The document discusses the optimal design of a natural gas transmission pipeline from Calabar to Ajaokuta, Nigeria. Steady state gas flow analysis was performed using Pipesim software to determine the optimal pipe size. The analysis showed a 46/56 inch pipe combination requiring a single compressor station would produce the lowest compression ratio and lowest cost. A cost analysis confirmed this 46/56 inch design as the most optimal and economically viable option for delivering 2000mmscfd of natural gas over 490km.

1. O P T I M A L D E S I G N O F A N A T U R A L G A S
T R A N S M I S S I O N S Y S T E M
B Y C I N I G E K P E
C A S E S T U D Y : C A L A B A R - A J A O K U T A G A S P I P E L I N E
P R O J E C T

2. TABLE OF CONTENT
•Abstract
•Introduction
•Methodology
•Results and Analysis
•Conclusion and Recommendations
•References

3. ABSTRACT
The transportation of natural gas is more complex than that of oil as it is more volatile
and unstable. Therefore, in designing a gas pipeline it is important to consider the gas
behaviour in the pipe, pipeline characteristics and transmission distance.
In this dissertation, the Calabar-Ajaokuta pipeline project (CAP) in Nigeria was adopted
as the case study. Steady state gas flow analysis was carried out using Schlumberger’s
Pipesim software with the gas capacity at 2000mmscfd, gas temperature at 30ºC, delivery
pressure at 68barg and MAOP at 100barg to obtain the optimal pipe size required to
deliver natural gas from Calabar to Ajaokuta (a distance of 490km) , safely, efficiently and
economically.
The result showed that the 46/56 inches pipes combination was an optimal design as it
produced the lowest compression ratio requiring a single compressor station and also
costing the least amongst other options.

4. INTRODUCTION
The transportation of petroleum products from production regions to the market is key
in meeting with the energy needs of the end users. It is important that these products
arrive safely and meet the demand requirements of the consumer. Therefore, the
transporters would have to ensure that the transportation system employed is designed
to meet these demands and at the same time cost effective. The efficient and effective
movement of these products require extensive and elaborate transportation system
(Rajnauth, et al., 2008).
The different modes of natural gas transportation which are currently been exploited,
researched and planned for future applications are pipeline, liquefied natural gas (LNG),
gas to wire, gas to liquid (GTL), gas to product, compressed natural gas (CNG), and
natural gas hydrates (NGH) (Rajnauth, 2008). Pipeline transportation is the most
matured and trusted mode accounting for 70% of the world’s gas supply
Natural gas transportation systems are categorised under two types which are
transmission and distribution system (Nasr and Connor, 2014). In transmission system,
gas is transported in pipes under high pressure.

5. EXISTING AND PROPOSED GAS PIPELINE PROJECTS IN NIGERIA
Figure 1.1: Schematics of the West African
Gas Pipeline (EIA, 2015)
Figure 1.2: Map showing the
routes of Trans Saharan Gas
Pipeline (Green, 2009)

6. • The Western gas pipeline network (WGPN) is the domestic gas pipeline that feeds
Lagos, western area of Nigeria and the West African Gas Pipeline (WAGP) for export to
neighbouring Gulf of Guinea countries as shown in Figure 1.1. Nigeria has been
exporting natural gas through WAGP since 2011 with initial capacity of 170mmscf/d
with and expected capacity of 460mmscf/d (Nwaoha and Wood, 2014).
• The Trans Saharan Gas pipeline Project (TSGP) is a joint venture agreement between
the Nigerian and Algerian government. It was signed in 2005 by the Nigerian National
Petroleum Corporation (NNPC) and Algerian national oil and gas company Sonatrach
with the aim of supplying natural gas to Europe. The gas will move from Nigeria’s
Niger Delta region to the north (Kano) of Nigeria through the East-North Gas pipeline
network to Niger, then to Algeria from which it moves into Europe through Algeria’s
Beni Saf and El Kala export terminal

7. • The Trans Nigerian Gas Project (TNGP) which is designed to link up with the Trans
Saharan Gas Project is considered here. The TNGP is a project designed to deliver gas
from the south-eastern region to the north region of Nigeria (Calabar-Umuahia-
Ajaokuta-Abuja-Kano) spanning about 1037km (Bello, 2013). This has further been
divided into two parts which are (Nwaoha and Wood, 2014):
• Eastern Gas Pipeline Network: Qua Ibo/ Calabar Ajaokuta Pipeline (CAP)
• East-North Gas Pipeline Network: Ajaokuta- Kaduna-Kano (AKK)
• This report focuses on the Calabar Ajaokuta Pipeline which is part of the Eastern Gas
Pipeline Network System.

