This document provides the design and calculations for an impressed current cathodic protection system for an offshore pipeline and underground plant piping. It includes definitions, location data of the pipeline, design concepts, calculation methods, formulas used, and considerations for interference mitigation. The cathodic protection system will use mixed metal oxide anodes in a deep well groundbed configuration to provide corrosion protection for the pipeline and meet criteria for protective potentials outlined in relevant standards.

1. GUNUNG MEGANG - SINGA
GAS COMPRESSION & PIPELINE FACILITY
DESIGN AND CALCULATION OF
IMPRESSED CURRENT CATHODIC PROTECTION
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DESIGN AND CALCULATION OF
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PROTECTION
TABLE OF CONTENT
1. INTRODUCTION ............................................................................................................ 2
2. REFERENCE.................................................................................................................. 3
3. DEFINITIONS................................................................................................................. 3
4. LOCATION DATA........................................................................................................... 4
5. DESIGN CONCEPT ....................................................................................................... 4
6. DESIGN METHOD IMPRESSED CURRENT CATHODIC PROTECTION .................... 5
7. CALCULATION FORMULA............................................................................................ 6
8. INTERFERENCE............................................................................................................ 9
8.1. Stray Current on Metallic Structures..................................................................... 9
8.1.1. At Area of Current Pick-Up ..................................................................... 10
8.1.2. Along the Structure Parallel Line (Parallel Line) ..................................... 10
8.1.3. At the Stray Current Discharge Location ................................................ 10
8.2. List of Instrument and Equipment......................................................................... 10
8.3. Criteria.................................................................................................................. 11
8.4. Interference Testing.............................................................................................. 11
8.5. Mitigation .............................................................................................................. 12
8.5.1. Install Interference Bond......................................................................... 12
8.5.2. Installation of Galvanic Anodes............................................................... 12
8.5.3. Using Coating to Mitigate Interference Effects........................................ 12
8.5.4. Install/burying a metallic shield next to the affected structure................. 13
1. INTRODUCTION
This document is to define the requirement of onshore pipeline cathodic protection system
for Gunung Megang – Singa pipeline and underground plant piping at Gunung Megang
Booster Station.

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DESIGN AND CALCULATION OF
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PROTECTION
The pipeline is coated with 1 layer epoxy primer and 2 layers PE wrapping tape. And also
protected by impressed current cathodic protection system with deep well groundbed type
installation.
2. REFERENCE
NACE RP-0169-2002 : Control of External Corrosion on Underground of
Submerged Metallic Piping System.
NACE RP-0177-95 : Mitigation of Alternating Current & Lightning Effect on
Metallic Structures & Corrosion Control System.
NACE RP-0572-2002 : Design, Installation, Operation and Maintenance of
Impressed Current Deep Ground beds.
NACE RP-0286-97 : Electrical Isolation of Cathodically Protected Pipelines.
DNV RP B401 : Cathodic Protection Design.
ASTM G57 : Standard Method for Field Measurement of Soil Resistivity
Using the Wenner Four- Electrode Method.
ISO 15589-1 : Petroleum and Natural Gas Industries Cathodic Protection
of Pipeline Transportation Systems Part 1 - On-land
Pipeline.
BS-7361-Part1-1991 : Cathodic Protection of Buried or Immersed Metallic
Structures. General Principles and Application for Pipelines.
AW PEABODY : Control of Pipeline Corrosion (Second Edition)
GMC-L-RE-001 Rev.A : Design Basis for 10” Gunung Megang – Singa Pipeline
3. DEFINITIONS
The following terms shall have the meaning stated:
Employer : PT. Mitra Energi Gas Sumatera

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DESIGN AND CALCULATION OF
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PROTECTION
Contactor : PT. Citra Panji Manunggal
Subcontractor : PT. Pelinkar Iso Mandiri
4. LOCATION DATA
The pipeline and in-plant pipeline is outline in the following table
SECTION PIPELINE DATA
Pipe Diameter Wall Thickness
AREA KP Start KP End
(Inch) (mm)
Pipe
Length
(m) (Inch) (mm)
Coating
System
Pipe
Status
MUARA
ENIM
KP.
0+000
(G.
Megang
Station)
Pig
Receiver
MEP
Metering
G.
Megang
KP.
17+500
(Singa
Station)
Slug
catcher
G.
Megang
Pig
Receiver
G.
Megang
10.75
10.75
6.625
10.75
273.0
273.0
168.23
273.0
17500
175.0
205
Equal to
123
0,365
0.365/
0.592
0.432
9,271
9.271/
15.04
10.973
1 Layer
Primer
Coating +
2 Layer
Wrapping
Tape
Buried
Pipeline material: 10” API 5L X56 and API 5L X65
In Plant piping material: 10” API 5L X56 and 6” API 5L Grade B
5. DESIGN CONCEPT
The external surface of pipeline shall be protected with combination an anti corrosion
coating and Cathodic Protection; where Impressed Current Cathodic Protection will be
used.
Design of the Impressed Current Cathodic Protection shall be 20 years life time.

