1 Copyright © 2011 by ASME

Proceedings of the ASME 2011 International Design Engineering Technical Conferences & Computers and Information in Engineering Conference

IDETC/CIE 2011 August 29-31, 2011, Washington, DC, USA

DETC2011-47192

MODELING OF IMPRESSED CURRENT CATHODIC PROTECTION ANODE ARRANGEMENTS FOR STORAGE TANK BOTTOMS

Robert Adey BEASY

25 Bridge Street, Billerica, MA 01821. USA

John Baynham CM BEASY Ltd

Southampton, SO407AA. UK

Cristina Peratta CM BEASY Ltd

Southampton, SO407AA. UK

ABSTRACT Above ground tanks are frequently used for the storage of

Oil & Gas products and they can present a challenge to design

an optimum corrosion control system. The base of the tank lies

on or near the surface of the ground and is in contact with the

material used to support the tank and therefore presents a

corrosion challenge.

One method of protecting the bottom of a tank is by use of

an impressed current cathodic protection (ICCP) system. There

are a number of types of Cathodic Protection systems which are

designed to protect the tank base in these circumstances. The

details of the design of such a system are very important for

optimal performance and also the cost, if a number of such

systems are to be installed The consequence of a poor design

can be uneven distribution of protection potential on the tank

base or in the worst cases regions where corrosion of the tank

base can take place. An over designed system on the other hand

can have significant economic consequences both in terms of

installation cost and running costs.

Computer modeling is now widely used to optimize CP

Designs and verify that the design of the cathodic protection

(CP) system meets the design requirements. In this paper a

modeling tool is presented which enables corrosion engineers to

evaluate the performance of tank base CP systems by predicting

the protection provided to the tank for a given CP design. The

paper describes and discusses all aspects of the modeling

methodology, which it then applies to several different design

concepts.

INTRODUCTION Above ground tanks are frequently used for the storage of

Oil & Gas products and they can present a challenge to design

an optimum corrosion control system. Frequently a membrane

is installed below the tank to prevent soil contamination in the

case of leakage which has the consequence that the tank base is

electrically insulated from the surrounding soil

One method of protecting the bottom of a tank is by use of

an impressed current cathodic protection (ICCP) system. There

are a number of types of Cathodic Protection systems designed

to protect the tank base in these circumstances. They can range

from a simple rectangular grid of anodes, a single spiral anode

or a number of anode ribbons arranged in a circular grid

connected to distribution bars

The details of the design of such a system are very

important for optimal performance, and the consequences of:

• too large a spacing between the anodes and/or between

the current distribution bars

• an insufficient number of power feed connections

• or poorly chosen connection points for the cables

can be uneven distribution of protection potential on the tank

base or in the worst cases regions where corrosion of the tank

base can take place. An over designed system on the other hand

can have significant economic consequences both in terms of

installation cost and running costs.

The optimal design of a CP system for a particular structure

and environmental condition is not trivial, and may not

necessarily be achieved by incremental changes of a prior

design. The use of computer simulation (3,4,5,6,7), in

conjunction with suitable data obtained from the study of

previous tank systems, allows the consideration of many design

options and the effects of different soil or electrolyte conditions.

Such simulation allows selection in a systematic and predictable

2 Copyright © 2011 by ASME

way of the most suitable design which provides good protection

of the tank whilst minimizing cost.

For an already installed system, or as part of the design

process, simulation can be applied to investigate the most

suitable remedial measures when faults occur. For an existing

ICCP system this investigative activity may help avoid more

costly repairs, while if performed at the design stage, fault

tolerance can be built into the system.

Similarly simulation can be used to determine the most

suitable set point for the Transformer Rectifier Unit (TRU)

control system, based on the potential at the reference electrode

position (or positions if more than one Reference Electrode(

RE) which corresponds to the most desirable distribution of

potential on the tank base. In the event of failure of a

connection under the tank this process provides informed

control of operation of the ICCP system.

The driving force of an ICCP system is the electric current

flowing from the power supply to the anodes. A single discrete

anode supplied by a power supply will have a current output

which can easily be quantified. However this is not the case

where anodes are distributed or cable runs are of variable

length. In a tank base anode grid, power loss in cables,

distribution bars or anodes themselves may have a significant

impact on performance of the ICCP system.

The robustness of the CP Design can be determined by

performing what-if studies investigating different damage

scenarios, for example failure of one or more welds, or of a

cable connection. Effects of adding resistances to some of the

power supply cables can be determined, so that use of this

method as a possible remedial technique (or some other form of

power splitting) can be investigated.

The main objectives of this work are to show that

• Simulation during the design stage of a tank-base ICCP

system can be of considerable benefit to the designer

• Simulation results can assist in initial set-up of a

system

• Simulation can be used to investigate the fault-

tolerance of a design, its ability to perform adequately

despite occurrence of faults, and the effects of any

planned remedial actions.

