Key Process Considerations for Pipeline Design Basis

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Key Process Considerations for Pipeline Design Basis
Jayanthi Vijay Sarathy, M.E, CEng, MIChemE, Chartered Chemical Engineer, IChemE, UK
Prior to venturing into an oil & gas pipeline
project, the project team would require a
design basis, based on which the project is to
proceed. Oil & Gas Pipeline design begins
with a route survey including engineering &
environmental assessments. The following
document provides a few key considerations
for process engineers to keep in mind, the
factors that matter when preparing a pipeline
design basis from a process standpoint.
1. Well Production Data/Profile
Well production profiles are required as this
determines the size of the pipeline required
to transport volume/time of fluid. Gases are
highly compressible and cannot be treated
the same as liquids such as, crude oils &
petroleum distillates. The operating
pressures & temperatures are required to be
known as they determine the design
conditions of the pipeline.
2. Fluid Physical Properties
The physical properties of the materials being
transported dictate the design and operating
parameters of the pipeline. Specific gravity,
compressibility, kinematic & dynamic
viscosity, pour point, and vapour pressure of
the material are the primary considerations.
The pour point of a liquid is the temperature
at which it ceases to pour. The pour point for
oil can be determined under protocols set
forth in the ASTM Standard D-97.
In general, crude oils have high pour points.
When transported hydrocarbons operate
below their pour point, auxiliary measures
such as heating, diluting with lighter
hydrocarbons that are miscible & allows
lowering the viscosity & pour point
temperature, mixing with water to allow the
waxes to slide through the pipe walls, or
modifying the chemical composition of the
hydrocarbon. It is to be noted that, in the case
of finished products, (e.g., gas oil or Jet A1
fuel), many of the auxiliary measures like
addition of water or mixing with lighter
hydrocarbons becomes infeasible, since they
affect the product specification.
Vapour Pressure of a liquid is its capacity to
vaporize/evaporate into its gaseous phase. In
pipeline operations, slack flow is a situation
where due to the elevational & pipeline
pressure drops, a portion of the hydrocarbon
experiences pressure below its vapour
pressure. As a result, a portion of the liquid
vaporizes & reaches the high points in the
pipeline. Upon restarting the pipeline, the
vapour pockets experience a compressive rise
in pressure due to the upstream &
downstream liquid pockets, only to collapse &
release energy that can rupture pipelines.
Reid vapour pressures are critical to liquid
petroleum pipeline design, since the pipeline
must maintain pressures greater than the
Reid vapour pressure of the material in order
to keep the material in a liquid state.
Pipelines that handle finished products are
preferred to be operated with single phase
flow regime & fully filled pipes. This ensures
there is no scope for volatilization that
reduces the scope for fire hazards.
3. Pumping Costs
Viscous fluids require more power to deliver
required motive force to the hydrocarbons to
transport them across the pipeline. Waxy
crudes can be pumped below their pour point
However if the flow is stopped, for e.g., after a
pipeline shutdown, the energy required to
restart the pipeline would be much higher
than what was required to keep it flowing.
Pipelines also suffer from the formation of
hydrates & asphaltenes. Waxes can form

