site backfill materials, evaluation of specialised imported material, etc. The soil box can be utilised in providing resistivity measurements of water samples including seawater, ground water, process water, effluent, etc.



SURVEY METHODS & TECHNIQUES TESTI NG OF CATHODI(I SOLATI C PROT ON E CTI ONNTS INSULATION F LANGE S J OI ) The testing of insulation flanges or isolation joints before installation is relatively easy, the problems come when an electrical short is suspected on an installed unit. Testing methods can be divided into two categories; A.

Conclusive tests.

B.

Indicative or supportive tests.

The only conclusive tests are those, which simulate the performance of the flanges or joints under cathodic protection operating conditions. It is of vital importance to remember that, in the electrical testing of pipeline or piping insulation flanges, there is invariably an alternative earth path for current to take.

A.

Conclusive Tests

Apply cathodic protection by permanent or temporary (current drain test) means to the pipework on the “protective “ side of the flange/joint. For completely satisfactory results, a negative change should be recorded of the order of 300 mV on the protected side; zero or a positive potential change should accompany this on the unprotected side. For the above tests, the unprotected pipework must be earthed. Provide a good temporary earth if necessary. When employing a temporary groundbed for the test, the location of the groundbed can be critical and the tests may produce a cathodic (negative) potential change on both sides of the flange/joint. If such results are obtained it will be necessary to make use of a nearby “earth” structure such as an iron/steel water main, sheet piling, or power earthing system in the following manner: Prove that the nearby structure is truly a good earth by conducting a current drain test on it - it is necessary to show that a significant amount of current is required to produce a reasonable potential change.


Having satisfied the above, set up the drainage test on the protected side of the flange/joint under test. Now connect the proven earthy structure to the unprotected side whilst observing the effect of this on the protected side - if the joint is working satisfactorily the potential on the protected side of the joint should not change. For further evidence of satisfactory working of the flange/joint, connect the proven earthy structure to the protected side and this should result in immediate loss of protection. Always take into account the resistance of test cables used in conjunction with temporary “earth” and connections. It is no good proving that a structure is “earthy” and then using a large coil of small diameter test cable. Keep short and use larger or multiple cables if necessary. On existing established cathodic protection systems it is normally acceptable to observe static potential differences between protected and unprotected sides of a flange/joint. Where it is necessary or desirable to observe potential change, it is more readily achieved by locally, “shorting” of the flange/joint, and observing the loss of protection on the protected side. Note again, that the degree of loss in protection caused by the short will depend on how well earthed the unprotected pipework is.

B.

I ndicative or Supportive Tests

For exposed insulated flanges and where insulating washers have been fitted under all bolts heads/nuts i.e., two washers per bolt or stud, a continuity test may be carried out between each bolt or stud and the flanges. Typically, the “DC ohms” range on an avometer, multimeter or similar, may be used for this test a 500 V Megger insulation tester should not be used for field-testing. A specified “Pass” value of ohms cannot be given because of the variety of circumstances arising in the field (e.g. moisture content of the insulation material). The test should, therefore, be used on a comparative basis and as a means of determining that the insulation of certain bolts/studs is suspected by comparison with others. This test may alternatively be undertaken using a bulb and battery instead of an ohmmeter. A relatively crude but quick indicative test consists of connecting a high capacity battery, (e.g. car battery), directly across the flange or joint and observe the intensity of spark obtained by momentary connection. This test is should be no sparking at all if the insulation is satisfactory. This test is not highly recommended as it cannot be used in class 1 areas or on high pressure pipelines or pipework, which may be, have its mechanical integrity damaged by arc burns etc. This test should not be undertaken under any circumstances without the prior written approval of the client or the responsible engineering manager of the plant under test. An extension of test 2 above, which may be used when the pipe is earthed on both sides of the flange/joint, involves the measurement of pipe/soil potentials in conjunction with a battery. There, TRAINING PROGRAM Prepared ByR J WATERHOUSE, MICorr78 of 113

the positive terminal of the battery is connected to one side of the insulation flange/joint, the negative on the other side.

Placing the half-cell above the buried section of the pipe and connecting the negative lead of the voltmeter, a pipe/soil potential reading should show a significant positive change if the flange/joint is adequately insulated. Whether or not insulation is satisfactory, the test will invariably result in a significant current flow and rapid discharge of the battery. Momentary connection only of the battery is recommended with observation of instantaneous potential change.


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES DETE RMI NE CURRENT FLOW MEASU RE MENTS ON PI PELI NES Purpose Current measuring techniques are required to determine the distribution, magnitude and direction of current along a cathodically protected pipeline. Current measurements can be used to identify and locate underground contacts, determine the presence and effects of interference, and to analyse the current spread from impressed current systems.

Equipment The equipment required to perform current measurements is:   

Battery Test leads Voltmeter

  

High sensitivity galvanometer (or voltmeter plus shunt) Ammeter Swain current clamp

Procedure 1.

Where possible, a Swain current clamp or other suitable current clamp should be used to measure the current flow through pipelines, flow lines, CP cables, etc. Such clamps are faster and easier to use than the two other alternative techniques discussed in this Attachment.

2.

Permanent 4 Wire Test Point Technique a.

The equipment shall be set up as shown in Figure 1A (below). potential drop, E1, across terminals 2 and 3.

b.

