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.
Springer Verlag Berlin, Heidelberg NY, Tokyo. 1984.
2 Adey RA, Niku S: “Computer Modeling of Galvanic
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
and Potential Distribution Modeling For Cathodic
Protection Of Tank Bottoms”. Corrosion Vol 67, No 2.
2011
5 Peratta AB, Baynham JMW, and Adey RA: A
Computational Approach for Assessing Coating
Performance in Cathodically Protected Transmission
Pipelines. CORROSION 2009, Paper 6595 Atlanta,
Georgia. NACE International 2009.
6 Adey RA, Baynham JMW: “Design And Optimization Of
Cathodic Protection Systems Using Computer
6 Copyright © 2011 by ASME
Simulation”. CORROSION 2000, Paper 723. Houston,
Texas. NACE International, 2000.
7 Adey RA, Baynham JMW: “Simulation Assisted Design of
Storage Tank Base ICCP” CORROSION 2010, San
Antonio, Texas . NACE International 2010.
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