 |
| Source: Duke Energy
Gas Transmission Canada |
The efficient and effective movement of
natural gas from producing regions to consumption regions requires an
extensive and elaborate transportation system. In many instances,
natural gas produced from a particular well will have to travel a great
distance to reach its point of use. The transportation system for
natural gas consists of a complex network of pipelines, designed to
quickly and efficiently transport natural gas from its origin, to areas
of high natural gas demand. Transportation of natural gas is closely
linked to its storage: should the natural gas being transported not be
immediately required, it can be put into storage facilities for when it is needed.
There are three
major types of pipelines along the transportation route: the gathering
system, the interstate pipeline system, and the distribution system.
The gathering system consists of low pressure, small diameter pipelines
that transport raw natural gas from the wellhead to the processing
plant. Should natural gas from a particular well have high sulfur and
carbon dioxide contents (sour gas), a specialized sour gas gathering
pipe must be installed. Sour gas is corrosive, thus its transportation
from the wellhead to the sweetening plant must be done carefully.
Review the treatment and processing of natural gas.
Pipelines can be characterized as interstate
or intrastate. Interstate pipelines are similar to in the interstate
highway system: they carry natural gas across state boundaries, in
some cases clear across the country. Intrastate pipelines, on the other
hand, transport natural gas within a particular state. This section
will cover only the fundamentals of interstate natural gas pipelines,
however the technical and operational details discussed are essentially
the same for intrastate pipelines.
Interstate Natural Gas Pipelines
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| Interstate Natural Gas Pipelines |
| Source: National
Energy Technology Laboratory, DOE |
The interstate natural gas pipeline network
transports processed natural gas from processing plants in producing
regions to those areas with high natural gas requirements, particularly
large, populated urban areas. As can be seen, the pipeline network
extends across the entire country.
Interstate pipelines are the 'highways' of natural gas transmission.
Natural gas that is transported through interstate pipelines travels at
high pressure in the pipeline, at pressures anywhere from 200 to 1500
pounds per square inch (psi). This reduces the volume of the natural
gas being transported (by up to 600 times), as well as propelling
natural gas through the pipeline.
This section will cover the components of the interstate pipeline system, the
construction of pipelines, and
pipeline inspection and safety. For more information on interstate pipelines in general, click
here to visit the website of the Interstate Natural Gas Association of America.
Pipeline Components
Interstate
pipelines consist of a number of components that ensure the efficiency
and reliability of a system that delivers such an important energy
source year-round, twenty four hours a day, and includes a number of
different components.
Transmission Pipes
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| Pipes in Transit |
| Source: Duke Energy
Gas Transmission Canada |
Transmission pipes can measure anywhere from
6 to 48 inches in diameter, depending on their function. Certain
component pipe sections can even consist of small diameter pipe, as
small as 0.5 inches in diameter. However, this small diameter pipe is
usually used only in gathering and distribution systems. Mainline
transmission pipes, the principle pipeline in a given system, are
usually between 16 and 48 inches in diameter. Lateral pipelines, which
deliver natural gas to or from the mainline, are typically between 6
and 16 inches in diameter. Most major interstate pipelines are between
24 and 36 inches in diameter. The actual pipeline itself, commonly
called 'line pipe', consists of a strong carbon steel material,
engineered to meet standards set by the
American Petroleum Institute
(API). In contrast, some distribution pipe is made of highly advanced
plastic, because of the need for flexibility, versatility and the ease
of replacement.
Transmission pipelines are produced in steel
mills, which are sometimes specialized to produce only pipeline. There
are two different production techniques, one for small diameter pipes
and one for large diameter pipes. For large diameter pipes, from 20 to
42 inches in diameter, the pipes are produced from sheets of metal
which are folded into a tube shape, with the ends welded together to
form a pipe section. Small diameter pipe, on the other hand, can be
produced seamlessly. This involves heating a metal bar to very high
temperatures, then punching a hole through the middle of the bar to
produce a hollow tube. In either case, the pipe is tested before being
shipped from the steel mill, to ensure that it can meet the pressure
and strength standards for transporting natural gas.
