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 |
# | 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 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 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 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 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);
E°c the design protective potentials is -0.80V for sea water;
E°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 |