The test leads shall be re-connected as shown in Figure 1B. Measure the potential drop, E2, across terminals 2 and 3, and record the current, I1.

c.

Correct the potential drop obtained due to the resistance of the circuit in item (a), Ec = E1 - E2.


Measure the

d.

Divide the current, I1, measured in item (b) by the corrected potential drop, E c to obtain the calibration factor, C. C =

e.

I1 Ec

Disconnect the battery and measure the CP potential drop, E3, across terminals 2 and 3 using a high sensitivity galvanometer or a shunted voltmeter. Correct this potential drop for the circuit resistance in item (a) above. E = E1 - E3

f.

Calculate the current flow by multiplying E by the calibration factor C obtained in item (d) above. I = EC

DETE RMI NAT I ON OF CURRE NT FLO W ON PI PELI NES

A

V 1

2

3

4 TEST STATION

4'

200'

Figure 1A


4'

DETE RMI NAT I ON OF CURRE NT FLO W ON PI PELI NES A

V 1

2

3

4 TEST STATION

200'

4'

4'

Figure 1B

3.

Probe Rod (2 Wire) Technique a.

The equipment shall be set up as shown in Figure 2A. Measure the potential drop, E1, across terminals 1 and 2.

b.

Disconnect the battery and measure the potential drop, E2, across terminals 1 and 2.

c.

Calculate meter correction Factor, C as follows: C = E2 +

E1 E2

d.

The corrected potential, E, equals the measured potential, E2, plus the correction factor, C. E = E2 + C

e.

The resistancebetween of the pipeline span must be pipeline calculated described in Table The distance the connections to the of as terminals 1 and 2 must8.2. be known.

f.

Calculate the pipeline current flow by dividing the corrected potential drop, E, by the pipeline span resistance R. I =


E R

g.

If test stations are not installed, it is still possible to obtain line current measurements. Contact to the pipe can be made by sharp, pointed tungsten carbide tipped probe rods after the pipe has been located. Install the two probe rods at spacing 200 ft + 1 inch and firmly contact the pipe. To ensure that good electrical contact is made use the continuity circuit of the multi-meter. Proceed with items a to d above. This technique should not be used where pipe is buried at excessive depth or if the condition of the line is such that contact with the line by the probe bars is liable to result in failure of the line.

V TEST STATION

1

2

200'

Fig. 2A


DETE RMI NAT I ON OF CURRE NT FLO W ON PI PELI NES Table Steel Pipeline Resistances

PIPE SI ZE

WALL

WEI GHT P ER

RESISTANCE

(INCHES)

THICKNESS (INCHES)

FOOT (POUNDS)

(OHMFOOT) S * 10 P ER

6

2

0.154

3.65

79.2

4

0.237

10.80

26.8

6

0.280

19.00

15.2

8

0.322

28.60

10.1

10

0.365

40.50

7.13

12

0.375

49.60

5.82

14

0.375

54.60

5.29

16

0.375

62.60

4.61

18

0.375

70.60

4.09

20 22

0.375 0.375

78.60 86.60

3.68 3.34

24

0.375

94.60

3.06

26

0.375

102.60

2.82

Notes:

1.

Based on steel density of 489 pounds per cubic foot and steel resistivity of 18 Microhms-cm.

2.

R =

= 3.

16.061 x Resistivity in Microhms-cm Weight Per Foot Resistance of One Foot of Pipe in Microhms

Refer also to API 5L and API 5LX for weight per foot of other size and wall thickness pipe.


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES DETERMI NING REMO TE EARTH It may be desirable to ascertain at what distance from a structure the ground is electrically remote. Electrically remote is defined as being the point at which further changes in electrode position distant from the structure result in negligible changes in potential. This method can be used to select sites for impressed current ground beds, to optimise current distribution and attenuation, or to determine possible interference influences on proposed pipelines near an existing ground bed.

1.

Equipment required is a voltmeter, test leads, Cu-CuSO4 electrode and wire reel.

2.

The procedure involves obtaining electrical contact to the intended structure by test leads, and moving the Cu-CuSO4 electrode in one direction at known intervals. The suggested interval is 10 metres between initial measurements and changing to 3 metre intervals, as the change in potential becomes less than 5 mV between measurements. When a point is reached where little further increase in potential is noted (+1 mV), then that point is considered remote. It is critical to note that remote is not necessarily concentric (of equal distance) around the point contacted due to differences in soil resistivity and strata (See Figure 1A).

An electrode placed over the structure concerned, or any other structure

(ground beds, foreign pipelines, etc.), will make interpretation extremely difficult. These structures can effectively mask the desired potential.


DETERMI NING RE MOTE EARTH SEE NOTE V REF. ELEC. STRUCTURE

A

ALL POINTS FURTHER AWAY FROM A ARE REMOTE TO STRUCTURE AT B

B DISTANCE

Figure 1A

Note:

Typically, spacing should initially be 10m and then reduced to 2m, as the potential difference between readings becomes less than 5 mV.


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES HI GH SO I L RESI STIVI TY: VOLTM ETER CORRE CTI ON Certain types of soil have exceptionally high resistivities, which may affect the performance of pipe-to-soil potential measurements. In order to overcome this, a voltmeter correction calculation must be performed as described below. Corrected P/S Potential = Input Impedance

Measured P/S Potential x



External Circuit Re tan sis

ce

Input impedance

Measure the P/S potential on two different input impedance scales and set the two “Corrected P/S Potential” equations equal to one another (Corrected P/S Potential will be the same for each input impedance scale). Solve the equation for the unknown external circuit resistance (ECR).

