A 05.02.2014 ISSUED FOR INFORMATION M.C. Z.K. E.D.

Rev. No.

Date Description Prepared Checked Contr. By Onay EDOPEC

ORIGINATOR

Published by

.

EDOPEC ENERJİ PETROL MÜHENDİSLİK SANAYİİ VE TİCARET

LİMİTED ŞİRKETİ

Document Title CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK

EXTERNALS Head Office : Palmiye Mah. Adnan menderes Bulv. Oktay Sitesi No: 9/8 PK = 33100 Yenişehir /Mersin –TURKEY P:+90 324 3260595 F:+90 324 3260596 www.edopec.com. info@edopec.com

Document No.

EDO CAL COE COR INT XXX 014 190 218 Rev ACo. Org.

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17

Page 2 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

Table of Content

1.  DOCUMENT SCOPE .................................................................................................. 3 

2.  REFERENCE ................................................................................................................ 3 

2.1  Project Specifications ............................................................................................................... 3 

2.2  Codes ......................................................................................................................................... 4 

2.3  Object Datasheets ..................................................................................................................... 4 

2.4  Vendor Documents ................................................................................................................... 4 

3.  SITE CONDITIONS .................................................................................................... 5 

4.  GENERAL ASSUMPTIONS ....................................................................................... 5 

5.  CALCULATION AND SIZING  METHOD ................................................................ 6 

5.1.  Protection Current Demand ..................................................................................................... 6 

5.2.  Sizing of the Anode Groundbed, ............................................................................................. 7 

5.3.  Verification of Adequacy .......................................................................................................... 9 

5.4.  IR Drop & Circuit Resistance Calculation ............................................................................. 11 

5.5.  Sizing of the Power Supply .................................................................................................... 17 

 

 

Page 3 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

1. DOCUMENT SCOPE 

The purpose of this document is to define the parameters and describe the engineering calculation methods, to design effective and longtime Cathodic Protection (CP) system for external surfaces of at-grade storage tanks (Hereafter called Tank) for; ONSHORE FACILITIES (Hereafter called The Facility), by means of;

site and operational conditions,

design assumptions,

evaluation of relevant objects to be cathodically protected,

along formulations required for calculation of;

protection current demands,

sizing the groundbeds,

verification of adequacy,

IR drop calculation and

sizing of TR units

This document shall be considered as the main engineering calculation booklet for object-wise CP calculation notes and detail drawings in order to design most effective way to protect the Tanks bottom plates from the external electrochemical corrosion phenomenon.

2. REFERENCE 

2.1 Project Specifications 

The latest revision of Purchaser documents referred thereto has been considered as the main reference for this document:-

DB-1718-999-P332-0204 Basic Engineering Design Data for Onshore Facilities RP-1718-999-6300-5002 General Requirements for Cathodic Protection RP-1718-999-1630-0007 Onshore Electrical Design Criteria RP-1718-999-2500-0001 Specification for Storage Tanks

The clarification sheets submitted along Purchaser documents are also considered as an integral part of project specifications.

Page 4 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

2.2 Codes 

The applicable international standards listed on Purchaser documents no.;

DB-1718-999-P332-0203 List of Applicable Codes and Industry Standards

have been considered as the reference codes of this document. In case of conflict between requirements specified in purchaser’s documents and the requirements of any other referenced standard, the order of precedence shall be according to POR/MR document. Any conflict will be brought to Purchaser’s attention to receive a written clarification before proceeding with any work.

