Cathodic Protection Design

GAMBA EXPORT LOADING LINE (SHELL GABON) Cathodic Protection Design Issued Date 10/12/13

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PROJECT NUMBER: 13027 DOCUMENT NUMBER :

Z.205.1

REVISION CONTROL SHEET REV.NO

DATE

SECTION NO.

SUMMARY OF CHANGES

Rev. No. A

GAMBA EXPORT LOADING LINE (SHELL GABON) Cathodic Protection Design Issued Date 10/12/13

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TABLE OF CONTENTS 1.0

INTRODUCTION .............................................................................. 4 General ......................................................................................... 4 Scope ............................................................................................ 4

2.0

UNITS AND ABBREVIATIONS ....................................................... 5 System of Units ............................................................................. 5 Abbreviations ................................................................................ 5

3.0

SUMMARY OF RESULTS & RECOMMENDATIONS ...................... 6 Results Summary .......................................................................... 6 Recommendations ........................................................................ 8

4.0

DESIGN DATA ................................................................................ 9 Pipeline Data ................................................................................. 9 Environmental Data ....................................................................... 9 Anode Data ................................................................................. 10

5.0

DESIGN METHODOLOGY ............................................................ 11 General ....................................................................................... 11 Current Demand Calculation ....................................................... 11 Anode Mass Calculation .............................................................. 13 Calculation of Number of Anodes ................................................ 13 Design Assumptions .................................................................... 15

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RESULTS ...................................................................................... 16 Anode Properties......................................................................... 16 Cathodic Protection Requirement ................................................ 16

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REFERENCES ............................................................................... 18

APPENDIX A – SAMPLE OF CATHODIC PROTECTION CALCULATION .... 19

Rev. No. A

GAMBA EXPORT LOADING LINE (SHELL GABON) Cathodic Protection Design Issued Date 10/12/13

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PROJECT NUMBER: 13027 DOCUMENT NUMBER :

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Rev. No. A

INTRODUCTION General Shell Gabon is operating and producing oils from numerous fields located in the western part of Gabon. Crude is treated and exported from the Gamba Terminal to tankers through a 30 inch offshore export line connected to a SBM via a PLEM and 2 x 16 inch floating hose strings (risers). The total length of the existing export line is about 10.7km from the export pump in Gamba terminal to a PLEM (1.3km located onshore and 9.4km located offshore).

Figure 1-1 Project Location

Shell Gabon intends to replace the existing export pipeline. Zeetech B.V. has been awarded by Shell to perform a concept replacement study and Front End Engineering Design (FEED) for the selected concept. Scope This report presents cathodic protection design of new dual 22 inch Gamba export line for onshore and offshore sections including its expansion spools in accordance with Company Design Engineering Practices (Ref. 1 and Ref. 2).

GAMBA EXPORT LOADING LINE (SHELL GABON) Cathodic Protection Design Issued Date 10/12/13

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Rev. No. A

UNITS AND ABBREVIATIONS System of Units In accordance with Shell DEP 00.00.20.10, the International System (SI) of units is adopted as the main system of units unless noted otherwise. Abbreviations CP

Cathodic Protection

DEP

Design Engineering Practices

DNV

Det Norsk Veritas

FEED

Front End Engineering Design

KP

Kilometer Post

ISO

International Standard Organisation

PLEM

Pipeline End Manifold

w.r.t.

With relation to

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Rev. No. A

SUMMARY OF RESULTS & RECOMMENDATIONS Results Summary The required cathodic protection of the dual 22 inch Gamba export line from onshore Gamba terminal to offshore PLEM/SBM has been designed in accordance with Ref. 1 and Ref. 2 for onshore and offshore pipelines respectively.

The onshore pipeline will be buried from KP 0.0 to KP 1.1 outside Gamba Terminal area. Based on the On-bottom Stability report (Ref. 7), for the onshore pipeline which is buried underneath the water table will have a concrete coating of 50mm. Therefore, a bracelet sacrificial anode is used in-line with the offshore pipeline cathodic protection system. The offshore pipeline will be buried from KP 1.1 to KP 1.3 and the remaining section up to KP 10.6 will be laid on the seabed.

Anode quantities have been selected to satisfy the following criteria: 

Total anode net mass shall meet the current demand over the design life of the pipeline;



Anode current output shall meet the current demand at the end of design life;



Anode spacing shall not exceed the maximum value of 300m as specified by Sec. 7.1 of Ref. 4.

Table 3-1 below show the required anode quantities from the analysis to meet the design criteria stated above.

