CATHODIC PROTECTION SYSTEM (CPS) DESIGN CONCEPT

A. Preamble

The design of the cathodic protection system(CPS) shall be an integral part of the total pipeline design.

The CPS is one of the methods employed to minimise the corrosion of the pipeline and maintain its

integrity. The Cathodic protection is applied to the coated structure to provide corrosion control to areas where the

coating may be damaged.

Other methods include:

Effective coating of the pipeline and

Corrosion allowance in the thickness design

A combination of applying both a coating and cathodic protection will normally results in the most practical

and economic overall protection system.

The design, construction and commissioning of the CP scheme shall be in accordance with BS Code of

Practice for Cathodic Protection (CP 1021), NACE Standards NACE RP0169; SP0572 AND SP0177

and other acceptable international codes.

Before the definition of the cathodic protection design for the 110km pipeline system we shall undertake a

site survey including:

A soil resistivity measurement along the pipeline route to assess the apparent aggressiveness level and,

at points either side of the route, for the installation of impressed current anodes or otherwise.

The cathodic protection system shall be designed so that any external corrosion on the pipeline is

eliminated and any adverse stray current effect on the pipeline or on foreign structures/pipelines is

avoided.

Additional Factors to be considered in the design of the corrosion prevention includes:

i. Nature of the product to be transported. Working temperature or pressure of the piping system

and the tendency of backfill to cause soil stress on the pipeline.

ii. Location of the pipeline considering the population density of the route.

iii. Other installations around or along the same route.

iv. Economic factors such as installation cost and protection maintenance cost over the life span of

the pipeline.

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v. Since this route is known to have a number of existing CPS. A predesign natural potential survey

shall be required to determine the additional potential requirement for the pipeline especially at

the OB/OB end of the pipeline.

Cathodic protection can be achieved in two ways:

- By the use of galvanic (sacrificial) anodes, or

- By “impressed” current.

a. Galvanic anode systems employ reactive metals as auxiliary anodes that are directly

electrically connected to the steel to be protected. The difference in natural potentials between

the anode and the steel, as indicated by their relative positions in the electro-chemical series,

causes a positive current to flow in the electrolyte, from the anode to the steel. Thus, the

whole surface of the steel becomes more negatively charged and becomes the cathode. The

metals commonly used, as sacrificial anodes are aluminium, zinc and magnesium. These

metals are alloyed to improve the long-term performance and dissolution characteristics.

A galvanic system requires:

i) Sacrificial anodes

ii) Direct welding to the structure or a conductor connecting the anode to the structure

iii) Secure and minimum resistance connections between conductor and structure, and

between conductor and anode.

b. Impressed-current systems employ inert (zero or low dissolution) anodes and use an external

source of dc power(rectified ac) to impress a current from an external anode onto the cathode

surface.

An impressed-current system requires:

i) Inert anodes (clusters of which, connected together often in a backfill, are called the

“groundbed”).

ii) A dc power source.

iii) Electrically well insulated, minimum resistance and secure conductors between anodes

and power source.

iv) Secure and minimum resistance connections between power source and structure.

B. GENERAL ROUTE SOIL RESISTIVITY SURVEY

Soil Resistivity –In carrying out the route resistivity survey. The appropriate survey technique shall

be applied that will predict reliably where and to what extent corrosion of buried pipeline will occur

along the 110km route.

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For any buried structure the most important preliminary investigation is the measurement of soil

resistivity at various points, the main purpose being to locate suitable places for ground beds and

establish the soil resistance along the pipeline route.

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

indicating type of terrain and vegetation, physical and environmental conditions such as farm land,

forests, open fields, desert, swamps, rocks rivers crossings, railways, major and minor roads,

overhead power lines etc..)

i. SOIL RESISTIVITY MEASUREMENT TECHNIQUE /METHODS AND PROCEDURE

Soil resistivity can be measured by different methods, depending on the location and the purpose.

The most common method is the "Wenner" or 4-terminal method. This is an in-situ method using 4

pins driven into the ground. A known alternating current is passed through the ground and the

resulting voltage indicates the soil resistance.

Changing the electrode spacing can vary the influence of depth. The resistivity is calculated by

means of appropriate formulae. A description of the 4-pin Wenner Method and soil box method is

given in ASTM G 57.

Soil resistivity, .cm Soil Corrosivity

Under 1500 Very corrosive

1500 to 5000 Moderately corrosive

Above 5000 Slightly corrosive

In general,

in very corrosive soils cathodic protection shall be applied. In moderately corrosive soils additional tests shall be done to determine the requirement

for cathodic protection. In slightly corrosive soils no cathodic protection is required unless there is a known

corrosion history of similar installations under comparable soil conditions.

Some relatively high-resistivity soils may still be very corrosive, e.g. acidic peaty soils and

anaerobic soils containing sulphate-reducing bacteria.

