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PIPILINE CATHODIC PROTECTION REPORT

INTRODUCTION
1.1         General

The AFANG field is located in block-00 OML 419, approximately 45 km of the south-eastern coast of Wakanda in approximately 40 meters water depth. The field, initially brought into production in 1997 is owned by the Joint Venture APC/PDP and is operated by APPDPLPC Nigeria Limited (APPDPLPCNL). The OML 419 block is shown in the following figure.

Figure 1‑1: Project Scope

Figure 1‑2: AFANG Field Architecture

1.2         Objectives

The main goal of this report is to meticulously design the cathodic protection system for the 12-inch APC-2 to PDP-1 Crude Export Line, adhering to the following criteria:

  • DNVGL – RP – F103 Edition 2016
  • ISO 15589-2 Edition 2012

The cathodic protection design will be performed using Excel spread sheet. The excel spreadsheet will be    used to determine the total number of anodes required along with the spacing for a design life of 25 years.

1.3         Scope of Document

The scope of this document is to design a cathodic protection (CP) system for the 12-inch APC-2 to PDP-1 Crude Export Line. The objective is to supply the required current and anode mass to ensure the pipeline remains safeguarded against external corrosion over its intended lifespan. To achieve this, we will employ Al-Zn-In half-shell bracelet anode systems as the primary method of providing cathodic protection for the pipeline. Furthermore, an additional 5% anodes have been calculated as spares for contingency purpose.

1.4   Definitions
Term Definition
Al-Zn-In Aluminium-Zinc-Indium
AFT Anode Final Thickness
AFV Anode Final Volume
Al Aluminium
COMPANY  APPDPLPCC NIGERIA LIMITED
CP Cathodic Protection
DNVGL Det Norske Veritas and Germanischer Lloyd
ISO International Organization for Standardization
MLW Mean Low Water
NDT Non-Destructive Test
  • References
# Document
1 DNVGL-RP-F103- Cathodic protection of submarine pipelines Edition July 2016.
2 ISO 15589-2- Petroleum, petrochemical and natural gas industries Cathodic protection of pipeline transportation systems Part 2: Offshore pipelines 2nd edition 2012
3 LP-NG-HCD2023-RPT-017- Pipeline Design Basis
4 Metocean Data
5 LP-NG-HCD2023-RPT-018- Wall thickness Calculation Report
1.6         Symbol Definitions

AFT             : Anode final thickness

AFV             : Anode final volume

AS               : Anode spacing along pipeline

Am               : Total surface area of mill-applied coating section of pipeline

Afj               : Total surface area of field joint-applied coating section of pipeline

Af                : Final anode surface area

Afj               : Total surface area of field joint-applied coating section of pipeline

AL               : Anode length

Asmax           : Maximum anode spacing

AT               : Anode thickness

D                 : Pipeline outer diameter

Eoa               : Design closed circuit anode potential

Eoc               : Design protective potential

ε                 : Anode current capacity

FAL             : Final anode length

fi                 : Initial coating breakdown factor

fcm               : Mean coating breakdown factor for mill-applied coating

ffm               : Final coating breakdown factor for mill-applied coating

fcfj               : Mean coating breakdown factor for field joint-applied coating

fffj                : Final coating breakdown factor for field joint-applied coating

GW             : Gap width

im                : Mean current density

Imean            : Protective current demand

Ifinal              : Final current demand

Iai                : Initial anode current output

Iam               : Mean anode current output

Iaf                : Final anode current output

L                  : Pipeline length

M                : Total net anode mass

m                : Approximate anode mass

N                 : Number of anodes required based on joints

Nanodes         : Number of anodes required to cathodically protect pipeline

Nfinal            : Quantity of anodes required for end of life

NJA              : Number of joints per anode

Nmass           : Quantity of anodes required by mass

nfj                : Number of field joints

ρ                 : Anode density

ρw               : Seawater resistivity

Rai               : Initial anode resistance

Ram              : Mean anode resistance

Raf               : Final anode resistance

To                : Operating temperature

Ts                : Seawater temperature

TNC            : Total number of anodes required for crossing

tw                : Wall Thickness

tcc, tc            : Concrete coating thickness, Corrosion coating thickness

tf                 : Design life

U                 : Anode utilization factor

V                 : Anode volume

Dftf             : Average yearly increase in coating breakdown

 

 

 

 

 

 

 

2           RESULTS SUMMARIES, CONCLUSIONS, AND RECOMMENDATIONS
2.1         Analysis Summaries

The design of the pipeline cathodic protection systems for the 12-inch APC-2 to PDP-1 Crude Export Line are presented in this report.