GAMBA EXPORT LOADING LINE (SHELL GABON) Cathodic Protection Design Issued Date 10/12/13

PROJECT NUMBER: 13027 DOCUMENT NUMBER : Z.205.1

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Rev. No. A

Table 3-1 Summary of Cathodic Protection Design for a 22in Gamba Export Pipeline

Pipe Section

Onshore

Pipeline KP Range

Length (m)

Mass of Exposure Condition

one anode (kg)

Total

Required

anode net

Final

mass to

Individual

meet

Anode

current

Current

demand

Output

(kg)

(A)

Actual Individual Final Current Output (A)

Required

Required

number of

anode

anode to

spacing to

meet total

meet total

anode net

anode net

mass

mass

(ea)

(m)

Required

Required total

Recommended

number of

anode weight

Spacing of

anode based

based on

(1)

anode

(joints)

on

recommended

recommended

spacing

spacing (ea)

(kg)

0.20 – 1.07

1100

Buried

21

93

0.059

0.09

5

248

7

13

273

1.07 – 1.24

200

Buried

28

6

0.063

0.18

1

1050

24

1

28

1.24 – 10.6

9300

Un-buried

28

674

0.212

1.37

24

390

24

32

896

-

25

Un-buried

28

4

0.485

1.37

1

164

1

1

28

Offshore

Spool

Notes: 1. Anode spacing of 300m (24 joints) for offshore pipeline has been recommended in accordance with the requirements of Ref. 2. 2. Dual 22 in Gamba export pipeline will be routed parallel from Gamba terminal to the PLEM/SBM, hence the anode required above shall be applicable to both pipelines.

GAMBA EXPORT LOADING LINE (SHELL GABON) Cathodic Protection Design Issued Date 10/12/13

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Rev. No. A

Recommendations The following recommendations shall be addressed in the next design stage. 

The individual anode mass is selected based on minimum required number of anodes to meet total anode net mass. Anode dimension of the anode may be adjusted during detailed design based on the anode’s supplier data.



Onshore soil resistivity considered for the design is based on assumptions. It is recommended to update the value once the actual survey data becomes available.



To protect the un-buried onshore pipeline/piping section inside the Gamba Terminal from the offshore cathodic protection system to interfere, an isolation joint shall be introduced to separate these two systems.

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DESIGN DATA The following subsections presents design data used in the cathodic protection design. Pipeline Data The pipeline properties used in the design are presented in Table 4-1 below. This data is applicable to both pipelines. Table 4-1 – Gamba 22in Pipeline Data

Descriptions

Units

Pipeline Design Life

22in Gamba Export Lines

years

25

Onshore

m

1100

Offshore

m

9500

Spool

m

25

Outside Diameter

mm

558.8

Selected Pipeline Wall thickness

mm

10

Onshore

mm

50

Offshore

mm

65

Spool

mm

0

-

3LPE

mm

3.2

Pipeline Length

Selected Concrete Coating thickness

Anti-Corrosion Coating Material Anti-Corrosion Coating Thickness

Environmental Data The seawater resistivity of 0.22Ωm has been taken from Figure A.1 of Ref. 4. A salinity of 3% and seawater temperature of 25oC has been assumed in the analysis. A resistivity of the seabed sediments is considered as 1.5Ωm in accordance with Sec. A.9 of Ref. 2. For onshore section, a soil resistivity of 3.4Ωm has been assumed.

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Rev. No. A

Anode Data Refer to Ref. 2, aluminum alloy anodes has been used for sacrificial anodes material. A flush mounted bracelet anode has been selected.

Design values of the galvanic anode has been extracted from Table B of Ref. 2 and shown in Table 4-2.

Table 4-2 Anode material properties

Immersed in Seawater

Anode

Buried in Seawater Sediments

Surface

Potential

Electrochemical

Potential

Electrochemical

Temperature

Ag/AgCl/Seawater

Capacity, ε

Ag/AgCl/Seawater

Capacity, ε

(mV)

(A.h/kg)

(mV)

(A.h/kg)

30

-1050

2000

-1025

1600

60

-1050

1600

-1025

680

80

-1000

720

-1000

320

o

( C)

For non-buried pipeline, the anodes surface temperature has been taken as the external pipeline temperature, not the internal fluid temperature.

As per Sec. 7.4 of Ref. 2 a utilization factor of 0.85 has been used for the bracelet and flush mounted anode. Anode alloy density is assumed to be 2700kg/m3.

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Rev. No. A

DESIGN METHODOLOGY General Cathodic protection design for new dual 22in Gamba export line is in accordance with the Shell DEP 30.10.73.31 (Ref. 1) and Shell DEP 30.10.73.32 (Ref. 2) for onshore and offshore pipeline respectively.

Anode quantities are selected to satisfy the following criteria: 

Total anode net mass shall meet the current demand over the design life of the pipeline;



Anode current output shall meet the current demand at end of the design life;



For offshore section, anode spacing shall not exceed the maximum value of 300m as specified by Sec. 7.1 of Ref. 4.