Over the life of a facility conditions can change, e.g. changing water tables and changing

climates.

Chemical analyses of the soil samples shall collected to determine the concentrations of various

salts and the ph values of soil samples taken at points along the route.

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ii. INVESTIGATION OF SOIL COMPOSITION

The investigation shall include chemical analyses to determine the concentrations of

various salts and the pH values of soil samples taken at various points at the possible

groundbed site or along the pipeline route.

These soil characteristics must be identified in order to establish the requirements for the

protection. The presence or not of sulphides, sulphates, chloride, pH and other constituents

will affect the level of current requirements necessary for the effective protection of the

buried pipeline.

Corrosion can occur in fairly high-resistivity soils if there is a considerable variation in

composition and/or resistivity at different points at the construction site or along the route of

a pipeline, causing concentration-cell effects.

Differences in soil composition such as in the case of partial land fill or reclamation may

also cause concentration cell effects, which may require the installation of interference

protection system. For existing pipelines met along the pipeline route, soil variations can be

detected by measuring the natural potential of the pipeline at regular intervals.

Such analysis may indicate areas of high salt concentrations, bacterial activity and the

presence of acidic waste.

Generally, water pH less than 6.5 is associated with uniform corrosion, while pHs between

6.5 and 8.0 can be associated with pitting corrosion. Some studies have suggested that

systems using only pH to control corrosion shall maintain a pH of at least 9.0 to reduce the

availability of hydrogen ions as electron receptors.

However, pH is not the only factor in the corrosion equation; carbonate and alkalinity levels

affect corrosion as well.

Generally, an increase in pH and alkalinity can decrease corrosion rates and help form a

protective layer of scale on corrodible pipe material. Chemicals commonly used for pH and

alkalinity adjustment are hydrated lime (CaOH2 or calcium hydroxide), caustic soda (NaOH

or sodium hydroxide), soda ash (Na2CO3 or sodium carbonate), and sodium bicarbonate

(NaHCO3, essentially baking soda).

Care must be taken, however, to maintain pH at a level that will control corrosion but not

conflict with optimum pH levels for disinfection and control of disinfection by-products.

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High salt concentrations in original soil are usually indicated by the soil resistivity

measurements.

iii. Bacterial activity

In soils and water, bacterial activity is a common phenomenon. Bacteria produce

substances that may influence corrosivity. The absence of oxygen, particularly in

waterlogged soils, may provide a corrosive environment for iron and steel through the

growth of sulphate reducing bacteria (SRB), which generate hydrogen sulphide. These

microorganisms can exist in active form only in the absence of free oxygen and obtain their

energy from the reduction of sulphates into sulphides. Bacterial corrosion of iron and steel

under damp anaerobic conditions is usually rapid and severe. This type of attack can often

be recognised by the bright (unoxidized) appearance of the corroded surfaces and the

emission of hydrogen sulphide. The presence of sulphate reducing bacteria creates special

requirements for cathodic protection systems

The resistivity of soil can vary greatly with its water content and with the electrolyte

dissolved in the water. Thus the soil resistivity at a given location may vary with the season

of the year and the rainfall. However, if the soil is well drained and washed free of

electrolytes by frequent heavy rain it maintains a fairly high resistivity even when moist.

A professional analysis of the soil chemical composition and redox potential is requires to

determine the effective corrosion control method to achieve a 25years corrosion prention

for the pipeline.

Laboratory methods to determine soil resistivity are mostly based on soil box methods. A

sample of soil is placed in a calibrated box and the resistance is measured. A

4-terminal-type measurement can also be made on (undisturbed) soil samples obtained

from cores of test boreholes

C. CPS BASIC DESIGN

At this design stage of the cathodic-protection scheme, a decision must be made as to whether the

scheme will be a galvanic or impressed-current system. In specific circumstances, the use of both

types of systems may be appropriate.

Galvanic systems have the advantage of being –

i. Simple to install

ii. Independent of a source of external electric power

iii. Suitable for localised protection

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iv. Less liable to cause interaction on neighbouring structures.

However, the current output available from the practical size and weight of galvanic anodes is

relatively small and depends principally on the electrical resistivity of the local environment of the

buried pipe.

The anodes are usually self-regulating because their current output is usually less than their maximum

output capability and is controlled by the difference in potential between the two metals. The current

from the anodes is not normally controllable; thus changes in the structure, such as the deterioration of

a coating, that causes an increase in protection current demand, may necessitate the installation of

further sacrificial anodes to maintain protection.

Impressed-current installations have the advantage of being –

i. Able to supply a relatively large current

ii. Able to provide of high dc driving voltages (up to 50V).

iii. Enables it to be used in most types of electrolytes(high and low resistivity soils)

iv. Able to provide a flexible output that may accommodate changes in, and additions to, the

structure being protected.