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DESIGN AND CALCULATION OF
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PROTECTION
Protective Potential Requirement shall be in accordance with GMC – L – RE – 001 Section
6.8 of Design Basis to maintain at least -900 mV and not more than -1.050 mV measured
relatively to Ag/AgCl. This value is equal to -950 mV and not more than -1.100 mV
measured relative to a Cu/CuS04 reference electrode (CSE).
The other criteria and requirement of Clause 6.2 of NACE RP-0169 may be applied to
determine whether cathodic protection has been successfully achieved. These other
criteria shall be utilized only when it is not possible to achieve a polarized potential of -900
mV (CSE)
With specific regard to pipeline sections in high – resistivity aerated sandy soil condition,
less negative values may be acceptable – refer to Clause 6.2.2.3 of NACE PR-0169.
Acceptable lesser negative values may clearly laid out in ISO 15589-1 and reproduced
below for convenience.
Polarized Protection Potential
(Cu/CuSO4)
Soil Resistivity
(ohm meters)
-750 mV
-650 mV
100< ρ < 1000
1000 < p
Soil Resistivity Measurement required for Cathodic Protection design shall be obtained by
Contractor.
Construction drawings shall be prepared by Contractor showing details of Impressed
Current Cathodic Protection Proposed.
At any pipeline crossing or where pipeline running in parallel; where interference may
occured, the existing pipelines at crossing or parallel to be protected shall be either
insulated or shall be taken into account in the Cathodic Protection design.
6. DESIGN METHOD IMPRESSED CURRENT CATHODIC PROTECTION
The Impressed Current Cathodic Protection will include the following main material:
• Transformer Rectifier
• Mixed Metal Oxide Anode
• Cables
• Test Station

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DESIGN AND CALCULATION OF
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• Current Measurement Test Station
• All accessories required for the system installation
• Junction box.
The methodology used for the Impressed Current Anode design is in accordance with the
recommendation stipulated in NACE RP-0169. Based on this methodology a calculation
sheet will be made to estimate the Cathodic Protection requirements.
The design of Impressed Current Cathodic Protection System for Gunung Megang – Singa
pipeline will be based on document no: GMG-L-RE-001.
• Pipeline Diameter 0.273 M
• Length of Pipeline 17,500 M + In Plant piping 300 M
• Maximum Design Temperature 200o
F = 93.33o
C
• Current Density at Maximum Temperature 88.33 mA
• Design Life 20 years
• Safety Factor 25%
• Positive Limit Protection Level w.r.t. Cu/CU SO4 -0.950 mV
• Negative Limit Protection Level w.r.t. Cu/CU SO4 -1.100 mV
Based on the above mentioned data, a calculation will be made to obtain the following:
• Surface Area
• Current requirement
• Quantity of anodes
• Resistance of groundbed
• Resistance of cable
• Total Resistance
• Transformer Rectifier DC Output Capacity Requirement
• Attenuation calculation
7. CALCULATION FORMULA
a. Surface Area to be protected (m2)
The protected area for pipeline is calculated according to the following formula:
SA = π x OD x Lp

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Where:
SA = Surface Area to be protected (m2)
OD = Outside diameter of pipe (m)
Lp = Length of pipe (m)
π = 3.1415
b. Cathodic Protection Current Requirement
The total protective current required is calculated according to the following formula:
Im = (Cr x SA x I )
Where:
Im = Current requirement (A)
Cr = Mean coating breakdown factors ( 3%)
SA = Surface area to be protected (sqm)
I = Current density in milliampere per square meter (A/sqm)
c. Total Minimum Anode by current requirement method (Method for MMO
Requirements)
Quantity of anode by current requirement is calculated by following equation:
Q = Ip / la
Where:
Q = Quantity of anode required (pcs)
Ip = Current requirement (A)
Ia = Current output of each anode
d. Resistance of Anode Groundbed
Resistance of Anode Groundbed is calculated by the following formula:
Rg = Ra + Rb x Inf
Where:
Rg = Total Resistance of Groundbed
Ra = Resistance of anode to the calcined petroleum backfill and is calculated by the
following formula:
Ra=
Where:
Ra = Resistance anode to backfill (ohm-m)
P = Calcined Petroleum coke breeze backfill Resistivity (ohm-cm)
NA = Number of Anode