The modeling approach is based on the fundamental

equations governing current flow in the electrolyte,

electrochemical polarization effects at the surfaces of the active

electrodes in contact with the electrolyte, Ohm’s law and charge

conservation equations for the electrical supply and return

circuit connecting the power supply units, anodes, discrete

resistors and so on.

COMPUTER MODELING The simulation tool described in this work (BEASY)

performs direct simulation of CP systems with ICCP anodes.

The main objective of the simulation is to obtain quantitative

results for levels of protection against corrosion on the structure

by considering the physical configuration of the surrounding

environment and design parameters of the system, i.e. anode

geometry, type, electrolyte conductivity, etc.

In general the input data for a model of a CP system

consists of the following:

• physical and geometrical properties of the electrolyte

• anode geometry (sizes and locations) and surface

coating

• reference electrode set points and locations

• condition of any coatings/paints on the tank base

• polarization properties of the materials involved as

active electrodes

The outcomes of the simulation are the current densities

and protection-potentials on the metallic surfaces, electric

potential and gradient values at any point in the electrolyte, and

voltage, current and power loss in the components of the circuit.

The simulation involves solution of two coupled problems: the

electrolyte and the external circuit. The former involves the

electrolyte itself, and all the surfaces surrounding it, including

the thin layer on the active electrodes and any other insulating

surface bounding the electrolyte, while the latter involves the

resistive network composed of discrete electrical components

such as resistors, transformer-rectifier units (TRU’s), diodes,

shunts, etc.

In the problem defined by the external circuit, the TRU

maintains a voltage difference between the metallic structure

and the anodes circuitry. The voltage distribution in the anode

grid is determined using the Kirchhoff equations for electrical

networks.

Boundary Element Methods (BEM) have been used to

simulate the behavior of cathodic protection systems since the

late 70’s (1, 2). As the name implies, the method requires

creation of elements, but only on the boundary (i.e. surfaces) of

the problem geometry. The BEM is used to mathematically

model the potential drop in the electrolyte represented by the

Laplace equation.

Boundary discretization combined with the collocation

technique leads to an algebraic linear system of equations, in

which the unknowns are potentials and current densities normal

to the boundary evaluated on the surfaces of the electrolyte.

The boundary conditions applied to surfaces of the

electrolyte in contact with active electrodes consist of

polarization curves, which relate the normal current density

flowing through the surface to the potential drop across the

interface between the metal and the electrolyte. The relationship

between current density and potential difference is in general

non-linear, so that the solution is obtained in an iterative way.

CASE STUDY The study investigated the design of the CP system for a

42m diameter tank sitting on a 0.23m thick layer of sand with a

3 Copyright © 2011 by ASME

membrane separating the sand from the surrounding soil. Figure

1 shows the cylindrical base of the tank and the concentric

circles of the anode ribbons. The lines crossing the anode

ribbons show the possible locations of the power distribution

cables. (Note. In the design only one line of the cables was

used. See Figure 5). The sand resistivity was initially assumed

to be 50,000 Ohm-cm.

Figure 1 Schematic view of the tank base showing the ribbon anodes as concentric circles positioned 0.17m below the tank base

The CP system consisted of the following:

� Mixed Metal Oxide (MMO) coated titanium ribbon

anodes arranged in concentric rings symmetrically

distributed below the tank bottom with fixed

separation between them. MMO anodes were used in

this case because of their durability.

� The anodes are located 0.06m above the membrane.

� The ribbon anodes have cross-section 6.35mm by

0.635mm, and linear resistance 0.15 Ohm/m.

� Each anode is assumed to be cylindrical with a

diameter of 4.4468x10-3 meters, giving a cross

sectional area equal to the ribbon anode of dimensions

6.35mmx0.635mm.

� Distance from the tank edge to the first ring is 0.23m

The initial design considered an anode spacing of 2m as

shown in Figure 2 which shows the anodes located in the sand

between the membrane and the tank base.

A key element in predicting the performance of the CP

system is the electrical connections between the power supply

(TRU) the distribution cables and the anode ribbon as shown in

Figure 3. Ro is calculated using the resistivity of copper and

cable dimensions. The ribbon anode 1 is the outermost one.

Ribbon anode 2 is the following ring anode and subsequently

RN is the smallest and last ring. Ri, for a ring anode “i” is

calculated using the resistivity of the ribbon anodes and the

distance to the connector. Hence R1<R2<..<RN

Figure 2 Configuration of anode rings below the tank bottom

Figure 3 Electrical circuit diagram showing the connections between the TRU and the anode ribbons below the tank

The feeder cables were connected as shown in Figure 4.

The anode ribbons were distributed in a circular pattern and

connected along a line cutting the circle in half as shown in

Figure 5.

Figure 4 Feeder cables were attached to both ends of each half ring of the anode ribbons

4 Copyright © 2011 by ASME

Figure 5 Detailed view showing the anode ribbon and its connection to the feeder cables

The remaining data required for the simulation is the

electrode kinetics on the metallic surfaces of the tank bottom

and the anode ribbons. This data is supplied in the form of a

polarization curve as shown in Figure 6. This is typical data for

the type of tank being modeled.

