The identification of natural gas as a cleaner and more environmentally friendly source makes it even more attractive in this age of sensitivity to the well being of the environment. Significant amounts of Nigeria's non-associated gas (NAG) reserves are located in the remote offshores. Nigeria's current proven gas reserves are approximately 182TCF (trillion cubic feet), making the country's gas reserves the seventh in the world. Apart from the recoverable reserve estimate, there are yet-to-be-found gas reserves in Nigeria, given largely unexplored position of the resources. The gas quality is high, virtually without sulphur, low in carbondioxide, CO2 and rich in liquids (condensate) content. [10] Its natural gas reserves/production is estimated at 109years" [8] and its unproven reserve are about 300,600tcf. [9]
However, Nigeria is among the top gas flaring countries of the world, accounting for about 16% of global gas flaring. [10] Official data for 2004 indicates that Nigeria lost over 8.5TCF of natural gas as a result of flaring during that year alone. [24] This volume of gas is equivalent to about 5% of the country's total proven reserves.
Fig. 1.1: National gas resource and demand balance. [14]
Fig. 1.2: Natural Gas Flaring, 2010. [11]
The non-utilization and flaring of natural gas made the government develop the Nigeria Master Plan which was approved on February 13, 2008. "It aims at growing the Nigerian economy with gas by pursuing three key strategies:
Stimulate the multiplier effect of gas in the domestic economy.
Position Nigeria competitively in high value export market
Guarantee the long term energy security of Nigeria." [12]
As part of the Master Plan is a construction of the first South to North gas transmission line that will take dry gas through the Akwa-Ibom/Calabar facility to Ajaokuta, Abuja, Kano and Katsina.[13]
The maintechnologies currently used or planned in the realization of "stranded" natural gas resource are given below: [14]
Gas re-injection or recycle.
Pipeline.
Liquefied Natural Gas (LNG).
Gas-to-liquids (GTL) diesel and synfuels.
Gas-to-methanol.
Compressed Natural Gas.
Gas-to-hydrates.
The new industrial projects being considered to increase gas utilization are: [14] Nigerian Liquefied Natural Gas Project, West African Gas Pipeline Project (Escravos), The Trans-Saharan Gas Pipeline, Equatorial Guinea Gas Plan, Nigerian Gas Project, Industrial Gas Project (Ajaokuta-Abuja-Kano and Aba-Enugu-Gboko), Escravos Gas-to-liquids Project.
Fig. 1.3: Total Energy consumption in Nigeria by type, IEA. [11]
1.2 Aim and Objectives of the Analysis
1.2.1 Aim
The aim of the dissertation is to analyze the natural gas pipeline from Ajaokuta to Kano, through the Nigerian Federal Capital, Abuja.
1.2.2 Objectives
The objectives of the analysis include to:
determine the flows in all the pipes, and
determine the pressures at the pipe junctions.
1.3 Scope of Work
The network analysis of the pipeline covers the main pipeline from Ajaokuta to Kano via Abuja. Every other pipelines connected (spur lines and offtakes) to the main line are considered as nodes, and the flow to these nodes are also analyzed including the pressure at the nodes.
A computer model is built and the simulation of the model is carried out. The results of the simulation are verified by comparing with the designed capacities and pressures of the pipeline to validate the model.
1.4 Thesis Structure
Chapter One: gives few description of natural gas: its properties, uses, composition etc., describes the general overview of the Ajaokuta-Abuja-Kano Gas Pipeline and the circumstances that necessitates the design of this pipeline, states the aim and objectives of the research and its justification.
Chapter Two: discusses gas transportation in pipelines, derives equations that governs gas flows in a pipeline and some analytical tool for these flows are considered.
Chapter Three: describes the methodology used to build the model, simulate the model and the validating of the model using existing data.
Chapter Four: lists the results obtained from the analysis and gives detailed discussion of the results.
Chapter Five: concludes the research work and offers necessary recommendations for future works.
1.5 Benefit of the Analysis
The analysis of the pipeline which has been designed is important because of the following reasons:
flow in each pipe will be determined.
pressure at each nodes will also be determined.
The determination of the flows and pressures ensures that the pipeline designed is capable of performing the purpose it is meant for and also meet future redevelopment if required.
1.6 Problem Description
Nigeria has an estimated 187 trillion cubic feet of proven natural gas reserve giving the country one of the top ten natural gas endowments in the world. Due to lack of gas utilization infrastructure, Nigeria still flares more than half of the natural gas she produces and re-injects 12% to enhance oil recovery.
The nation's main gas transmission system currently consist of three (3) major pipeline namely: Escravos - Lagos trunk pipeline (LP) for supplies to the western parts of the country and to Oben - Ajaokuta pipeline, which is the backbone for supplies to the North and Alakiri - Obigbo - Ikot - Abasi for the Eastern trunk.
The Nigeria Gas Company installed pipelines have a total system capacity of over 2.0BSCFTD. In Nigeria, new initiatives are arising within the gas sector; the Federal Government of Nigeria in 2008 approved new gas pricing and domestic supply obligation regulations that include short-term, medium-term and long term gas supply targets.
The short-term target is aimed at doubling domestic gas supply to 140 MMSCFTD by the end of 2008 and triples it to about 2,050 MMSCFTD by the end of 2009. Also in the plan, the gas availability to existing domestic industrial users is expected to triple
to about 450 MMSCFTD in 2009.
The medium-term include development of three major domestic gas transmission system that consist:
Western system comprising of the existing Escravos - Lagos Pipeline system
(ELPS) and a new offshore extension to Lagos
2. The first South-North gas transmission line that will take dry gas through the Akwa - Ibom /Calabar facility to Ajaokuta, Abuja, Kano and Katsina. The line will also serve the South - East states of Anambra, Abia, Ebonyi, Enugu and Imo and
3. The interconnector, which will link the eastern gas reserve centre with the other two transmission system.
The 740 km Ajaokuta - Abuja - Kaduna - Kano 42" gas pipeline project is taking place against the backup of domestic supply context in Nigeria.
It comprises the construction, commissioning and operation of approximately 740 km
Ajaokuta - Abuja - Kaduna - Kano 42" Natural Gas Pipeline supply system project [13]
CHAPTER TWO
LITERATURE REVIEW
2.1 Natural Gas
Natural gas importance as a source of energy to the world and its economy cannot be over-emphasized. It is one of the cleanest, safest and most useful of all energy sources. [1] Natural gas can be defined as "any gaseous material, usually combustible, and normally emerging from the ground either without outside assistance, purely under its own pressure, or from a bore hole drilled from the surface into an underground reservoir. [2]
The main constituent of natural gas is methane, it also usually contain ethane, and sometimes propane and butanes. Ethylene, propylene, butylenes, hydrogen, carbon dioxide, hydrogen sulphide, ammonia, and inert gases such as helium, argon, xenon etc. can also be present.
