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Cathodic Protection Co. Limited Project Ref: 12/P16227

CATHODIC PROTECTION

TETCO GADARIF STRATEGIC DEPOT PROJECT

EXTERNAL TANK BASE SYSTEM AND INTERNAL TANK SYSTEMS

DETAILED DESIGN CALCULATIONS Document Number: 16227-CP-03-DD

0

28/03/2012

Issued for client approval

DM

Rev.

Date

Description

Prepared

No.

SF

DM

Checked Approved

ORIGINATOR

CLIENT

Venture Way, Grantham, Lincs, NG31 7XS, UK. Tel: +44 (0) 1476 590666 Fax: +44 (0) 1476 570605 E-mail: [email protected] Website: www.cathodic.co.uk Registered Office: Minalloy House, Regent Street, Sheffield S1 3NJ, UK Vat No. 116 8408 71 Registered in England No. 478098

TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

TABLE OF CONTENTS 1 1.1 1.2

1.2.1 2 2.1 2.2 2.3

2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 3 3.1 3.2 4 4.1 4.2 4.3 4.4 4.5

4.5.1 4.5.2 4.5.3 4.5.1 4.6 5 6 6.1 6.2 6.3

6.3.1 6.3.2 6.3.3 6.3.4 6.3.5 7 7.1 7.2 8 8.1 8.2 8.3 8.4 9

Document Title Document No CPCL Project No

INTRODUCTION........................................................................................................................ 3 GENERAL .................................................................................................................................. 3 REFERENCE DOCUMENTS..................................................................................................... 3

INTERNATIONAL STANDARDS ........................................................................ 3 CORROSION CONTROL METHODS – EXTERNAL CP ......................................................... 4 GENERAL .................................................................................................................................. 4 COATING SYSTEMS ................................................................................................................ 4 CATHODIC PROTECTION ....................................................................................................... 4

DESIGN LIFE ..................................................................................................... 4 COATING BREAKDOWN AND DESIGN CURRENT DENSITIES ...................... 4 CURRENT DRAIN .............................................................................................. 5 CURRENT DEMAND ......................................................................................... 5 CP PROTECTION POTENTIAL RANGES ......................................................... 5 DESIGN CONSIDERATIONS – EXTERNAL CP ...................................................................... 6 DESIGN PARAMETERS ........................................................................................................... 6 CATHODIC PROTECTION PHILOSOPHY ............................................................................... 6 DESIGN CALCULATIONS – EXTERNAL CP .......................................................................... 8 CURRENT REQUIREMENTS ................................................................................................... 8 MMO ANODE RIBBON CALCULATIONS ............................................................................... 9 RIBBON ANODE BED RESISTANCE .................................................................................... 11 Ti CONDUCTOR BAR CALCULATIONS ............................................................................... 13 EQUIVALENT CIRCUIT RESISTANCE .................................................................................. 14

GROUNDBED VOLT DROP CALCULATION ................................................... 15 MMO RIBBON VOLT DROP ............................................................................ 15 CONDUCTOR BAR VOLT DROP .................................................................... 16 CABLE VOLT DROP ........................................................................................ 17 CALCULATION SUMMARY ................................................................................................... 18 SUMMARY OF INSTALLATION – EXTERNAL CP ............................................................... 19 CORROSION CONTROL METHODS – INTERNAL CP......................................................... 20 GENERAL ................................................................................................................................ 20 COATING SYSTEMS .............................................................................................................. 20 CATHODIC PROTECTION ..................................................................................................... 20

DESIGN LIFE ................................................................................................... 20 COATING BREAKDOWN AND DESIGN CURRENT DENSITIES .................... 20 CURRENT DRAIN ............................................................................................ 21 CURRENT DEMAND ....................................................................................... 21 CP PROTECTION POTENTIAL RANGES ....................................................... 21 DESIGN CONSIDERATIONS – INTERNAL CP ..................................................................... 22 DESIGN PARAMETERS ......................................................................................................... 22 CATHODIC PROTECTION PHILOSOPHY ............................................................................. 22 DESIGN CALCULATIONS – INTERNAL CP ......................................................................... 23 CALCULATION SUMMARY ................................................................................................... 23 CURRENT REQUIREMENTS ................................................................................................. 24 NUMBER OF ANODES BY CURRENT OUTPUT .................................................................. 25 NUMBER OF ANODES BY WEIGHT ..................................................................................... 26 SUMMARY OF INSTALLATION – INTERNAL CP................................................................. 27

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

1

INTRODUCTION 1.1

GENERAL This document is the design philosophy and detailed calculations that shall be adopted to provide the permanent cathodic protection system installed to protect the new tanks at the Gadarif Strategic Depot project. The requirements in this document apply to permanent corrosion protection by impressed current cathodic protection to the external surfaces of the tank base and by sacrificial anode current cathodic protection to the internal surfaces of the tank.

1.2

REFERENCE DOCUMENTS This section lists the codes, standards and project documents / drawings, which are applicable to the detailed design.

1.2.1 INTERNATIONAL STANDARDS All CP equipment shall be designed, manufactured, tested and supplied in accordance with applicable codes of practice, British Standards, NACE and other applicable standards listed below:

Document Title Document No CPCL Project No

Document Number

Title

BS 7361:Part1

Cathodic Protection, Code of Practice for Land and Marine Applications

NACE-RP-0193:1993

External Cathodic Protection of On-Grade Metallic Storage Tank Bottom

EDS-508 Revision A

Cathodic Protection Specifications

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

2

CORROSION CONTROL METHODS – EXTERNAL CP

2.1 GENERAL The corrosion protection system selected shall be based on a high integrity coating system in combination with a cathodic protection system. 2.2 COATING SYSTEMS The tank base plates shall be coated in line with the project specification. 2.3 CATHODIC PROTECTION The permanent CP system for the tank will be an impressed current cathodic protection (ICCP) system. The design criteria for the cathodic protection system are outlined in the sections below:

2.3.1

DESIGN LIFE The design life shall be 25 years for permanent CP system.

2.3.2

COATING BREAKDOWN AND DESIGN CURRENT DENSITIES The current density below is taken from CPCL experience and international specifications:

Structure Surface Tank Bottom

2

Minimum Current Density (mA/m ) 20

For the protection of structure with elevated operating temperatures the minimum design current densities given above shall be increased by 25% per 10 °C rise in temperature above o 30 °C. All tanks will be at an operating temperature at 48 C therefore, a design current 2 density of 31.25 mA/m is considered suitable. For the purposes of the design the coating breakdown has been assumed to be a maximum of 50 %: Structure Surface Tank Bottom

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

Coating Breakdown (%) 50

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

2.3.3

CURRENT DRAIN The tank does not need to be electrically isolated from any foreign structures using isolation joints / flanges and polarisation cells as this is a close anode system. Where possible the tank should be electrically isolated from earthing / grounding systems and any re-bar used in construction.

2.3.4

CURRENT DEMAND The current demand for each tank is calculated based on the surface area and the applicable final current density for steel as given in section 2.3.2.

2.3.5

CP PROTECTION POTENTIAL RANGES The effectiveness of the cathodic protection system should be determined by potential shift. The following “instant off” or IR free potentials should apply in the case of all tank bases. The protection criteria for items in contact with soil are in line with international specifications and in summary: •

Steel in Soil -0.850 to -1.200 Volts with respect to a Cu/CuSO4 reference electrode.

Document Title Document No CPCL Project No

•

100 mV polarisation shift.