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crystalline structures that tend to
agglomerate & is referred to as gelling.
Gelling is also a phenomenon that is found in
storage tanks in production facilities where
the fluid sits motionless for hours or even
days, resulting in operational difficulties.
Hence to attend to these limitations, pour
point estimation becomes vital to determine
if external heating is required. In some cases,
if the waxy crude does not gel enough, it can
get transported to the pump where shear
forces & rise in temperature allow the waxy
crudes to stay above the pour point.
4. Thermal Stresses
Petroleum pipelines are normally buried
unless local regulation prevents them. To do
so, trenches are dug & are laid below
grade/frost line level. Such measures also
provide the advantage of maintaining
relatively constant temperature in line with
the ambient/season soil temperature, thereby
ensuring the pipeline expansion does not
occur to the point of deflection. Expansion
joints as well as in some cases, trenches are
dug extra wide to accommodate any lateral
movement. In case of river crossings, the
pipeline is to be laid above ground. In
locations that are prone to landslides, buried
pipelines option is preferred to avoid direct
impact of rock structures. But this does not
necessarily mean buried pipelines are free
from structural damage, since the weight of
the soil/rock structures deposited above the
pipeline can also crush the buried pipelines.
5. Pipeline Pressure Drop
Pipelines are designed keeping in mind, the
material & construction costs as well
operating costs. Material costs are
determined by the pipeline weight, whereas
operational costs are largely impacted by the
pressure drop experienced which is a
function of the flow regime. The two key
forces dictating the pipeline total pressure
losses are – Hydrostatic pressure drop due to
the pipeline elevation & frictional pressure
drop which depends on the flow rate. In
multiphase pipelines across hilly terrains,
hydrostatic pressure drop decreases while
frictional pressure drop increases with flow.
The sum of both these pressure losses gives
the total pressure loss. The pipeline size
chosen should be preferably, the point at
which the total pressure loss is the least.
Fig 1. Pipeline Total Pressure Loss – Hilly Terrain
From the above figure it can be seen that
operating a multiphase line at a lower
flowrate can actually cost more to pump.
6. Max Hydro Test – ASME B31.8
For a class1 div 1, the test pressure is limited
to 125% of the design pressure. For class 3 or
4, test pressure is up to 1.4  MAOP.
Therefore, for an 8 hour min. test pressure,
with a design factor of 0.72 for class 1, the
test would cause the hoop reaching 72% of
Pipeline’s SMYS. Testing at 125% of MAOP
will result in 1.25  0.72 = 0.9 or 90% of
SMYS. Hence by hydro testing the pipeline at
1.25 times the operating pressure, the
pipeline is stressed out to 90% of its SMYS.
The hydro test pressure is based on the
location class and maximum test pressure, &
becomes the lower value of 8 hour minimum
test pressure & test pressure at low point.
7. Valve Spacing
Pipelines need valves to placed & spaced
taking into consideration – Rapid
Isolation/Shutdown of pipeline sections to

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minimize inventory breach, maintain pipeline
design integrity, and facilitate maintenance,
repairs & hot tapping operations. Pipelines
would also be subjected to pigging & hence
the valve placement must enable recovery of
stuck pigs.
8. Hydrocarbon Flares
Pipelines would sometimes have to be blown
down of any hydrocarbons (liquid or liquid
mixed vapours) during events of over
pressure. Burn pit lines serve this purpose. It
is important to monitor pilot flames and
provide pilot flame failure alarms. Since burn
pit lines are a source of open flame, they are
to be located at least 150 m away from
roadways, process & storage facilities. In
cases of pipeline in remote locations
requiring maintenance or repair, mobile flare
units can be used.
However not all occasions would allow open
flaring, as a result of which, enclosed ground
flares can be used. These conform to the
requirements of flaring & disposal in
populated areas or process facilities that are
in close proximity to the flare system. The
flaring is smokeless with no visible flame &
noiseless due to insulation of the combustion
chamber. To attend to the flare capacities
required, a flare study report is to be made
part of the design basis.
9. Pipeline Standards/Codes1
ASME has been a pioneer in developing
industry codes & standards for oil & gas
pipelines. The scope of the first draft of the
ASME Code for Pressure Piping, which was
approved by the American Standards
Association in 1935, included the design,
manufacture, installation, and testing of oil
and gas pipelines (ASME B31.4). As the needs
of the industry evolved over the years, rules
for new construction have been enhanced,
and rules for operation, inspection, corrosion
control, and maintenance have been added. In
addition to ASME, several other
organizations, including the API and NACE
International, also developed standards used
by the pipeline industry. Some of the
ASME/API/ANSI standards are,
1. “Gas Transmission and Distribution Piping
Systems,” ASME B31.8, 1999.
2. “Pipeline Transportation Systems for Liquid
Hydrocarbons and Other Liquids,” ASME
B31.4, 1998.
3. “Power Piping”, ASME B31.1, 1998;
Addenda B31.1A, 1999; Addenda B31.1B,
2000
4. “Process Piping” ASME B31.1, 1999;
Addenda B31.3A, 1999
5. “Slurry Transportation Piping Systems”
ASME B31.11, ‘89; Addenda B31.11A, 1991
6. “Pipeline Transportation Systems for Liquid
Hydrocarbons and Other Liquids” ASME
B31.4, 2002
7. “Gas Transmission and Distribution
Systems,” ASME 31.8, 2003
8. “Specification for Line Pipe”, API 5L, Mar
2004 / Errata 1, Jan 2005
9. “Steel Pipelines Crossing Railroads and
Highways” API 1102 (1993)
10. “Specification for Pipeline Valves (Gate,
Plug, and Check Valves)”, 21st edition, API
6D1, June 1998 Supplement 2
11. Pipeline wall thickness (API B31.G)
Velocity Considerations3
Gas line velocities should be less than 60 to
80 ft/s to minimize noise & allow for
corrosion inhibition. A lower velocity of 50
ft/s should be used in the presence of known
corrosives such as CO2. The minimum gas
velocity should be between 10 and 15 ft/s,
which minimize liquid fallout. The minimum
fluid velocity in multiphase systems must be
relatively high to keep the liquids moving in
order to prevent/minimize slugging. The
recommended minimum velocity is 10 to 15