The sacrificial anodes for the pipelines will be of aluminum (Al-Zn-In) alloy, specifically designed in a bracelet configuration with half-shell components. This design complies with DNVGL-RP-F103 and ISO 15589-2 standards, ensuring a designated service life of 25 years.

The allocation and placement of these anodes for the offshore pipeline are determined in such a way that the quantity needed by the end of the design life (referred to as Nfinal) is equal to or less than the quantity required by mass (referred to as Nmass). Furthermore, the spacing between the anodes adheres to the specified maximum of 300 meters (equivalent to 24 joints) as outlined in DNVGL-RP-F103.

A contingency of 5% of the number of anodes calculated was considered in order to determine the total number of anodes required for this project.

The anode unit mass, anode total mass, anode spacing along the pipeline and total number of anode quantity required for the offshore pipeline and expansion spools sections are listed in Table 2‑1.

Table 2‑1: Summary of Cathodic Protection Design

Description Unit Values
Pipeline Length (Target box – Target box) M 4240.04
Spool Length M 158.757
Type of bracelet anodes     Half Shell (See Figure 2-1)
Outside diameter Inch (mm) 12.75 (323.9)
Anode length Inch (mm) 15.98 (406)
Anode thickness Inch (mm) 1.14 (29)
Single anode mass required Kg 34.72
Required Anode spacing along pipeline m 163.08
Required Anode spacing along the spool m 99.2
Selected number of joints per anode along the offshore pipeline Joints 14
Selected number of joints per anode along the spool Joints 8
Quantity of anodes required for the pipeline based on selected joints Anodes 28
Quantity of anodes required for the spool based on selected joints Anodes 3
Contingency (Ref. 6) Anodes 3
Total number of Anodes required Anodes 34

Figure 21: Typical Half Shell Bracelet Anode

2.2         Conclusions

According to the cathodic protection design calculations, it is recommended that a total of 34 units of Al-Zn-In half-shell bracelet anodes be employed to ensure effective external corrosion protection for the 12.75-inch APC-2 to PDP-1 crude export line throughout its designated design life.

2.3         Recommendations

The following are recommended:

It is recommended that the anode spacing should not exceed a maximum of 14 joints for the pipeline and 13 joints for the spool.

It is recommended to utilize a half-shell bracelet anode, as depicted in Figure 2-1, with the specified parameters outlined in Table 2-1 above.

A total of 28 anodes are calculated and recommended to be installed on the pipeline, with additional 3 anodes recommended for the expansion spool.

 

3           DESIGN DATA
3.1             Assumptions

The following assumptions have been adopted:

  • The design mean current density is based on the non-buried exposure condition seen in 1 DNVGL-RP-F103 Table 6-2.
  • Half shell bracelet anodes are assumed because the pipeline is not concrete weight coated.
  • 5% of the calculated quantity of anode were applied to calculate the total anode required for the 12-Inch APC-2 to PDP-1 Crude export line.
  • An anode thickness of 29mm, anode length 406mm, anode gap width 51mm, and anode material density 2660 kg/m3 are used for this calculation. Manufacturer shall provide the final dimensions for COMPANY review/approval.
  • Mean seawater salinity is 35.6ppt [Ref.4]
  • Anodes required for destructive testing are not included in the design quantity; these shall be provided separately by the anode Manufacturer.
3.2         Design Data

The cathodic protection design data are given in Table 3-1.

Anode chemical composition shall be per DNVGL-RP-F103, Section 6.1.7, Table 6-1

Table 3‑1: Design Data for Cathodic Protection Calculation

Parameter Unit Values Ref.
Outer diameter inch (mm) 12.75 (323.9) Ref. 3
Pipeline length m (km) 4240.04 (4.240) Ref. 3
Expansion spool length m (km) 158.757 (0.158) Ref. 3
Wall thickness inch (mm) 0.500 (12.75) Ref. 5
Corrosion coating (3LPP) inch (mm) 0.039 (2.7) Ref. 2
Maximum Operating temperature °F (0C) 176 (80) Ref. 3
Sea water Mean temperature °F (0C) 72.68 (22.60) Ref. 3
Sea water resisitivity1 Ωm 0.20 Ref. 1
Maximum anode spacing   m (ft) 300 (984.25) Ref. 1
Constants for Coating Breakdown Factors Linepipe 3LPP coating a 0.004 Ref. 2
b 0.0002
Design current density A/m2 0.075 Ref. 1
Design Life Year 25 Ref. 3
Anode type Al-Zn-In Ref. 3
Gap width inch (m) 2.00 (0.051) Per vendor data
Anode thickness inch (m) 1.14 (0.029) Per vendor data
Anode length inch (m) 15.98 (0.406) Per vendor data
Density of Anode alloy kg/m3 2660 Typical Value
Design protective potential V -0.8 Ref. 1
Design closed circuit anode potential V -1.0 Ref. 1
Electrochemical capacity Ahr/kg 720 Ref. 1
Utilization factor2 0.8 Ref. 1
Note: 1. See Figure B-2 of [Ref.1]. Resistivity was selected using curve 35% at temperature of 22.6oC being the range of design parameter.