Impressed current systems require regular maintenance and monitoring.

i. DETERMINING THE BOUNDARIES OF CPS

The scope potential boundaries shall be provided with electrical isolation to prevent interaction

between different systems. This shall consider:

the installation of isolation facilities ,

Where electrical isolation is not possible or not practicable, a localized cathodic protection

system may be used.

By determining the scope of a system, the type of structure, surface condition and total surface

area are defined and the boundaries for the design and monitoring are laid down.

In addition to the structure to be protected and the electrolyte (soil, water, etc.), impressed current

cathodic protection systems consist of the following essential components:

a. the impressed current anodes, buried in soil or immersed in (sea) water;

b. the anode groundbed

c. the current source, such as transformer/rectifiers, solar generators, etc.;

d. the interconnecting cables

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ii. CATHODIC PROTECTION SYSTEM COMPONENTS

CHOICE OF ANODES

a. ANODE CHARACTERISTICS

Each anode material has its typical electrochemical potential. This potential determines the

driving voltage of the anode, i.e., the voltage difference between the anode and the (protected)

steel. The more negative the open anode potential, the greater the driving voltage and the more

powerful the anode system.

The current output of an anode depends on the driving voltage and on the circuit resistance.

Assuming that the resistance of metallic connections is low, this resistance is mainly determined

by the soil or water resistivity, and the shape of the anode.

Current output determines the rate of consumption, i.e. the service life of an anode. This can be

expressed as anode consumption rate (kilograms of anode material consumed per Ampere. year,

kg/A.y) or as the anode Capacity (the number of Ampere-hours (Ah) that can be supplied by one

kg of anode material, Ah/kg)..

All these parameters are considered in the sacrificial anode system design.

b. CURRENT OUTPUT OF ANODES

The current output of sacrificial anodes depends on the driving force available and the circuit

resistance. The driving force is the difference in potential between anode and protected steel. The

potential of the protected steel is the minimum requirement.

The circuit resistance can be calculated and is the sum of cable resistance and anode-in-medium

resistance.

For the calculation of the latter a number of formulae are available each covering a type of

medium or position of anode.

The cathodic protection system shall permanently modify the electrochemical free potential of the

entire protected surface of the pipeline in the negative sense, and over the upper limit as defined

below.

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850 mV/Cu/CuSO4 for steel in an aerated soil

950 mV/Cu/CuSO4 for steel in a de-aerated soil with confirmation of the presence of active

sulphate reducing bacteria

c. GROUNDBEDS

The groundbed of the impressed current cathodic protection system shall be designed such that:

i. its mass and quality is sufficient to last for the design life of the system

ii. its resistance to earth allows the maximum predicted current demand to be met

at 80% or less of the voltage capacity of the DC source during the design life of

the system

iii. its location is remote from the pipeline and any other buried structure, to provide

a regular distribution of current along the pipeline

iv. the risk of causing harmful interference on other buried structures is minimized.

v. The selection of the location and the type of groundbed shall depend on local conditions

such as:

vi. Soil conditions and resistivity at various depths

vii. Groundwater levels and resistivity

viii. Strong seasonal changes in surface soil conditions

ix. Available terrain (for surface groundbeds)

x. Risk of shielding (specially for parallel pipelines)

xi. Risk of damage by excavation (surface groundbeds).

d. DEEPWELL/SHALLOW GROUNDBEDS

Deepwell groundbeds shall be used:

- if the soil conditions at the required depth are suitable to meet the requirements

- if there is a risk of shielding by other pipelines or buried structures

- if the available space is limited

- if there is a risk of stray currents on adjacent installations.

The basic design and calculation of the groundbed resistance shall be based on the most accurate soil resistivity data taken during the resistivity survey, using established methods and formulae.

Deepwell groundbeds shall be provided with adequate venting pipes to prevent gas blocking of

the well. Vent pipe material shall be chlorine resistant and non conductive. Filter gravel shall be

used to avoid blockage of the vent pipe by coke backfill.

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In a shallow groundbed, the anodes may be installed horizontally or vertically. The choice

depends on the soil resistivity distribution at various depths.

The anodes or the highest point of the carbonaceous backfill shall be not less than 1 metre below

ground level.

e. DC POWER /CURRENT SUPPLY

DC VOLTAGE SOURCES

The preferred DC voltage source for the 50km pipeline is a transformer/rectifier unit, fed by an AC

power supply. However where the power source is not available, an alternative solar powered dc

rectifier shall be used.

The maximum DC output voltage of a DC power source shall be 50 volts.

TRANSFORMER/RECTIFIERS

Transformer/rectifiers shall comply with IEC 146.

Transformer/rectifiers shall be of a special design for cathodic protection service

Transformer/rectifiers shall be suitable to operate under the prevailing service conditions unattended.

The output voltage shall be adjustable from zero to the maximum rated output when on load.