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ft/s. The maximum recommended velocity is
60 ft/s to inhibit noise & 50 ft/s for
CO2 corrosion inhibition. In two-phase flow, it
is possible that the flow stream’s liquid
droplets can impact the pipe wall causing
erosion of the corrosion products. Erosion of
the pipe wall itself could occur if solid
particles, particularly sand, are entrained in
the flow stream.
Pipeline Mechanical Design
As an example to perform Gas Pipeline
mechanical design, ASME B31.8 is used. The
requirement to be met for pipeline wall
stresses as per ASME B31.8 is Design factor
[F], Temperature De-rating [T], Longitudinal
Joint Factor [E] for the chosen pipeline joining
methods. This is shown below as follows,
Table 1. Reference Mechanical Design Parameters
Design Factors [F] - Gas Pipeline Location
Class Description F
Class 1, Div 1 Deserted 0.80
Class 1, Div 2 Deserted 0.72
Class 2 Village 0.60
Class 3 City 0.50
Class 4 Densely Populated 0.40
Temperature De-rating [T] for Gas Pipelines
T [0F] T [0C] T
 250  120 1.00
300 150 0.97
350 175 0.93
400 200 0.91
450 230 0.87
Abbreviation Joining Method E
SMLS Seamless 1.0
ERW Electric Resistance Weld 1.0
EFW Electric Flash Weld 1.0
SAW Submerged Arc Weld 1.0
BW Furnace Butt Weld 0.6
EFAW Electric Fusion Arc Weld 0.8
The pipeline specification requirement as per
API 5L plain end line pipe specifications,
ranges from 6” ND to 80” ND. The product
pipeline specification (PSL) with its
respective Specified Minimum Yield Strength
(SYMS) to be used as per API 5L are PSL 1 and
PSL 2. The pipeline grades are as follows,
Table 2. Product Specification Level (PSL)
Grade
SMYS
Grade
SMYS
MPa MPa
PSL 1 Gr A25 172 PSL 2 Gr B 241
PSL 1 Gr A 207 PSL 2 X42 290
PSL 1 Gr B 241 PSL 2 X46 317
PSL 1 X42 290 PSL 2 X52 359
PSL 1 X46 317 PSL 2 X56 386
PSL 1 X52 359 PSL 2 X60 414
PSL 1 X56 386 PSL 2 X65 448
PSL 1 X60 414 PSL 2 X70 483
PSL 1 X65 448 PSL 2 X80 552
PSL 1 X70 483 - -
Location of the Gas Pipelines
1. Class 1 location - A Class 1 location is any
1-mile pipeline section that has 10 or
fewer buildings intended for human
occupancy including areas such as,
wastelands, deserts, rugged mountains,
grazing land, farmland, sparse populations.
2. Class 1, division 1 Location – A Class 1
location where the design factor, F, of the
pipeline is greater than 0.72 but equal to,
or less than 0.80 and which has been
hydrostatically tested to 1.25 times the
maximum operating pressure.
3. Class 1, division 2 Location - This is a
Class 1 location where the design factor, F,
of the pipeline is equal to or less than 0.72,
and which has been tested to 1.1 times the
maximum operating pressure.
4. Class 2 Location - This is any 1-mile
section of pipeline that has more than 10

