NIGER DELTA PETROLEUM RESOURCES
ENGINEERING DESIGN OF
12 x 20 KM OGBELE RUMUJI PIPELINE PROJECT
CATHODIC PROTECTION REPORT
OG0108-OGB-SR-C-001
Rev.
No.
Date
30 Oct., 2009
Description
Approved for
Construction
Author
Reviewed
by
Approved
by
V. Ebesunun
I. Mbonu
K. Okanigbe
POINT
POINT ENGINEERING LIMITED
This document is controlled electronically and is uncontrolled when printed.
Approved
by NDPR
Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
TABLE OF CONTENT
1.0 INTRODUCTION
1.1 PROJECT BACKGROUND
1.2 SCOPE
2.0 CATHODIC PROTECTION REPORT
2.1 GENERAL
2.2 TRANSFORMER/RECTIFIER
2.3 DEEPWELL GROUND BED
2.4 CABLES AND ACCESSORIES
2.5 JUNCTION BOX
2.6 INSPECTION AND TESTING
2.7 CALCULATION
2.8 CURRENT REQUIREMENT FOR THE NEW FLOWLINE
2.9 DEEPWELL GROUND BED SIZING
2.9.1
ANODE REQUIREMENT
2.9.2
GROUNDBED RESISTANCE
2.9.3
QUANTITY OF BACKFILL
3.0 CONCLUSION
4.0 REFERENCES
Cathodic Protection Report
4
4
4
5
5
5
5
6
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7
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12
13
Oct, 2009
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Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
Abbreviations
AC
Alternating Current
CP
Cathodic Protection
DC
Direct Current
HMW-PE
High Molecular Weight polyethylene
KM (Km)
Kilometer
NDEP
Niger Delta Exploration and Production
NDPR
Niger Delta Petroleum Resources Limited
NLNG
Nigerian Liquefied Natural Gas
PEFS
Process Engineering Flow Scheme
PFS
Process Flow Scheme
POINT
Point Engineering Limited
ROW
Right of Way
SWA
Single Wire Armoured
SOW
Scope of Work
SPDC
Shell Petroleum Development Company
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Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
1.0 INTRODUCTION
1.1
PROJECT BACKGROUND
The client, Niger Delta Petroleum Resources (NDPR) Limited desires to deliver Lean
Gas via a 12 x 20km pipeline from its field in Ogbele/Otari/Obumeze Community to
the Nigeria Liquefied Natural Gas (NLNG) Company Pipeline Node (Manifold) at
Rumuji.
1.2
SCOPE
The overall purpose of this report is to evaluate the adequacy of the existing Cathodic
Protection system on the site for the new 20km Ogbele Rumuji pipeline using the
existing rectifier/ transformer.
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Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
2.0 CATHODIC PROTECTION REPORT
2.1
GENERAL
The Cathodic protection system that will operate in 12 x 20 Km Ogbele Rumuji
pipeline project is the existing impressed current type. The existing cathodic
protection system is supplied power from Obele diesel generating sets and supply
230V AC voltage, 26.7A, 50Hz frequency at the input and 50V DC, 50A at the output.
The system has an operating temp of 45 0C, oil cooled case. There are three cable
routes from the transformer/rectifier, the 1c x 70mm2 HMW-PE cable connected to
ground bed Junction Box JB-1. The proposed 1c x 70mm2 HMW-PE cable is
connected from proposed ground bed to Junction Box JB-4 which is in turn hookedup to the existing Rectifier system. There are two existing 1c x 35mm2 HMW-PE
cables, each connected to JB-2 and JB-3 as shown on drawing OG0108-OGB-ED-C001. Protection connections shall be made from JB-3 by 1c x 16 mm2 HMW-PE cable
to the 12 new pipeline as shown on drawing OG0108-OGB-ED-C-001 through the
spare rheostat. The new pipeline will be connected to JB-3 spare rheostat as shown
on drawing OG0108-OGB-ED-C-004.
2.2
TRANSFORMER/RECTIFIER
The existing transformer/rectifier units at the Ogbele flow stations shall be utilized for
the impressed current CP system.
The DC output of the transformer/rectifier is 50V, 50A and input is 230V AC voltage,
50Hz single phase system.