Current Demand Calculation The mean current demand, Icm, and final demand is calculated from: 𝐼𝑐 = 𝐴𝑐 ∗ 𝑓𝑐 ∗ 𝑖𝑐 Where: 𝐼𝑐 – current demand for specific pipeline section (A) 𝐴𝑐 – total surface area for specific pipeline section (m2) 𝑓𝑐 – coating breakdown factor, calculated for mean and final conditions 𝑖𝑐 – current density, selected for mean and final conditions (A) The coating breakdown factor is calculated based on equation (1) and (2) of Ref. 4. For mean coating breakdown factor is, 𝑓𝑐 = (1.75 ∗ 𝑓𝑖 ) + (0.5 ∗ ∆𝑓 ∗ 𝑡𝑑𝑙 ) For final coating breakdown factor is, 𝑓𝑓 = (1.75 ∗ 𝑓𝑖 ) + (∆𝑓 ∗ 𝑡𝑑𝑙 )

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Where: 𝑓𝑖 – initial coating breakdown factor at start of pipeline operation; To account for coating damage during fabrication and installation or workmanship damage, factor of 1.75 is used as recommended by Ref. 2 Sec. 6.5. ∆𝑓 – average yearly increase in the coating breakdown factor; 𝑡𝑑𝑙 – design life of the pipeline (years). For offshore pipeline, the initial coating breakdown factor are extracted from Table 3 of Ref. 4. Table 5-1 Coating breakdown factor – Offshore Section

Coating Type

Multilayer (incl. FBE primer) polyethylene (PE) and polypropylene (PP) anti-corrosion Multilayer

(incl.

FBE

primer)

PE/PP

corrosion + concrete

Factor 𝑓𝑖

∆𝑓

0.005

0.0002

0.002

0.0001

The coating breakdown factors given refer to both pipelines exposed to seawater and pipelines buried in the seabed.

Recommended mean current density has been extracted from Figure 2 of Ref. 4 using the upper conservative curve. Mean current density of 0.068 A/m2 for 25oC of seawater temperature is used in the analysis.

For pipeline fully buried in sediments or artificially covered (e.g. rock dumping), a design current density (mean and final) of 0.02 A/m2 is used.

For onshore pipelines, the current densities which has been multiplied by coating breakdown factor is extracted from Table-1 of Ref. 3.

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Rev. No. A

Table 5-2 Design current density & coating breakdown factor for Onshore Pipeline (𝒊𝒄 *𝒇𝒄 )

Design current density(1) Pipeline coating

10-yr design

20-yr design

30-yr design

life

life

life

0.08

0.1

0.4

3-layer epoxy-polyethylene 3-layer epoxy polypropylene (1) Refer to Ref. 3, for pipelines operating at elevated temperatures, the current density value shall be increased by 25% for each 10oC rise in temperature above 30oC.

Based on Table 5-2 above, for fluid temperature of 55oC, design current density which has been multiplied by coating breakdown factor is taken to be 0.000375 A/m2.

Anode Mass Calculation The total net anode mass required to maintain cathodic protection throughout the design life is calculated based on equation below. 𝑚 = 𝐼𝑐𝑚 ∗ 𝑡𝑑𝑙 ∗

8760 𝑢∗𝜀

Where: 𝑚 – total net anode mass (kg) 𝐼𝑐𝑚 – mean current (Ampere) 𝑡𝑑𝑙 – design life (years) 𝜀 – electrochemical capacity of the anode material (A.h/kg) 𝑢 – utilization factor (0.85 for bracelet flush mounted anodes – Sec. 7.4 of Ref. 4)

Calculation of Number of Anodes The number of the anodes required can be derived from the following equation, 𝑛= Where: 𝑛 – number of anode

𝑚 𝑚𝑎

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𝑚 – total net mass anodes (kg) 𝑚𝑎 – individual mass anode (kg) A boundary on the number of anodes is given by the maximum allowable spacing. The maximum spacing based on Ref. 4 is 300m. To provide the required current, the actual anode current output shall be greater than or equal to the required current output as given below, 𝐼𝑎𝑓 ≥ 𝐼𝑓

Where: 𝐼𝑓 =

𝐼𝑐𝑓

𝐼𝑎𝑓 =

𝑛

– required end of life individual anode current output (Ampere)

𝐸𝑐 −𝐸𝑎 𝑅𝑎

– actual individual anode current output at the end of life (Ampere)

𝐼𝑐𝑓 – final current demand at the end of life (Ampere) 𝑛 – number of anodes to be installed 𝐸𝑎 – design closed-circuit potential of the anode from Table 4-2 of Sec. 4.3 (Volts) 𝐸𝑐 – design protection potential, minimum negative potential (-0.80 Volts, Table 1 of Ref. 4) 𝑅𝑎 – total circuit resistance (Ohms) The anode resistance to be applied for pipeline bracelet, 𝑅𝑎 , is calculated based on equation below. 𝑅𝑎 = 0.315 ∗

𝜌 √𝐴

Where: 𝜌 – environmental resistivity (ohm.m) – see Sec. 4.2. 𝐴 – exposed surface area of the anode (m2).

