Design of Natural Gas Transmission Pipeline A Case Study of A Typical Marginal Oil and Gas Field in Niger Delta Nigeria
Design of Natural Gas Transmission Pipeline A Case Study of A Typical Marginal Oil and Gas Field in Niger Delta Nigeria
Design of Natural Gas Transmission Pipeline A Case Study of A Typical Marginal Oil and Gas Field in Niger Delta Nigeria
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951
ABSTRACT
The natural gas transmission pipeline for a typical marginal oil and gas field in Niger delta has
been designed. A secondary data of the field gas was obtained and empirical study was carried
out to determine the nature of the gas. The gas flow rate, pressure, temperature was collated and
used to design the gas pipeline.
Hydraulic design equations from ASME B31.8 and API 5L standard codes and specifications
were used to estimate some of the parameters used in the design. The sizing, design pressure,
collapse pressure, burst pressure, hydrotest pressure. Pipe diameter, schedule number and
thickness etc were calculated using appropriate design equations. Aspen HYSYS version 8.8
(34.0.0.8909) software was used to carry out simulation of the process flow and it all converged.
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INTRODUCTION
Natural Gas due to its storage difficulties needs to be transported to its needed destination as
soon as it is produced from the reservoir. There are a number of options for transporting natural
gas energy from oil and gas fields to market (Guo Buoyan et al., 2005). This includes:
Pipeline Natural Gas (PNG), Compressed Natural Gas (CNG), Liquefied Natural Gas (LNG) and
Gas To Solid (GTS). Pipelines are connected pipes which are installed for the purpose of
transmitting gases, liquids, slurries, etc., from sources of supply to one or more distribution
center(s) or to one or more large-volume customer(s); a pipe installed to interconnect source(s)
of supply to one or more distribution center(s) or to one or more large-volume customer(s); or a
pipe installed to interconnect source(s) of supply. (Lyons W; 2011). Pipes are tube with a round
cross section conforming to the dimensional requirements for nominal pipe size. An average of
over 12,000 miles per year of newly constructed gas pipeline has been completed in the last
decades; most of which are transnational. It is a common belief that if political instabilities and
other variables are guaranteed, then pipelines use may be able to provide a long lasting solution
for transportation of natural gas.
Marginal Field is an oil /gas field which is incapable of producing the desired net income that
will trigger its development at a given point in time. However, if technical / economic conditions
changes, such field may become commercially viable. It is usually associated with small pockets
of hydrocarbons with a few years of plateau. Marginal fields have some issues or parameters that
affects them. They include the following: Environmental concerns, political stability, access,
remoteness etc. In short, marginal fields are oil and gas reserves usually too small for production
to be economically viable for large oil companies. These fields are usually awarded to
indigenous companies to explore, considering the size of the reserves.
Gas pipeline design is a process or plan to show the look and function of gas pipeline before it
is constructed. The design consists mainly of four interrelated areas, that is, hydraulic design,
mechanical design, geothermal design and operating/maintenance design (Mike Y, 2010). They
are all geared towards figuring out suitable pipeline that will safely transport the fluid from the
oil and gas field to the place of storage or utilization. Several codes and standards have been
developed as a guide for the design, construction and operations of pipelines. The objectives of
this codes and standards are to ensure the safety of the personnel and the general public by
minimizing the risk of high pressure pipelines (Cult et al, 2008).
The properties and compositions of the gas must be determined in order to design an appropriate
gas pipeline. Gas reservoir properties such as carbon contents, gas specific gravity, gas
compressibility factor, gas viscosity, critical temperature and pressure, gas density, etc. must be
determined to select the grade of the pipeline.
Pipeline route and profile survey is also required to ascertain the minimum and maximum
elevation of the pipeline, to know if there will be creek/river crossing and to understand the
environmental condition of such terrain.
In this paper, a case study of a typical marginal field will be undertaken to design a natural gas
transmission pipeline. The terrain is fairly flat and accessible. A secondary data of the field gas
will be obtained and empirical study will be carried out to determine the nature of the gas. The
gas flow rate, pressure, temperature will be collated and used to design the gas pipeline.
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METHODOLOGY
Data collection and analysis for pipeline design
A secondary data of the gas sample from the field were collected for analysis as shown below.