EXAMPLE: Measured P/S potential @ 200M input impedance on 2V scale = 1100 mV Note: Effective input impedance becomes 200M x 2 = 400M Measured P/S potential @ 50M input impedance on 2 V scale = 900 mV Note: Effective input impedance become 50M x 2 = 100M Solving for the external circuit resistance (ECR): 1100 x

400M



ECR

400M

= 900 x

6 6.25 x 10 (ECR)

100M



ECR

100M

= 200

ECR

= 32M

Substituting this back into the srcinal equation will yield the corrected P/S potential: Corrected P/S potential

= 1100 x

400M

= 1188 mV




32 M

400M

CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES PI PE -TO-SOI L (P/S) POTE NTI AL ME ASURE ME NTS 1.

Ensure that the digital voltmeter and reference cell are in working order and calibrated. The Cu-CuSO4 electrode should have an excess volume of CuSO 4 crystals.

2.

Place Cu-CuSO4 electrode over structure for “close” reading, or at a distance for “remote” reading. Remove cap from electrode and place in firm contact with moist ground. In very dry conditions, moisten ground with fresh water. If meter is a type with variable input resistance, changing input resistance setting should not affect the reading. A high input resistance meter (10 megohms or greater) will reduce electrode contact resistance problems.

3.

Electrode is connected to negative terminal of instrument and test lead from pipe to positive terminal, as shown in Figure 9.1.

4.

If direct-reading indicating voltmeters are used, the test circuit should always be checked for the effect of high circuit resistance. When readings are being taken, switch to next higher range. If there is a substantial difference between readings, test leads may be defective or contact to structure may be poor.

5.

The “instant-off” technique is preferred wherever possible. This involves interrupting the influential rectifiers at known cycles and obtaining the P/S potential when all influential rectifiers have been momentarily turned off. Due to response time and depolarisation, the reading will slowly continue to decay until the rectifiers come back on. For digital meters, the instant off potential is the reading recorded one seconds after the potential reading indicates that all rectifiers are off.


PIPE -TO- S OIL (P/S ) POTENTIAL MEA S URE MENTS

TEST POST +

V

VOLTMETER -

-

+

TRANSFORMER RECTIFIER Cu-CuSO4 ELECTRODE

IMPRESSED CURRENT ANODES

CABLE

PROTECTIVE CURRENT PIPELINE

Figure 1 - Pipe-to-Soil (P/S) Potential Measurements


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES CL OSE I NTER VAL ( PI PE TO SOI L) P OTENTI AL S URVE YS (CI PS) Overview The close interval pipe-to-soil potential survey (CIPS) is a technique, which provides a detailed profile of the potential difference between the pipeline and the soil. This testing method provides a pipe-soil potential profile of the structure under test, which is used to determine the performance and effectiveness of the applied Cathodic Protection system. Furthermore, it has the capability of detecting active corrosion cells, interactive effects, metallic contacts anomalies, coating defects, which may have an impact upon the Cathodic Protection afforded to the pipeline.

Normal pipe-to-soil surveys take measurements at test stations located at intervals of between approximately 1 and 3 km along the pipe. Based on the results of such a survey, an interpolative judgement could be made on the performance of the CP system, and the condition of the pipeline coating. However, if a defect exists at a point remote from a test station, can be as little of a few metres, then it is likely that this defect will remain undetected. Therefore, a larger sample population of pipe-to-soil potential measurements must be taken in order to detect remote defects (between test points) and to ensure that pipeline Cathodic Protection systems are operating properly over the entire structure and without any external interference. This is achieved by undertaking a close interval pipe-to-soil potential survey by obtaining pipe-to-soil potential measurements at between 1 and 5 metre intervals, thus providing a pseudo-continuous pipe-to-soil potential profile. (Figure 1) P/S potential measurements are recorded with the Cathodic Protection systems switched on and then immediately afterwards, switched off. Surveys where the potential is recorded at the point where the CP current is switched off are often termed IR drop error free, instantaneous off, or polarised potential surveys.

Equipment The equipment required to perform CIPS consists of voltmeter, Cu-CuSO4 electrode, test leads and a reel of wire. In addition, a cyclic timer must be fitted to the transformer rectifier (TR) unit or other DC power sources in order to interrupt the CP current. This eliminates measurement of the IR drop error caused by the flow of Cathodic Protection applied current in the soil. The current interrupters must have the facility to switch at high speeds, cause minimum transient switching surges, have a selection of commonly used time cycles and have an error of synchronisation between devices of less than 100 ms which must be maintained for a minimum of one working day.


Conducting the Survey 1. a. The current interrupter can be placed in either the AC or DC circuit of the TR unit in order to cyclically switch the CP current and must be sufficiently rated to the power of the TR unit. b. Should there be any other current sources which could influence the potential at the point being measured, then it is vitally important and they be switched in unison with the structure under tests Cathodic Protection system. This is achieved by using synchronised cyclic switching units. c.