Finally the simulation is controlled by specifying the

voltage from the TRU. The voltage is increased until the

simulation predicts that the potential on the tank base is more

negative than the target value (in this case -850 mV vs Ag Ag Cl

reference electrode.)

Figure 6 Polarization curve for the tank base. The polarization curve describes the relationship between the potential and current density on the metal surface.

INITIAL SIMULATION In the initial simulation for the tank standing on sand with a

resistivity of 50,000 Ohm-cm the ring spacing was set at 2m

which gave 11 rings under the tank bottom. A series of

simulations were performed in which the TRU voltage was

increased from 10V to 50V and the results evaluated to

determine if the potential on the tank base was within the target

range.

Figure 7 shows the predicted potential on the tank base for

an applied TRU voltage of 10 V. As can be seen in the close up

shown in Figure 8 the potential varies significantly over the

base between the tank surface nearest to the anode and the mid

point between the anodes.

While the tank is protected at the locations near the anodes

it is significantly under protected at the mid points. Therefore

the voltage was increased in an attempt to improve the

protection however increasing the voltage to 50V only shifted

the most positive potentials by 20% which was inadequate to

meet the design goals.

Figure 7 Two views of the potential predicted on the tank base for a TRU voltage of 10 V

Figure 8 Close up view of the potential on the tank base. The blue indicates the potentials on the base closest to the anode and the red the more positive potentials in the gaps between the anodes

REVISED DESIGN As it was impossible to achieve the desired potentials with

the initial design with anode spacing of 2m a new design was

proposed with a spacing of 0.5m. Therefore there were now 41

anode rings under the tank and the design was simulated as

before to determine the TRU voltage required to achieve the

required potentials on the tank base. Results are shown in

Figure 9 for the case of TRU 20v which can be seen now

achieves the required potential on the tank base.

5 Copyright © 2011 by ASME

Figure 9 Predicted potentials on the tank base for the anode spacing of 0.5m and TRU 20V.

Revised Design With Reduced Sand Resistivity The design cases considered in the initial study and revised

design assumed the sand resistivity was 50,000 Ohm-cm. In

order to test the sensitivity/range of application of the revised

design a new case was considered where the sand resistivity was

reduced to 5,000 Ohm-cm. The same procedure was followed to

determine the TRU voltage necessary to achieve the required

potentials and the results are shown in Figure 10.

Figure 10 Predicted potential on the tank base with TRU 5V for the sand resistivity of 5,000 Ohm-cm

The model also provides insights into how the internal

workings of the system are behaving which can used to

optimize the design. Such data includes the IR drop in the

supply cables, feeder cables, junctions and in the ribbon anodes.

This data can also be used to simulate the impact of a failure of

part of the system on the overall system performance. For

example the voltages in the anodes can be visualized as shown

in Figure 11 as well as the numerical values displayed. The

figure clearly illustrates how the voltages reduce with distance

from the feeder cable connection points.

Figure 11 Predicted voltages in the ribbon anodes under the tank.

In this study the location of the reference electrodes was

not defined as the objective was to identify the best location for

the reference cells and the values of the set points to achieve the

best overall protection. For example good locations for the

reference cells would be where the model predicts the tank base

is least protected. Alternatively the modeling strategy could

have been to define the reference cell locations and allow the

model to automatically adjust the ICCP voltage to achieve the

required “set point” potentials.

The figure also clearly shows the interaction between IR

drop in the anode ribbons, feeder cables and the polarization of

the tank bottom. The shorter anode ribbons in the center of the

tank results in an area where the losses in the feeder cables are

greater but compensated by the losses through the anodes being

less. Whereas the longer ribbon anodes show a significant drop

in the voltage at the further distance from the connection points

to the feeder cables. The resulting reduced protection to the

tank base can be seen in Figure 10.

SUMMARY A computational model has been introduced which

combines a numerical model of the physics of a galvanic

corrosion system with an electrical circuit model. The model is

capable of simulating the interaction between the electrode

kinetics on the metallic surfaces in contact with the electrolyte,

the IR drop through the electrolyte and the current flow through

the TRU and feeder cables.

The model has been applied to predict the protection

provided to a tank base by a cathodic protection system.

The use of the model to optimize the design and test its

robustness under a range of conditions has been demonstrated.

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Techniques – Theory and Application in Engineering.

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Corrosion, in “Galvanic Corrosion”. Harvey P. Hack,

editor. ASTM Committee G-1 on Corrosion of Metals.

ASTM International, 1988

3 Peratta AB, Baynham JMW, Adey RA: “Modeling

Impressed Current Cathodic Protection of Storage

Tanks”. Eurocorr 2009.

4 Bazzoni, B, Lorenzi. S, Marcassoli. P, Pastore. T. “Current

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Protection Of Tank Bottoms”. Corrosion Vol 67, No 2.

2011

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6 Copyright © 2011 by ASME

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