8. FIGURE 1.3: SEQUENCE OF PIPELINE DESIGN (KADIR, 2014)
The sequence of design shows the important considerations made before an optimal
design is selected and it also shows how there are interlinked. These are steps
considered by the design engineer and design team when choosing a desired design
for either a transmission or distribution system.

9. METHODOLOGY
• The optimal design will be gotten by undertaking a simulated analysis of the gas in the
pipeline, considering gas temperature, delivery pressure, outer diameter, gas flow rate
and velocity. Also, the frictional component, inlet pressure, inner diameter, wall
thickness and other parameters will be obtain theoretically using some basic design
formulas dependent on the design code adopted.
• The approach adopted was an iterative method, where a set of parameters goes
through a process to meet a desired condition (output) and if not met, it would be
remodelled till the output is met.
• In this case, the parameters are; the inlet pressure, outer and inner diameter, wall
thickness, gas composition and gas flowrate, while the desired conditions are; gas
flowrate and delivery pressure.
• The pipe sizes varied was between 46” and 56” to obtain the desired condition with or
without the need for compressors.
• After the desired condition is satisfied, then stress calculations, tonnage calculations,
overall pipeline cost and result analyses were carried out.

10. PIPELINE AND GAS DATA
PARAMETER DATA
Maximum Allowable Operating Pressure
(MAOP)
100 barg
Delivery Pressure 68 barg
Gas flow capacity (rate) 2000 MMSCFD
Pipeline length 490 km
Gas temperature 30º C
Ambient temperature 25º C
Pipeline diameter range 48-56 inches
Design code ASME B 31.8
Design factor 0.72
Material Grade API 5L X60
These were the parameters used in carrying out the analysis. Some of which were fixed
and others varied. The fixed parameters are: gas composition, gas flow rate, while the
variable parameters are: the pipe size parameters (diameters and wall thickness), inlet
pressure which is influenced by the inner diameter.
Table 1.1: Given and obtained pipeline and gas data

11. PIPELINE DESIGN FORMULAS USED
• 𝑓 =
64
𝑅𝑒
• 𝑅𝑒 =
ū 𝑑 𝜌 (𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑜𝑟𝑐𝑒)
𝜇 (𝑉𝑖𝑠𝑐𝑜𝑢𝑠 𝑓𝑜𝑟𝑐𝑒)
• 𝑄 =
7.574
104
𝑇𝑠
𝑃𝑠
1
𝑓
𝑃1
2−𝑃2
2 𝑑5
𝑆 𝐿 𝑍 𝑇
0.5
• 𝐶. 𝑅 =
𝑃 𝑑
𝑃𝑠
Where 𝑇𝑠 is the base temperature (288.15K), Q is gas flow
rate (m3/hr), f is frictional factor with
1
𝑓
as transmission
factor, 𝑃𝑠 is the base pressure (1.01325 barg), P1 and P2 are
upstream and downstream pressure (bar) respectively, L is
the pipe length (m), T is the average temperature of the
gas (K), Z is compressibility factor, S is gas gravity, Ps is gas
suction pressure (psia) and Pd is gas discharge pressure
(psia).
The different regimes of fluid flow is shown below;
Re < 2000, flow is Laminar
Re > 4000, flow is Partially Turbulent
Re > 107, flow is Fully Turbulent

12. RESULTS AND ANALYSIS
• The analysis and discussion of the results obtained from following the procedures in the as
described in the methodology is presented here. Before the detailed steady state analysis of
gas flow, important pipeline and gas parameters data need to be given, obtained or
derived.
The derived data are as follows:
– Inlet pressure
– Specific gravity
– Wall thickness
– Pipe roughness
– Compressibility factor
– Reynolds number
– Relative roughness
• Design calculations are done using steady gas flow equations, moody charts and pipeline
codes. After these are done the simulation process begins using iterative techniques.
• Different approaches to gas hydraulic analysis is carried out using the flow analysis
software. These approaches were;
– Free flow analysis
– Obtaining Inlet Pressure from Delivery Pressure
– Inclusion of Compressors at Optimal Locations

13. PIPE SIZES
(Inches)
COMPRESSION RATIO
46 4.07
48 2.66
46 and 56 2.17
48 and 56 2.27
From the different approaches adopted, the following results were obtained
considering the pressure and velocity profile of gas flow within a pipe. It was
observed that to deliver 2000mmscfd over 490km, a network including a
compressor(s) is necessary.
Table 1.2 shows the pipe size combination of 46 and 56 inches as the best
possible gas transmission pipeline design. The gas flow analysis showed this
but a cost analysis was carried out to obtain the optimal design for the
transmission of natural gas from Calabar terminal to Ajaokuta.
Before assuming the optimal design a stress and cost analysis are carried out
to ascertain the economically viable option.
Table 1.2: Summary of C.R for different pipe
sizes

