Cathodic Protection Design

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GAMBA EXPORT LOADING LINE (SHELL GABON)

Cathodic Protection Design Issued

Date PROJECT NUMBER: 13027

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10/12/13 DOCUMENT NUMBER : Z.205.1 2 of 19 A

REVISION CONTROL SHEET

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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

6.0 RESULTS ... 16

Anode Properties ... 16

Cathodic Protection Requirement ... 16

7.0 REFERENCES ... 18

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1.0 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.

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2.0 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

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3.0 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.

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Cathodic Protection Design

Issued Date PROJECT NUMBER: 13027 Sht. No. Rev. No.

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

Pipe Section KP Range Pipeline Length (m) Exposure Condition Mass of one anode (kg) Total anode net mass to meet current demand (kg) Required Final Individual Anode Current Output (A) Actual Individual Final Current Output (A) Required number of anode to meet total anode net mass (ea) Required anode spacing to meet total anode net mass (m) Recommended Spacing of anode(1) (joints) Required number of anode based on recommended spacing (ea) Required total anode weight based on recommended spacing (kg) Onshore 0.20 – 1.07 1100 Buried 21 93 0.059 0.09 5 248 7 13 273 Offshore 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 Spool - 25 Un-buried 28 4 0.485 1.37 1 164 1 1 28 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.

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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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4.0 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 22in Gamba

Export Lines

Pipeline Design Life years 25

Pipeline Length

Onshore m 1100

Offshore m 9500

Spool m 25

Outside Diameter mm 558.8

Selected Pipeline Wall thickness mm 10

Selected Concrete Coating thickness

Onshore mm 50

Offshore mm 65

Spool mm 0

Anti-Corrosion Coating Material - 3LPE

Anti-Corrosion Coating Thickness mm 3.2

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 25o_{C 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.

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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

Anode Surface Temperature

(o_C)

Immersed in Seawater Buried in Seawater Sediments

Potential Ag/AgCl/Seawater (mV) Electrochemical Capacity, ε (A.h/kg) Potential Ag/AgCl/Seawater (mV) Electrochemical Capacity, ε (A.h/kg) 30 -1050 2000 -1025 1600 60 -1050 1600 -1025 680 80 -1000 720 -1000 320

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.

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5.0 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:

𝐼_𝑐 = 𝐴_𝑐∗ 𝑓_𝑐∗ 𝑖_𝑐

Where:

𝐼_𝑐 – current demand for specific pipeline section (A) 𝐴_𝑐 – total surface area for specific pipeline section (m2₎

𝑓_𝑐 – 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,

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Where:

𝑓_𝑖 – 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; 𝑡_𝑑𝑙 – 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 Factor 𝑓𝑖 ∆𝑓

Multilayer (incl. FBE primer) polyethylene

(PE) and polypropylene (PP) anti-corrosion 0.005 0.0002 Multilayer (incl. FBE primer) PE/PP

corrosion + concrete 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 25}o_{C of seawater}

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

Pipeline coating

Design current density(1)

10-yr design life 20-yr design life 30-yr design life 3-layer epoxy-polyethylene 3-layer epoxy polypropylene

0.08 0.1 0.4

(1) Refer to Ref. 3, for pipelines operating at elevated temperatures, the current density value shall be increased by 25% for each 10o_{C rise in temperature above 30}o_C.

Based on Table 5-2 above, for fluid temperature of 55o_{C, 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) 𝐼_𝑐𝑚 – mean current (Ampere) 𝑡_𝑑𝑙 – 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 = 𝑚

𝑚_𝑎 Where:

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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,

𝐼_𝑎𝑓≥ 𝐼_𝑓

Where:

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

𝐼_𝑎𝑓=𝐸𝑐−𝐸𝑎

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

𝑅_𝑎 – total circuit resistance (Ohms)

The anode resistance to be applied for pipeline bracelet, 𝑅𝑎, is calculated based on

equation below.

𝑅_𝑎= 0.315 ∗ 𝜌 √𝐴

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Design Assumptions

The following assumptions have been considered in this cathodic protection analysis.  The coating breakdown factor, 𝑓_𝑐, 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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6.0 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 Value Anode thickness (mm) Onshore 50 Offshore 65 Anode length (mm) 90

Half shell gap (mm) 100

Anode density (kg/m3₎ ₂₇₀₀

Cathodic Protection Requirement

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

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

Pipe Section KP Range Pipeline Length (m) Exposure Condition Mass of one anode (kg) Total anode net mass to meet current demand (kg) Required Final Individual Anode Current Output (A) Actual Individual Final Current Output (A) Required number of anode to meet total anode net mass (ea) Required anode spacing to meet total anode net mass (m) Recommended Spacing of anode(1) (joints) Required number of anode based on recommended spacing (ea) Required total anode weight based on recommended spacing (kg) Onshore 0.20 – 1.07 1100 Buried 21 93 0.059 0.09 5 248 7 13 273 Offshore 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 Spool - 25 Un-buried 28 4 0.485 1.37 1 164 1 1 28 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.

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7.0 REFERENCES

1. 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). 2. 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). 3. ISO 15589-1, Petroleum and Natural Gas Industries – Cathodic Protection of

Pipeline Transportation System – Part 1: On-land Pipelines.

4. ISO 15589-2, Petroleum and Natural Gas Industries – Cathodic Protection of Pipeline Transportation System – Part 2: Offshore Pipelines.

5. Z.100.0, Design Basis Report. 6. Z.202.1, Wall Thickness Report. 7. Z.203.1, On-Bottom Stability Analysis.

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