2.3 Object Datasheets 

The latest revisions of Mechanical datasheets of Tanks have been used to evaluate need of Cathodic protection as the main reference for this document:-

DW-1718-101-2510-0101 101-T-101/201/301/401 Amine Surge Tank

SP-1718-121-2500-0101-3 Mechanical Data Sheet for Steam Condensate Storage Tank

SP-1718-125-2500-0101-6 Mechanical Data Sheet for Sea Water Storage Tank

SP-1718-128-0800-0101-3 Mechanical Data Sheet for Potable Water Storage Tank

SP-1718-129-2501-0101-4 Mechanical Data Sheet for Waste Caustic Soda Storage Tank

SP-1718-130-2500-0101-8 Mechanical Data Sheet for Fire Water Storage Tank

SP-1718-144-2500-0101-3 Mechanical Data Sheet for Liquid Sulfur Storage Tank

SP-1718-146-2501-0102-5 Mechanical Data Sheet for Fresh MEG Storage Tank

SP-1718-146-2501-0104-5 Mechanical Data Sheet for Fresh Amine Storage Tank

SP-1718-146-2501-0105-6 Mechanical Data Sheet for Fresh Caustic Soda Storage Tank

2.4 Vendor Documents 

The vender print document no.;

VP- PO-1718-1186-001 Cathodic Protection System Project Scope of Work

VP- PO-1718-1186-101 Cathodic Protection System Design Philosophy and Considerations

are also pre-requisite and integral parts of this document that have been derived accordingly.

4

Page 5 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

3. SITE CONDITIONS In accordance to Doc. No.: DB-1718-999-P332-0204-1, recorded ambient temperature and meteorological site conditions within entire Facility are as follows:-

– Highest Air Temperature: 47ºC – Lowest Air Temperature: 2ºC – Design Temperature

o for outdoor equipment: 2-48ºC o for indoor equipment : Max 40 ºC

– Maximum relative humidity: 91% January – Design relative humidity

o for outdoor equipment: 80 to 100% o for indoor equipment : up to 80%

– Altitude: varies between 24 to 64 meters above sea level over the site locations

4. GENERAL ASSUMPTIONS 

With regard to the project specifications and “Cathodic Protection System Design Philosophy and Considerations” document No. VP- PO-1718-1186-101, the following assumptions shall be considered to formulate all the relevant details of calculation.

a) Cathodic protection system for tanks is assumed to be in full integration within the further installation of CP system for future buried piping system on said facility.

b) External Cathodic protection for all applicable Tanks assumed to be the type Impressed Current system (ICCP)

c) The soil under the tank base assumed NOT containing of asphalt, oily sand or bitumen.

d) The connecting pipes including inlet/outlet, fire fighting, cooling, and close drainage system along earthing system do not require galvanic isolation from Tanks due to the usage of HDPE geo-membrane acting as a dielectric shield underneath the tank.

e) Tanks which of those operating temperature are below 120°C shall be externally subjected of cathodic protection. However, the temperature effect on increasing the protective current demand over 80°C is assumed to be negligible.

f) Tanks which of those operating temperature are minimum of 120°C does not require external cathodic protection. Thus, those current demands assumed being nil in such a way.

g) Where the tank base shall be constructed on full concrete pad foundations the tank bottom plate is not required to be protected by external cathodic protection system. The current demand assumed null and void. Provision of chloride extraction for such a concrete pad shall be considered while calculating polarization current demand for rebar in concrete.

h) All the tanks which the need of cathodic protection is considered to null and void shall be bonded via Cathode Bonding boxes to the relevant Piping system.

Page 6 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

5. CALCULATION AND SIZING  METHOD 

Individual Impressed Current Cathodic Protection System (ICCP) shall be sized for various Tanks within the boundary of Facility. The sizing of individual ICCP’s shall be addressed on object-wise Calculation Notes and Installation Details.

The Calculation Note for each of subjected Tanks shall show the following details as minimum;

protection current demands, sizing of the anode groundbed, verification of adequacy, IR drop & circuit resistance calculation and sizing of Power supply Unit

Hence the formulations as described below have been derived from the project reference documents.