The switching frequency of the Cathodic Protection system and associated equipment may be determined by the following:   

The degree of synchronisation that can be achieved between the switching units The measurement response time of the data collected The measurement interval of pipe-to-soil potentials in automatic time dependant surveys

d. It is important to select a time cycle such that degradation of the polarised potential is eliminated during the “off” period, thus minimising the measur ement of inaccurate and unreliable potentials. e. For this reason a minimum on: off ratio of 3:1 seconds is common practice. This ratio is selected on the basis that it will not exceed the response time of the measuring equipment or the synchronisation error of the switching units. 2. a.

The equipment is set up and a suitable connection made with the pipeline, such as at a test station.

b.

Potentials are read by placing the electrode in the soil at set intervals along and directly above the pipeline.

c.

Pipelines of diameter 600mm (24 inches) or less should have potentials read at a minimum spacing of 1m. For pipelines with diameter in excess of 600mm, potentials should be read at 1m intervals, although this may be increased to 2m.

d. It may be necessary to reduce the spacing between potential intervals in order to determine particular characteristics such as the extent of a perceived defect. In general, an abrupt change in potential indicates an area, which should be subject to closer examination. 3.

Pipe-to-soil potential measurements taken at each point along the pipeline should be recorded in tabular form. A graph can then be created plotting potential measurements along the length of the line.


Automated Data logger Techniques Where feasible, portable data logging equipment, which can be downloaded to a computer to perform analysis, together with accurate, synchronised high current interruption devices should be used to achieve larger sample populations in shorter duration surveys.

REF. ELEC.

MV

PIPELINE

ON OFF

DISTANCE (M)

Fig. 1


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES DC VO LTAGE GR ADI ENT SU RVE Y (DCVG) Purpose DC Voltage Gradient survey (DCVG) is used to determine the size and extent of a coating defect in a buried pipeline. The sensitivity and stability of the measuring equipment allows very accurate soil voltage gradient measurements, even where stray currents exist.

Equipment 1.

One fast response, high input resistance, centre zero voltmeter. The voltmeter has voltage ranges form 25 - 10000mV full-scale deflection (FSD). The meter has a centre zero indicator and a left hand zero scale. Special precautions are taken to ensure that the meter is not affected by static, which can be a problem in desert conditions.

2.

Two ground contact probes, which are non-polarisable. The probe handles each have a meter bias control ranged from -1.4 to +1.4V. This enables offset voltages to be introduced to the circuit to maintain the indicator on the voltmeter at the centre zero position.

3

One solid-state cyclic timer switch which is connected into the DC negative output of the TR unit to provide a DC pulse for the survey. The unit is self contained and powered by rechargeable batteries.

Procedure 1.

The solid-state switch unit interrupter should be incorporated into the DC negative output circuit of an existing CP TR unit. The switch unit should be set to turn the output from the TR unit on and off with a ratio of approximately 1:3 within a period of one second. This will establish the pulsed DC signal necessary to perform the survey.

2.

Once the DC signal has been established, the meter and probes must be calibrated. Placing the probes in the ground, approximately 1m apart, and adjusting the bias control on one of the probes achieve this. If the system is operating correctly and the ground probes are properly placed, this adjustment will cause the needle to move on the voltmeter.

3.

To perform the survey, the operator walks along the pipeline placing the probes in the ground approximately every metre. In dry soil conditions, the DCVG path should be watered with fresh water to ensure that accurate readings are obtained.


4.

Only at the point at which a defect is detected will there be a significant current flow to and from the pipe. Detection of a defect will be shown by deflection of the voltmeter needle and further distinguished by the pulse from the TR unit. The movement of the needle will always be in the direction of the defect. a.

As the operator approaches the defect, the potential gradient becomes steeper and deflection on the voltmeter dial increases. The potential gradient should be followed until it becomes zero, at which point the operator will be directly above the coating defect.

b.

Precise defect location is established by progressively moving the probes closer together whilst maintaining a zero reading on the voltmeter. Probe separation down to 300mm is practical, giving a defect location accurate to within 150mm. At this point the defect will be exactly at the mid-point between the probes.

c.

Once the defect is accurately located, the potential gradient from remote earth to the defect location is measured using the equation: %IR

=

OL / RE x 100 PS

Where: OL/RE = potential gradient from defect location to remote earth in mV. PS

= pipe-to-soil potential in mV.

Calculation of potential gradient takes into account the defect size and the available cathodic protection current. Evaluating %IR establishes a measure of the severity of the defect. This alone is not usually sufficient to determine the size of the defect accurately due to variations in soil conditions and pipeline depth. As a guide, 6% IR represents a defect of approximately 10cm 2 bare steel in contact with the soil at a depth of 1 metre where the soil resistivity is 1000 ohm cm. 5.

A combined CIS and DCVG survey may be carried out in order to collate sufficient data to prepare detailed pipeline rehabilitation plans. The DCVG survey results are entered into the data logger used by the CIPS operator. The CIS is performed directly after the DCVG survey. When this data is processed, it is possible to establish the priority areas that require coating refurbishment and areas where cathodic protection current should be increased.

6.

Experience has shown that the following criteria may be used to establish the severity of coating defects from the % IR calculated: Areas with values: a.

Less than 15% IR would not normally require coating repair work and would be sufficiently protected by the CP system.


b.

Areas showing IR values between 15% and 45% would require attention under general maintenance within 2-4 years.

c.

Areas with values in excess of 50% IR require immediate attention.

These values are, realistically, rules of thumb for guidance only. Accurate interpretation of defect severity from % IR would require an empirical benchmark to be established for each pipeline. This is accomplished by excavation and examination at a number of selected locations along the line showing varying % IR.