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but fewer than 46 buildings intended for
human occupancy including fringe areas
around cities and towns, industrial areas,
and ranch or country estates.
section of pipeline that has 46 or more
buildings intended for human occupancy
except when a Class 4 Location prevails,
including suburban housing developments,
shopping centres, residential areas,
industrial areas & other populated areas
not meeting Class 4 Location requirements
section of pipeline where multi-storey
buildings are prevalent, traffic is heavy or
dense, and where there may be numerous
other utilities underground. Multi-storey
means four or more floors above ground
including the first, or ground, floor. The
depth of basements or number of
basement floors is immaterial.
Line Specification of Gas Pipelines – API 5L
1. PSL1 pipes are available through size 2/5”
to 80” whereas the smallest diameter pipe
available in PSL2 is 4.5” & the largest
diameter is 80”. PSL1 pipelines are
available in different types of ends, such as
Plain end, Threaded end, Bevelled end,
special coupling pipes whereas PSL2
pipelines are available in only Plain End.
2. For PSL2 welded pipes, except continuous
welding & laser welding, all other welding
methods are acceptable. For electric weld
welder frequency for PSL2 pipeline is
minimum 100 kHz whereas there is no
such limitation on PSL1 pipelines.
3. Heat treatment of electric welds is
required for all Grades of PSL2 pipes
whereas for PSL1 pipelines, grades above
X42 require it. All kinds of welding method
are acceptable to manufacture PSL1;
however, continuous welding is limited to
Grade A25.
Gas Pipeline Wall Thickness Estimation
The B31.8 code is often used as the standard
of design for natural gas piping systems in
facilities, such as compressor stations, gas
treatment facilities, measurement &
regulation stations & tank farms. The B31.8
wall-thickness formula is stated as,
𝑡 =
𝐷𝑃×𝑂𝐷
2×𝐹×𝐸×𝑇×𝑆𝑀𝑌𝑆
(1)
Where,
t = Minimum design wall thickness [in]
DP = Pipeline Design Pressure [psi]
OD = Pipeline Outer Diameter [in]
SMYS = Specific Minimum Yield Stress [psi]
F = Design Factor [-]
E = Longitudinal Weld Joint Factor [E]
T = Temperature De-rating Factor [-]
References
1. “Standard for Gas Transmission and
Distribution Piping Systems”, ANSI/ASME
Standard B31.8, 1999
2. “Overview of the Design, Construction, and
Operation of Interstate Liquid Petroleum
Pipelines”, ANL/EVS/TM/08-1, Argonne
National Laboratory
3. https://petrowiki.org
About the Author
Vijay Sarathy holds a Master’s
Degree in Chemical Engineering
from Birla Institute of Technology
& Science (BITS), Pilani, India and
is a Chartered Engineer from the Institution
of Chemical Engineers, UK. His expertise over
12 years of professional experience covers
Front End Engineering, Process Dynamic
Simulation and Subsea/Onshore pipeline flow
assurance in the Oil and Gas industry. Vijay
has worked as an Upstream Process Engineer
with major conglomerates of General Electric,
ENI Saipem and Shell.

Key Process Considerations for Pipeline Design Basis

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Key Process Considerations for Pipeline Design Basis