5.1. Protection Current Demand 

The protection current demand is the current required to maintain an efficient protection level during the design life, and is calculated by the following Equation;

DTBTTP FIICFI )( (Eq.1.1)

where:

PI :tank protection current demand (mA)

TF :tank foundation type constant (where the tank is on soil infill =1 & 0 for Concrete Pad)

TC : corrosion type constant (when operating temperature is below 120°C =1 & 0 for above)

BI : tank basic current demand in at 30°C operating temperature (See Eq.1.2)

TI : tank additional current demand of operating temperature above 30°C (See Eq.1.3)

DF :current drainage factor (considered being Unit Value when di-electric shield is present)

5.1.a) Basic Current Demand  

In order to calculate the tank basic current demand ( BI ), the coated and bare surfaces of tank base plate extracted for coating efficiency, in conduction to relevant current density is used as per equation1.2;

PCPBB SCESCEI )()1( (Eq.1.2)

where:

PS :total surface of tank bottom plate to be protected (See Eq. 1.4)

B :protection current density for bare Tank Bottom (20mA/m2)

C :protection current density for coated Tank Bottom (4mA/m2)

CE :coating efficiency (considered to be the value of 0.5 for coated tank bottom and 0 for bare tank bottom )

Page 7 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

5.1.b) Temperature Rise Current Demand  

Protection current demand of tanks has a nature of inflation, affected from the temperature rise on

surfaces subjected to be protected in direct conduction of Tank operating temperature. Hence TI shall be calculated and added to the basic current demand, which is calculated from;

F

T

FT III

1025.1 (Eq.1.3)

where: T :temperature rise over 30°C up-to 80°C in accordance to the tank operating temperature

PS : tank bottom plate surface area in contact with infill material (m2)

5.1.c) Protection Surface Area 

The Tank base plate planned to be erected on foundation infill material with specific diameter is the certain area of the object to be protected. However the base plat is lap welded within a proper slope in order to drain fluid impurities and debris through internal sump pit.

Regardless of either inward or outward slope of the tank base plate the exact slant surface area subjected to be protected is calculated as follows;

2

2

2

122 dhd

dl SP

(Eq.1.4)

In which;

l :slant height of cone

h :height of cone

d :base diameter of cone

Figure 1- Schematic for Slope of Tank Bottom Plate

5.2. Sizing of the Anode Groundbed, 

MMO Titanium anodes with excellent current output and long lifetime are proven to have the following advantages:

Availability of Large Driving Potential

High Current Output Capable of Protecting Large Structure

Capability of Variable Current Output

Applicability To Almost Any Soil Resistivity

Page 8 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

5.2.a) Anode Shape

MMO Titanium anodes to be installed underneath the tank shall be in the shape of ribbon with standard dimensions available in market. A MMO Ribbon anode with 6.35 mm width and 0.635 mm thickness has been considered for use on this project.

5.2.b) Anode Quantities  

Anode quantities shall be subject of calculation by the following means;

Required quantities of anode to provide essential current of designated anodes design life,

Adequacy of current distribution underneath the tanks by means of earth potential shift criteria. (See clause no. 5.3)

The required length of anode for each tank shall be calculated by dividing total protection current demand by maximum current output of the ribbon in fine sand; i.e.,

A

PM

IL

(Eq.2.1)

where:

PI :tank protection current demand as per Eq.1.1 (mA)

A :anode current density per meter length (from the 3 A/m2for 50+ years of design life, assumed to be 42 mA/m for standard ribbon 6.35 mm width and 0.635 mm thick as per data sheets)

ML :minimum required length of anode in meters

However the actual anode length shall be subject of verification of adequacy factor ( AF ) on distribution of current by means of Anodic Voltage Cone, underneath of entire bottom plate of tank meeting the following criterion;

)(

)()(

SU

SLSU

Max

MinMaxA PP

PPPP

V

VVF

(Eq.2.2)

where:

MaxV :the maximum level of Anodic Voltage Cone calculated at a certain point underneath the tank

MinV : the minimum level of Anodic Voltage Cone calculated at a certain point underneath the tank

UP :upper limit of protective potential allowed as per project specifications (-1200mV wrt Cu/CuSo4)

LP : lower limit of protective potential allowed as per project specifications (-850mV wrtCu/CuSo4)

SP :natural potential of Steel bottom plate (considered to be -550mV wrt Cu/CuSo4)

Hence the actual length, the quantities of segments and spacing between parallel ribbon anodes shall be determined in a way to ensure the even current distribution underneath the tank have enough adequate for protection of tank bottom plate under within project protective potential rages.