•

All above potentials are IR free or “OFF” potentials.

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

3

DESIGN CONSIDERATIONS – EXTERNAL CP

3.1 DESIGN PARAMETERS

Structure to be protected

:

Two Gas Oil Tanks Two Gasoline Tanks One Fire Water Tank

Diameter

:

25 m 17.5 m 17.5 m

Coating

:

50 % Coating Breakdown

Design Life

:

25 Years – Permanent CP

Design Current Density

:

31.25 mA/m²

CP Protection Criteria, ECU

:

-0.85 V (IR Free)

Soil Resistivity

:

100 Ohm.m

3.2 CATHODIC PROTECTION PHILOSOPHY The cathodic protection system for all external surfaces will be based on an impressed current cathodic protection (ICCP) system and the design life for the permanent impressed current system shall be 25 years. All cable will be XLPE/PVC where running underground. Structure to cable connections will be by bolted connections. All junction boxes will be GRP and have a minimum of IP67 protection and be ATEX certified for use in hazardous areas. The permanent cathodic protection system for the tank base will be based by a “close” anode grid arrangement with a ribbon type anode. Temporary protection is not required as the permanent system can be energised upon completion of the tank.

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

The tank will be electrically continuous from the remainder of any piping and earthing but as this is “close” anode system and there will be a containment layer – hence will not be affected by any current drains. DC Power supply shall be transformer rectifiers to CPCL specifications. The current required is less than 5 A for the 17.5 m tanks and 10 A for the 25 m tanks and it is recommended that a 5 A or 10 A CP station is installed for each tank. Due to the high surface soil resistivity anticipated it is recommended that a grid type anode is installed at a minimum depth of 300 mm and maximum depth of 550 mm below the tank base in a mesh arrangement with MMO coated ribbon running in one direction and Titanium conductor bar in the other. The maximum current output of the anode is based upon a soil resistivity of up to 100 Ohm.m which would be as anticipated in this method of installation and backfilling. Impressed current anodes shall consist of a Mixed Metal Oxide (MMO) / Titanium ribbon anode with Titanium conductor bar to carry the current and connected via cable and a junction box to the transformer rectifier. The MMO ribbon is to be spot welded to the conductor bar at each intersection. Various power feed connectors are then connected to each of the conductor bars and to a cable which exits the tank to the junction box. The negative cable will be connected to tank via a junction box from the transformer rectifier unit o

at two points 180 from each other. Permanent reference electrodes and slotted monitoring tube shall be installed under the tank to allow effective monitoring.

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4

DESIGN CALCULATIONS – EXTERNAL CP A grid of MMO / Ti ribbon and Ti conductor bar is the selected design for the installation of an anode below the bottom of the aboveground storage tank. The design of such a grid will be done by taking a simplified approach, which is based on the diameter of the tank, depth from the location of the anode bed, the ribbon anode width, and the cathodic protection current requirement for the given environment. The equations derived enabled the determination of the length of anodes and the anode bed resistance.

4.1 CURRENT REQUIREMENTS The current required for the external tank bases to be protected is shown below. The external surface area of the structure is calculated using the following formula:

SA = Π.r 2 Where for Gas Oil in first column then Gasoline and Firewater in second column:

Π

=

Pi

r

=

Radius

SA

=

Surface Area (m²)

3.142

3.142

12.5

8.75

490.87

240.53

m m²

The total current required can then be calculated using the following:

Itot = (SA.J .Fc ) Where,

Document Title Document No CPCL Project No

SA

=

Surface Area

J

=

Current Density

Fc

=

Coating Breakdown Factor

Itot

=

Total Current

Detailed Design Calculations 16227-CP-03-DD 12/P16227

490.87

240.53

31.25

31.25

50

50

%

7.67

3.76

A

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.2 MMO ANODE RIBBON CALCULATIONS The minimal total ribbon length, L, is determined by the current rating for the size and type of anode material selected:

Lmin =

I tot Ia

Where, Itot

=

Total Current

Ia

=

Anode Current Output (A/linear m)

Lmin

=

Minimum Ribbon Length

1

7.67

3.76

A

0.019

0.019

A

410.0

201.0

m

The space between anodes should be adjusted to achieve uniform current distribution under the tanks. The MMO ribbon spacing is calculated by:

S mmo =

Ia J1

Where, Ia

=

Anode Current Output (A/linear m)

12

12

mA

J1

=

Applied Current Density

10

10

mA/m²

S1

=

MMO Ribbon Spacing

1.20

1.20

m

MMO spacing restricted to 1.2 m for correct spacing.

1

2

The ribbon anode output is considered to be a maximum of 3 A / m then for standard ribbon this is

equivalent to 0.035 A (35 mA) / linear m for a lifetime of 25 years and therefore any value lower is acceptable. Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

The lengths of the MMO ribbon anodes are calculated below using the following formula: Initial Spacing = 0.5 x (MMO ribbon spacing m) Subsequent Spacing = initial spacing + (MMO ribbon spacing m)

L1 = 4.S1 .(d − S1 ) Where, S1

=

0.5 x MMO Ribbon Spacing

0.6

0.6

m

D

=

Diameter of Tank

25

17.5

m

L1

=

MMO Ribbon Length

7.65

6.39

m

The length of cord in each row is calculated from the above equation.

Row Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Total

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

Spacing (m) 0.60 1.79 2.98 4.17 5.36 6.55 7.74 8.93 10.12 11.31 12.50 13.69 14.88 16.07 17.26 18.45 19.64 20.83 22.02 23.21 24.40

Length (m) 7.62 12.88 16.19 18.63 20.52 21.98 23.11 23.96 24.54 24.89 25.00 24.89 24.54 23.96 23.11 21.98 20.52 18.63 16.19 12.88 7.62 413.65

Spacing (m) 0.58 1.75 2.92 4.08 5.25 6.42 7.58 8.75 9.92 11.08 12.25 13.42 14.58 15.75 16.92

Length (m) 6.28 10.50 13.04 14.80 16.04 16.87 17.34 17.50 17.34 16.87 16.04 14.80 13.04 10.50 6.28

207.26

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.3 RIBBON ANODE BED RESISTANCE The total resistance between the tank bottom and the bed of grid ribbon anodes without taking into account the mutual interference between the anodes is:

ρ 4 L2 R= (ln − 2) 2πL hw Where

ρ

=

Soil Resistivity

100

100

Ohm.m

L

=

Length of Anode

7.62

6.28

m

h

=

Distance between Anode and Tank

0.5

0.5

m

w

=

Ribbon Anode Equivalent Diameter

0.005

0.005

m

R

=

Resistance

20.78

24.23

Ohm

The resistance of each cord is calculated from the above equation and the total resistance calculated by summing the resistance of each cord. Thus the total anode resistance is calculated:

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

Row

Spacing

Resistance

Spacing

Resistance

Number

(m)

(ohm)

(m)

(ohm)

1

0.60

20.78

0.58

24.23

2

1.79

13.59

1.75

16.05

3

2.98

11.26

2.92

13.45

4

4.17

10.03

4.08

12.13

5

5.36

9.26

5.25

11.35

6

6.55

8.74

6.42

10.89

7

7.74

8.38

7.58

10.64

8

8.93

8.13

8.75

10.56

9

10.12

7.97

9.92

10.64

10

11.31

7.88

11.08

10.89

11

12.50

7.85

12.25

11.35

12

13.69

7.88

13.42

12.13

13

14.88

7.97

14.58

13.45

14

16.07

8.13

15.75

16.05

15

17.26

8.38

16.92

24.23

16

18.45

8.74

17

19.64

9.26

18

20.83

10.03

19

22.02

11.26

20

23.21

13.59

21

24.40

20.78

Total (Ohm)