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L = Length of anode in feet
d = Diameter of anode in feet
Rb = Resistance of Calcined Petroleum Coke Breeze backfill to Soil and is
calculated by the following formula:
Rb =
Where:
Rb = Resistance backfill to soil (ohm-m)
Ps = Soil resistivity (ohm-cm)
L = Length of column Calcined Petroleum coke breeze backfill in feet
d = Diameter of Column Calcined Petroleum coke breeze backfill in feet
Inf = Interferrence factors is safety factors + 1.
e. Resistance of Cable
The resistance of cable is calculated as follows:
Rc = 0.0172 x (Lc : Csq)
Where:
Rc = Resistance of Cable (ohm-m)
Lc = Length of Cable (m)
Csq = Cable Cross Section (mm2)
f. Total Resistance
The total resistance is calculated by the following equation:
R Total = Rg + Rc
R Total = Total Circuit Resistance (ohm-m)
g. Transformer Rectifier DC Output Capacity Requirement
Total DC voltage rating of the power supplies to achieve the desire DC current
output is calculated by the following equation:
T/R Volt = I x R total x (1+ SF) + back emf
I = Current Requirement in amperes
R total = Rg + Rc (ohm-m)
h. Attenuation Calculation

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DESIGN AND CALCULATION OF
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The potential attenuation of a pipeline is calculated using the following formula:
Lp =
∆P
∆P0
α
. IdA. Cosh
A = л x E x (OD –E) [cross section area of pipe]
∆P = Positive Limit Protection Level
∆Po = Negative Limit Protection Level
Id = Current Density of Steel Coated at 25ºC (Normal Temperature)
α = Attenuation Factor, will calculated by the following formula:
α =
r
L = Longitudinal Resistance of pipeline. This will Calculated by following formula:
r
L = ra / a
ra = r25 x [1 + δ (T – To)]
r25 = Average steel resistivity for grade API 5L – X56 steel at 25o
C (ohm-m)
d = Temperature coefficient of resistance = 0.00306
T = 93.33o
C
To = 25
o
C
a = Surface area of pipe per meter length = OD x π x 1
E = Wall thickness of pipe
r
T = Transversal resistance of pipe ( ). This calculated by following formula:
r
T = rs / a
rs = Coating resistance after installation and 20 years life (Ω/m2)
8. INTERFERENCE
8.1. Stray Current on Metallic Structures
Stray currents are currents through electrical paths other than the intended circuit.
Stray current is not galvanic corrosion current between anodes on the same
structure. Stray current or interference current can refer to either alternating current
(AC) or direct current (DC). AC stray current is more of a safety hazard than a
corrosion problem. DC stray current causes significant corrosion of most metals.

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PROTECTION
Specifically, the subject matter dealt within this document relates to stray current
caused by operation of Impressed Current Cathodic Protection System for Gunung
Megang – Singa pipeline to foreign crossing and/or parallel pipelines in the area.
Stray currents will occur if foreign pipeline crosses a groundbed voltage gradient and
it will promote current pick-up on the foreign pipeline within the area of influence.
Because current is picked up on the foreign pipeline, then current must discharge
outside the area of influence.
The effects of stray current on metallic structures can be harmful, beneficial or
innocuous depending on the magnitude of the current density, type o structure and
location of current pick-up and discharge areas.
8.1.1. At Area of Current Pick-Up
At the area of current pick-up, a negative shift will result in cathodic
polarization, and if the foreign structure is mild steel then there is a beneficial
effect as the structure is receiving some measure of cathodic protection. If
the structure is coated and has its own cathodic protection system, the
additional polarization from the stray current pick-up may result in cathodic
blistering of the coating.
8.1.2. Along the Structure Parallel Line (Parallel Line)
Stray current in metallic structure does not usually cause damage between
the stray current pick-up and discharge location unless the current is very
large (close to anode groundbed)
8.1.3. At the Stray Current Discharge Location
Considerable attention is given to identifying the site of current discharge in
stray current investigations because this is where corrosion damage is most
like to occur on all metallic structures. When a current transfers from a
metallic structure to earth, it must do so via an oxidation reaction which
converts electronic current to ionic current.
8.2. List of Instrument and Equipment
The Interference testing will be carried out using the following the
instrument/equipment:
1) GPS Synchronised Current Interrupters
2) Portable copper/copper sulphate reference electrodes
3) Reel of test wired