Natural gas can be found as "associated" with crude oil or as "dry" gas, this is due to the migration of hydrocarbon in a reservoir after a long period of time.
Dry natural gas contain majorly methane and ethane, other higher hydrocarbons are in trace quantities, while the associated gas contains propane and butane, which are liquefiable under pressure and also at times pentane and other heavier hydrocarbons
Table 2.1: Typical Composition of Natural Gas [1]
Gases
Molecular Formula
% Composition
Methane
CH4
70-90%
Ethane
C2H6
0-20%
Propane
C3H8
Butane
C4H10
Carbon dioxide
CO2
0-8%
Oxygen
O2
0-0.2%
Nitrogen
N2
0-5%
Hydrogen Sulphide
H2S
0-5%
Rare Gases
Ar, He, Ne, Xe
trace
Natural gas is deposited in several reservoirs all around the world, the "global natural gas reserves by country" is given below:
Fig. 2.1: Global natural gas reserves by country [3]
Fig. 2.2: World Natural Gas Reserves by Region, 2006 [4]
ProvedNaturalGasReserves2010 ENERGY INDEPENDENCE THE BIG LIE
Fig. 2.3: World Natural Gas Reserves by Region [5]
Natural gas has several uses which could be commercial, domestic, industrial and even for transportation.
Residential or domestic uses: cooking, heating, gas-powered appliances etc.
Commercial uses: space heating, water heating and cooling of office buildings, schools, churches, hotels, restaurants etc., and for cooking etc.
Industrial uses: production of plastic, fertilizers, anti-freeze, fabrics etc., and for lighting, waste-treatment and incinerations, metals preheating, drying and dehumidification, glass smelting, food processing etc.
It is also used for power generation and as fuels in vehicles.
2.1.1 Properties of Natural Gas
Natural gas is a combustible gas that is a mixture of simple hydrocarbon compounds. It is a fossil fuel that contains primarily methane, along with small amounts of ethane, butane, pentane, and propane. Natural gas does not contain carbon monoxide. The by-products of burning natural gas are primarily carbon dioxide and water vapour.
Natural gas is a colourless, tasteless, odourless, and non-toxic gas. Because it is odourless, a powerful chemical called mercaptan is added to the gas, in very small amounts, to give the gas a distinctive smell of rotten eggs. This strong smell can be helpful in detecting the source of any gas leak.
Natural gas is about 40% lighter than air, so should it ever leak, it can dissipate into the air. Other positive attributes of natural gas are a high ignition temperature and a narrow flammability range, meaning natural gas will ignite at temperatures above 1,100 degrees and burn at a mix of 5 - 15% volume in air.
Natural gas is found in rocks beneath the earth's surface, in sedimentary rock that is porous. Production companies explore, drill, and bring the natural gas to the surface.[6]
Table 2.2: Gross heating values of gases [7]
Component
Gross heating values (MJ/m3)
Nitrogen
0.0
Methane
37.697
Ethane
66.032
Propane
93.972
i-butane
121.426
n-butane
121.779
Pentane+
163.521
Gross heating value of mixture = ∑(mole fraction x gross heating value) of pure components
Table 2.3: Limit of flammability of gases with air at 1atm.(volume or mol%) [7]
Constituents
Lower limit
Higher limit
Methane
5.0
15.0
Ethane
2.9
13.0
Propane
2.1
9.5
n-butane
1.8
8.4
i-butane
1.8
8.4
n-pentane
1.4
8.3
i-pentane
1.4
8.3
Hexane
1.2
7.7
Hydrogen sulphide
4.3
45.5
2.2 Natural Gas Transport
The utilization of natural gas is usually predominant in regions/areas that are far away from the production sites. Most times, the gas produced travels through quite a long distance from the production wells to the point of use. "The efficient and effective movement of natural gas from producing regions to consumption regions requires an extensive and elaborate transportation system." [1] Natural gas is usually transported through pipelines which can be transmission or distribution lines.
2.3 Pipeline Segments
Pipeline segments are parts of a pipeline system; these include the main lines, spur lines, offtakes, etc.
The mainline is the segment of the of the pipeline that is connected to the source or upstream, it is the major line in a network of pipelines to which other pipes are connected to deliver gas to the final destination. The mainline is usually very long and comprises of larger diameter pipes.
The spur lines, also called branch lines, at the name connotes, are secondary lines that are connected to the mainline. The spur lines are usually shorter compared to the mainline and with lower diameter pipes which can be as low as six inches (6").
The figure below shows the Northern Leg Gas Pipeline (NLGP) system which consists of a 20 inch mainline (trunkline) which begins at the Magnus platform in the Northern North Sea and transports 80km (50miles) to the Brent A platform. There are three connecting spurline from Murchison, Thistle A and Statfjord B platforms. [15]
http://www.bpnsi.com/images/1/PageImages/NLGP%20picture.jpg
Fig. 2.4: Northern Leg Gas Pipeline showing mainline and spurlines. [15]
An offtake can be a pipeline transporting fluid from a larger pipeline transporting fluid from one point to another. [16] Hence, an offtake can be said to be the connection between a transmission and a distribution line or the point of connection between two transmission or distribution lines.
5244937109_8bd41dbf99
Fig. 2.5: An Offtake [17]
2.4 Gas Flow in Pipelines
For gas flow in pipes, the parameters such as velocity, pressure and density, which describe the behavior and state of a fluid, are not constant. They vary from one point in the pipe to another and from one instant in time to another. [18] The pipeline throughput (flow rate) will depend upon the gas properties, pipe diameter and length, initial gas pressure and temperature, and the pressure drop due to friction. [19]
2.4.1 Types of Flow
Steady-state Flow: This is a flow in which the various parameters at any point do not vary with time, i.e. they are assumed constant throughout the pipe length.
Unsteady-state Flow: This is a flow in which the various parameters at all points vary with time, i.e. they are not constant throughout the entire length of the pipe.
Uniform Flow: This is a flow in which the various parameters at any particular instant do not change with position.
Non-uniform Flow: This is a flow in which the various parameters change with position.
Other types of flows include:
Compressible/Incompressible flows.
Viscous/Inviscid flows.
Laminar/Turbulent flows.
Subsonic/Supersonic flows.
Also, the factors that affect the pressure drop of gas flowing between two points in a pipe include: Rate of gas flow, internal pipe diameter, pipe length, gas temperature, gas properties (e.g. viscosity, density), properties of the pipe wall, and initial pressure of the gas. [18]
2.4.3 Ideal and Real Gases
An ideal gas is defined as a gas in which:
the volume of the gas molecules is negligible when compared to the volume occupied by the gas,
the attraction or repulsion between individual gas molecules and the container is negligible,
the molecules are considered to be perfectly elastic, and
there is no internal energy loss resulting from collision between the molecules.