Line pipe is also covered with a specialized
coating to ensure that it does not corrode once placed in the ground.
The purpose of the coating is to protect the pipe from moisture, which
causes corrosion and rusting. There are a number of different coating
techniques. In the past, pipelines were coated with specialized coal
tar enamel. Today, pipes are often protected with what is known as a
fusion bond epoxy, which gives the pipe a noticeable light blue color.
In addition, cathodic protection is often used; which is a technique of
running an electric current through the pipe to ward off corrosion and
rusting.
Compressor Stations
As mentioned, natural gas is highly
pressurized as it travels through an interstate pipeline. To ensure
that the natural gas flowing through any one pipeline remains
pressurized, compression of this natural gas is required periodically
along the pipe. This is accomplished by compressor stations, usually
placed at 40 to 100 mile intervals along the pipeline. The natural gas
enters the compressor station, where it is compressed by either a
turbine, motor, or engine.
 |
| A Compressor Station |
| Source: Duke Energy
Gas Transmission Canada |
Turbine compressors gain their energy by
using up a small proportion of the natural gas that they compress. The
turbine itself serves to operate a centrifugal compressor, which
contains a type of fan that compresses and pumps the natural gas through
the pipeline. Some compressor stations are operated by using an
electric motor to turn the same type of centrifugal compressor. This
type of compression does not require the use of any of the natural gas
from the pipe, however it does require a reliable source of electricity
nearby. Reciprocating natural gas engines are also used to power some
compressor stations. These engines resemble a very large automobile
engine, and are powered by natural gas from the pipeline. The
combustion of the natural gas powers pistons on the outside of the
engine, which serves to compress the natural gas.
In addition to
compressing natural gas, compressor stations also usually contain some
type of liquid separator, much like the ones used to dehydrate natural
gas during its processing. Usually, these separators consist of
scrubbers and filters that capture any liquids or other unwanted
particles from the natural gas in the pipeline. Although natural gas in
pipelines is considered 'dry' gas, it is not uncommon for a certain
amount of water and hydrocarbons to condense out of the gas stream
while in transit. The liquid separators at compressor stations ensure
that the natural gas in the pipeline is as pure as possible, and usually
filter the gas prior to compression.
Metering Stations
In addition to compressing natural gas to
reduce its volume and push it through the pipe, metering stations are
placed periodically along interstate natural gas pipelines. These
stations allow pipeline companies to monitor the natural gas in their
pipes. Essentially, these metering stations measure the flow of gas
along the pipeline, and allow pipeline companies to 'track' natural gas
as it flows along the pipeline. These metering stations employ
specialized meters to measure the natural gas as it flows through the
pipeline, without impeding its movement.
Valves
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| A Ground Valve |
| Source: Duke Energy
Gas Transmission Canada |
Interstate pipelines include a great number
of valves along their entire length. These valves work like gateways;
they are usually open and allow natural gas to flow freely, or they can
be used to stop gas flow along a certain section of pipe. There are
many reasons why a pipeline may need to restrict gas flow in certain
areas. For example, if a section of pipe requires replacement or
maintenance, valves on either end of that section of pipe can be closed
to allow engineers and work crews safe access. These large valves can
be placed every 5 to 20 miles along the pipeline, and are subject to
regulation by safety codes.
Control Stations and SCADA Systems
Natural gas pipeline companies have
customers on both ends of the pipeline - the producers and processors
that input gas into the pipeline, and the consumers and local gas
utilities that take gas out of the pipeline. In order to manage the
natural gas that enters the pipeline, and to ensure that all customers
receive timely delivery of their portion of this gas, sophisticated
control systems are required to monitor the gas as it travels through
all sections of what could be a very lengthy pipeline network. To
accomplish this task of monitoring and controlling the natural gas that
is traveling through the pipeline, centralized gas control stations
collect, assimilate, and manage data received from monitoring and
compressor stations all along the pipe.
 |
| Pipeline Control Station |
| Source: Duke Energy
Gas Transmission Canada |
Most of the data that is received by a
control station is provided by Supervisory Control and Data Acquisition
(SCADA) systems. These systems are essentially sophisticated
communications systems that take measurements and collect data along the
pipeline (usually in a metering or compressor stations and valves) and
transmit it to the centralized control station. Flow rate through the
pipeline, operational status, pressure, and temperature readings may
all be used to assess the status of the pipeline at any one time. These
systems also work in real time, meaning that there is little lag time
between the measurements taken along the pipeline and their
transmission to the control station.