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES WE LL CAS I NG PO TENTIAL PRO F I LE Purpose A common method to determine Cathodic Protection current requirement for a casing string is by running a downhole casing potential profile (CPP) log. Its use is limited by high cost and lack of suitable wells but it is the most useful tool to ensure that protection has been achieved on the casing string. The method is also limited to wells where the surface casing is not set overly deep thus shielding the production casing from CP protection. Due to similarities in strata and completion depths in a given field, one to two CPP logs may be used to predict requirements of other similar wells in the same field. In addition, an E log I survey should be performed for comparison purposes. There are two down hole logging companies who provide the tools and service for evaluating the Cathodic Protection current requirements of well casings. Company

Tool

Dresser

Casing Potential Profile Survey (CPP)

Schlumberger

Corrosion Protection Evaluation Tool (CPET)

Listed below are the advantages and disadvantages of the CPP and CPET logging tools:

Tool

CPP

Advantages  

Simple tool Faster logging time than a CPET

Disadvantages 

Usually takes readings every 50-100 feet.



Requires the production casing to be isolated with a packer and the hole filled with oil.



CPET



Usually takes readings 


Assumes constant production casing resistivity. Slightly more difficult

every 2 feet. 

Can be run in water, so  casing isolation with a packer is not required  resulting in lower workover costs.



Measures the actual production casing resistivity every 2 feet.

to interpret than a CPP. More complex tool than a CPP tool. Slower logging time than a CPP tool.

Procedure 1.

Performing a CPP or CPET Log a.

In preparation for running the profile tool, a suitable well should be selected such that as much useful information as possible can be obtained while the tool is in the hole. This information includes not only protection (Amps DC) requirements but also interference effects from other nearby structures. The production casing can be shielded from CP currents by other larger diameter casing strings set at shallower depths. This effect needs to be accounted for when interpreting the log data.

b.

The selected well will require a CP current source from either its own ground bed or a temporary ground bed installed for the test.

c.

The interior surface of the casing should be scraped, isolation packer installed aboveto the perforations crude oil, and inhibited diesel used to exclude moisture in order maintain good or electrical contact. Note: The CPET tool does not require the use of an isolation packer or oil.

2.

d.

The type and weight per foot of casing must be known so that resistance per foot of casing can be calculated (CPP tool only).

e.

The initial value of the first run of the CPP or CPET is to determine the individual anodic points on the casing and the direction of current flow. In many cases, separate runs of the tool are made with increasing current applied in an attempt to define the current requirement for the production casing of the well. The correct CP current is achieved when all anodic points are removed and the current returns up the casing. See Figure 12.1. Complete polarisation on deep wells may require up to 3 months of constant uninterrupted current.

Interpretation of CPP Results a.

Figure 1below shows a plot of typical potential versus depth and interpretations of the results. Positive slopes indicate current flowing on to the casing; negative slopes show current flowing off the casing, which is an indication that external casing corrosion is occurring at that point. A negative potential indicates current flowing down the casing, whilst a positive potential indicates current flowing up the casing. At the point where the production casing enters the surface casing, there


will always be some effect on the CPP log. This “surface casing effect” can normally be disregarded in analysing the CP current requirements of well casings. b.

As indicated in Figure 1 below, the selected current output may not be sufficient to achieve full protection along the entire string. In this case, current output is increased and the CPP or CPET tool rerun until the entire curve indicates a positive potential. A completely protected casing would show a curve with all, or almost all, positive slopes and the entire curve indicating a positive potential. This indicates that current is being picked up along the entire length of the of the string, and all current is returning to the current source.


TYPICAL CASING POTENTIAL PROFILE LOG

RUN A

RUN B

Surface Surface Casing Effect

Depth

Positive Slope

Bottom

Negative Slope O

Potential Fig 1


+

( V)

CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES WE LL CASING CURRE NT-POTENTI AL PRO FI LE S (E LO G I ) Purpose Current versus potential profiles can be used to determine the current required to achieve Cathodic Protection on a structure. This technique may be used both on pipelines or well casings, although this technique is usually reserved for well casings. 1.

2.

Equipment required for this test is: a.

Multimeter, electrode and wire reel.

b.

Current source (rectifier, generator or storage battery).

c.

Temporary or permanent ground bed.

a.

Current is applied in increments to the structure, and potentials measured to a remote electrode.

b.

An adjustable current source is connected to a permanent or temporary groundbed and energised. The output is adjusted to 0.5 amp and left to polarise (at least overnight). If a temporary ground bed is used it should be located at the anticipated location where a permanent bed would be installed.

c.

A remote Cu-CuSO4 electrode is connected to the structure and meter. “Instant off” potentials are recorded for each change in current applied.

d.

Current is changed in increments on a logarithmic scale and allowed to polarise for a minimum of 15 minutes, or until the potential is no longer fluctuating.

e.

Record potential versus log of current.

f.

The current required to protect the structure is the point of interception of the two straight-line portions of the curve. However, for very deep wells use the point of linearity (high current tangency point) to indicate current required.

g.

Extreme care must be taken when using this technique as many factors influence the outcome. It is advisable to compare the results of this test with all other available data concluding what current requirement appears to be. For well casings, E log I results should be compared with Casing Potential Profile log data (Attachment 12) in order to determine CP current requirement.