The transformer/rectifier shall be provided with an isolator or Moulded Case Circuit Breaker (MCCB)

on its incoming circuit and, where applicable, on its AC sub-circuits. Additionally, suitably sized fuses

shall be installed on the transformer/rectifier's phase AC sub-circuits and negative DC output circuits.

The transformer/rectifier shall be capable of withstanding a short circuit of up to 15 seconds duration

at the output terminals without damage to any of its devices.

The preferred type of transformer/rectifier is oil cooled, the incoming cables shall terminate in

separate non oil filled cable boxes and penetration into the tank shall be via bushings above oil level.

A sight glass and thermometer shall be provided.

The transformer/rectifier shall be properly sized to meet the load requirement of the Cathodic

protection system so designed.

SOLAR POWERED RECTIFIERS

Where AC power is not available solar system powered rectifier shall be used in place of transformer/rectifiers.

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The solar system shall ensure regular power supply to provide a continuous source of cathodic protection current.

Solar system shall be rated to recharge the batteries in less than 48 hours from a partially discharged state due to extended period of no sun.

Solar system shall be designed to maintain the design capacity at the highest ambient temperature.

f. MONITORING FACILITIES

The protection potential is the most important criterion for full protection. Facilities shall be installed

to allow electrical contact with the pipeline structure and to measure other parameters that may be

useful to monitor the system performance.

Monitoring procedures shall be prepared during the detail design and shall be related to the type of

monitoring facilities. They are part of the operating procedures of a cathodic protection system.

TEST POINTS

In order to verify the level of protection of the buried pipeline, the anodes shall as much as possible

at a maximum of two (2) kilometres apart be connected to installed test posts and where need be in

the case of possible interference and road crossings location another test posts shall be installed to

monitor the pipeline potentials.

TESTS POSTS AND DISTRIBUTION BOXES

An adequately sized weather-proof junction box shall be provided to terminate the anode lead wires

and rectifier positive lead. A nonconductive panel equipped with individual shunts, lead wire terminal

connections, provision for resistors, and a common bus bar facilitates testing. The junction box shall

be designed to dissipate heat generated by resistors.

Junction box selection shall be in compliance with fire and safety code requirements.

Distribution boxes must be weather proofed in accordance with the minimum degree of protection (IP

54) in accordance with IEC 529.

All cables must be connected to individual terminals.

Test posts and distribution boxes must be accessible during all seasons and designed such that they

are not prone to vandalism or damage.

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POSITIVE JUNCTION BOX

A suitably sized weather-proof junction box shall be installed for individual termination of each anode

and rectifier positive lead wires.

A shunt can be installed in each anode circuit to monitor the current output.

Resistors may be installed in individual anode circuits to balance anode outputs.

Sealing the cable entry may be necessary to prevent entry of corrosive gases.

Sealing the anode wires to prevent capillary action between insulation layers may be necessary to

prevent corrosive elements from entering the junction box.

g. CABLE CONNECTIONS

In order to avoid harmful effects to the pipeline to be protected, at the connection point, cable-to-pipe

connections must be carried out by means of brazing welding method. All underground cable

connections must be protected by resin splicing kits.

h. TEMPORARY PROTECTION ANODES

If cathodic protection is not immediately installed after the pipeline is laid, temporary protection is to

be provided by magnesium or zinc or aluminium anodes in accordance with the conditions hereafter.

Ground Resistivity (ρ) Ohms

per Metre

Maximum period without protection

(Months

ρ < 20 0.5

2 < ρ < 50 1.5

50 < ρ < 100 3

ρ > 100 6

i. LIGHTNING PROTECTION

Surge arrestors suitable lightning protection shall be installed to protect the pipeline isolation and

cathodic protection equipment. Surge arrestors shall be mounted across isolating joints/flanges and

across the output terminals of DC voltage sources.

j. SURGE ARRESTORS

Surge arrestors required to prevent elevated voltages due to faults in adjacent electrical power

systems or lightning shall be of the spark gap type and shall be designed such that:

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- the impulse breakdown voltage of the electrodes is lower than that of the isolating joint across

which they are mounted

- the spark gap is capable of discharging the expected lightning currents without sustaining

damage

- the spark gaps are fully encapsulated to prevent sparks in open atmosphere and to protect the

spark gaps from moisture.

k. ELECTRICAL EARTHING

The pipeline under cathodic protection shall be electrically isolated from common or plant earthing

systems to avoid a loss of current especially at the tie point.

D. SUMMARY

Protection system chosen shall be capable to fully control corrosion on the pipeline. This means that:

The protection shall be limited to the structure(s) defined in the scope;

The whole structure shall be polarised to comply with the minimum protection potential ;

The system shall be able to supply sufficient current for protection during the design life of

25yrs.

The CPS system shall be reliable;

The installation shall be as cost effective as possible through its design life;

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