The data is relayed to a centralized control station, allowing pipeline
engineers to know exactly what is happening along the pipeline at all
times. This enables quick reactions to equipment malfunctions, leaks,
or any other unusual activity along the pipeline. Some SCADA systems
also incorporate the ability to remotely operate certain equipment
along the pipeline, including compressor stations, allowing engineers
in a centralized control center to immediately and easily adjust flow
rates in the pipeline.
Pipeline Construction
As natural gas use
increases, so does the need to have transportation infrastructure in
place to supply the increased demand. This means that pipeline
companies are constantly assessing the flow of natural gas across the
U.S., and building pipelines to allow transportation of natural gas to
those areas that are underserved.
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| Surveying the Right-of-Way |
| Source: Duke Energy
Gas Transmission Canada |
Constructing natural gas pipelines requires a
great deal of planning and preparation. In addition to actually
building the pipeline, several permitting and regulatory processes must
be completed. In many cases, prior to beginning the permitting and
land access processes, natural gas pipeline companies prepare a
feasibility analysis to ensure that an acceptable route for the
pipeline exists that provides the least impact to the environment and
public infrastructure already in place.
Assuming a pipeline company obtains all the
required permits and satisfies all of the regulatory requirements,
construction of the pipe may begin. Extensive surveying of the intended
route is completed, both aerial and land based, to ensure that no
surprises pop up during actual assembly of the pipeline.
Installing a
pipeline is much like an assembly line process, with sections of the
pipeline being completed in stages. First, the path of the pipeline is
cleared of all removable impediments, including trees, boulders, brush,
and anything else that may prohibit the construction. Once the
pipeline's path has been cleared sufficiently to allow construction
equipment to gain access, sections of pipes are laid out along the
intended path, a process called 'stringing' the pipe. These pipe
sections are commonly from 40 to 80 feet long, and are specific to
their destination. That is, certain areas have different requirements
for coating material and pipe thickness.
 |
| 'Stringing' the Pipe |
| Source: Duke Energy
Gas Transmission Canada |
Once the pipe is in place, trenches are dug
alongside the laid out pipe. These trenches are typically five to six
feet deep, as the regulations require the pipe to be at least 30 inches
below the surface. In certain areas, however, including road crossings
and bodies of water, the pipe is buried even deeper. Once the trenches
are dug, the pipe is assembled and contoured. This includes welding the
sections of pipe together into one continuous pipeline, and bending it
slightly, if needed, to fit the contour of the pipeline’s path.
Coating is applied to the ends of the pipes. The coating applied at a
coating mill typically leaves the ends of the pipe clean, so as not to
interfere with welding. Finally, the entire coating of the pipe is
inspected to ensure that it is free from defects.
Once the pipe is
welded, bent, coated, and inspected it can be lowered into the
previously-dug trenches. This is done with specialized construction
equipment acting to lift the pipe in a level manner and lower it into
the trench. Once lowered into the ground, the trench is filled in
carefully, to ensure that the pipe and its coating retain their
integrity. The last step in pipeline construction is the hydrostatic
test. This consists of running water, at pressures higher than will be
needed for natural gas transportation, through the entire length of the
pipe. This serves as a test to ensure that the pipeline is strong
enough, and absent of any leaks of fissures, before natural gas is
pumped through the pipeline.
 |
| Lowering Pipe |
| Source: Duke Energy
Gas Transmission Canada |
Laying pipe across streams or rivers can be
accomplished in one of two ways. Open cut crossing involves the digging
of trenches on the floor of the river to house the pipe. When this is
done, the pipe itself is usually fitted with a concrete casing, which
both ensures that the pipe stays on the bottom of the river and adds an
extra protective coating to prevent any natural gas leaks into the
water. Alternatively, a form of directional drilling may be employed,
in which a 'tunnel' is drilled under the river through which the pipe
may be passed. The same techniques are used for road crossings - either
an open trench is excavated across the road and replaced once the pipe
is installed, or a tunnel may be drilled underneath the road.