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES PIPE LI NE ST E E L CASI NG/SLEE VI NG E VALUA TION Initial Test Procedure 1.

Measure and record the casing-to-soil (C/S) potential during routine surveys.

2.

If the C/S potential is less negative than -850mV and the C/S is 200mV less negative than the pipeline when referenced to a Cu-CuSO4 reference cell, then no further action is required.

3.

If C/S potential is equal or more negative than -850mV, when referenced to a Cu-CuSO4 reference cell, then identify the casing as “potentially shorted” (S), on the relevant pipe -tosoil survey reporting form. Conduct a casing evaluation test using the casing depolarisation test procedure outlined below to determine whether the road casing/pipe contact is electrical (S) or electrolytic (ES).

Casing Depolarisation Test Procedure The purpose of this test is to verify isolation status by applying a positive DC current to the casing. A significant potential difference will occur between the casing and the pipe if the two structures are not electrically connected by a metal-to-metal contact. 1.

Construct a temporary metallic structure laterally to and spaced an appropriate distance from the carrier and casing. (A spacing of 50 feet is usually an adequate distance). Steel rods driven into the earth or sheets of aluminium foil in contact with the earth can provide an adequate temporary structure.

2.

Connect the negative terminal of a variable DC power source to the temporary metallic structure.

3.

Connect the positive terminal of a variable DC power source to the casing.

4.

Position a reference electrode over the carrier pipe near the casing end.

5.

Use an appropriate DC voltmeter to measure and record the carrier pipe-to-soil, and casingto-soil (C/S) potentials.

6.

Apply a small increment of current (0.1 ampere is a satisfactory first increment of current) to the casing for a short period of time such as 1 or 2 seconds.


7.

Measure and record the P/S and C/S potentials to determine the effect of the applied current. Record the increment of current.

8.

Repeat steps 6 and 7 using additional increments of current such as 0.2, 0.3 ampere, etc. The use of a minimum of three different values of test current and measurement of the effects is recommended. The amount of current required for an effective evaluation will vary due to the size of the structure and condition of any coating present. A maximum of 10 amperes of DC current is usually adequate to develop significant potential shifts.

Follow-up Actions 1.

If the C/S potential is less negative than -850mV and the C/S is 200mV less negative than the pipeline when referenced to a Cu-CuSO4 reference cell, then no further action is required.

2.

If the results of the road casing depolarisation test procedure indicate that the casing/carrier contact is electrolytic (ES), then indicate on the P/S survey form and re-evaluate annually to ascertain if the C/S potential is still an electrolytic short.

3.

If the results of the road casing depolarisation test procedure indicate that the road casing/pipe contact is an electrical short (S), then indicate so on the annual P/S survey form and initiate one of the actions as defined below: 

Undertake casing short location test to determine short position.



Clear the road casing/pipe contact by either cutting back the steel sleeve to the shorted position or separate the two structures and install suitable supports to prevent reoccurrence.



Fill the road casing/pipe annular space with dielectric filler.



Inspect the road casing end seals and replace if required.



Install low pressure (10 psig) burst plates on the road casing vents and then purge the road casing/pipeline annular space with argon (inert) gas.



Monitor the casing vents for gas leakage every three months.


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES STRUCTUR E TO WATE R POTE NTI AL ME ASUR E ME NTS 1.

Remove cap from the Ag-AgCl electrode and submerge it in the water where the desired reading is to be obtained.

2.

Connect reference electrode to negative terminal and structure to positive terminal of a high resistance voltmeter. Contact with the structure can be made using test station leads, or other device contacting an oil and rust-free metallic surface. For measurement of riser potentials, ensure that contact is made on waterside (protected side) of any insulators, which may be installed.

3.

For structures with impressed current systems, the instant-off potential should be obtained. Where impressed current systems are used for protection on another nearby structure, such as an offshore pipeline, that rectifier should be interrupted and its effect on the structure of interest obtained.

Note: Reference electrode contact resistance is not encountered due to the low resistance of the environment therefore a potentiometer circuit is not normally required .


CATHODIC PROTECTION TRAINING COURSE SURVEY METHODS & TECHNIQUES FI ELD E NGINE ER I NSTRU MENT & EQ UI PM ENT LIST Test Instruments Basic Requirement           

High Impedance Multimeter 2 x Copper/copper sulphate reference electrode Copper sulphate crystals Distilled water Silver/silver chloride reference electrode Test cable kit Ammeter Megger Cable loom for resistivity measurement (Wenner 4 pin Method) Pen knife Cable crimper, suitable for all sizes up to 35mm cable

      

Basic tool kit – various screwdrivers, spanners, cable stripper, etc. Soil box, including leads 2 x cable reels with 100m of 4mm cable on each Spare fuses AC & DC Crimps, lugs (various sizes including 10mm, 16mm & 35mm) Insulation tape (various widths) Swain current clamp

Specialist Field Equipment     

Pearson Survey kit Portable DC generator (i.e. Honda type unit – welding set) Portable air-cooled transformer rectifier - rheostat controlled (i.e. 100A – 24V) Various adjustable resistors & rheostats Heavy-duty cables for current drain tests, e-log I etc.