Generally, ideal gases obey several gas laws, like Boyle's, Charles's and ideal gas laws.
For ideal gases
where p = absolute pressure of the gas (Pa),
V = volume occupied by the gas (m3),
n = number of moles (moles) and it is given as:
R = universal gas constant (J/molK),
T = absolute temperature of gas (K).
Most gases will obey the ideal gas law when the pressures are close to the atmospheric pressure. When pressures are higher, the ideal gas equation will not be accurate for most real gases. The error in calculations at high pressures using ideal gas equation may be as high as 500% in some instances, compare to 2 to 3% at low pressure. [18]
For real gases, the ideal gas equation is modified by including a modifying factor called "compressibility factor" or "deviation factor", Z. the compressibility factor is the ratio of the volume occupied by a gas at a given temperature and pressure to the volume occupied at ideal state. Z is a dimensionless number usually less than 1.0 and varies with temperature, pressure and composition of the gas.
where p = absolute pressure of the gas (Pa),
V = volume occupied by the gas (m3),
n = number of moles (moles),
Z = gas compressibility factor (dimensionless),
R = universal gas constant (J/molK),
T = absolute temperature of gas (K).
The compressibility factors can be obtained from charts using the phenomenon of reduced pressure and temperature which is derived from the critical pressure and temperature, respectively.
where P = absolute pressure of gas (Pa),
T = absolute temperature of gas (K),
Tr = reduced temperature (dimensionless),
Pr = reduced pressure (dimensionless),
Tc = critical temperature (K),
Pc = critical pressure (Pa).
https://www.e-education.psu.edu/files/png520/graphics/figure0801.gif
Fig. 2.6: Compressibility Chart. [19]
Or pseudo-reduced states for gas mixtures:
where Tpr = pseudo-reduced temperature (dimensionless),
Ppr = pseudo-reduced pressure (dimensionless),
Tpc = pseudo-critical temperature (K),
Ppc = pseudo-critical pressure (Pa).
For a given mixture, the apparent molecular mass is given as:
where Ma = apparent molecular mass of gas (g/mol)
yi = mole fraction of gas component i (dimensionless)
Mi = molecular mass of gas component i (g/mol).
Similarly, Kay's rule can be used to calculate the average pseudo-critical properties of the gas mixtures.
There are other methods of calculating the compressibility factor which include:
Standing-Katz method (most popular).
Dranchuk, Purvis and Robinson method.
American Gas Association (AGA) method.
California Natural Gas Association (CNGA) method.
2.4.4 Reynolds Number
To differentiate between different flow regimes, a dimensionless parameter known as Reynolds Number is used. It is the ratio of the inertial forces to the viscous forces and given mathematically as:
where Re = Reynolds Number (dimensionless),
= average velocity (m/s),
d = characteristic linear dimension; diameter for a circular conduit (m),
Ï = density of fluid (Kg/m3),
µ = dynamic viscosity of fluid (Pas or Ns/m2 or Kg/ms).
For natural gas, since both density and velocity change with decrease in pressure, Reynolds number is usually calculated with the assumption that all flows occurred at Metric Standard Condition (MSC).
The mass flow rate of a gas entering a pipe, for a steady-state gas flow, is equal to the mass flow rate of gas leaving the pipe, so the continuity equation
where , , and = densities of fluid at point 1, point 2 and standard condition respectively,
, and = cross-sectional area of pipe at point 1, point 2 and standard condition respectively,
, and = average velocity of fluid at point 1, point 2 and standard condition respectively.
is valid not only for continuity of mass flow in the pipe but also for expressing the mass flow rate of the same gas at MSC.
From equation 2.11,
Since = = (same cross-sectional area).
Substituting equation 2.12 into equation 2.10
Also,
and
where Qs = gas flow rate at MSC (m3/s),
A = cross-sectional area of pipe (m2),
d = pipe diameter (m),
= density of gas at MSC (Kg/m3),
= density of air (Kg/m3),
S = relative density of gas (dimensionless).
Substituting equations 2.14, 2.15 and 2.16 into equation 2.13
but = 1.225Kg/m3
µ = 10.38 x 10-6Kg/m.s
and S = 0.6
Assuming Qs is in m3/h and d is in mm,
since 1m3/s = 3600m3/h
and 1m = 1000mm.
Note, when
Re < 2000, the flow is laminar,
2000 < Re < 4000, the flow is in the critical zone,
Re > 4000, the flow is partially turbulent (where most gas transmission takes place),
Re > 107, the flow is fully turbulent.
2.4.5 The Effect of Friction on Flow
Friction in fluid flow, as with any other process, results in a loss of energy. The total energy equation developed by Euler shows that a moving fluid possesses energy, and the equation is given as:
where x = elevation above a given datum (m),
p = absolute pressure (Pa),
Ï = density of fluid (Kg/m3),
u = velocity of fluid (m/s),
g = acceleration due to gravity (m/s2).
For incompressible fluids, where Ï is constant, by direct integration equation 2.19 becomes:
This is called the Bernoulli's equation.
For a fluid transmitted from a point 1 to another point 2, assuming no loss of energy (ideal system),
Due to friction, there are losses in the system, and these losses are represented with
where = frictional head loss (m).
For laminar flow, the frictional head loss is obtained from the Hagen-Poiseuille equation.
but
From equation 2.10,
Substituting equations 2.24, 2.25 and 2.26 into equation 2.23
Rearranging,
where = pipe diameter head loss,
= velocity head loss,
= friction factor for laminar flow.
For general flow,
where f = general friction factor.
Equation 2.28 is known as the Darcy-Weisbach equation.
The friction factor f, can be calculated from several equations or read on the Moody chart if the Reynolds Number and pipe relative roughness is known.
http://www.mathworks.com/matlabcentral/fx_files/7747/1/moody.png
Fig. 2.7: Moody Chart. [20]
2.5 General Flow Equation
The following assumptions are made in the derivation of the General Flow Equation or Fundamental Flow Equation:
the flow process is isothermal,
changes in the kinetic energy of the gas are negligible,
the pipeline is horizontal,
no mechanical work is done by or on the gas,
the energy loss due to friction is given by the Darcy-Weisbach equation (2.28), and
the flow is steady state.