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

0.458

0.860

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.4 Ti CONDUCTOR BAR CALCULATIONS The total length of titanium conductor bar is based upon an assumption that conductor bar separation should not exceed 4.0 m. The lengths of the individual Ti Conductor bar chords are calculated as below using the following formula: Initial Spacing = 0.5 x (Conductor bar spacing m) Subsequent Spacing = initial spacing + (Conductor bar spacing m)

L2 = 4.S 2 .(d − S 2 ) Where, S2

=

0.5 x Conductor Bar Spacing

d

=

Diameter of Tank

L2

=

Initial Conductor Bar Length

1.75

1.75

m

25

17.5

m

12.88

10.5

m

The length of cord in each row is calculated from the above equation. Row Number

Spacing (m)

Length (m)

Spacing (m)

Length (m)

1

1.79

12.88

1.75

10.5

2

5.36

20.52

5.25

16.04

3

8.93

23.96

8.75

17.5

4

12.50

25

12.25

16.04

5

16.07

23.96

15.75

10.5

6

19.64

20.52

7

23.21

12.88

Total (m)

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

86.6

70.6

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.5 EQUIVALENT CIRCUIT RESISTANCE This section proves through calculation that there is sufficient driving voltage in the CP circuit to enable the system to operate at its design current after the groundbed voltage, cable volt drops and other losses have been taken into consideration. The total system volt drop is the sum of the following components: •

Groundbed volt drop

•

MMO Ribbon volt drop

•

Conductor bar volt drop

•

Cable volt drop

•

Back EMF and other losses

For the CP to be effective this total must be equal to or less than the voltage of the Transformer Rectifier. This can be expressed in the following formula:

V (vd1)+V (vd 2)+V (vd 3)+V (vd 4)+Vlosses ≤ VTR

Where, V(vd1)

=

Groundbed Volt Drop

V(vd2)

=

V(vd3)

4.6

4.3

V

MMO Ribbon Volt Drop

0.005

0.005

V

=

Conductor Bar Volt Drop

0.192

0.092

V

V(vd4)

=

Cable Volt Drop

5.45

2.73

V

Vlosses

=

Back EMF and Structure Losses

4.0

4.0

V

VTotal

=

Total Voltage

14.23

11.13

V

VTR

=

Transformer Rectifier Voltage

24

24

V

See below for calculations of each component

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.5.1

GROUNDBED VOLT DROP CALCULATION

The groundbed volt drop is a combination of the following components: •

Groundbed to earth resistance

•

Groundbed current

This can be expressed in the following formula:

V ( vd1) = RGB. Itot Where,

4.5.2

RGB

=

Groundbed Resistance to Earth

Itot

=

V(vd1)

=

0.458

0.860

Ohm

Total Current

10

5

A

Groundbed Volt Drop

4.6

4.3

V

MMO RIBBON VOLT DROP

The MMO ribbon volt drop is calculated using the following formula:

V ( vd 2 ) = I .R.L Where, I

=

MMO Ribbon Load / m

R

=

Resistance of MMO Ribbon

L

=

Length of MMO Ribbon

V(vd2)

=

MMO Ribbon Volt Drop

2

0.021

0.02

A

0.138

0.138

1.79

1.75

m

0.005

0.005

V

Ohm/m

Where the load is the current required for half the distance between two conductor bars.

I = I tot / L1 /(0.5.S 2 )

2

Where, Itot

=

Total Current

L1

=

Total Length of MMO Ribbon

S2

=

Spacing of Conductor Bar

I

=

MMO Ribbon Load / m

10

5

A

413.65

207.26

m

3.5

3.5

m

0.014

0.014

A

Data from MMO anode manufacture

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.5.3

CONDUCTOR BAR VOLT DROP

The Conductor Bar volt drop is calculated using the following formula:

V ( vd 3) = I .R.L Where, I

=

Load

R

=

Resistance of Conductor Bar

L

=

V(vd2)

=

0.223

0.152

A

0.069

0.069

Ohm/m

Length of Conductor Bar

12.50

9.75

m

MMO Ribbon Volt Drop

0.192

0.092

V

3

Where the load is the current required for half the length of the longest conductor bar (worst case).

I = I tot / L2 /(0.5.L3 ) Where,

3

Itot

=

Total Current

L2

=

Total Length of conductor Bar

L3

=

Length of Longest conductor Bar

I

=

Conductor Bar Load / m

10

5

A

139.7

70.6

m

25

17.5

m

0.006

0.008

A

Data from conductor bar manufacture

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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4.5.1

CABLE VOLT DROP

Cable volt drops are calculated using the following formula:

V(vd 4) = V1 + V2 + V3 + V4 Where, V1

=

Main Positive Cable Volt Drop

1.05

V

V2

=

Main Negative Cable Volt Drop

1.05

V

V3

=

Powerfeed Cable Volt Drop

2.3

V

V4

=

Negative Cable Volt Drop

1.05

V

V(vd4)

=

Cable Volt Drop

5.45

V

Individual cable volt drops are calculated using the following formula:

Vvd = (Rcab / 1000).Lcab .I cab Where, RCab

=

Resistance of Cable

Lcab

=

Icab Vvd

4

0.524

0.524

Ohm/km

Length of Cable

200

200

m

=

Current in Cable

10

5

A

=

Cable Volt Drop

1.1

0.524

V

The volt drop of each cable is calculated from the above equation.

4

RCab

Lcab

Icab

Vvd

V1

0.524

100

10

1.05

V2

0.524

100

10

1.05

V3

1.15

100

10

2.3

V4

0.524

100

10

1.05

From cable manufacturers

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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4.6 CALCULATION SUMMARY Parameter

Document Title Document No CPCL Project No

25 m

17.5 m

Unit

SA

=

Surface Area (m²)

490.87

240.53

m²

Itot

=

Total Current

10

5

A

Lmin

=

Minimum Ribbon Length

414

208

m

L1

=

Actual Ribbon Length

414

208

m

S1

=

MMO Ribbon Spacing

1.2

1.2

m

L2

=

Conductor Bar Length

139.7

70.6

m

S2

=

Conductor Bar Spacing

3.6

3.5

m

VTotal

=

Total Voltage Drop

15

12

V

VTR

=

Transformer Rectifier Voltage

24

24

V

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

5

SUMMARY OF INSTALLATION – EXTERNAL CP In summary the CP system and groundbed will be sized as follows:

Item

25 m

17.5 m

2

3

Transformer Rectifier Voltage

24 V

24 V

Transformer Rectifier Current

10 A

5A

414 m

208 m

7

5

Permanent CP Station

MMO Ribbon Length Powerfeed Number Powerfeed Cable Size Powerfeed Cable Length Feeder Cable Size Total Feeder Length (including 20 % spare for routing) Number of Reference Electrodes

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

16 mm

2

150 m 35 mm

2

16 mm

2

100 m 35 mm

200

200

5

5

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6

CORROSION CONTROL METHODS – INTERNAL CP

6.1 GENERAL The corrosion protection system selected shall be based on a high integrity coating system in combination with a cathodic protection system. 6.2 COATING SYSTEMS The tank shall be coated internally in line with project specifications. 6.3 CATHODIC PROTECTION The permanent CP system for the tanks will be a sacrificial anode cathodic protection (SACP) system. The design criteria for the cathodic protection system are outlined in the sections below:

6.3.1

DESIGN LIFE The design life shall be 25 years for sacrificial anodes.