Page 9 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

5.3. Verification of Adequacy 

In order to evaluate the adequacy of current distribution underneath the tank (item ‘b’ in sec.6.3.), the “voltage cone” and “superposition” concepts shall be taken into account.

When the protection current is transmitted to the soil via the anode bed, a voltage gradient is produced in the soil which exponentially decreases with increasing distance from anode bed, being the greatest at the vicinity of the anode.

At a distance from the anode bed where no appreciable field strength due to the protection current is detectable, the soil potential approaches zero. This potential is termed that of the remote ground.

The voltage between the remote ground and the anode bed is the anode voltage. Because of the cone-shaped curve of the voltage distribution on the soil surface (Figure 2), this is called the “Voltage Cone” of the anode bed. The height of the voltage cone depends on the anode voltage and its shape depends on the arrangement of the anodes.

For a single ribbon anode placed horizontally in the ground, the equi-potential lines in the soil around the anode bed may be shown as Figure 3.

The earth potential shift depends on the following parameters.

a) soil resistivity value, b) depth and length of the anode, c) horizontal distance from the anode, d) current transmitted to the soil by the ribbon anode.

Figure 3- Voltage Cone in soil for a Horizontal Single Anode

Ribbon Anode 

Current Drain Point (Single Vertical Anode, 

Deep Well, etc.) 

Figure 2- Single Vertical Anode Voltage Cone Schematic

Page 10 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

This will be calculated by using the following formula from Handbook of Cathodic Corrosion Protection by W. von Baeckmann.

22

222 2

22

222

lltr

lltr

l 

I Vr

ln

(Eq.3.1)

where:

rV : earth potential shift at distance "r" from the anode

: soil resistivity value for tank infill material

I : current flow from ribbon anode l :anode length t : vertical distance between tank bottom and anode r : horizontal distance of the anode to a certain point of the tank bottom surface

At the stage of calculating the potential shift of the soil underneath the tank bottom plate by use of the above formula, the superposition effect of all segments of MMO ribbon anodes shall be considered. For instance, for calculating the potential shift at the point of the tank bottom immediately above segment 1 in Figure 4, the superposition effects of segments 2, 3, 4, … , N shall be obtained and added to the potential shift emanates from segment 1 itself.

Similarly, for segment 2, the effects of segments 1, 3, 4, … , N shall be calculated as per segment 1 that shall be individually calculated for all of the segment of anodes up to the segment N.

n

iir

njjS jiVV

11 ),()()(

(Eq.3.2) where:

SV : earth potential shift with superposition effects of all anode segments (repeated for individual

locations at the point of the tank bottom plate immediately above each segment of anode) ),( jiVr : function of Earth potential shift effect arisen from individual segments of anode to a certain location of tank bottom plate

In which with respect to the Eq.3.1, the function of earth potential shift arisen from individual segments of anode is defined as below;

22

222 2

22

222

iis

iis

i

ir

lltrji

lltrji

l 

I jiV

ln),(

(Eq.3.3)

Figure 4- Location of Ribbon Anode underneath

the Tank Bottom Plate

Page 11 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

where: j : the variable number of a certain location affected by earth potential shift of individual anode

segments i : the variable number of anode segment that its effect at a certain (J) location is under study

iI : current flow from the segment of anode that its effect at a certain (J) location is under study

il : length of the segment of anode that its effect at a certain (J) location is under study

t : vertical distance between tank bottom and anode( Typically 0.4 meters)

sr : horizontal distance between segments of anode (shall be adjusted during verification for

optimum usage of anode and protection adequacy)

Hence the maximum and minimum value of earth potential shift with superposition effects of all anode segments beneath the tank bottom plate can be addressed as follows;

);...;;( 321 SNSSSMax VVVVMaxV

(Eq.3.4)

);...;;( 321 SNSSSMin VVVVMinV

(Eq.3.5)

That shall be used in order to verify the of adequacy factor ( AF ) indicated on pervious section as per

Eq.2.2. Hence the distance between the segment of anodes from each ( sr ) shall be adjusted for

optimum installation by both means of efficient usage of anode and protection adequacy.