Euler's equation for an ideal system is
Incorporating the head loss due to friction for a real system, it becomes
but,
For an elemental length, dL, the equation becomes:
For a horizontal pipeline with negligible kinetic energy changes,
For compressive fluids such as gases, the density and velocity changes. Therefore, using the continuity equation
Substituting this in equation 2.29
Multiply through by
To deal with the variable density, an isothermal condition is assumed (i.e. gas temperature is constant). From the real gas equation:
Substituting 2.31 into 2.30
Multiply through by p
But = 1.0
Integrating between p1 and p2 at a distance L apart
But
and
where Ra = gas constant for air (J/molK)
S = relative density of gas (dimensionless)
Equation 2.33 then becomes:
From equation 2.15
If L is in meters (m)
p is in bar
d is in millimeters (mm)
Qs is in m3/h
T is in K
Since Ra = 287.06J/molK
This is the general flow equation.
Assuming ideal gas (Z =1.0), and temperature remains constant (T = 288.15K) and the flow is expressed at MSC i.e.
Ts = 288.15K
ps = 1.01325bar(abs)
Equation 2.34 becomes
This is the medium pressure gas flow equation.
For low pressure gas flow,
Substituting equation 2.36 into equation 2.35
but ps = 1.01325bar(abs)
To obtain in mbar
Since 1000mbar = 1bar
This is low pressure gas flow equation.
Generally, the general flow equation is:
Where KN = Numerical constant
KG = Gas properties constant
KP = Pipe constant
2.5.1 Effect of Pipe Elevations
When elevation difference between the ends of a pipe segment is included, the General flow equation is modified as follows:
where
The equivalent length,, and the term take into account the elevation difference between the upstream and downstream ends of the pipe segment. The parameter s depends upon the gas gravity, gas compressibility factor, the flowing temperature and the elevation difference.
where s = elevation adjustment parameter (dimensionless)
H = elevation difference (m).
2.5.2 Average Pipe Segment Pressure
In the General Flow Equation, the compressibility factor Z, is used and it is calculated at the gas flowing temperature and average pressure in the pipe segment. The average pressure, pavg, in a pipe segment is:
2.5.3 Explicit Flow Equations
These incorporate Blasius-type transmission factor which gives the transmission factor in terms of Reynolds Number only. For example, the Panhandle 'A' flow equation uses,
Substituting this into the General flow Equation (2.34)
All equations of this type may be represented by:
where n = flow exponent (dimensionless)
K = resistance factor (dimensionless)
The low pressure gas flow equation is represented by
This is known as the Gas Deliverability Equation.
2.5.4 Gas Velocity
The velocity of gas flow in a pipeline is the speed at which the gas molecules move from one point to another. Unlike a liquid flow in pipeline due to compressibility, the gas velocity depends upon the pressure and hence, will vary along the pipeline length even if the cross-sectional area remains constant. The highest velocity will be at the downstream end, where the pressure is the least. Correspondingly, the least velocity will be at the upstream end, where the pressure is higher. [19]
The velocity of gas is given by:
2.5.5 Erosional Velocity
The gas velocity in a pipeline varies directly as the flow rate. As the flow rate increases, the gas velocity increases. It is important to know how high the gas velocity can be in a pipeline. This is because as the velocity increases, vibration and noise become more evident. Higher velocities result in erosion of the pipe interior over a long period of time.
The maximum gas velocity or erosional velocity is usually approximately calculated from the equation:
Where = maximum or erosional velocity (m/s)
= gas density at flowing temperature (Kg/m3)
But
Therefore,
The velocity of gas is usually less than 20m/s to prevent erosion of the walls of the pipeline.
2.5.6 Compressor Stations
The compressor stations compensate for the pressure drops due to friction in the pipeline, valves and other joints, as well as those due to elevation changes. In a pipeline network, compressor stations consume a small fraction of transported gas. [22]
As the gas flow through the network pressure (and energy) is lost due to friction between the gas and the pipes' inner wall, and heat transfer between the gas and its environment. The loss of energy of the gas is periodically restored at the compressor stations, which are installed in the network. These compressor stations typically consume about 3-5% of the transported gas. [23]
For a given length of pipeline with constant diameter, if compressors stations are installed at equal intervals along the line, the relationship between the capacity of the pipeline and the number of compressor stations is given as:
where Q = pipeline capacity (m3/s),
N = number of compressor stations at equal intervals along the pipeline (dimensionless),
L = pipeline length (m).
2.6 Network Analysis of Gas Pipelines
Network analysis is a technique of solving networks - flow and pressure. Previously, networks are solved using trial-and-error methods, which later was developed by using the Hardy Cross method and others like it. Recently, computer programs have been developed to model, analyze and simulate networks of pipeline.
pipe flow simulation diagram
Fig. 2.8: Various reasons to simulate pipe flow
Pipe flow simulation schematics
Fig. 2.9: Various parameters affecting pipe flow computational complexity
2.6.1 Objectives of Network Analysis
The objectives of network analysis when applied to gas supply systems are to:
determine the flows in all the pipes/capacity,
determine the pressures at the pipe junctions.
Usually these objectives can be met when the values of the following are known:
the fixed pressures at all supply points,
the loads or offtakes from the system,
other possible constraints such as the minimum pressure at any points in the network.
The input requirements for network analysis include: Flow equation, pipe diameter, pipe length, source pressure, and load data.
2.6.2 Application of Network Analysis to Gas Systems
To determine the minimum pressure areas within an existing network and hence the requirement for reinforcement of these areas.
To optimize pressures within a network.
For operational planning, i.e. simulating the effect of plant failures, pipe breakage, or maintenance shut downs, under varying condition and prepare contingency plan accordingly.
To test the system under specific conditions.
Network design for new housing development.
2.6.3 Network Analysis Laws and Techniques
There are several network analysis techniques, but it is important, first and foremost to define a couple of terms used to describe the elements of a network.
2.6.3.1 Definition of Terms
(1) Source: This is any point where gas enters the network at a fixed pressure.
(2) Node: This is a junction of two or more pipe sections and can also include the free end of a spur pipe. Nodes exist at:
The junction between two or more pipes.
The free end of a spur pipe.
Where a change of diameter occurs.
Where a change in pipe material occurs
An input to a network - source node.
The offtake from a network - a load node.
The inlet or outlet of valves, regulators, compressors, etc. [21]
(3) Tree: This is a network in which each pipe is connected to a source via a route only.
(4) Loop: This is a condition where two or more paths exist to supply a particular pipe or node within a network. It is a closed path which begins and ends at the same point.
2.6.3.2 Kirchhoff's Laws
The principles that govern the quantitative volume of gas flow through nodes and around loops are known as Kirchhoff's laws.
Kirchhoff's First Law
It states that the volume of flow entering a given node is equal to the volume of flow leaving that node in a specified period of time, that is, input flow equals output flow at any given time.
Mathematically, the algebraic sum of all flows entering and leaving a given node is equal to zero, i.e.:
Kirchhoff's Second Law
It states that at a given instant in time, the pressure difference between any two nodes in a network is fixed and is the same for every flow path between those two nodes.