           2. per DNVGL-RP-103 section 6.4.2

 

 

                                 Figure 32: Typical Anode with 3LPP Coated Pipeline

 

 

4           CALCULATIONS METHODOLGY

The process employed to calculate the mean current demand, final current demand, and the total anode mass required for the entire system’s design life involved the use of a proprietary Excel calculation tool. This tool has been internally validated and aligns with the methodology outlined in DNV-RP-F103.

The spreadsheet checks the following criteria:

  • Quantity of Anodes required for end of life (Nfinal) ≤ Quantity of Anode required by Mass (Nmass)
  • Anode Spacing along pipeline ≤ Maximum anode spacing
4.1       Mean Current Demand

Mean current demand throughout the pipeline lifetime is calculated by multiplying the contributions of coated linepipe, field joints, exposed pipeline surface area and design mean current density.

……………………………………….1

 

Where:

Icm             Mean current demand;

Ac              Pipeline surface area;

fcm             Mean coating breakdown factor;

icm              Design mean current density

k            Design factor = 1.1 per DNVGL-RP 103

 

fcm = a + 0.5 ⋅ b ⋅ tf……………………………………………………………..……2

Where:

tf        design life (Year).

a and b in equation (2) are constants given in DNV-RP-F103 Table A.1 and A.2 in Annex 1 give recommendations for constants to be used for specific combinations of linepipe coating and Field Joint Coating systems.

4.2         Final Current Demand

Current demand at the end of the design life is calculated in a similar way to the mean current demand and assumes the mean current density requirement remains, yet considers end of life conditions of linepipe coating and field joints.

   ……………………………………..3

Where:

Icf Final current demand;

Ac              Pipeline surface area;

fcf              Final coating breakdown factor;

icm             Design mean current density.

k            Design factor = 1.1 per per DNVGL-RP 103

fcf = a + b ⋅ tf …………………………………………4

4.3         Mass Requirement to Meet Mean Current Demand

Total net anode mass required to maintain CP throughout the design life has been calculated for each section of the pipeline using the formula given below.

 

………………………………5

 

Where:

M              The total net anode mass (kg);

Icm             Total mean current demand (A);

tf             the design life (year),

u             Utilization factor (dimension less);

ε             Electrochemical capacity (A.hr/kg).

4.4         Anode Dimensions to Meet Final Demand

Based on the total net anode mass (M) determined in Section 4.3, a tentative pipeline anode may be selected.  The ‘final anode current output’ of the selected anode has been calculated using the formula below.

 

…………………………….6

 

Where:

Iaf                        the final anode current output (A);

c         the design protective potentials is -0.80V for sea water;

a             the design closed circuit anode potential = -1.050 V

Raf             Final anode resistance (Ω)

 

Final anode resistance for bracelet anodes has been calculated from the following formulation.

……………………………7

Where:

The environmental Resistivity (ohm.m)

Exposed surface area of the anode (m2)

 

From the final (individual) anode current output (Iaf) calculated above, and the total final current demand (Icf), the required number (N) of anodes becomes:

………………………………………8

 

5        RESULTS

The cathodic protection design has been carried out in accordance with the methodology given in section 4. Table 5-1 shows the results of the design calculations. The detailed calculations including the required number of anodes and required anode mass for the protection of the pipeline against corrosion are given in the Appendix 1.

Table 5‑1: Cathodic Protection Calculations Details

Summary of Results Units Offshore Pipeline Spool
Selected Number of joints Joints 14 13
Total Net Anode Mass kg 879.69 32.94
Final current Demand Amp 3.20 0.12
Protective Current demand Amp 2.31 0.09
Final Anode Output Amp 1.99 1.98
Maximum Anode spacing m 300 300
Anode Spacing along pipeline m 163.08 158.757
Maximum Anode Spacing; Anode Spacing along pipeline <= Maximum Anode Spacing Comply  

Comply

Total Number of Anodes required Anodes 30 4
Net Weight of a Single Anode kg 34.72 32.94
Anode Length mm 406 406