6.3.2

COATING BREAKDOWN AND DESIGN CURRENT DENSITIES The current density below is taken from CPCL experience and international specifications: Structure Surface

2

Minimum Current Density (mA/m )

Tank Internals

100

For the protection of structure with elevated operating temperatures the minimum design current densities given above shall be increased by 25% per 10 °C rise in temperature above 30 °C. o

All tanks will be at an operating temperature at 48 C therefore, a design current density of 2 156.25 mA/m is considered suitable. The current density below is taken from Project Document:

Document Title Document No CPCL Project No

Structure Surface

Coating Breakdown (%)

Gasoline / Gas Oil

5

Firewater

10

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

6.3.3

CURRENT DRAIN The tanks do not need to be electrically isolated from any foreign structures using isolation joints / flanges and polarisation cells as this is an internal system. Where possible the tank should be electrically isolated from earthing / grounding systems and any re-bar used in construction.

6.3.4

CURRENT DEMAND The current demand for each tank is calculated based on the surface area and the applicable final current density for steel as given in section 6.3.2.

6.3.5

CP PROTECTION POTENTIAL RANGES The effectiveness of the cathodic protection systems should be determined by potential shift. The following “instant off” or IR free potentials should apply in the case of all tank bases. The protection criteria for items in contact with soil are in line with international specifications and in summary: •

Steel in Water -0.800 to -1.050 Volts with respect to a Ag/AgCl reference electrode.

Document Title Document No CPCL Project No

•

100 mV polarisation shift.

•

All above potentials are IR free or “OFF” potentials.

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

7

DESIGN CONSIDERATIONS – INTERNAL CP

7.1 DESIGN PARAMETERS

Structure to be protected and

:

Diameter / Height considered for CP

Gas Oil: 25 m / 1 m Gasoline: 17.5 m / 1 m Firewater: 17.5 m / 12.5 m

Coating

:

10 % Coating Breakdown Factor 5 % for Firewater only

Design Life

:

25 Years – Permanent CP

Current Density

:

156.25 mA/m² at 30 C

CP Protection Criteria, Eag

:

-0.80 V (IR Free)

Water Resistivity

:

200 Ohm.cm or less

o

7.2 CATHODIC PROTECTION PHILOSOPHY The cathodic protection system for the internal surfaces will be based on a sacrificial anode cathodic protection (SACP) system and the minimum design life for the permanent system shall be 25 years. The permanent cathodic protection system for the internal of the tanks will be based on aluminium sacrificial anodes which shall be bolted to welded support brackets installed at various locations around the base of each tank. Temporary protection is not required as the permanent system can be energised upon completion of the tank.

Document Title Document No CPCL Project No

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

8

DESIGN CALCULATIONS – INTERNAL CP The design of the system is based on the diameter and height of the tank bottom, anode size and weight, and the cathodic protection current requirement for the given environment. The equations derived enabled the determination of the number of anodes and the anode resistances.

8.1 CALCULATION SUMMARY The table below is an executive summary of the calculations in this section:

Tank

Gas Oil

Gasoline

Firewater

Unit

SA

=

Surface Area (m²)

570

296

928

m²

If

=

Current

8.90

4.62

7.25

A

Ia

=

Individual Anode Current

0.25

0.25

0.25

A

An1

=

Anode Number for Current

36

19

29

No

Ma

=

Individual Anode Mass

25.2

25.2

25.2

kg

An2

=

Anode Number for Mass

37

19

30

No

At

=

Anode Number Required

40

24

34

No

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

8.2 CURRENT REQUIREMENTS The current required for the internal tank surface to be protected is shown below: The internal surface area of the structure (GAS OIL TANK ONLY) is calculated using the following formula:

SA = π .r 2 + π .d .h Where for tank,

π

=

Pi

R

=

Radius

12.5

m

D

=

Diameter

25.0

m

H

=

Height

1.0

m

SA

=

Surface Area (m²)

3.142

569.41

m²

The current required for GAS OIL TANK can then be calculated using the following:

If = (SA.J .Fc ) Where for tank, SA

Surface Area

596.41

m²

J

Current Density

156.25

mA

Fc

Coating Breakdown Factor

If

Document Title Document No CPCL Project No

=

Final Current

Detailed Design Calculations 16227-CP-03-DD 12/P16227

10

%

8.90

A

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

8.3 NUMBER OF ANODES BY CURRENT OUTPUT The anode resistance is calculated using the following equation:

Rf =

4l f ρ (ln − 1) 2πl f rf

Where,

ρ

=

Water Resistivity

lf

=

Final Anode Length (90%)

rf

=

Rf

=

2

Ohm.m

112.5

cm

Final Effective Anode Radius (50%)

49.2

cm

Final Anode Resistance to Earth

1.19

Ohm

Then, the maximum current output per anodes is calculated using Ohm’s Law:

Ia =

V Rf

Where, V

=

Anode Driving Potential

Rf

=

Ia

=

5

0.30

V

Final Anode Resistance to Earth

1.19

Ohm

Individual Anode Current

0.25

A

The minimum number of anodes is calculated from the total current required divided by the maximum current output for each individual anode.

An1 =

If Ia

Where for tank, If

=

Final Current

8.90

A

Ia

=

Individual Anode Current

0.25

A

An1

=

Anode Number (Current)

36

No.

5

Anode Driving Potential is the potentials difference between the protected potential and the open circuit anode potential Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

8.4 NUMBER OF ANODES BY WEIGHT The total anode weight is calculated using the following equation:

Mt =

I m .t.8760 Z .UF

Where for the tank, Im

=

Current

t

=

Design Life

Z

=

Alloy Anode Amp/Hour Capacity

UF

=

Utilisation Factor

Mt

=

Total Anode Mass

8.90 25 2500

A Years A.hours/kg

85

%

917.6

kg

Finally, the minimum number of anodes is calculated from the total mass required divided by the mass of each individual anode.

An 2 =

Mt Ma

Where for tank,

Document Title Document No CPCL Project No

Mt

=

Total Anode Mass

Ma

=

An2

=

Detailed Design Calculations 16227-CP-03-DD 12/P16227

917.6

kg

Individual Anode Mass

25.2

kg

Anode Number

37

No.

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9

SUMMARY OF INSTALLATION – INTERNAL CP From the design calculations above the number of anodes required for the Permanent CP will be the greater of An1 and An2: Gas Oil Tanks:

An1

=

Anode Number (Current)

36

No.

An2

=

Anode Number (Mass)

37

No.

An2

=

Anode Number (Distribution)

40

No.

At

=

Total Anode Number

40

No.

An1

=

Anode Number (Current)

19

No.

An2

=

Anode Number (Mass)

19

No.

An2

=

Anode Number (Distribution)

24

No.

At

=

Total Anode Number

24

No.

An1

=

Anode Number (Current)

30

No.

An2

=

Anode Number (Mass)

29

No.

An2

=

Anode Number (Distribution)

34

No.

At

=

Total Anode Number

34

No.