Mathematically, the algebraic sum of the pressure drops around any given loop is equal to zero:
This law also applies to the difference in the pressures squared, i.e.:
Since only one pressure value exists at a given node in a network.
2.6.3.3 The Hardy Cross Method
The Hardy Cross equation is given by:
or if n =2,
where Qij = flow between nodes i and j (m3/s)
() = (r +1)th iteration (dimensionless)
r = rth iteration (dimensionless)
= resistance factor of pipe between nodes i and j (dimensionless)
= length of pipe between nodes i and j (m)
n = flow exponent (dimensionless).
The Hardy Cross method is simple and convenient, especially when dealing with large steady state networks such as low pressure distribution systems. Its limitation is that it cannot be used for transient analysis because Kirchhoff's laws, on which Hardy Cross is based, are not applicable. [21]
Fig. 2.10: Algorithm of the analysis of a simple network.
2.6.3.4 The Nodal Method
The nodal method is applied to nodes only and loops are not considered although loops are present in most networks. The advantage of the nodal method is that it does not require the network loops to be defined, a process which can take up a significant proportion of the overall solution time on a computer. The method does have a significant disadvantage which is that convergence problems can occur making the solution highly dependent on the initial estimate for the nodal pressures. [21]
The nodal equation is given by:
OR
where Qij = flow between nodes i and j (m3/s)
() = (r +1)th iteration (dimensionless)
r = rth iteration (dimensionless)
= load from node i (m3/s)
= algebraic sign (, if is +ve,, if
= constant which is the inverse of resistance factor (dimensionless), i.e
(bar or Pa)
= initial estimate of (bar or Pa).
Fig. 2.11: Chart for Network Analysis Methods
CHAPTER THREE
MODELLING AND SIMULATION
3.1 Design Parameters
The total approximate length of the proposed pipeline route from Ajaokuta to Kano TGS is 585km, the spur line from the Abuja Node to the Abuja TGS is 15.5km long and the spur line to the Kaduna TGS is 0.5km.
The following diameters have been defined and recommended for use recommended for use on the following sections:
From Ajaokuta Tie-in Station to Kano TGS 2 x 42" pipelines.
From Abuja Node to Abuja TGS 2 x 20" pipelines.
From Kaduna Node to Kaduna TGS 2 x 20" pipelines.
The pipeline is sized for an ultimate design capacity of 3,000MMSCFD. The Abuja and Kaduna spurs are sized for 500MMSCFD each leaving up to 2,000MMSCFD available at Kano for local distribution and export to the future Trans-Saharan pipeline.
The design basis specifies that the minimum supply pressure at Ajaokuta and the minimum delivery at Kano are to be 68.95barg (1,000psig).
To satisfy this requirement booster compressor stations along the length will be installed to compensate unavoidable pressure losses due to friction. It is proposed that each station comprises two turbo compressor trains in operation with one spare train. The hydraulic simulation indicates that the optimum locations of the Booster Compressor stations are at 60km, 147 km and 440 km from Ajaokuta.
Summarily, a 42" dual pipeline installation with 3 booster compressor stations is proposed for cost effectiveness.
Fig. 3.1: Selected Pipeline Route
3.2 Building the Model
The model of the pipeline was built using Pipesim (Steady-state Multiphase Flow Simulation) software package. The model comprises of a source (Ajaokuta tie-in) and three sinks (Abuja TGS, Kaduna TGS and Kano TGS) with two nodes (Abuja and Kaduna nodes). The mainline consists of the pipeline between Ajaokuta to Kano TGS (flowlines B1, B3 and B5) and spurlines to Abuja TGS(flowline B2) and Kaduna TGS (flowline B4).
Fig. 3.2: Model of the Ajaokuta-Abuja-Kano Gas Pipeline
3.3 Input Data
The following data were inputted:
Table 3.1: Input data for nodes
Name
Type
Temperature (°C)
Gas Rate (mmscfd)
Pressure (barg)
Ajaokuta
Source
37
1500
Abuja TGS
Sink
-
250
Kaduna TGS
Sink
-
250
Kano TGS
Sink
-
-
68.95
Note: 1500mmscfd =42.48mmsm3/d
Table 3.2: Input data for flowlines
Flowlines
Distance (km)
Elevation (m)
Diameter (inches/mm)
Temp. (°C)
Ajaokuta-Abuja
233
150
42/1066.8
37
Abuja - Kaduna
244
405
42/1066.8
37
Kaduna - Kano
244
-10
42/1066.8
37
Abuja node - Abuja TGS
19
80
20/508.0
37
Kaduna node - Kaduna TGS
0.75
10
20/508.0
37
Note: 1inch = 25.4mm
3.4 Fluid Composition
Natural gas is a compositional fluid consisting of the following gases. The composition (in mol%) used are given below:
Fig. 3.3: Compositional properties of natural gas
G:\Dissertation Original\Network Analysis\Phase envelope plots.BMP
Fig. 3.4: Phase envelope for the natural gas
3.5 Simulation
After the building of the pipeline model and inputting of the data, the following simulations were made:
Horizontal pipeline with no elevation effects and compressors.
Elevation effects.
Fig. 3.5: Elevation against pipeline route for the pipeline
Compressor effects.
Increase flow effects.
Decrease flow effects.
3.6 Model Validation Using Existing Data
The model built and simulated was compared with the existing data and analyzed.
CHAPTER FOUR
RESULTS AND DISCUSSIONS
4.1 Horizontal Pipeline
The following results were obtained for the simulation of the horizontal pipeline model without considering elevation effects:
Table 4.1: Parameters at the nodes for horizontal pipeline
Name
Type
Temperature (°C)
Pressure (barg)
Gas Flow (mmscfd)
Abuja node
Junction
31.45
104.68
1500
Abuja TGS
Sink
31.28
103.40
250
Ajaokuta
Source
37.00
126.66
1500
Kaduna node
Junction
29.36
85.54
1250
Kaduna TGS
Sink
29.34
85.40
250
Kano TGS
Sink
29.56
68.95
1000
Table 4.2: Parameters in the pipes for horizontal pipeline
Name
Temp. (°C)
Press. (barg)
Gas Flow (mmscfd)
Max. mean vel. (m/s)
Max. erosional vel. (m/s)
IN
OUT
IN
OUT
B1
37.00
31.45
126.66
106.06
1500
3.79
10.44
B2
31.45
31.28
106.06
103.40
250
2.87
10.59
B3
31.45
29.36
104.68
85.56
1250
4.05
11.81
B4
29.36
29.34
85.56
85.40
250
3.58
11.82
B5
29.36
29.56
85.54
68.95
1000
4.22
13.47
Fig. 4.1: Graph of Pressure (barg) against Total Distance (km) for horizontal pipeline.