14. PIPE SIZES LINEPIPE COST
(US DOLLARS)
NUMBER OF
COMPRESSOR(s)
REQUIRED
COMPRESSION
RATIOS
46 184,664,340 2 2.09 and 1.98
48 206,458,560 1 2.66
56 272,873,160 - -
46 and 56 232,732,152.40 1 2.17
48 and 56 234,984,579.50 1 2.27
Table 1.3: Cost Analysis for pipe design
PIPELINE DESIGN COST ANALYSIS
The pipe unit weight was obtained from standard charts and with the pipe length of
490km, the tonnage was gotten. The unit price per tonne of API 5L X60 carbon steel pipe
material was obtained from a steel manufacturing company called Heibei Shenzhoul in
China as US $ 600/tonne. With these data the total cost of the pipeline is obtained.

15. CONCLUSION AND RECOMMENDATION
• the optimal design of a transmission system for the delivery of natural gas from
Calabar to Ajaokuta as part of the Eastern gas pipeline network of the Trans Nigeria
pipeline project was modelled and optimised using Schlumberger's Pipesim simulator.
• A steady state free flow analysis was carried out from which various approaches were
adopted to determine the optimal design. One of such approach, gave the possibility
of running a 56 inch pipeline over the full 490km pipe length to give the required
delivery pressure of 68barg at an inlet pressure of 92.8135barg very close to the MAOP
of 100barg. It was looked at as a possible pipe design but the cost analysis proved that
it was not an economical option especially because there is no room for increase in gas
capacity.
• It is best to select the option that requires less compression and a low compression
ratio. This conditions gave rise to the optimal design chosen. This design was the
46”/56” configuration with one compressor station operate on a compression ratio of
2.17. This design was selected because it had the lowest compression ratio and the
cost analysis proved the material cost to be $233 million dollars with a compressor
station of about $140 million, making this option the most economical.

16. In determining the optimal design for this case study which is Calabar-Ajaokuta pipeline
(CAP) for the Trans Nigeria pipeline project, the following were observed for further
recommendation as follows:
• The use of two pipe size configuration as the optimal design showed, were 46 inch and
56 inch pipe lengths were used at optimal compressor locations.
• The adoption of the velocity profile in determining the optimal compressor location. In
this study, the velocity profile was used to determine where compression should begin
as its no expected that the gas velocity should reach the erosional velocity of 20m/s.
So before this velocity is reached a compressor should be installed.
• In this study, steady flow Analysis was adopted for the simulation. But in future
designs, transient flow analysis is recommended to determine the time factor in flow
analysis because it is suitable for gas leakage determination. And also the concept of
gas storage in the pipe as it relates to linepack should be studied.
• The use of coolers after compression has occurred to reduce the gas temperature as
noticed in this simulation that the temperature rises really high just after compression.
• The utilisation of Schlumberger's Pipesim simulation software as a useful computer
simulation package for gas flow analysis.

17. REFERENCES
• Bello, G. (2013). The Trans-Saharan Gas Pipeline Project. Infrastructure Concession
Regulatory Commission, Nigeria.
• Energy Information Administration EIA. (2015). Country Analysis Brief: Nigeria, US Dept. of
Energy, Washington D.C.
• Green, M. (2009). Total to back Trans-Sahara Gas Pipeline. Available:
http://www.ft.com/cms/s/0/23d401e6-0338-11de-b405-
000077b07658.html#axzz3Wp1C12CA. Last accessed 9th April 2015.
• Kadir, A. (2015). Transmission Lecture note. Salford: University of Salford Press
• Nasr, G.G and Connor, N.E. (2014). Natural Gas Engineering and Safety Challenges:
Downstream Process, Analysis, Utilization and Safety. Switzerland: Springer International
Publishing.1-3 and 17-46.
• Nwaoha, C. and Wood, D.A. (2014). A review of the utilization and monetization of Nigeria's
natural gas resources: Current realities. Journal of Natural Gas Science and Engineering,
18(0), 412-432.
• Rajnauth, J. J., Ayeni, K. B., and Barrufet, M. A. (2008). Gas Transportation: Present and
Future. Society of Petroleum Engineers. Doi: 10.2118/114935-MS