Gasoline Tanks:

Firewater Tank:

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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View more...
CATHODIC PROTECTION

TETCO GADARIF STRATEGIC DEPOT PROJECT

EXTERNAL TANK BASE SYSTEM AND INTERNAL TANK SYSTEMS

DETAILED DESIGN CALCULATIONS Document Number: 16227-CP-03-DD

0

28/03/2012

Issued for client approval

DM

Rev.

Date

Description

Prepared

No.

SF

DM

Checked Approved

ORIGINATOR

CLIENT

Venture Way, Grantham, Lincs, NG31 7XS, UK. Tel: +44 (0) 1476 590666 Fax: +44 (0) 1476 570605 E-mail: [email protected] Website: www.cathodic.co.uk Registered Office: Minalloy House, Regent Street, Sheffield S1 3NJ, UK Vat No. 116 8408 71 Registered in England No. 478098

TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

TABLE OF CONTENTS 1 1.1 1.2

1.2.1 2 2.1 2.2 2.3

2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 3 3.1 3.2 4 4.1 4.2 4.3 4.4 4.5

4.5.1 4.5.2 4.5.3 4.5.1 4.6 5 6 6.1 6.2 6.3

6.3.1 6.3.2 6.3.3 6.3.4 6.3.5 7 7.1 7.2 8 8.1 8.2 8.3 8.4 9

Document Title Document No CPCL Project No

INTRODUCTION........................................................................................................................ 3 GENERAL .................................................................................................................................. 3 REFERENCE DOCUMENTS..................................................................................................... 3

INTERNATIONAL STANDARDS ........................................................................ 3 CORROSION CONTROL METHODS – EXTERNAL CP ......................................................... 4 GENERAL .................................................................................................................................. 4 COATING SYSTEMS ................................................................................................................ 4 CATHODIC PROTECTION ....................................................................................................... 4

DESIGN LIFE ..................................................................................................... 4 COATING BREAKDOWN AND DESIGN CURRENT DENSITIES ...................... 4 CURRENT DRAIN .............................................................................................. 5 CURRENT DEMAND ......................................................................................... 5 CP PROTECTION POTENTIAL RANGES ......................................................... 5 DESIGN CONSIDERATIONS – EXTERNAL CP ...................................................................... 6 DESIGN PARAMETERS ........................................................................................................... 6 CATHODIC PROTECTION PHILOSOPHY ............................................................................... 6 DESIGN CALCULATIONS – EXTERNAL CP .......................................................................... 8 CURRENT REQUIREMENTS ................................................................................................... 8 MMO ANODE RIBBON CALCULATIONS ............................................................................... 9 RIBBON ANODE BED RESISTANCE .................................................................................... 11 Ti CONDUCTOR BAR CALCULATIONS ............................................................................... 13 EQUIVALENT CIRCUIT RESISTANCE .................................................................................. 14

GROUNDBED VOLT DROP CALCULATION ................................................... 15 MMO RIBBON VOLT DROP ............................................................................ 15 CONDUCTOR BAR VOLT DROP .................................................................... 16 CABLE VOLT DROP ........................................................................................ 17 CALCULATION SUMMARY ................................................................................................... 18 SUMMARY OF INSTALLATION – EXTERNAL CP ............................................................... 19 CORROSION CONTROL METHODS – INTERNAL CP......................................................... 20 GENERAL ................................................................................................................................ 20 COATING SYSTEMS .............................................................................................................. 20 CATHODIC PROTECTION ..................................................................................................... 20

DESIGN LIFE ................................................................................................... 20 COATING BREAKDOWN AND DESIGN CURRENT DENSITIES .................... 20 CURRENT DRAIN ............................................................................................ 21 CURRENT DEMAND ....................................................................................... 21 CP PROTECTION POTENTIAL RANGES ....................................................... 21 DESIGN CONSIDERATIONS – INTERNAL CP ..................................................................... 22 DESIGN PARAMETERS ......................................................................................................... 22 CATHODIC PROTECTION PHILOSOPHY ............................................................................. 22 DESIGN CALCULATIONS – INTERNAL CP ......................................................................... 23 CALCULATION SUMMARY ................................................................................................... 23 CURRENT REQUIREMENTS ................................................................................................. 24 NUMBER OF ANODES BY CURRENT OUTPUT .................................................................. 25 NUMBER OF ANODES BY WEIGHT ..................................................................................... 26 SUMMARY OF INSTALLATION – INTERNAL CP................................................................. 27

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

1

INTRODUCTION 1.1

GENERAL This document is the design philosophy and detailed calculations that shall be adopted to provide the permanent cathodic protection system installed to protect the new tanks at the Gadarif Strategic Depot project. The requirements in this document apply to permanent corrosion protection by impressed current cathodic protection to the external surfaces of the tank base and by sacrificial anode current cathodic protection to the internal surfaces of the tank.

1.2

REFERENCE DOCUMENTS This section lists the codes, standards and project documents / drawings, which are applicable to the detailed design.

1.2.1 INTERNATIONAL STANDARDS All CP equipment shall be designed, manufactured, tested and supplied in accordance with applicable codes of practice, British Standards, NACE and other applicable standards listed below:

Document Title Document No CPCL Project No

Document Number

Title

BS 7361:Part1

Cathodic Protection, Code of Practice for Land and Marine Applications

NACE-RP-0193:1993

External Cathodic Protection of On-Grade Metallic Storage Tank Bottom

EDS-508 Revision A

Cathodic Protection Specifications

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

2

CORROSION CONTROL METHODS – EXTERNAL CP

2.1 GENERAL The corrosion protection system selected shall be based on a high integrity coating system in combination with a cathodic protection system. 2.2 COATING SYSTEMS The tank base plates shall be coated in line with the project specification. 2.3 CATHODIC PROTECTION The permanent CP system for the tank will be an impressed current cathodic protection (ICCP) system. The design criteria for the cathodic protection system are outlined in the sections below:

2.3.1

DESIGN LIFE The design life shall be 25 years for permanent CP system.

2.3.2

COATING BREAKDOWN AND DESIGN CURRENT DENSITIES The current density below is taken from CPCL experience and international specifications:

Structure Surface Tank Bottom

2

Minimum Current Density (mA/m ) 20

For the protection of structure with elevated operating temperatures the minimum design current densities given above shall be increased by 25% per 10 °C rise in temperature above o 30 °C. All tanks will be at an operating temperature at 48 C therefore, a design current 2 density of 31.25 mA/m is considered suitable. For the purposes of the design the coating breakdown has been assumed to be a maximum of 50 %: Structure Surface Tank Bottom

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

Coating Breakdown (%) 50

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

2.3.3

CURRENT DRAIN The tank does not need to be electrically isolated from any foreign structures using isolation joints / flanges and polarisation cells as this is a close anode system. Where possible the tank should be electrically isolated from earthing / grounding systems and any re-bar used in construction.

2.3.4

CURRENT DEMAND The current demand for each tank is calculated based on the surface area and the applicable final current density for steel as given in section 2.3.2.

2.3.5

CP PROTECTION POTENTIAL RANGES The effectiveness of the cathodic protection system should be determined by potential shift. The following “instant off” or IR free potentials should apply in the case of all tank bases. The protection criteria for items in contact with soil are in line with international specifications and in summary: •

Steel in Soil -0.850 to -1.200 Volts with respect to a Cu/CuSO4 reference electrode.

Document Title Document No CPCL Project No

•

100 mV polarisation shift.

•

All above potentials are IR free or “OFF” potentials.