Fig. 4.2: Graph of Temperature (K) against Total Distance (km) for horizontal pipeline
Fig. 4.3: Graph of Gas Velocity (m/s) against Total Distance (km) for horizontal pipeline
Fig. 4.4: Graph of Erosional Velocity (m/s) against Total Distance (km) for horizontal pipeline
4.2 Elevation Effect
The following results were obtained for the simulation of the horizontal pipeline model considering elevation effects:
Table 4.3: Parameters at the nodes considering elevation effect
Name
Type
Temperature (°C)
Pressure (barg)
Gas Flow (mmscfd)
Abuja node
Junction
31.73
115.58
1500
Abuja TGS
Sink
30.96
111.87
250
Ajaokuta
Source
37.00
136.90
1500
Kaduna node
Junction
27.09
87.78
1250
Kaduna TGS
Sink
27.02
88.12
250
Kano TGS
Sink
28.02
68.95
1000
Table 4.4: Parameters in the pipes considering elevation effect
Name
Temp. (°C)
Press. (barg)
Gas Flow (mmscfd)
Max. mean vel. (m/s)
Max. erosional vel. (m/s)
IN
OUT
IN
OUT
B1
37.00
31.73
136.90
115.58
1500
3.43
9.93
B2
31.73
30.96
115.58
111.87
250
2.60
10.08
B3
31.73
27.09
115.58
87.79
1250
3.84
11.50
B4
27.09
27.02
88.40
88.12
250
3.37
11.47
B5
27.09
28.02
87.78
68.95
1000
4.17
13.39
Fig. 4.5: Graph of Pressure (barg) against Total Distance (km) considering elevation effects
Fig. 4.6: Graph of Temperature (K) against Total Distance (km) considering elevation effect
Fig. 4.7: Graph of Gas Velocity (m/s) against Total Distance (km) considering elevation effect
Fig. 4.8: Graph of Erosional Velocity (m/s) against Total Distance (km) considering elevation effect
4.3 Increase Flow
4.3.1 2000MMSCFD Capacity (4000MMSCFD in Total)
Table 4.5: Parameters at the nodes for increased flow at 2000mmscfd
Name
Type
Temperature (°C)
Pressure (barg)
Gas Flow (mmscfd)
Abuja node
Junction
30.89
122.90
2000
Abuja TGS
Sink
27.31
112.06
500
Ajaokuta
Source
37.00
153.56
2000
Kaduna node
Junction
23.85
87.55
1500
Kaduna TGS
Sink
26.68
86.93
500
Kano TGS
Sink
29.56
68.95
1000
Table 4.6: Parameters in the pipes for increased flow at 2000mmscfd
Name
Temp. (°C)
Press. (barg)
Gas Flow (mmscfd)
Max. mean vel. (m/s)
Max. erosional vel. (m/s)
IN
OUT
IN
OUT
B1
37.00
30.89
153.56
123.37
2000
4.20
9.51
B2
30.89
27.31
123.37
112.06
500
5.01
9.89
B3
30.89
23.85
122.90
87.56
1500
4.50
11.37
B4
23.85
23.64
87.56
86.93
500
6.67
11.41
B5
23.85
26.68
87.55
68.95
1000
4.12
13.33
Fig. 4.9: Graph of Pressure (barg) against Total Distance (km) 2000MMSCFD Capacity
Fig. 4.10: Graph of Temperature (K) against Total Distance (km) 2000MMSCFD Capacity
Fig. 4.11: Graph of Gas Velocity (m/s) against Total Distance (km) 2000MMSCFD Capacity
Fig. 4.12: Graph of Erosional Velocity (m/s) against Total Distance (km) 2000MMSCFD Capacity
4.3.2 2500MMSCFD Capacity (5000MMSCFD in total)
Table 4.7: Parameters at the nodes for increased flow at 2500mmscfd
Name
Type
Temperature (°C)
Pressure (barg)
Gas Flow (mmscfd)
Abuja node
Junction
28.98
145.50
2500
Abuja TGS
Sink
27.67
137.36
500
Ajaokuta
Source
37.00
184.29
2500
Kaduna node
Junction
21.22
102.86
2000
Kaduna TGS
Sink
21.05
102.44
500
Kano TGS
Sink
18.46
68.95
1500
Table 4.8: Parameters in the pipes for increased flow at 2500mmscfd
Name
Temp. (°C)
Press. (barg)
Gas Flow (mmscfd)
Max. mean vel. (m/s)
Max. erosional vel. (m/s)
IN
OUT
IN
OUT
B1
37.00
28.98
184.29
144.95
2500
4.44
8.74
B2
28.98
27.67
144.95
137.36
500
4.06
8.91
B3
28.98
21.22
145.50
102.96
2000
4.76
10.12
B4
21.22
21.05
102.96
102.44
500
5.28
10.15
B5
21.22
18.46
102.86
68.95
1500
5.83
12.93
Fig. 4.13: Graph of Pressure (barg) against Total Distance (km) 2500MMSCFD Capacity
Fig. 4.14: Graph of Temperature (K) against Total Distance (km) 2500MMSCFD Capacity
Fig. 4.15: Graph of Gas Velocity (m/s) against Total Distance (km) 2500MMSCFD Capacity
Fig. 4.16: Graph of Erosional Velocity (m/s) against Total Distance (km) 2500MMSCFD Capacity
4.4 Decrease Flow
4.4.1 1000MMSCFD Capacity
Table 4.9: Parameters at the nodes for decreased flow at 1000mmscfd
Name
Type
Temperature (°C)
Pressure (barg)
Gas Flow (mmscfd)
Abuja node
Junction
32.21
91.39
1000
Abuja TGS
Sink
31.14
86.94
250
Ajaokuta
Source
37.00
108.79
1000
Kaduna node
Junction
31.95
74.90
750
Kaduna TGS
Sink
31.84
74.62
250
Kano TGS
Sink
35.25
68.95
500
Table 4.10: Parameters in the pipes for decreased flow at 1000mmscfd
Name
Temp. (°C)
Press. (barg)
Gas Flow (mmscfd)
Max. mean vel. (m/s)
Max. erosional vel. (m/s)
IN
OUT
IN
OUT
B1
37.00
32.21
108.79
91.39
1000
3.06
11.47
B2
32.21
31.14
91.39
86.94
250
3.55
11.78
B3
32.21
31.95
91.39
75.17
750
2.90
12.91
B4
31.95
31.84
74.90
74.62
250
4.30
12.96
B5
31.95
35.25
74.90
68.95
500
2.19
13.74
Fig. 4.17: Graph of Pressure (barg) against Total Distance (km) 1000MMSCFD Capacity
Fig. 4.18: Graph of Temperature (K) against Total Distance (km) 1000MMSCFD Capacity
Fig. 4.19: Graph of Gas Velocity (m/s) against Total Distance (km) 1000MMSCFD Capacity
Fig. 4.20: Graph of Erosional Velocity (m/s) against Total Distance (km) 1000MMSCFD Capacity
4.4.2 750MMSCFD Capacity
Table 4.11: Parameters at the nodes for decreased flow at 750mmscfd
Name
Type
Temperature (°C)
Pressure (barg)
Gas Flow (mmscfd)
Abuja node
Junction
33.77
82.09
750
Abuja TGS
Sink
31.91
76.89
250
Ajaokuta
Source
37.00
94.99
750
Kaduna node
Junction
34.93
70.91
500
Kaduna TGS
Sink
34.78
70.62
250
Kano TGS
Sink
36.87
68.95
250
Table 4.12: Parameters in the pipes for decreased flow at 750mmscfd
Name
Temp. (°C)
Press. (barg)
Gas Flow (mmscfd)
Max. mean vel. (m/s)
Max. erosional vel. (m/s)
IN
OUT
IN
OUT
B1
37.00
33.77
94.99
82.09
750
2.65
12.34
B2
33.77
31.91
82.09
76.89
250
4.15
12.73
B3
33.77
34.93
82.09
71.49
500
2.10
13.44
B4
34.93
34.78