2.3
DEEPWELL GROUND BED
This section provides a guideline for design of impressed current ground bed. For this
project, one vertical deep well ground bed is proposed to boost the performance of
the existing cathodic protection system in order to accommodate the new 12 x 20 Km
pipeline. However, this proposition is to maintain the initial design of two ground beds
for the system for which one of the ground beds was not in use presently. The ground
bed shall be installed as shown on drawing OG0108-OGB-ED-C-003. During the
ground bed installation, the following should be taken into consideration:
The ground bed should be sited at least 100m from all buried metallic
structures within the area of influence surrounding the ground bed. This is to
prevent the metallic structures from picking up current from the ground bed and
create a stray current interference problem that will require corrective measures
when using the site.
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Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
The current source should be taken from the existing rectifier-powered
impressed current on the site.
The ground bed site should be reasonably accessible for construction and
maintenance purposes.
The ground bed must be free from further building construction (new
pipelines, highway development, or other similar work) that will make the site
untenable in the near future.
2.4
CABLES AND ACCESSORIES
Cables shall be single core multi-stranded copper conductors, HMW-PE
insulated, SWA, and over sheathed, with the color indicated on construction
drawings.
The cable cross-sections and routes are shown on the drawings OG0108OGB-ED-C-003.
The cable laying should be done with optimum care to avoid kink.
The cable should be laid to ensure that their allowable bending radius is
not exceeded.
Test point cables connection to the pipelines shall be by thermite welding
according to Standard Construction Specification (S17 of Shell Dep. 30.10.73.10Gen. Nov 2003 pg 79).
Electrical connections between anode and leads and header cable are
critical part of impressed current ground bed construction. Anode leads must be
connected to the header cable by a method that will have permanently low
resistance.
The connection, once made, must be insulated so that it will be waterproof
to present current leakage and cable corrosion.
2.5
JUNCTION BOX
The proposed Junction box (JB-4) shall be designed to provide electrical
continuity, voltage regulation via rheostats and potential balancing for the cathodic
protection system as shown on the drawing OG0108-OGB-ED-C-003.
JB-4 shall be installed such that offer ease of maintenance, and also
provide with special looking system.
The installed junction box shall be complete with resistors and current
shunts.
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Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
2.6
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
INSPECTION AND TESTING
After complete installation of the cathodic protection system, it is necessary to carry
out tests to ensure and confirm satisfactory installation and operation of all parts of
the system.
2.7
CALCULATION
The soil data used below for the analysis are based on the Final Report on Subsoil
Investigation for the proposed NDPR Gas Plant dated July 2007
Length of existing pipe line
7.5 Km
Diameter of existing pipe
6 (15.24 cm)
Length of new flow line
20 Km
Total length of distribution (l)
27.5 Km
Diameter of new pipe (d)
12 (30.5 cm)
Total diameter of pipe (D)
18 (45.74 cm)
Average soil resistivity
1500 cm
Corrosion Coating Thickness (tc)
3 mm
Average leakage conductance (G)
2.14 x 10-3s/m2 in 1000 cm
Average potential shift to remote earth
-0.250V
Total surface area of the installation pipe (A)
dl ( = 3.142)
3.142 x 45.74/100 x 27.5
39.522m
Estimated leakage conductance of new pipe in 100 cm (g)
=
GxA
2.14 x 10-3 x 39.522
0.0846 siemens
Resistance to remote earth
1/conductance
1/0.0846
11.824
Estimated resistance to remote earth in 1500 cm soil
=
11.824 x 1.5
17.736
Estimated current to shift pipe potential to remote earth -0.250V
I
Cathodic Protection Report
=
=
E/R
0.250/17.736
0.0141A
Oct, 2009
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Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
2.8
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
CURRENT REQUIREMENT FOR THE NEW FLOWLINE
Pipeline surface Area
=
=
=
(D + 2tc)L
(12 x 25.4 + 2(10)) x 20
2.0423 x 104 m2
The minimum design current density for PE coating for up to 30 - year life is
0.01mA/m2. Using a design current density, J = 0.5mA/m2,
Protection Current,
Ip
2.9
(A x J) / 1000
0.5 x 2.0423 x 10 / 1000
10.212 A
DEEPWELL GROUND BED SIZING
In the following section, ground bed sizing calculations are detailed. Note that the
sizing is based on Dwights formula. In addition, due to the unavailability of soil
resistivity data, the ground bed resistance calculation is based on assumed value of
1500 ohm.cm, which is reasonable at the great depth considered in this design.