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Rev. No. A

Design Assumptions The following assumptions have been considered in this cathodic protection analysis.  The coating breakdown factor, 𝑓𝑐 , and the electrochemical capacity, ε, used in the CP design is assumed to be same over the life of the pipeline.  Attenuation of protection is considered not required to be performed as the selected anode spacing does not exceed 300m in accordance with the Annex A.10 of Ref. 4.  The coating breakdown factor on field joint coating is as per the requirements of Shell DEP guideline.  As there is no data available, resistivity of the onshore soil is assumed to be 3.40Ωm.

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RESULTS Results of the cathodic protection design are presented in the following subsections. Anode Properties The anode dimension and density of the alloy used in cathodic protection design is presented in Table 6-1. Table 6-1 Anode dimension

Property Anode thickness (mm)

Value

Onshore

50

Offshore

65

Anode length (mm)

90

Half shell gap (mm)

100

Anode density (kg/m 3)

2700

Cathodic Protection Requirement Table 6-2 shows the minimum anode mass required to meet the required total anode net mass.

GAMBA EXPORT LOADING LINE (SHELL GABON) Cathodic Protection Design Issued Date 10/12/13

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A

Table 6-2 Summary of Cathodic Protection Design for a 22in Gamba Export Pipeline

Pipe Section

Onshore

Pipeline KP Range

Length (m)

Mass of Exposure

one

Condition

anode (kg)

Total

Required

anode net

Final

mass to

Individual

meet

Anode

current

Current

demand

Output

(kg)

(A)

Actual Individual Final Current Output (A)

Required

Required

number of

anode

anode to

spacing to

meet total

meet total

anode net

anode net

mass

mass

(ea)

(m)

Required

Required total

Recommended

number of

anode weight

Spacing of

anode based

based on

anode(1)

on

recommended

(joints)

recommended

spacing

spacing (ea)

(kg)

0.20 – 1.07

1100

Buried

21

93

0.059

0.09

5

248

7

13

273

1.07 – 1.24

200

Buried

28

6

0.063

0.18

1

1050

24

1

28

1.24 – 10.6

9300

Un-buried

28

674

0.212

1.37

24

390

24

32

896

-

25

Un-buried

28

4

0.485

1.37

1

164

1

1

28

Offshore

Spool

Notes: 1. Anode spacing of 300m (24 joints) for offshore pipeline has been recommended in accordance with requirements of Ref. 2. 2. Dual 22 in Gamba export pipeline will be routed parallel from Gamba terminal to the PLEM/SBM, hence the anode required above shall be applicable to both pipelines.

GAMBA EXPORT LOADING LINE (SHELL GABON) Cathodic Protection Design Issued Date

PROJECT NUMBER: 13027 DOCUMENT NUMBER : Z.205.1

10/12/13

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Rev. No. A

REFERENCES 1. 2. 3. 4. 5. 6. 7.

Shell DEP 30.10.73.31-Gen, Design of Cathodic Protection Systems for Onshore Buried Pipelines and Plot Piping (Amendments/Supplements to ISO 15589-1:2003). Shell DEP 30.10.73.31-Gen, Design of Cathodic Protection Systems for Onshore Buried Pipelines and Plot Piping (Amendments/Supplements to ISO 15589-1:2003). ISO 15589-1, Petroleum and Natural Gas Industries – Cathodic Protection of Pipeline Transportation System – Part 1: On-land Pipelines. ISO 15589-2, Petroleum and Natural Gas Industries – Cathodic Protection of Pipeline Transportation System – Part 2: Offshore Pipelines. Z.100.0, Design Basis Report. Z.202.1, Wall Thickness Report. Z.203.1, On-Bottom Stability Analysis.

GAMBA EXPORT LOADING LINE (SHELL GABON) Cathodic Protection Design Issued Date 10/12/13

PROJECT NUMBER: 13027 DOCUMENT NUMBER : Z.205.1

APPENDIX A – Sample of Cathodic Protection Calculation

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Rev. No. A

CP calculation – Gamba

Page 1 of 4

CP calculation – Gamba

Page 2 of 4

CP calculation – Gamba

Page 3 of 4

CP calculation – Gamba

Page 4 of 4