# Design & Calculation Cathodic Protection Impressed Cureent System

September 16, 2017 | Author: LuthfiSiradj | Category: Electric Current, Materials, Force, Electrical Engineering, Chemistry

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GUNUNG MEGANG - SINGA GAS COMPRESSION & PIPELINE FACILITY

DESIGN AND CALCULATION OF IMPRESSED CURRENT CATHODIC PROTECTION

0

13/07/09

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Issued For Approval

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18/05/09

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DATE

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APVD

DESCRIPTION

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MEPI

STATUS: A = Issued for Review; B = Issued for Approval; C = Approved for Construction TOTAL OR PARTIAL REPRODUCTION AND/OR UTILIZATION OF THIS DOCUMENT ARE FORBIDDEN WITHOUT PRIOR WRITTEN AUTHORIZATON OF THE OWNER

DOC. NO GMS-40-CS-014 REVISION

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DESIGN AND CALCULATION OF IMPRESSED CURRENT CATHODIC PROTECTION

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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.

8.2. 8.3. 8.4. 8.5.

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 List of Instrument and Equipment......................................................................... 10 Criteria .................................................................................................................. 11 Interference Testing.............................................................................................. 11 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

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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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.

3.

REFERENCE NACE RP-0169-2002

:

Control of External Corrosion on 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 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

DEFINITIONS The following terms shall have the meaning stated: Employer

:

PT. Mitra Energi Gas Sumatera

Underground

Maintenance

of

of

DESIGN AND CALCULATION OF IMPRESSED CURRENT CATHODIC PROTECTION

4.

Contactor

:

PT. Citra Panji Manunggal

Subcontractor

:

PT. Pelinkar Iso Mandiri

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LOCATION DATA The pipeline and in-plant pipeline is outline in the following table SECTION

Pipe Pipe Diameter AREA KP Start KP End Length (Inch) (mm) (m) KP. MUARA KP. ENIM 0+000 17+500 (Singa 10.75 273.0 (G. 17500 Megang Station) Station) Slug Pig Receiver catcher 10.75 273.0 G. MEP Megang

175.0

6.625 168.23 Pig Metering 205 Receiver G. Equal to 10.75 273.0 G. Megang 123 Megang

PIPELINE DATA Coating Wall Thickness System (Inch) (mm) 0,365

0.365/ 0.592

0.432

Pipe Status

9,271

9.271/ 15.04

1 Layer Primer Coating + 2 Layer Wrapping Tape

Buried

10.973

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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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) -750 mV -650 mV

Soil Resistivity (ohm meters) 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

DESIGN AND CALCULATION OF IMPRESSED CURRENT CATHODIC PROTECTION

• • •

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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 Length of Pipeline Maximum Design Temperature Current Density at Maximum Temperature Design Life Safety Factor Positive Limit Protection Level w.r.t. Cu/CU SO4 Negative Limit Protection Level w.r.t. Cu/CU SO4

0.273 M 17,500 M + In Plant piping 300 M 200oF = 93.33oC 88.33 mA 20 years 25% -0.950 mV -1.100 mV

Based on the above mentioned data, a calculation will be made to obtain the following: • • • • • • • •

7.

Surface Area Current requirement Quantity of anodes Resistance of groundbed Resistance of cable Total Resistance Transformer Rectifier DC Output Capacity Requirement Attenuation calculation

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 d Rb

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= Length of anode in feet = Diameter of anode in feet = 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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The potential attenuation of a pipeline is calculated using the following formula: Lp =

∆P ∆P0

A. Cosh

. Id

α

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

r

L

ra r25 d T To a E r T r

T

rs

8.

= Longitudinal Resistance of pipeline. This will Calculated by following formula:

= ra / a = r25 x [1 + δ (T – To)] = Average steel resistivity for grade API 5L – X56 steel at 25oC (ohm-m) = Temperature coefficient of resistance = 0.00306 = 93.33oC o = 25 C = Surface area of pipe per meter length = OD x π x 1 = Wall thickness of pipe = Transversal resistance of pipe ( ). This calculated by following formula: = rs / a = Coating resistance after installation and 20 years life (Ω/m2)

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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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 instrument/equipment:

will

be

carried

out

using

1) GPS Synchronised Current Interrupters 2) Portable copper/copper sulphate reference electrodes 3) Reel of test wired

the

following

the

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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.