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

3

DESIGN CONSIDERATIONS – EXTERNAL CP

3.1 DESIGN PARAMETERS

Structure to be protected

:

Two Gas Oil Tanks Two Gasoline Tanks One Fire Water Tank

Diameter

:

25 m 17.5 m 17.5 m

Coating

:

50 % Coating Breakdown

Design Life

:

25 Years – Permanent CP

Design Current Density

:

31.25 mA/m²

CP Protection Criteria, ECU

:

-0.85 V (IR Free)

Soil Resistivity

:

100 Ohm.m

3.2 CATHODIC PROTECTION PHILOSOPHY The cathodic protection system for all external surfaces will be based on an impressed current cathodic protection (ICCP) system and the design life for the permanent impressed current system shall be 25 years. All cable will be XLPE/PVC where running underground. Structure to cable connections will be by bolted connections. All junction boxes will be GRP and have a minimum of IP67 protection and be ATEX certified for use in hazardous areas. The permanent cathodic protection system for the tank base will be based by a “close” anode grid arrangement with a ribbon type anode. Temporary protection is not required as the permanent system can be energised upon completion of the tank.

Document Title Document No CPCL Project No

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

The tank will be electrically continuous from the remainder of any piping and earthing but as this is “close” anode system and there will be a containment layer – hence will not be affected by any current drains. DC Power supply shall be transformer rectifiers to CPCL specifications. The current required is less than 5 A for the 17.5 m tanks and 10 A for the 25 m tanks and it is recommended that a 5 A or 10 A CP station is installed for each tank. Due to the high surface soil resistivity anticipated it is recommended that a grid type anode is installed at a minimum depth of 300 mm and maximum depth of 550 mm below the tank base in a mesh arrangement with MMO coated ribbon running in one direction and Titanium conductor bar in the other. The maximum current output of the anode is based upon a soil resistivity of up to 100 Ohm.m which would be as anticipated in this method of installation and backfilling. Impressed current anodes shall consist of a Mixed Metal Oxide (MMO) / Titanium ribbon anode with Titanium conductor bar to carry the current and connected via cable and a junction box to the transformer rectifier. The MMO ribbon is to be spot welded to the conductor bar at each intersection. Various power feed connectors are then connected to each of the conductor bars and to a cable which exits the tank to the junction box. The negative cable will be connected to tank via a junction box from the transformer rectifier unit o

at two points 180 from each other. Permanent reference electrodes and slotted monitoring tube shall be installed under the tank to allow effective monitoring.

Document Title Document No CPCL Project No

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4

DESIGN CALCULATIONS – EXTERNAL CP A grid of MMO / Ti ribbon and Ti conductor bar is the selected design for the installation of an anode below the bottom of the aboveground storage tank. The design of such a grid will be done by taking a simplified approach, which is based on the diameter of the tank, depth from the location of the anode bed, the ribbon anode width, and the cathodic protection current requirement for the given environment. The equations derived enabled the determination of the length of anodes and the anode bed resistance.

4.1 CURRENT REQUIREMENTS The current required for the external tank bases to be protected is shown below. The external surface area of the structure is calculated using the following formula:

SA = Π.r 2 Where for Gas Oil in first column then Gasoline and Firewater in second column:

Π

=

Pi

r

=

Radius

SA

=

Surface Area (m²)

3.142

3.142

12.5

8.75

490.87

240.53

m m²

The total current required can then be calculated using the following:

Itot = (SA.J .Fc ) Where,

Document Title Document No CPCL Project No

SA

=

Surface Area

J

=

Current Density

Fc

=

Coating Breakdown Factor

Itot

=

Total Current

Detailed Design Calculations 16227-CP-03-DD 12/P16227

490.87

240.53

31.25

31.25

50

50

%

7.67

3.76

A

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.2 MMO ANODE RIBBON CALCULATIONS The minimal total ribbon length, L, is determined by the current rating for the size and type of anode material selected:

Lmin =

I tot Ia

Where, Itot

=

Total Current

Ia

=

Anode Current Output (A/linear m)

Lmin

=

Minimum Ribbon Length

1

7.67

3.76

A

0.019

0.019

A

410.0

201.0

m

The space between anodes should be adjusted to achieve uniform current distribution under the tanks. The MMO ribbon spacing is calculated by:

S mmo =

Ia J1

Where, Ia

=

Anode Current Output (A/linear m)

12

12

mA

J1

=

Applied Current Density

10

10

mA/m²

S1

=

MMO Ribbon Spacing

1.20

1.20

m

MMO spacing restricted to 1.2 m for correct spacing.

1

2

The ribbon anode output is considered to be a maximum of 3 A / m then for standard ribbon this is

equivalent to 0.035 A (35 mA) / linear m for a lifetime of 25 years and therefore any value lower is acceptable. Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

The lengths of the MMO ribbon anodes are calculated below using the following formula: Initial Spacing = 0.5 x (MMO ribbon spacing m) Subsequent Spacing = initial spacing + (MMO ribbon spacing m)

L1 = 4.S1 .(d − S1 ) Where, S1

=

0.5 x MMO Ribbon Spacing

0.6

0.6

m

D

=

Diameter of Tank

25

17.5

m

L1

=

MMO Ribbon Length

7.65

6.39

m

The length of cord in each row is calculated from the above equation.

Row Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Total

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

Spacing (m) 0.60 1.79 2.98 4.17 5.36 6.55 7.74 8.93 10.12 11.31 12.50 13.69 14.88 16.07 17.26 18.45 19.64 20.83 22.02 23.21 24.40

Length (m) 7.62 12.88 16.19 18.63 20.52 21.98 23.11 23.96 24.54 24.89 25.00 24.89 24.54 23.96 23.11 21.98 20.52 18.63 16.19 12.88 7.62 413.65

Spacing (m) 0.58 1.75 2.92 4.08 5.25 6.42 7.58 8.75 9.92 11.08 12.25 13.42 14.58 15.75 16.92

Length (m) 6.28 10.50 13.04 14.80 16.04 16.87 17.34 17.50 17.34 16.87 16.04 14.80 13.04 10.50 6.28

207.26

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.3 RIBBON ANODE BED RESISTANCE The total resistance between the tank bottom and the bed of grid ribbon anodes without taking into account the mutual interference between the anodes is:

ρ 4 L2 R= (ln − 2) 2πL hw Where

ρ

=

Soil Resistivity

100

100

Ohm.m

L

=

Length of Anode

7.62

6.28

m

h

=

Distance between Anode and Tank

0.5

0.5

m

w

=

Ribbon Anode Equivalent Diameter

0.005

0.005

m

R

=

Resistance

20.78

24.23

Ohm

The resistance of each cord is calculated from the above equation and the total resistance calculated by summing the resistance of each cord. Thus the total anode resistance is calculated:

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

Row

Spacing

Resistance

Spacing

Resistance

Number

(m)

(ohm)

(m)

(ohm)

1

0.60

20.78

0.58

24.23

2

1.79

13.59

1.75

16.05

3

2.98

11.26

2.92

13.45

4

4.17

10.03

4.08

12.13

5

5.36

9.26

5.25

11.35

6

6.55

8.74

6.42

10.89

7

7.74

8.38

7.58

10.64

8

8.93

8.13

8.75

10.56

9

10.12

7.97

9.92

10.64

10

11.31

7.88

11.08

10.89

11

12.50

7.85

12.25

11.35

12

13.69

7.88

13.42

12.13

13

14.88

7.97

14.58

13.45

14

16.07

8.13

15.75

16.05

15

17.26

8.38

16.92

24.23

16

18.45

8.74

17

19.64

9.26

18

20.83

10.03

19

22.02

11.26

20

23.21

13.59

21

24.40

20.78

Total (Ohm)