5.4. IR Drop & Circuit Resistance Calculation 

The size and current capacity of the ICCP system shall be designed within calculation of IR and drop equivalent circuit resistance of the system.

The following parameters are affecting the total circuit resistance and resulting IR drop on various system components subsequently;

The transition resistance between the surface of the anode and the electrolyte, i.e., the resistance of the interface of anode and its surrounding soil (see clause no. 5.4.a)

The resistance of soil resulting IR drop to anode to a certain point of earth (tank bottom plate), as an interfering function of close distribution of anode instead of remote earth criterion. (effects of Vx described in clause no. 5.4.b)

Structure to soil resistance as a function of cathode surface area and resistivity of foundation infill material

Linear/internal resistance of structure as a result of tank material conductance and section area (which is deeply small and negligible due to the mechanical nature of Tanks)

The internal resistance of the anode groundbed (i.e., Tint described in clause no.5.4.e),

Internal resistance of cabling resulting IR drop on various segment of system conductors

Reverse Electromagnetic Force resulting 2 volts of deduction on driven Voltage of CP system as a result of voltage repercussion.

Page 12 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

An equivalent electrical circuit may be outlined and the overall resistance of the circuit shall be considered as follows.

E = max impressed current driven voltage Rc+= resistance of positive cable RAint= Total internal resistance of anode grid and feeders RA =anode to soil resistance RE =structure resistance to soil RSt = linear resistance of structure Rc = resistance of negative cable IP= cathodic protection current demand in ampere Vx= IR Drop to soil for close anode installation (Eq.4.2) Vs = 2 volts for volt repercussion (back EMF)

Hence, the output driven voltage of ICCP system shall be calculated from equivalent circuit resistance through the following equation.

sxPcStEAAcVVIRRRRRRE

int

(Eq.4.1)

5.4.a) Transition Resistance of Anode Groundbed 

In order to include the effects of IR drop of anode groundbed to earth, the transient resistance between anode grids to the soil shall be calculated, as the equivalent resistance ( AR )shown in equation no. 4.2.

SUMRA

1

(Eq.4.2)

which is the reverse of the total conductance of parallel running of the segments of ribbon anodes in soil as follows;

n

n

i i RRRRSUM

1111)(

211

(Eq.4.3)

Hence the resistance of individual segment of ribbon anodes (from one to n) shall be calculated individually using equation no. 4.4

d t

l 

l R i

i

i

2

2ln

(Eq.4.4)

in which, : soil resistivity(for the soil infill surrounding the anode grid)

il : length of "i" th. ribbon anode (from table 3)

d : equivalent diameter of MMO ribbon anode (4.44mm for 6.35mm×0.635mm ribbons) t : depth of the MMO ribbon anode with respect to tank bottom plate (i.e., 0.40m from Sec.6.4)

 

Figure 5- Equivalent Electrical Circuit

Page 13 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

5.4.b) IR Drop to Earth  

Due to the close installation of anode grid to the structure, current density (current per unit of cross-sectional area of the earth) flowing away from the anode ground bed is high at close area and exponentially decreases with increase of distance. Where the current density is highest, the greatest point-to-point potential shift can be observed in the earth. The net result of this effect is that most of the IR drop in the earth for a single anode normally is encountered within the first few centimeters.

In order to calculate the maximum IR drop of the groundbed to earth, arisen from interfering anode current flowing to the cathode (Bottom Plate) from electrolyte (tank infill material), the resistance between anode grid to the soil immediately beneath the steel bottom plate shall be considered as a deteriorative effect on reducing driven voltage of ICCP system.

Hence the IR drop to earth shall be calculated for unit length of ribbon anode (Say one meter length) to the closest distance to the tank bottom plate (Say horizontal zero distance from anode, but at a vertical distance between anode and tank bottom plate), under maximum applicable current density per unit length of anode, by using the following formula from Handbook of Cathodic Corrosion Protection by W. von Baeckmann).