   

Current interrupter(s), (min 100A), capable of 1 sec interval interruption CIPS kit & software Pipe locator (basic version) Geonics EM 31 or EM 34 (hire as when necessary – deep well resistivity)


CATHODIC PROTECTION TRAINING COURSE CONSTRUCTION PRACTICE PI PELI NE CABLE I DENTI F I CAT I ON COLOUR CODI NG S YSTEM PIPELINE TEST POINT:

GREEN CABLE ONLY - PIPELINE

INSULATION FLANGE:

RED CABLE - DEAD SIDE BLACK CABLE - LIVE SIDE

IN-LINE INSULATION FLANGE: RED CABLE - UP STREAM RED CABLE - BOND BLACK CABLE - DOWN STREAM BLACK CABLE - BOND

If necessary to BOND across INSULATION FLANGE - connect RED/BLACK cables together in Marker Post direct or through resistor. SACRIFICIAL ANODE INSTALLATION: RED CABLE - ANODE GREEN CABLE - PIPELINE BLACK CABLE - PIPELINE

Bond BLACK/RED cables together direct or through resistor. FOREIGN SERVICE CROSSING: GREEN CABLE BLACK CABLE - PIPELINE BROWN CABLE - SERVICE

- PIPELINE

If BOND required - connect BROWN/BLACK cables together in M28 post or through resistor. PERMANENT REFERENCE ELECTRODE: RED CABLE GREEN CABLE - PIPELINE

- REFERENCE

Reference cable size shall be 10mm RED. EARTHING CELL:

GREEN PIPE BLACK PIPE

YELLOW/GREEN CABLE - CELL - PIPE - PIPE

Connect the YELLOW/GREEN & BLACK cablestogether. PARALLEL PIPELINE:


BLUE CABLE

- PIPE

DRAIN POINT:

35mm BLACK CABLE 35mm BLACK CABLE - PIPE GREEN CABLE - PIPE BLACK CABLE - PIPE

-TR CONNECTION

All test cables to be brought above ground and to be terminated in M28 or similar marker post, 16mm cable to be installed only and used as a bond either direct or through a resistor when additional pipeline is laid parallel and is to be included in scheme.



CATHODIC PROTECTION DEFINITIONS Anaerobic The lack of free oxygen in the electrolyte adjacent to a metal structure Anode The electrode through which direct current enters an electrolyte Sacrificial Anode An anode used to protect a structure by galvanic action, also known as a ‘galvanic anode’ or ‘reactive anode’ Anode backfill A low-resistance moisture-retaining material immediately surrounding a buried anode for the purpose of decreasing the effective resistance of the anode to the soil Anodic area The part of the metal surface, which acts as an anode Bond A piece of metal conductor, either solid or flexible, usually copper, connecting two points on the same or on different structures, to prevent any appreciable change in the potential of the one point with respect to the other Continuity bond A bond designed and installed specifically to ensure the electrical continuity of a structure Drainage bond A bond to effect electrical drainage Remedial bond A bond installed between a primary and secondary structure in order to reduce or eliminate corrosion interaction Bond resistance The ohmic resistance of a bond including the contact resistance at the point of attachment of its extremities Cathode The electrode through which direct current leaves an electrolyte


Cathodic area The part of the metal surface, which acts as a cathode Cathodic disbonding The failure of adhesion between a coating and a metallic surface that is directly attributable to Cathodic Protection conditions and that is often initiated by a defect in the coating system, such as accidental damage, imperfect application or excessive permeability of the coating Cathodic Protection A means of rendering a metal immune from corrosive attack by causing direct current to flow from its electrolytic environment into the entire metal surface Cell A complete electrolytic system comprising of a cathode and an anode in electrical contact with an intervening electrolyte Conductor A substance (mainly a metal or carbon) in which electrical current flows by the movement of electrons Corrosion The chemical or electrochemical reaction of a metal with it environment, resulting in its progressive degradation or destruction Corrosion product The chemical compounds or compounds produced by the reaction of a corroding metal with its environment Corrosion interaction (corrosion interference) The increase or decrease in the rate of corrosion, or the tendency towards corrosion, of a buried or immersed structure caused by the interception of part of the Cathodic Protection current applied to another structure or current from other sources Crossing point A point where two or more buried or immersed structure cross each other when viewed in plan Current density The current per unit geometrical area of the protected structure, coated and uncoated, in contact

with the electrolyte Differential aeration The unequal access of air to different parts of a metal surface Electrical drainage A means by which protection of an underground or immersed (underwater) metallic structure against electrochemical corrosion is achieved by making an electrical connection between the structure and the negative return circuit (rail, feeder, bus bar) of a DC electric traction system TRAINING PROGRAM Prepared ByR J WATERHOUSE, MICorr108 of 113

Forced drainage A form of drainage in which the connection between a protected structure and a traction system includes an independent source of direct current Drainage test Tests in which current is applied for a short period, usually with temporary anodes and power source, in order to assess the magnitude of the current needed to achieve permanent protection

against electrochemical corrosion Driving potential (of a sacrificial anode system) The difference between the structure/electrolyte potential and the anode/electrolyte potential Earth (1) The conducting mass of earth or of any conductor in direct electrical connection therewith (include expanse of natural water) Earth (2) A connection, whether intentional or unintentional, between a conductor and earth Earth (3) To connect any conductor with the general mass of earth Electrode A conductor of the metallic class (including carbon) by means of which current passes to or from an electrolyte Electrolyte A liquid, or the liquid component in a composite material such as soil, in which electric current flows by the movement of ions Electronegative The state of a metallic electrode when its potential is negative with respect to another metallic electrode in the system Electropositive The state of a metallic electrode when its potential is positive with respect to another metallic electrode in the system Galvanic action A spontaneous chemical reaction, which occurs in a system comprising a cathode and an anode in electrical contact and with an intervening electrolyte, resulting in corrosion of the anode Groundbed A system of buried or submerged electrodes connected to the positive terminal of an independent source of direct current, in order to lead to earth the current used for the Cathodic Protection of a buried or immersed metallic structure