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4) Clamp ammeter
5) Electrician hand tools
All equipments will be calibrated and should have valid calibration certificates where
required.
8.3. Criteria
Three (3) criteria are commonly used in determining the adequacy of the mitigation:
1) The “NO SWING” criterion may be applied. This requires that the potential of the
effected pipeline does not shift in the positive direction when the foreign rectifier
cycles from off to on. This criterion is reasonably applicable to well coated
pipelines but maybe unnecessarily severe.
A “NATURAL POTENTIAL” criterion maybe applied. The affected pipe is to be
returned to the potential existing before the interference began by the installation by
mitigation measures such as an interference bond. In many cases the natural
potential may be very difficult to determine.
A “NO CORROSION” criterion may be applied. If it can be shown that the affected
pipeline is cathodically protected and meets the applicable potential criterion (criteria
for protection as defined ISO 15589-1:2003) for full cathodic protection then no
additional measures need by taken.
8.4. Interference Testing
Essentially where interference is suspected or there may be a possibility the rectifiers
on one line are cycled ON and OFF, measurements are made on both pipes at the
pipeline crossing and/or parallel to see if the potential on the foreign crossing and/or
parallel pipeline are being adversely affected. A positive shift on a foreign crossing
and/on parallel pipelines indicates interference.
1) Cycling the rectifiers ON and OFF will require the installation of GPS current the
interrupters on all cathodic protection current sources likely to influence the area
of the affected pipe crossing.
2) The pipe-to-soil potential measurements will require the installation a high
impedance DC voltmeter, portable copper/copper sulphate reference electrode
and a set of test lead wires.
3) A copper/copper sulphate reference electrode will be placed in the soil directly
over the pipeline. Make sure that there is good low resistant contact between the
soil & reference electrode. In dry soil it may be necessary to use some water to
wet a small area of soil.
4) Connect the lead wire of the reference electrode to the negative terminal of the
voltmeter. Connect the positive terminal of the voltmeter to the Pipelines via the
test station terminal (this will result a negative polarity on the DC display of digital
voltmeter).

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5) Adjust the voltmeter to the 2-Volt DC meter range and turn the meter on.
6) Observe and record the potential value at each test station location.
7) Records “ON” and “OFF” readings.
8.5. Mitigation
If the shift is more than allowed by BS-7361-Part 1: 1991 – 9.3.3.2, i.e. 20 mV, then a
number of methods can be used to lessen the harmful effects of stray currents as
listed below:
8.5.1. Install Interference Bond
An interference bond with an adjustable resistor allows the interference
current to be return from the foreign crossing pipeline via the bond cable and
the resistor. The resistor is adjusted so that just enough current flows to
remove the positive shift but not allow the foreign crossing to take too much
Cathodic Protection current. Not all crossing will require a bond. An
interference bond is a cable from one pipe to the other through a variable
resistor normally mounted in a junction box. The installation of variable
resistor is relatively simple. The variable resistor shall be slotted in the
junction box.
Condition on every crossing/parallel is different and the current drained and
the value of resistance is different. Generally speaking a small resistor say 0
to 1 ohm and 2 amperes is sufficient.
Upon installation of the interference bond, measurements as per section 8.4
shall be repeated.
8.5.2. Installation of Galvanic Anodes
When the area of stray current discharge is very localized, such as at
crossing with the interfering structure, the installation of galvanic anodes to
the foreign pipeline has considerable benefit. If the foreign pipeline is coated,
the path resistance through the galvanic anodes will be substantially less
than the coated pipeline.
8.5.3. Using Coating to Mitigate Interference Effects

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Applying a coating is an attempt to increase the resistance of the stray
current path, thus decreasing the stray current magnitude. As a stand alone
method, coating should only be applied at current pick-up locations. If the
discharge area of a structure is coated, there is a risk of corrosion failure due
to high discharge current density at a holiday in the coating.
8.5.4. Install/burying a metallic shield next to the affected structure
The intent of a buried metallic conductor is to intercept the stray current and
thus provide an alternative low resistance path for the stray current compared
to the metallic structure path. Connecting the metallic shield, which could be
a bare cable or pipe, directly to the negative terminal of the transformer
rectifier would more effective. This is applied to anodic Interference. i.e.
foreign pipeline crosses a groundbed voltage gradient.