70.91
70.62
250
4.68
13.52
B5
34.93
36.87
70.91
68.95
250
1.11
13.83
Fig. 4.21: Graph of Pressure (barg) against Total Distance (km) 1500MMSCFD Capacity
Fig. 4.22: Graph of Temperature (K) against Total Distance (km) 1500MMSCFD Capacity
Fig. 4.23: Graph of Gas Velocity (m/s) against Total Distance (km) 1500MMSCFD Capacity
Fig. 4.24: Graph of Erosional Velocity (m/s) against Total Distance (km) 1500MMSCFD Capacity
4.4.3 Compresses Effects
Table 4.13: Proposed designs for compressors
Compressors
Outlet Pressure (barg)
Power (hp)
∆p (bar)
1
100
8716.2
11.79
2
100
7726.1
10.38
3
100
132280
76.93
Table 4.14: Optimized designs for compressors
Compressors
Outlet Pressure (barg)
Power (hp)
∆p (bar)
1
100
21422
25.61
2
100
25894
29.37
3
100
23955
27-11
Table 4.15: Pipeline length between compressors
Names
Designed Length (km)
Optimized Length (km)
Source - Compressor 1
60
150
Compressor 1 - Compressor 2
87
155
Compressor 2 - Compressor 3
293
150
Compressor 3 - Sink
300
280
Fig. 4.25: Designed compressor profile plot
Fig. 4.26: Optimized compressor profile plot
4.5 Discussion of Results
4.5.1 Horizontal Pipeline
The horizontal pipeline is the simplest (fundamental or basic) pipeline to be considered. Without elevation effect, the pressure-elevation relationship is almost non-existent. For the Ajaokuta-Abuja-Kano gas pipeline to supply a total capacity of 3000MMSCFD/84.96mmsm3/day (1500MMSCFD in each of the parallel pipelines of equal diameters) and maintaining a minimum pressure of about 68.95barg (1000psig), the source pressure at Ajaokuta is expected to be 126.66barg (1837.1psig). This pressure is larger than the maximum allowable operating pressure in gas transmission pipeline which is about 100barg (1450psig), for safety purpose.
This high pressure, maximum allowable operating pressure and the minimum pressure to be allowed in the pipeline will generate no significant effect on the Ajaokuta-Abuja-Kano gas pipeline because of the three (3) compressor stations located along the pipeline, which are located 60km, 147km and 440km from the Ajaokuta source on the mainline. These positions cover the areas where the largest elevation differences are experienced. Therefore, if the gas from the Ajaokuta source is supplied at 100barg (instead of 126.66barg) with a pressure drop of 31.05barg being the maximum pressure drop allowed, recompressed after 60km, 147km and 440km respectively, it is expected the gas reaches its destinations (sinks at Abuja, Kaduna and Kano Terminal Gas Stations) at about 68.95barg.
The temperature difference in the pipeline is not significant and hence, hydrate formation in the gas pipeline is not expected. The maximum mean gas velocity is about 4.22m/s and the maximum erosional velocity is about 13.47m/s which is below 20m/s, the maximum allowed.
Hence, the pipeline, as designed without considering elevation effect, is capable of supplying 3000MMSCFD (84.96mmsm3/day) of natural gas from Ajaokuta to Abuja, Kaduna and Kano Terminal Gas Stations.
4.5.2 Elevation Effect
Also, considering a minimum pressure of 68.95barg (1000psig), the pipeline as designed will require a source pressure of 136.90barg (1985.5psig) at Ajaokuta which is greater than the 100barg (1450psig) maximum allowable pressure, but with the compressor stations situated along the pipeline, this should not be a significant problem, and it is expected that the pipeline as designed will meet the 3000MMSCFD (84.96mmsm3/day) capacity with a minimum pressure of 68.95barg (1000psig) along the pipeline.
The maximum mean velocity for the gas is about 4.17m/s and a maximum erosional velocity of 13.37m/s, also below the maximum of 20m/s allowed.
4.5.3 Increase Flow
If the pipeline is expected to deliver a higher capacity due to maybe an increase in demand without any structural or physical modification of the pipeline, a greater pressure is required.
For a 4000MMSCFD (113.26mmsm3/day) of gas to be supplied in the pipeline, a pressure of 153.56barg (2227.2psig) is expected to supply the gas from Ajaokuta source, and this is significantly higher than the 100barg (1450psig) maximum allowable operating pressure. The constraint imposed by the high pressure is reduced (minimized) by the three (3) compressor stations used along the pipeline. The maximum mean gas velocity is about 6.67m/s and a maximum erosional velocity of 13.33m/s.
It will however be extremely more difficult, to almost practically impossible to supply a higher capacity than this without modifications to the pipeline in terms of addition of more compressor stations, pipe diameter increase, or reinforcement. For example, to supply a capacity of 5000MMSCFD (141.58mmsm3/day) with a minimum pressure of 68.95barg (1000psig) allowed in the pipeline, the source pressure of about 184.29barg (2672.9psig) is expected at the Ajaokuta end which is about twice the maximum allowable operating pressure of 100barg (1450psig). This makes it practically impossible to supply this amount of gas with the same pipeline unless modifications are made. The maximum mean velocity of the gas is about 5.83m/s with a maximum erosional velocity of 12.93m/s, but the outrageously high pressure will serve as a major factor.
4.5.4 Decrease Flow
Vandalism, pipe rupture or leakages, accidents, political or social unrests etc. are some major factors that could affect the regular supply of gas from the South-South region of Nigeria towards the Northern part of the country. Therefore, if there is a significant decrease in gas supplied, the effects of such decrease are also considered.
