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 DY: C A L A B A R - A J A O K U TA G A S P I P E L I N E P R O J E C T

TABLE OF CONTENT

•Abstract• Introduction•Methodology•Results and Analysis•Conclusion and Recommendations•References

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.

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 supplyNatural 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.

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)

• 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

• 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.

F I G U R E 1 . 3 : S E Q U E N C E O F P I P E L I N E D E S I G N ( K A D I R , 2 0 1 4 )

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.

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.

PIPELINE AND GAS DATA

PARAMETER DATAMaximum Allowable Operating Pressure (MAOP)

100 barg

Delivery Pressure 68 bargGas flow capacity (rate) 2000 MMSCFDPipeline length 490 kmGas temperature 30º CAmbient temperature 25º CPipeline diameter range 48-56 inchesDesign code ASME B 31.8Design factor 0.72Material Grade API 5L X60

Table 3.2: Pipeline and Gas data

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

PIPELINE DESIGN FORMULAS USED

Where is the base temperature (288.15K), Q is gas flow rate (m3/hr), f is frictional factor with as transmission factor, 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 LaminarRe > 4000, flow is Partially TurbulentRe > 107, flow is Fully Turbulent

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– Compression with combined pipe sizes

PIPE SIZES

(Inches)

COMPRESSION RATIO

46 4.0748 2.66

46 and 56 2.1748 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

PIPE SIZES LINEPIPE COST

(US DOLLARS)

NUMBER OF COMPRESSOR(

s)

REQUIRED

COMPRESSION RATIOS

46 184,664,340 2 2.09 and 1.9848 206,458,560 1 2.6656 272,873,160 - -

46 and 56 232,732,152.40 1 2.1748 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.

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.

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.

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