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Ma = (Yi x Mi) = 19.8g/mol
The Apparent Molecular Weight of the gas =19.8g/mol
19.8008
G 0.68
28.9625
T 532.2 0 R
Tr 1.39
Tc 383.22 0 R
P 1104psi
Pr 1.65
Pc 669.86psi
m
PV ZnRT ; but n
Ma
PMa ZRT
PMa
ZRT
1104psi 19.8g/mol
4.4 Ib/ft 3
0.88 10.73ft psi/ibmol. R 532.2 R
3 0 0
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µ1=0.010 cp. µ/µ1 @ (Tpr and Ppr)= 1.21
µ=1.21 x µ1= 1.21 x 0.010= 0.0121cp
Pipeline sizing
Determination of pipe diameter
Superficial velocity equation for sizing gas stream pipes is given as follows;
3.5 Qh TA
D
PA
where D = Pipe Diameter, Qh= Flowing Capacity (MSCF/D)
TA = Absolute Temperature, PA = Absolute Pressure
Where, Qh= 35MSCF,
TA= 72.2oF + 460 =532.2oF, PA= 1104 + 14.7 =1,118.7Psia
1
3.5 35 532.2 2
D 7.6"
1,118.7
Where;
tNOM=Nominal wall thickness, mm, Pd= Design Internal pressure
Ca=Corrosion thickness allowance
£w= Weld efficiency factor. It is 1.0 for seamless, Arc weld (SAW or DSAW)
δy= Specified Minimum Yield strength, ή= design Factor, which is 0.72 for pipeline.
Ft= Temperature derating factor, which is 1 for temperature under 250oC
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Determination of Reynolds Number and frictional factors.
20qhG 20 35 0.68
N Re
μD 0.0121 8
= 4,917.355
Since NRe > 2,100. The flow is turbulent.
1
P12 P2 2
Q g 1.1d 2.67
LZGT1
Where, T1= (72.4 +460)oR,= 532.4 oR, P1= 1104psi, P2=? d= 8”-2(0.3937) =7.213”
G = 0.68, L= 14 KM = 14 x 3280.84ft= 45,931.8ft, Z = 0.88, Qg = 35MSCF
1
1104 2 P2 2
35 8 2.67
45,931.8 0.88 0.68 532.4
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Figure 7: PFD showing First elbow Figure 8: PFD showing second elbow
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5 Delta ( P) 32psi(0.2Mpa)
20 Pipeline Maximum Allowable Operating Pressure (MAOP) 4,251.96 Psi (29.32 Mpa)
22 Schedule number 60
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The gas sample from the marginal field showed that, it is predominantly made of methane gas
(C1), the processed gas from the field is a dry gas hence, no fear for hydrate formation along the
flow line. The calculated diameter is approximately 8 inches. according to ASME B31.3 pipe
sizing. The pipeline wall thickness is estimated as 10mm, which is in compliance with ASME
B31.8 code for 8”, X-60 grade. The physical properties of the gas are gas apparent molecular
weight (19.8g/mol), gas gravity (0.68 ), density(4.4Ib/ft3) and viscosity(0.0121CP). In other to
specify the ordering of pipes, pipe joints are produced based on their pressure rating(that is, the
amount of pressure) they can withstand, termed schedule number. The schedule number is a
function of the pipe wall thickness. In this research, it was calculated as 60 and from pipe size
chart, 8’’ nominal diameter and thickness of 10.33mm has a schedule of 60, which conformed to
the calculated schedule number considering the allowable stress and operating pressures of the
pipeline.
The Reynolds number which is used to predict the fluid pattern was calculated to be greater than
2100, an indication that the flow regime is turbulent. The design allowable pressure such as, the
maximum allowable pressure, bursting pressure, collapse pressure and the hydro test pressure
were calculated as 4,251.96, 8,858.26, 5,905.51 and 5,314.96psi respectively. The delivery
pressure was calculated manually using Weymouth equation as 998psi , while the delivery
pressure from the simulation is said to be 1072psi as shown in figure 5. For the purpose of
precision, the simulated figure (1072psi) was considered in this design. Therefore, the pressure
drop along the entire pipeline will be 32psia. Similarly, considering the Heat transfer from the
steel pipe and the pipe surrounding, the overall heat transfer coefficient was determined using the
simulation as 3.1054Btu/hr-ft2.F as shown in figure 4. The coating material was assumed to be
Epoxy coating of thickness of 5mm. The thermal conductivities of the steel pipe and epoxy
coating are 45W/mK and 0.21W/mK respectively, as shown in table 3. The units were manually
converted to W/moC so as to be consistent with the units in the software used for this design.
From the full simulation report, the power consumed by the compressors at the Base Station and
Booster station is said to be 3.68HP and 3.32HP respectively.
It is worthy to note that the Natural Gas Simulation processes converged. The conditions of the
natural gas changes slightly as it leave each sections and fittings along the gas pipeline. For the
purpose of clarity, different nomenclatures were used to represent the natural gas material
streams as it enters and leaves each pipeline sections and fittings along the pipeline.
CONCLUSION
A gas transmission pipeline was designed using appropriate design equations from ASME and
API codes/standard for calculating the appropriate diameter, wall thickness, schedule number,
pressure ratings etc. Some of the design parameters calculated for are diameter 8”, wall
thickness, schedule number X-60, maximum allowable pressure, burst pressure and, delivery
pressure . The gas properties and deviation factor was done using the appropriate equations. The
designed pipeline was simulated using Aspen HYSYS version 8.8, and it all converged.
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REFERENCES
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