Peradventure, if the gas supply decreases to about 2000MMSCFD (56.64mmsm3/day), the pressure to supply this is expected to be lower; from the simulations, a source pressure of 108.79barg (1577.9psig), a little higher than 100barg (1450psig) is expected at the Ajaokuta tie-in, and the compressor stations should be more than capable of supplementing for this minimal difference.
The temperature difference is also not significant; hence hydrate formation is not expected. A maximum mean gas mean gas velocity of 4.30m/s and maximum erosional velocity of 13.74m/s are expected; these are good enough.
Further reduction in capacity say 1500MMSCFD (42.48mmsm3/day), brings the pressure required lower to about 94.99barg (1377.7psig), maximum mean velocity of 4.68m/s and maximum erosional velocity of 13.83m/s. hence, the pipeline is capable of handling significant reduction in supply without necessarily affecting the integrity of the pipeline.
4.5.5 Phase Envelope
From the phase envelope (diagram), the Cricondentherm, which is the maximum temperature above which liquid cannot be formed, is 40.02°C, and the Cricondentherm pressure is 59.80bar(abs). Also, the Cricondenbar, which is the maximum pressure above which no gas can be formed regardless of temperature, is 118.37bar(abs), and the Cricondenbar temperature is 0.86°C.
The critical pressure is 103.5bar(abs) and critical temperature is -27.30°C.
This implies to transmit natural gas with the given composition and ensure that no liquid entrainment is present, the temperature of the gas should be above 40.02°C, and the pressure should not be above 118.37bar(abs). The gas transmitted is at 37°C, which means there is presence of a liquid phase in the gas, which needs to be gasify before it is allowed into a compressors to avoid damage of the impellers.
4.5.6 Compressor Effects
The compressors as designed are positioned 60km, 147km and 440km respectively from the source at Ajaokuta. From the compressor profile plot it is observed that the positioning is not adequate. A pressure drop of about 15barg is too small when double this value can be allowed. A pressure drop of about 80barg, also is too large between compressor two(2) and three(3).
An optimized compressors position that is 150km, 305km, and 455km from the source at Ajaokuta is suggested. With this optimized positioning, a maximum pressure drop of 30barg is allowed between compressors. It is notable, though, that the final pressure at the Kano TGS is less than 70barg (i.e. about 54barg), this is so because from the Terminal Gas Stations, the pressure reduced to about 31.03barg (450psig) to supply for domestic supply.
4.6 Final Analysis
From the velocity profile plots, it is evident that the maximum mean velocity of the gas is about one-third of the maximum erosional velocity, which implies that the pipeline is not adequately utilized. Hence, it could have been suggested that a lower-diameter pipeline should be used for the optimal use of the pipeline but it is also notable that the Ajaokuta-Abuja-Kano Gas Pipeline is a part of the proposed Trans-Saharan Gas Pipeline. With this information and considering the amount of gas propsed to be transmitted through the Trans-Saharan Gas Pipeline (20 - 30bcm/y), it is expected that the pipeline is adequate as proposed.
The compressors station positioning should also be adequate since it is only three (3) of the eighteen (18) to nineteen (19) compressors on the Trans-Saharan Gas Pipeline, this arrangement would have been considered during the design.
CHAPTER FIVE
CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion
Ajaokuta-Abuja-Kano Gas Pipeline, one of the projects highlighted by the Federal Government of Nigeria to improve gas utilization in the country and increasing domestic consumption of natural, is a laudable one. As designed, the pipeline is a 42" dual pipeline which total length is approximately 740km long with three (3) compressor stations located 60km, 147km and 440km from its source at Ajaokuta. The total approximate length from Ajaokuta to Kano Terminal Gas Station (TGS) is 585km, the spur line from Abuja Node to Abuja Terminal Gas Station is 15.5km and the spur line from Kaduna Node to Kaduna Terminal Gas Station is 0.5km.
The pipeline is sized for an ultimate design of 3000MMSCFD (42.48MMSCMD). The Abuja and Kaduna spur lines are sized for 500MMSCFD (7.08MMSCMD) each leaving up to 2000MMSCFD (28.32MMSCMD) available at Kano for local distribution and export to the Trans-Saharan Gas Pipeline. The minimum supply pressure at Ajaokuta and the minimum delivery pressure at Kano are 68.95barg (1000psig).
The natural gas composition used as the compositional fluid for the simulation contains minute amount of liquid hydrocarbon and from the phase envelope (diagram), the Cricondentherm is about 40°C. Hence, the selection of 37°C as the gas temperature is ineffective because liquid entrainment will be expected in the pipeline which exposes the pipeline to hydrates formation risk, and can cause damage to the impellers of the compressors during compression. It is therefore expected that the gas temperature should exceed 40°C to avoid these consequences.
The pipeline is quite capable of handling the 3000MMSCFD (42.48MMSCMD) and even higher volume of up to about 4000MMSCFD (56.63MMSCMD), just that the compressors power requirements will be higher to meet this capacity. Reduction in capacity due to vandalism, social unrest etc. that could hinder the optimum supply of gas will be adequately handled by the pipeline, though the velocity of the gas is expected to drop due to reduction in the volumetric flow rate.
It is also noteworthy the comparison between the maximum gas velocity in the pipeline and the maximum erosional velocity. Although it is observed that the gas velocity is less than 20m/s which makes it adequate to be transported, but the gas velocity is about one-third (1/3) of the erosional velocity. This reveals that the gas pipeline is not adequately utilized and that the gas movement is quite slow.
Another notable observation is compressor stations positioning which is ineffective , an optimized positioning is 150km, 305km and 455km from the source at Ajaokuta.
Nevertheless, since the Ajaokuta-Abuja-Kano Gas Pipeline is an integral part of the proposed Trans-Saharan Gas Pipeline, it is to be assumed that this is the reason for the above observations and as soon as the Trans-Saharan Gas Pipeline is incorporated, the pipeline will be used adequately and effectively to capacity.
5.2 Recommendations
This dissertation which involves the analysis of the Ajaokuta-Abuja-Kano Gas Pipeline network was quite interesting and challenging. Some of the challenges encountered include non-availability of required important data, short time frame to consider the design of the pipeline, use of a single simulation package that did not allow for comparison of results, etc.
As a result of these and so many other challenges, the following recommendations are necessary to ensure future works on network analysis are better:
Other simulation packages should be considered instead of Pipesim that was used for this dissertation and results generated should be compared.
Pipesim is an isothermal, multiphase and steady-state flow simulation package; a transient flow simulation package should be used.
All information concerning the pipeline should be obtained, including future plans being envisaged to supply other parts that were not included in the initial design.
