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MODELING OF IMPRESSEDCURRENT CATHODICPROTECTION ANODEARRANGEMENTS FOR

STORAGE TANK BOTTOMSRobert A. Adey, John M WRobert A. Adey, John M W BaynhamBaynham and Cristinaand Cristina PerattaPeratta

Computational Mechanics IncComputational Mechanics Inc25 Bridge Street, Billerica, MA 0182125 Bridge Street, Billerica, MA 01821

CM BEASY LtdCM BEASY LtdAshurst Lodge, Southampton, Hampshire, SO40 7AA, UKAshurst Lodge, Southampton, Hampshire, SO40 7AA, UK

Overview

Introduction Protecting tank bases

Modeling Technology Tank base case study Simulation based design Impact of sand resistivity Revised design

Conclusions

Introduction

Above ground tanks are frequently used for thestorage of Oil & Gas products and they canpresent a challenge to design an optimumcorrosion control system

CP Design OptionsDeep Anodes

Credit. John Fitzgerald. Corrpro, Freshwater Spills Symposium

CP Design OptionsShallow Anodes

Credit. John Fitzgerald. Corrpro, Freshwater Spills Symposium

CP Design OptionsAnode Grid

Frequently a membrane is installed below the tank toprevent soil contamination in the case of leakage whichhas the consequence that the tank base is electricallyinsulated from the surrounding soil

CP Design OptionsAnode Grid

Credit. John Fitzgerald. Corrpro, Freshwater Spills Symposium

Anode Grids One method of protecting the bottom of a tank is

by use of an impressed current cathodicprotection (ICCP) system

There are a number of types of CathodicProtection systems designed to protect the tankbase in these circumstances

They can range from a: Simple rectangular grid of anodes Single spiral anode Number of anode ribbons arranged in a circular grid

connected to distribution bars

Design Considerations The details of the design of such a system are very

important for optimal performance The consequences of:

Too large a spacing between the anodes and/or between thecurrent distribution bars

An insufficient number of power feed connections 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 ofinstallation cost and running costs

Design Optimization The optimal design of a CP system for a

particular structure and environmental conditionis not trivial, and may not necessarily beachieved by incremental changes of a priordesign.

The use of computer simulation in conjunctionwith suitable data obtained from the study ofprevious tank systems, allows the considerationof many design options and the effects ofdifferent soil or electrolyte conditions

Such simulation allows selection in a systematicand predictable way of the most suitable designwhich provides good protection of the tank whilstminimizing cost

Objectives The main objectives of this work are to

demonstrate that Simulation during the design stage of a tank-

base ICCP system can be of considerablebenefit to the designer

Simulation results can assist in initial set-up ofa system

Simulation can be used to investigate thefault-tolerance of a design, its ability toperform adequately despite occurrence offaults, and the effects of any planned remedialactions

Computer Modeling Computer modeling is

now widely used for awide variety ofapplications including Ships and boats Offshore Oil & Gas

structures Pipelines Concrete structures Storage tanks Wells Aircraft and Vehicles

Computer Modeling 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

Modeling Features:Different Materials, Coatings and

Breakdown Factors

Simply choosedifferent materials,polarisation curves

and coatings for eachcomponent

Enter the currentdensity or total

current required forICCP anodes

Enter the electricalresistance of the

material if it is notzero.

Modeling Features:Electrical Connections, Return Paths

and Attenuation “IR” voltage drop in the supply and return

path This is very important when there are non-

zero resistances in the connections betweenstructures, for example the resistances offeeder cable lengths, or if there are significantresistances in ribbon anodes (for example).

Attenuation In the supply cable from the TRU to the cable

connection point In the feeder cables from the cable

connection point to the individual anode rings. In the anode rings

Case Study The study

investigated thedesign of the CPsystem for a 42mdiameter tanksitting on a 0.23mthick layer of sandwith a membraneseparating thesand from thesurrounding soil

Model Description

There are MMO coated titanium ribbon anodes arranged in concentricrings symmetrically distributed with fixed separation between them.

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 modeled as a cylinder with diameter 4.4468x10-3

meters, giving a cross sectional area equal to the ribbon anode ofdimensions 6.35mmx0.635mm..