Once the pipeline has been installed and
covered, extensive efforts are taken to restore the pipeline's pathway
to its original state, or to mitigate any environmental or other
impacts that may have occurred during the construction process. These
steps often include replacing topsoil, fences, irrigation canals, and
anything else that may have been removed or upset during the
construction process. For more information on natural gas pipeline
construction, visit the website of the
Interstate Natural Gas Association of America.
Pipeline Inspection and Safety
 |
| Pig - Pipeline Inspection Tool |
| Source: Duke Energy
Gas Transmission Canada |
I
n order to ensure the efficient and safe
operation of the extensive network of natural gas pipelines, pipeline
companies routinely inspect their pipelines for corrosion and defects.
This is done through the use of sophisticated pieces of equipment known
as ‘smart pigs.’ Smart pigs are intelligent robotic devices that are
propelled down pipelines to evaluate the interior of the pipe. Smart
pigs can test pipe thickness, and roundness, check for signs of
corrosion, detect minute leaks, and any other defect along the interior
of the pipeline that may either impede the flow of gas, or pose a
potential safety risk to the operation of the pipeline. Sending a smart
pig down a pipeline is fittingly known as 'pigging' the pipeline.
In addition to inspection with smart pigs,
there are a number of safety precautions and procedures in place to
minimize the risk of accidents. In fact, the transportation of natural
gas is one of the safest ways of transporting energy, mostly due to the
fact that the infrastructure is fixed, and buried underground.
According to the
Department of Transportation
(DOT), pipelines are the safest method of transporting petroleum and
natural gas. While there are in excess of 100 deaths per year
associated with electric transmission lines, according to the DOT's
Office of Pipeline Safety in 2009, there were 0 deaths associated with
transmission pipelines, and 10 deaths associated with distribution
systems. To learn more about pipeline safety, visit the
DOT's Office of Pipeline Safety.
A few of the safety precautions associated with natural gas pipelines include:
- Aerial Patrols - Planes
are used to ensure no construction activities are taking place too
close to the route of the pipeline, particularly in residential
areas. Unauthorized construction and digging is the primary threat
to pipeline safety, according to INGAA
- Leak Detection - Natural
gas detecting equipment is periodically used by pipeline personnel
on the surface to check for leaks. This is especially important
in areas where the natural gas is not odorized.
- Pipeline Markers - Signs
on the surface above natural gas pipelines indicate the presence
of underground pipelines to the public, to reduce the chance of
any interference with the pipeline.
- Gas Sampling - Routine
sampling of the natural gas in pipelines ensures its quality, and
may also indicate corrosion of the interior of the pipeline, or
the influx of contaminants.
- Preventative Maintenance - This involves the testing of valves and the removal of surface impediments to pipeline inspection.
- Emergency Response -
Pipeline companies have extensive emergency response teams that
train for the possibility of a wide range of potential accidents
and emergencies.
- The One Call Program -
All 50 states have instituted what is known as a 'one call'
program, which provides excavators, construction crews, and anyone
interested in digging into the ground around a pipeline with a
single phone number that may be called when any excavation
activity is planned. This call alerts the pipeline company, which
may flag the area, or even send representatives to monitor the
digging. The national 3-digit number for one call is “811.”
While large interstate natural gas pipelines
transport natural gas from the processing regions to the consuming
regions and may serve large wholesale users such as industrial or power
generation customers directly, it is the
distribution system that actually delivers natural gas to most retail customers, including residential natural gas users.