22

22ln

2

2

22

222

lltr

lltr

l

IVx

(Eq.4.5)

where: : soil resistivity(for the soil infill surrounding the anode grid)

I : current flows from unit length of ribbon anode (i.e., 42mA lengthwise; actual current shall be less than this due to the over design for total length of ribbon anodes)

l : anode length (to be One meter for unit length) t : Vertical distance between tank bottom and anode (Typically 0.40meter) r : horizontal distance of the anode to a certain point of the tank bottom surface (here to be

taken "0" because the highest amount of the IR drop shall be at r=0)

5.4.c) Structure to soil resistance 

The structure resistance to soil shall be calculated considering the based on both of the bare and coated resistance of structure to soil. Structure specific polarization resistance for defected and bare area and structure specific coating resistance represented per meter square in Ohm ( 2.m ) shall be divided to the appropriate surface area with reference to the coating efficiency of the structure. The bare steel to soil have a specific polarization resistance value of 10 2.m , while the specific coating resistance is varies between 2000 to 5000 2.m .

The quality of the coating changes the specific coating resistance has a direct effect to the protective current density of the structure as well. The increase of the specific coating resistance will extremely decrease the protective current density of structure. However in order to calculate the highest value as the extreme edge for protective current demand, the coating efficiency has been considered to be 50% only resulting 50% of the tank bottom plate to be considered bare and in direct contact to the soil.

Page 14 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

Hence the resistance of structure to soil shall be calculated individually for bare and coated tank bottom plate surface area s follows;

EBREC

E

R

1 

R

1R

1

(Eq.4.6)

where: ER :total structure to soil resistance

ECR :bare structure to soil resistance

EBR : coated structure to soil resistance

that shall be calculated from the following equations;

)(CES

rR

P

UEB

(Eq.4.6)

)( CES

rR

P

PEC

1

(Eq.4.7)

where:

Ur : specific coating resistance

Pr :specific polarization resistance

PS : Structure surface area in contact to soil (as per Eq.1.4)

CE : Structure coating efficiency 5.4.d) Linear/internal  resistance of structure 

And because of huge surface area of the metal structure the linear resistance of the tank bottom plate, SR , is negligible.

5.4.e) Internal Resistance of Anode Groundbed 

Typically the internal anode groundbed resistance of the tank’s ICCP system is consisting of Anode Grid network and Anode feeder cable. Thereby the internal resistance of Anode grounded shall be calculated within the consideration of internal resistance of both of above items in series.

Either the internal resistance of anode feeder(s) or the internal resistance for anode grid itself will result IR Drop that shall be taken on account when calculating and sizing ICCP system of each tank.

In order to reduce the internal resistance of anode groundbed, enough quantities of individual anode feeder cables shall be connected to the anode grid and the tails shall be run through trenches into the either Anode Junction Box (AJB) or Anode/Cathode Bonding Box (ACBB) which is located at certain a distance from the tank wall.

Figure 6- Simulated Circuit of Groundbed Internal Resistance

Page 15 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

Increasing the quantities of anode feeder cable with an average of resistance on each (RFC) to be connected to the anode grid via several feeding points (F) will divide the current flow into the groundbed, resulting the portions of the anode grid to act as parallel circuits and facing lower internal resistance (Rint) as desired.

Hence, the total resistance anode groundbed (i.e. the resistance between anode junction box and the chosen point "C" underneath the tank within worst case condition) shall be calculated to be adjusted to reach to the proper internal resistance as below;

F

intFCAint N

RRR

(Eq.4.8)

where:

intAR : total internal resistance of anode groundbed

FCR :average internal resistance of each anode feeder cable

intR : Maximum internal resistance of each portion of anode grid

FN : numbers of anode groundbed feeding networks (Shall be adjusted in order to reach to the required internal resistance as desired)

that shall be calculated as an average of for all segment of feeder from feeder 1 to N as follows;

n

ic

icFC a

lAvgR

1

(Eq.4.9)

where:

c : specific resistivity of Copper conductor

il :length of individual anode feeder cable

ca : conductor section area

As far as total surface of the structure subjected to be cathodically protected “SP“, the efficient domain for each

feeding point “Fi“ will be F

P

N

Sin sq.m, or a circle as

depicted in Figure 7 will be simulated; in which “RAn“ is the linear internal resistance of each segment of MMO ribbon anodes laying between the distance of Ti conductor bars and “RTi” is the linear internal resistance of each segment of Titanium conductor bars laying between the distance of Ribbon anodes.