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

0.458

0.860

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.4 Ti CONDUCTOR BAR CALCULATIONS The total length of titanium conductor bar is based upon an assumption that conductor bar separation should not exceed 4.0 m. The lengths of the individual Ti Conductor bar chords are calculated as below using the following formula: Initial Spacing = 0.5 x (Conductor bar spacing m) Subsequent Spacing = initial spacing + (Conductor bar spacing m)

L2 = 4.S 2 .(d − S 2 ) Where, S2

=

0.5 x Conductor Bar Spacing

d

=

Diameter of Tank

L2

=

Initial Conductor Bar Length

1.75

1.75

m

25

17.5

m

12.88

10.5

m

The length of cord in each row is calculated from the above equation. Row Number

Spacing (m)

Length (m)

Spacing (m)

Length (m)

1

1.79

12.88

1.75

10.5

2

5.36

20.52

5.25

16.04

3

8.93

23.96

8.75

17.5

4

12.50

25

12.25

16.04

5

16.07

23.96

15.75

10.5

6

19.64

20.52

7

23.21

12.88

Total (m)

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

86.6

70.6

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.5 EQUIVALENT CIRCUIT RESISTANCE This section proves through calculation that there is sufficient driving voltage in the CP circuit to enable the system to operate at its design current after the groundbed voltage, cable volt drops and other losses have been taken into consideration. The total system volt drop is the sum of the following components: •

Groundbed volt drop

•

MMO Ribbon volt drop

•

Conductor bar volt drop

•

Cable volt drop

•

Back EMF and other losses

For the CP to be effective this total must be equal to or less than the voltage of the Transformer Rectifier. This can be expressed in the following formula:

V (vd1)+V (vd 2)+V (vd 3)+V (vd 4)+Vlosses ≤ VTR

Where, V(vd1)

=

Groundbed Volt Drop

V(vd2)

=

V(vd3)

4.6

4.3

V

MMO Ribbon Volt Drop

0.005

0.005

V

=

Conductor Bar Volt Drop

0.192

0.092

V

V(vd4)

=

Cable Volt Drop

5.45

2.73

V

Vlosses

=

Back EMF and Structure Losses

4.0

4.0

V

VTotal

=

Total Voltage

14.23

11.13

V

VTR

=

Transformer Rectifier Voltage

24

24

V

See below for calculations of each component

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Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.5.1

GROUNDBED VOLT DROP CALCULATION

The groundbed volt drop is a combination of the following components: •

Groundbed to earth resistance

•

Groundbed current

This can be expressed in the following formula:

V ( vd1) = RGB. Itot Where,

4.5.2

RGB

=

Groundbed Resistance to Earth

Itot

=

V(vd1)

=

0.458

0.860

Ohm

Total Current

10

5

A

Groundbed Volt Drop

4.6

4.3

V

MMO RIBBON VOLT DROP

The MMO ribbon volt drop is calculated using the following formula:

V ( vd 2 ) = I .R.L Where, I

=

MMO Ribbon Load / m

R

=

Resistance of MMO Ribbon

L

=

Length of MMO Ribbon

V(vd2)

=

MMO Ribbon Volt Drop

2

0.021

0.02

A

0.138

0.138

1.79

1.75

m

0.005

0.005

V

Ohm/m

Where the load is the current required for half the distance between two conductor bars.

I = I tot / L1 /(0.5.S 2 )

2

Where, Itot

=

Total Current

L1

=

Total Length of MMO Ribbon

S2

=

Spacing of Conductor Bar

I

=

MMO Ribbon Load / m

10

5

A

413.65

207.26

m

3.5

3.5

m

0.014

0.014

A

Data from MMO anode manufacture

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.5.3

CONDUCTOR BAR VOLT DROP

The Conductor Bar volt drop is calculated using the following formula:

V ( vd 3) = I .R.L Where, I

=

Load

R

=

Resistance of Conductor Bar

L

=

V(vd2)

=

0.223

0.152

A

0.069

0.069

Ohm/m

Length of Conductor Bar

12.50

9.75

m

MMO Ribbon Volt Drop

0.192

0.092

V

3

Where the load is the current required for half the length of the longest conductor bar (worst case).

I = I tot / L2 /(0.5.L3 ) Where,

3

Itot

=

Total Current

L2

=

Total Length of conductor Bar

L3

=

Length of Longest conductor Bar

I

=

Conductor Bar Load / m

10

5

A

139.7

70.6

m

25

17.5

m

0.006

0.008

A

Data from conductor bar manufacture

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.5.1

CABLE VOLT DROP

Cable volt drops are calculated using the following formula:

V(vd 4) = V1 + V2 + V3 + V4 Where, V1

=

Main Positive Cable Volt Drop

1.05

V

V2

=

Main Negative Cable Volt Drop

1.05

V

V3

=

Powerfeed Cable Volt Drop

2.3

V

V4

=

Negative Cable Volt Drop

1.05

V

V(vd4)

=

Cable Volt Drop

5.45

V

Individual cable volt drops are calculated using the following formula:

Vvd = (Rcab / 1000).Lcab .I cab Where, RCab

=

Resistance of Cable

Lcab

=

Icab Vvd

4

0.524

0.524

Ohm/km

Length of Cable

200

200

m

=

Current in Cable

10

5

A

=

Cable Volt Drop

1.1

0.524

V

The volt drop of each cable is calculated from the above equation.

4

RCab

Lcab

Icab

Vvd

V1

0.524

100

10

1.05

V2

0.524

100

10

1.05

V3

1.15

100

10

2.3

V4

0.524

100

10

1.05

From cable manufacturers

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

4.6 CALCULATION SUMMARY Parameter

Document Title Document No CPCL Project No

25 m

17.5 m

Unit

SA

=

Surface Area (m²)

490.87

240.53

m²

Itot

=

Total Current

10

5

A

Lmin

=

Minimum Ribbon Length

414

208

m

L1

=

Actual Ribbon Length

414

208

m

S1

=

MMO Ribbon Spacing

1.2

1.2

m

L2

=

Conductor Bar Length

139.7

70.6

m

S2

=

Conductor Bar Spacing

3.6

3.5

m

VTotal

=

Total Voltage Drop

15

12

V

VTR

=

Transformer Rectifier Voltage

24

24

V

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

5

SUMMARY OF INSTALLATION – EXTERNAL CP In summary the CP system and groundbed will be sized as follows:

Item

25 m

17.5 m

2

3

Transformer Rectifier Voltage

24 V

24 V

Transformer Rectifier Current

10 A

5A

414 m

208 m

7

5

Permanent CP Station

MMO Ribbon Length Powerfeed Number Powerfeed Cable Size Powerfeed Cable Length Feeder Cable Size Total Feeder Length (including 20 % spare for routing) Number of Reference Electrodes

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

16 mm

2

150 m 35 mm

2

16 mm

2

100 m 35 mm

200

200

5

5

Date Revision Page

2

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

6

CORROSION CONTROL METHODS – INTERNAL CP

6.1 GENERAL The corrosion protection system selected shall be based on a high integrity coating system in combination with a cathodic protection system. 6.2 COATING SYSTEMS The tank shall be coated internally in line with project specifications. 6.3 CATHODIC PROTECTION The permanent CP system for the tanks will be a sacrificial anode cathodic protection (SACP) system. The design criteria for the cathodic protection system are outlined in the sections below:

6.3.1

DESIGN LIFE The design life shall be 25 years for sacrificial anodes.

