INTRODUCTION
1.1 General
The AFANG field is located in block00 OML 419, approximately 45 km of the southeastern 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 12inch APC2 to PDP1 Crude Export Line, adhering to the following criteria:
 DNVGL – RP – F103 Edition 2016
 ISO 155892 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 12inch APC2 to PDP1 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 AlZnIn halfshell 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 
AlZnIn  AluminiumZincIndium 
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  NonDestructive Test 
#  Document 
1  DNVGLRPF103 Cathodic protection of submarine pipelines Edition July 2016. 
2  ISO 155892 Petroleum, petrochemical and natural gas industries Cathodic protection of pipeline transportation systems Part 2: Offshore pipelines 2nd edition 2012 
3  LPNGHCD2023RPT017 Pipeline Design Basis 
4  Metocean Data 
5  LPNGHCD2023RPT018 Wall thickness Calculation Report 
1.6 Symbol Definitions
AFT : Anode final thickness
AFV : Anode final volume
AS : Anode spacing along pipeline
A_{m} : Total surface area of millapplied coating section of pipeline
A_{fj} : Total surface area of field jointapplied coating section of pipeline
A_{f} : Final anode surface area
A_{fj} : Total surface area of field jointapplied coating section of pipeline
AL : Anode length
As_{max} : Maximum anode spacing
AT : Anode thickness
D : Pipeline outer diameter
E^{o}_{a} : Design closed circuit anode potential
E^{o}_{c} : Design protective potential
ε : Anode current capacity
FAL : Final anode length
fi : Initial coating breakdown factor
f_{cm} : Mean coating breakdown factor for millapplied coating
f_{fm} : Final coating breakdown factor for millapplied coating
f_{cfj} : Mean coating breakdown factor for field jointapplied coating
f_{ffj} : Final coating breakdown factor for field jointapplied coating
GW : Gap width
i_{m} : Mean current density
I_{mean} : Protective current demand
I_{final} : Final current demand
I_{ai} : Initial anode current output
I_{am} : Mean anode current output
I_{af} : Final anode current output
L : Pipeline length
M : Total net anode mass
m : Approximate anode mass
N : Number of anodes required based on joints
N_{anodes} : Number of anodes required to cathodically protect pipeline
N_{final} : Quantity of anodes required for end of life
N_{JA} : Number of joints per anode
N_{mass} : Quantity of anodes required by mass
n_{fj} : Number of field joints
ρ : Anode density
ρ_{w} : Seawater resistivity
R_{ai} : Initial anode resistance
R_{am} : Mean anode resistance
R_{af} : Final anode resistance
T_{o} : Operating temperature
T_{s} : Seawater temperature
TNC : Total number of anodes required for crossing
t_{w} : Wall Thickness
t_{cc, }t_{c} : Concrete coating thickness, Corrosion coating thickness
t_{f} : 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 12inch APC2 to PDP1 Crude Export Line are presented in this report.
The sacrificial anodes for the pipelines will be of aluminum (AlZnIn) alloy, specifically designed in a bracelet configuration with halfshell components. This design complies with DNVGLRPF103 and ISO 155892 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 DNVGLRPF103.
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 21) 
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 2‑1: 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 AlZnIn halfshell bracelet anodes be employed to ensure effective external corrosion protection for the 12.75inch APC2 to PDP1 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 halfshell bracelet anode, as depicted in Figure 21, with the specified parameters outlined in Table 21 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 nonburied exposure condition seen in 1 DNVGLRPF103 Table 62.
 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 12Inch APC2 to PDP1 Crude export line.
 An anode thickness of 29mm, anode length 406mm, anode gap width 51mm, and anode material density 2660 kg/m^{3 }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 31.
Anode chemical composition shall be per DNVGLRPF103, Section 6.1.7, Table 61
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 (^{0}C)  176 (80)  Ref. 3  
Sea water Mean temperature  °F (^{0}C)  72.68 (22.60)  Ref. 3  
Sea water resisitivity^{1}  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/m^{2}  0.075  Ref. 1  
Design Life  Year  25  Ref. 3  
Anode type  –  AlZnIn  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/m^{3}  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 factor^{2}  –  0.8  Ref. 1  
Note: 1. See Figure B2 of [Ref.1]. Resistivity was selected using curve 35% at temperature of 22.6^{o}C being the range of design parameter.
2. per DNVGLRP103 section 6.4.2

Figure 3‑2: 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 DNVRPF103.
The spreadsheet checks the following criteria:
 Quantity of Anodes required for end of life (N_{final}) ≤ Quantity of Anode required by Mass (N_{mass})
 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:
I_{cm} Mean current demand;
A_{c} Pipeline surface area;
f_{cm} Mean coating breakdown factor;
i_{cm} Design mean current density
k Design factor = 1.1 per DNVGLRP 103
f_{cm} = a + 0.5 ⋅ b ⋅ t_{f……………………………………………………………..……2}
Where:
t_{f} design life (Year).
a and b in equation (2) are constants given in DNVRPF103 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:
I_{cf} Final current demand;
A_{c} Pipeline surface area;
f_{cf} Final coating breakdown factor;
i_{cm} Design mean current density.
k Design factor = 1.1 per per DNVGLRP 103
f_{cf} = a + b ⋅ t_{f} …………………………………………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);
I_{cm} Total mean current demand (A);
t_{f} 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:
I_{af }the final anode current output (A);
E°_{c} the design protective potentials is 0.80V for sea water;
E°_{a} the design closed circuit anode potential = 1.050 V
R_{af} 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 (m^{2})
From the final (individual) anode current output (I_{af}) calculated above, and the total final current demand (I_{cf}), 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 51 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 