Then, “internal resistance” of each segment will be calculated accordingly.

r

rtAna

lR

(Eq.4.10)

t

ttTia

lR

(Eq.4.11)

Figure 7- Anode Grid Simulation to a Resistance Network

Page 16 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

where: t : specific resistivity of Titanium conductor

ra : ribbon anode section area

ta : Ti conductor bar anode section area

rl :distance of ribbon anodes to each

tl :distance of conductor bars to each (to be adjusted in order to equalize RTi to RAn)

In order to distribute the protective current within an equi-potentially designed anode grid consist of the cells from segments of Ribbon anodes and Ti conductor bars the linear internal resistance of both shall be equalized to each at a value of “Ri”.

There by the distance of conductor bar to each shall be adjusted in order to meet the following criteria;

AnTii RRR

(Eq.4.12)

Hence, the distance of conductor bar (lt) is used to address the total length of Ti Conductor bar required to be used on anode grid.

Additionally if an arbitrary point of “C“ is chosen under the tank bottom plate, the current flowing from to the anode grid and through the soil to the structure, will pass a path like what is depicted in figure 7.

Then, the internal resistance of point “Fi“ to “C“ will be calculated using “Delta-Star Conversion Law” of the circuits resulting to the value of “Rint“ on an extreme edge:

iint RR 3

5.4.e) Internal Resistance of Main Cables 

As indicated above various parameters are involved to calculate different resistance in equivalent circuit of ICCP system such as;

FN : numbers of anode groundbed feeding networks

tl :distance of conductor bars to each

Hence the adjustments of those values are resulting to minimize the IR drop to its optimum value that shall be in-line to the driven voltage of ICCP system.

Additionally the internal resistances of main positive “ cR “ and main negative “ c

R “ cables have the

same roll that the sizing of the cable shall be calculate in a way that the calculated equivalent circuit resistance eqR to meet below criteria;

P

sxeq

I

VVER

In which;

cStEAAceq RRRRRRR

int

Page 17 of 17

Project: Prj. Identification:

Document Title: CALCULATION METHODOLOGY FOR AT-GRADE STORAGE TANK EXTERNALS

Document No.: EDO-CAL-COE-COR-INT-XXX-014-190-218-REV-A- CP CALCULATION_METHODOLOGY FOR TANK BOTTOM EXTERNALS

Revision: A Date: 05.02.2014

Hence the value of cR and c

R is calculated as properly sized as per below equation;

c

cc

a

lR

c

(Eq.4.13)

where:

cR : resistance of the cable under study (applicable for both of cR and c

R )

c : specific resistivity of Copper conductor

il :length of cable under study

ca : conductor section area

5.5. Sizing of the Power Supply 

Selection of the T/R unit will be based on the following parameters.

ambient conditions and environmental data,

execution area and presentation of hazardous gases and dusts,

AC power to be used and required voltage and current ratings,

maximum real-time controllability of protection parameters,

installation and maintenance requirements.

Considering the calculated maximum current demand for the protection of the tank to be “ PI “and an additional 50% margin to shall be added in order to address the output current capacity of power supply unit.

The output voltage of the power supply unit shall be in rating to meet all the requirements describe on section 5.4 of this document but limited to the maximum value of the 50 Volts.

Within the consideration that AC power will be easily provided in facility, Transformer-Rectifier units (TRU) or individual channel Transformer-Rectifier units (TRC) shall be used to generate required DC current as the power supply unit. TRU/TRC shall be installed outside of classified area preferably insides of substations.

Where the installation within the substations is not feasible, the TRU may be subject of outdoor installation under company approval.