2.9.1 ANODE REQUIREMENT
Anode Type
Mixed Metal Oxide
Anode Dimensions
100 cm x 2.5 cm
Net Weight
0.36 kg
Nominal Consumption Rate
0.0027kg/Ayr
Maximum Output per Anode
(at 1mA/cm2 Current Density)
8.0 A
Utilisation Factor
70%
Required Lifetime
25 years
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Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
Required Number of Anodes n
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
= Current Required for Protection
Current output/anode x Utilisation factor
=
Say
Anodes Group Weight
20 x 0.36
100 x 100
8 x 70
=
=
17.857
20 Anodes.
7.2 kg
Anodes consumption at maximum ground bed current output for the entire 25 years is
=
25 x 100 x 0.0027
=
6.75 kg
This is less than the anodes group weight and hence meets the anodes requirements
for the specified life of the pipelines.
The resistance to earth of the ground bed must allow the maximum predicted current
demand to be met at 80% or less of the voltage capacity of the DC source. This
minus the 2 V back voltage of the coke backfill divided by the current output from the
rectifier gives a maximum circuit resistance, Rmax, of
Rmax
Idc
100
Vdc x 80% - 2
50 x 0.8 2
= 0.38.
Where Vdc and I dc are the DC output ratings of voltage and current respectively of
the transformer rectifier.
2.9.2
GROUNDBED RESISTANCE
For a single anode placed vertically in soil, its resistance to the backfill medium (in
this case of 5 ohm resistance) is
8L
Ra
ln
1
2 L d
Where,
Ra= anode resistance in ohm
= medium resistivity in ohm.cm
L= anode length in cm
d= anode diameter in cm
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Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
8 100
5
ln
1
2 100 2.5
0.038
For an anode surrounded by coke backfill column of 600 cm length and 20 cm
diameter, the resistance of the backfill column to earth becomes
8 600
5
ln
1
2 600 20
0.006
Hence anode internal resistance is
0.038 0.006 0.032
Resistance of one anode installation in backfill of the given dimensions and soil of
1500 ohm.cm resistivity becomes
8 600
1500
ln
1 0.032 1.815
2 600 20
The combined resistance of the anodes and hence the anode ground bed resistance,
Rgb, taking into consideration the mutual interference is obtained from
Rgb
ln 0.66 n
n
nS
Where n = number of anodes
S
= anode spacing (500cm)
= single anode resistance
Rgb
1.221
1500
ln 0.66 20
20
20 500
0.184
Hence the ground bed configuration will be 20 no. anodes surrounded by
carbonaceous backfill material in a 200mm casing with anodes at 0.5 m spacing, the
total active length of the ground bed will be
(0.5+1) x 20 = 30 m
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Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
Giving a total deep well ground bed depth of 20 + 30 = 50 m
2.9.3 QUANTITY OF BACKFILL
The maximum volume occupied by the backfill in the well bore is taken, for ease of
calculation, to be the volume of the active length of the well bore neglecting the
volume occupied by anodes, cables etc.This volume is calculated from
Vb
D 2 L 3
m
4
Where L = length of active section
D
diameter of bore hole
Using the ground bed active section length of 30m and well bore of 20cm internal
diameter; the backfill volume is obtained from
Vb
30 0.2 2
m3
4
Vb
0.943 m3
Therefore volume of backfill will be Vb
0.943 m3
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Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
3.0 CONCLUSION
The existing rectifier/transformer on site has an output of 50V, 50A DC. From the
calculation above, the required protection current to supply the pipelines for the next
30 years is 10.212A, when added to the existing current gives 13.212A which is far
below the output current of the existing rectifier/transformer. The existing
rectifier/transformer is able to supply the required current to the new flowline.
Therefore the existing rectifier/transformer should be used as source of the impressed
current.
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Cathodic Protection Report
12 x 20 Km Ogbele Rumuji Pipeline Project
POINT ENGINEERING LIMITED
OG0108-OGB-SR-C-001
4.0 REFERENCES
1. Cathodic Protection system sizing report for zinc anodes of gas transmission
system (Ndele-Bonny pipeline) T-10.131.212-A
2. Cathodic protection system installation detail for deep well ground bed T10.131.100-8
3. Cathodic protection EPT 08-T-02 November 1992 Draft
4. Control of pipeline corrosion by A.W. Peabody (second edition) edited by Ronald
L. Bianchetti
5. Standard Construction Specification (S17 of SPDC).
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