Grounding cell Two electrodes of zinc or other sacrificial material that are separated by insulating spacers and packaged in a low-resistivity backfill material Holiday A defect in a protective coating at which the metal is exposed Impressed current The current supplied by a rectifier or other direct current source (specifically excluding a sacrificial anode) to a protected structure in order to attain the necessary protection potential Instantaneous-off potential The structure/electrolyte potential measured immediately after the synchronous interruption of all sources of applied Cathodic Protection current Insulation joint A joint or coupling between two lengths of pipe, inserted in order to provide electrical isolation (discontinuity) between them Insulation flange A flange joint between adjacent lengths of pipe in which the nuts and bolts are electrically insulated from one or both of the flanges and the jointing gasket is non-conducting, so that there is an electrical discontinuity in the pipeline at that point Interaction testing (interference testing) A test to determine the severity of corrosion interaction between two buried or immersed structures Ion An atom, or group of atoms, carrying a positive or negative electrical charge Joint Cathodic Protection scheme A scheme in which different structures are bonded together and protected by a common Cathodic Protection installation Passivity The state of the surface of a metal or alloy susceptible to corrosion where its electrochemical behaviour becomes that of a less reactive metal and its corrosion rate is reduced Pitting A non-uniform corrosion of a metal whereby a number of cavities, not in the form of cracks, are formed in the surface Polarisation Change in the potential of an electrode as the result of current flow


Polarisation cell A device inserted in the earth connection of a structure that drains only a small amount of direct current from the source used to provide Cathodic Protection for the structure, but provides a low resistance path to voltages to currents from high DC voltages and all AC voltages carried by the structure Protection current

The current made to flow into a metallic structure from its electrolytic environment in order to effect Cathodic Protection of the structure Protective potential The more negative level to which the potential of a metallic structure, with respect to a specified reference electrode in an electrolytic environment, has to be depressed in order to effect Cathodic Protection of the structure Protective coating A dielectric material adhered to or bonded to a structure to separate it from its environment in order to prevent corrosion Reaction (anodic, cathodic) A process of chemical or electrochemical change, particularly taking place at or near an electrode in a cell Reference electrode An electrode the potential of which is accurately reproduced and which serves as a basis of comparison in the measurement of other electrode potentials Copper/copper sulphate reference A reference electrode consisting of copper in a saturated copper sulphate solution Silver/silver chloride A reference electrode consisting of silver, coated with silver chloride, in an electrolyte containing chloride ions Stray Current Current flowing in the soil or water environment of a structure and arising mainly from Cathodic Protection, electric power or traction installations, and which can pass from the environment into the structure and vice versa Protected structure A structure to which Cathodic Protection is applied Unprotected structure A structure to which Cathodic Protection is not applied


Primary structure A buried or immersed structure cathodically protected by a system that may constitute a source of corrosion interaction with another (secondary) structure Secondary structure A buried or immersed structure that may be subject to corrosion interaction arising from the Cathodic Protection of another (the primary) structure Structure/electrolyte potential (pipe/soil) The difference in potential between a structure and a specified reference electrode in contact with the electrolyte at a point sufficiently close to (but without actually touching) the structure to avoid error due to the voltage drop associated with any current flowing in the electrolyte Sulphate reducing bacteria A group of bacteria found in most soils and natural water, but active only in conditions of near neutrality and freedom from oxygen, which reduce sulphates in their environment with the production of sulphides Utilization factor That proportion of anode material on an anode that may be consumed before the anode ceases to provide a current output as required in the design


CATHODIC PROTECTION TRAINING COURSE ACKNOWLEDGMENTS & REFERENCES British Standard BS 7361 Part 1

Cathodic Protection, part 1, Code of Practice for Land and Marine Application.

NACE Standards & Publications NACE SP-0169 NACE RP-0186 NACE SP-0572 NACE SP-0286

Control Of External Corrosion on Underground or Submerged Metallic Piping Systems. Application of Cathodic Protection for External Surfaces of Steel Well Casings. Design, Installation, Operation, and Maintenance of Impresses Current Deep Groundbeds. The Electrical Isolation of Cathodically Protected Pipelines

NACE RP-0675

Control of External Corrosion on Offshore Steel Pipelines

NACE SP-0388

Impressed Current Cathodic Protection of Internal Submerged Surfaces of Steel Storage Tanks

NACE RP-0193

External Cathodic Protection of On-Grade Metallic Storage Tank Bottoms

NACE SP-0575

Design, Installation, Operation and Maintenance of Internal Cathodic Protection Systems in Oil Treating Vessels

NACE SP-0186 NACE Publication

Application of Cathodic Protection for Well Casings Control of Pipeline Corrosion by A. W. Peabody

British Gas Standard BGC/PS/ECP1

Cathodic Protection of Buried Steel Pipelines, Associated Pipework and Fittings.


NACE-CP L-I (1)

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