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

R1 R0

RN

TankBase

RibbonAnode N

RN R0

Nribbonanodes

R0

RibbonAnode 1

R1

Circuit Model

A key element in predicting theperformance of the CP system is theelectrical connections between thepower supply (TRU) the distributioncables and the anode ribbon as shown

Ro is calculated using the resistivityof copper and cable dimensions.

The ribbon anode 1 is the outermostone. Ribbon anode 2 is the followingring anode and subsequently RN isthe smallest and last ring.

Ri, for a ring anode “i” is calculatedusing the resistivity of the ribbonanodes and the distance to theconnector.

Feeder Cables

Feeder cables were attached to both ends of eachhalf ring of the anode ribbons

Polarization Data For The TankBase

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0.10 1.00 10.00 100.00 1000.00 10000.00

Current density [mA/m**2]

Pote

ntia

l [m

V]

Initial Simulation In the initial simulation for the tank

standing on sand with a resistivity of50,000 Ohm-cm the ring spacing was setat 2m which gave 11 rings under the tankbottom

A series of simulations were performed inwhich the TRU voltage was increasedfrom 10V to 50V and the results evaluatedto determine if the potential on the tankbase was within the target range (-850mVand -950mV on the tank base)

Potential On The Tank Base

This result shown that2m of ring separationis not adequate whenthe soil resistivity is50,000 Ohm-cm

The blue indicates thepotentials on the baseclosest to the anodeand the red the morepositive potentials inthe gaps between theanodes

Potential On The Tank Base

This view of the sameresult shows thepotential valuesplotted with variationby colour and height.

The TRU wasincreased from 10 to50V and although themost negative valuechanged significantly,the most positivevalues did not changeby more than 20%.

Revised Design

As it was impossible to achieve thedesired potentials with the initial designwith anode spacing of 2m a new designwas proposed with a spacing of 0.5m

Therefore there were now 41 anode ringsunder the tank and the design wassimulated as before to determine the TRUvoltage required to achieve the requiredpotentials on the tank base

Revised Design

Results are shown for the case of TRU Voltageof 20v which can be seen now achieves therequired potential (< -850) on the tank base

Design Robustness

The design cases considered in the initialstudy and revised design assumed thesand resistivity was 50,000 Ohm-cm

In order to test the sensitivity/range ofapplication of the revised design a newcase was considered where the sandresistivity was reduced to 5,000 Ohm-cm.

Sensitivity Study: Resistivity

In this case the required protection wasachieved with a TRU setting of 5V

Predicted potential on the tank base with TRU5V for the sand resistivity of 5,000 Ohm-cm

Design Robustness The model also provides insights into how the

internal workings of the system which can usedto optimize the design

Such data includes the IR drop in the: Supply cables Feeder cables Junctions Ribbon anodes

This data can also be used to simulate theimpact of a failure of part of the system on theoverall system performance

Ribbon Anode Voltages The voltages in the ribbon anodes can be predicted by

the model to see the impact of the IR drop The simulation clearly shows how the voltages reduce

with distance from the feeder cable connection points

Understanding The Behavior OfThe Design

Area of leastprotection where

losses through theanodes are greatest

Area wherelosses in supplycables are least

Area where losses insupply cables is

greater butcompensated by thelosses through theanodes being less

CONNECTIONPOINTS

CONNECTIONPOINTS

Reference Electrodes

In this case the reference electrodes were notincluded in the model

However, it is possible to define them at userselected positions and the modeling processused to identify potentials at those chosenreference electrode locations.

These calculated potentials at referenceelectrode locations, can then be used by the CPdesigner in order to establish the system ‘setpoints’ in order to achieve the desired targetpotential on the tank base.

Coating Breakdown Factors In this study the tank base was modeled

as bare steel However by applying a breakdown factor

to the tank base coating, the model can beused to assess theperformance/robustness of the CP systemat the start and end of life

The impact of temperature can also beconsidered by modifying the polarizationdata

Summary A computational model has been introduced which

combines a numerical model of the physics of a galvaniccorrosion system with an electrical circuit model

The model is capable of simulating the interactionbetween the electrode kinetics on the metallic surfaces incontact with the electrolyte, the IR drop through theelectrolyte and the current flow through the TRU andfeeder cables.

The model has been applied to predict the protectionprovided to a tank base by a cathodic protection system.

The use of the model to optimize the design and test itsrobustness under a range of conditions has beendemonstrated