6.3.2

COATING BREAKDOWN AND DESIGN CURRENT DENSITIES The current density below is taken from CPCL experience and international specifications: Structure Surface

2

Minimum Current Density (mA/m )

Tank Internals

100

For the protection of structure with elevated operating temperatures the minimum design current densities given above shall be increased by 25% per 10 °C rise in temperature above 30 °C. o

All tanks will be at an operating temperature at 48 C therefore, a design current density of 2 156.25 mA/m is considered suitable. The current density below is taken from Project Document:

Document Title Document No CPCL Project No

Structure Surface

Coating Breakdown (%)

Gasoline / Gas Oil

5

Firewater

10

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

6.3.3

CURRENT DRAIN The tanks do not need to be electrically isolated from any foreign structures using isolation joints / flanges and polarisation cells as this is an internal system. Where possible the tank should be electrically isolated from earthing / grounding systems and any re-bar used in construction.

6.3.4

CURRENT DEMAND The current demand for each tank is calculated based on the surface area and the applicable final current density for steel as given in section 6.3.2.

6.3.5

CP PROTECTION POTENTIAL RANGES The effectiveness of the cathodic protection systems should be determined by potential shift. The following “instant off” or IR free potentials should apply in the case of all tank bases. The protection criteria for items in contact with soil are in line with international specifications and in summary: •

Steel in Water -0.800 to -1.050 Volts with respect to a Ag/AgCl reference electrode.

Document Title Document No CPCL Project No

•

100 mV polarisation shift.

•

All above potentials are IR free or “OFF” potentials.

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

7

DESIGN CONSIDERATIONS – INTERNAL CP

7.1 DESIGN PARAMETERS

Structure to be protected and

:

Diameter / Height considered for CP

Gas Oil: 25 m / 1 m Gasoline: 17.5 m / 1 m Firewater: 17.5 m / 12.5 m

Coating

:

10 % Coating Breakdown Factor 5 % for Firewater only

Design Life

:

25 Years – Permanent CP

Current Density

:

156.25 mA/m² at 30 C

CP Protection Criteria, Eag

:

-0.80 V (IR Free)

Water Resistivity

:

200 Ohm.cm or less

o

7.2 CATHODIC PROTECTION PHILOSOPHY The cathodic protection system for the internal surfaces will be based on a sacrificial anode cathodic protection (SACP) system and the minimum design life for the permanent system shall be 25 years. The permanent cathodic protection system for the internal of the tanks will be based on aluminium sacrificial anodes which shall be bolted to welded support brackets installed at various locations around the base of each tank. Temporary protection is not required as the permanent system can be energised upon completion of the tank.

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

8

DESIGN CALCULATIONS – INTERNAL CP The design of the system is based on the diameter and height of the tank bottom, anode size and weight, and the cathodic protection current requirement for the given environment. The equations derived enabled the determination of the number of anodes and the anode resistances.

8.1 CALCULATION SUMMARY The table below is an executive summary of the calculations in this section:

Tank

Gas Oil

Gasoline

Firewater

Unit

SA

=

Surface Area (m²)

570

296

928

m²

If

=

Current

8.90

4.62

7.25

A

Ia

=

Individual Anode Current

0.25

0.25

0.25

A

An1

=

Anode Number for Current

36

19

29

No

Ma

=

Individual Anode Mass

25.2

25.2

25.2

kg

An2

=

Anode Number for Mass

37

19

30

No

At

=

Anode Number Required

40

24

34

No

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

8.2 CURRENT REQUIREMENTS The current required for the internal tank surface to be protected is shown below: The internal surface area of the structure (GAS OIL TANK ONLY) is calculated using the following formula:

SA = π .r 2 + π .d .h Where for tank,

π

=

Pi

R

=

Radius

12.5

m

D

=

Diameter

25.0

m

H

=

Height

1.0

m

SA

=

Surface Area (m²)

3.142

569.41

m²

The current required for GAS OIL TANK can then be calculated using the following:

If = (SA.J .Fc ) Where for tank, SA

Surface Area

596.41

m²

J

Current Density

156.25

mA

Fc

Coating Breakdown Factor

If

Document Title Document No CPCL Project No

=

Final Current

Detailed Design Calculations 16227-CP-03-DD 12/P16227

10

%

8.90

A

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

8.3 NUMBER OF ANODES BY CURRENT OUTPUT The anode resistance is calculated using the following equation:

Rf =

4l f ρ (ln − 1) 2πl f rf

Where,

ρ

=

Water Resistivity

lf

=

Final Anode Length (90%)

rf

=

Rf

=

2

Ohm.m

112.5

cm

Final Effective Anode Radius (50%)

49.2

cm

Final Anode Resistance to Earth

1.19

Ohm

Then, the maximum current output per anodes is calculated using Ohm’s Law:

Ia =

V Rf

Where, V

=

Anode Driving Potential

Rf

=

Ia

=

5

0.30

V

Final Anode Resistance to Earth

1.19

Ohm

Individual Anode Current

0.25

A

The minimum number of anodes is calculated from the total current required divided by the maximum current output for each individual anode.

An1 =

If Ia

Where for tank, If

=

Final Current

8.90

A

Ia

=

Individual Anode Current

0.25

A

An1

=

Anode Number (Current)

36

No.

5

Anode Driving Potential is the potentials difference between the protected potential and the open circuit anode potential Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

8.4 NUMBER OF ANODES BY WEIGHT The total anode weight is calculated using the following equation:

Mt =

I m .t.8760 Z .UF

Where for the tank, Im

=

Current

t

=

Design Life

Z

=

Alloy Anode Amp/Hour Capacity

UF

=

Utilisation Factor

Mt

=

Total Anode Mass

8.90 25 2500

A Years A.hours/kg

85

%

917.6

kg

Finally, the minimum number of anodes is calculated from the total mass required divided by the mass of each individual anode.

An 2 =

Mt Ma

Where for tank,

Document Title Document No CPCL Project No

Mt

=

Total Anode Mass

Ma

=

An2

=

Detailed Design Calculations 16227-CP-03-DD 12/P16227

917.6

kg

Individual Anode Mass

25.2

kg

Anode Number

37

No.

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TETCO – Gadarif Strategic Depot Project Tank External Base and Internal Cathodic Protection Detailed Design Calculations

9

SUMMARY OF INSTALLATION – INTERNAL CP From the design calculations above the number of anodes required for the Permanent CP will be the greater of An1 and An2: Gas Oil Tanks:

An1

=

Anode Number (Current)

36

No.

An2

=

Anode Number (Mass)

37

No.

An2

=

Anode Number (Distribution)

40

No.

At

=

Total Anode Number

40

No.

An1

=

Anode Number (Current)

19

No.

An2

=

Anode Number (Mass)

19

No.

An2

=

Anode Number (Distribution)

24

No.

At

=

Total Anode Number

24

No.

An1

=

Anode Number (Current)

30

No.

An2

=

Anode Number (Mass)

29

No.

An2

=

Anode Number (Distribution)

34

No.

At

=

Total Anode Number

34

No.

Gasoline Tanks:

Firewater Tank:

Document Title Document No CPCL Project No

Detailed Design Calculations 16227-CP-03-DD 12/P16227

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