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According to the United States Department of Transportation's
Office of Pipeline Safety, corrosion was the cause of 25%
of all natural-gas pipeline accidents in a recent reporting
period. Still, thanks to modern methods of protection against
corrosion, the actual number of pipeline failures due to corrosion
is relatively small.
Cathodic protection, in conjunction with protective coatings,
is the primary method of defense against corrosion on major
cross-country pipelines. Cathodic protection requires the
constant delivery of a steady electrical charge. In a cathodic
protection system, the pipeline is negatively charged, acting
as a cathode. A positively charged sacrificial anode completes
the circuit. In this system, oxidation occurs in the anode
while the cathode—that is, the pipeline—is protected
from corrosion.
For Transwestern Pipeline Company, cathodic protection is
essential for the maintenance of 2,600 mi. of pipeline that
transport natural gas to markets in Texas, Arizona, New Mexico,
California, and the Midwest. John Gormley, a former Transwestern
corrosion specialist, describes the threat of corrosion as
"always waiting to reduce the metal back to its origins."
Gormley's job was to ensure the constant protection
of Transwestern's pipelines against corrosion. The remoteness
of the pipeline's route through the southwestern US makes
that job difficult because the necessary electrical supply
is not readily available. In populated areas, AC power can
be pulled off of overhead electrical lines and converted into
the DC power needed for the cathodic protection system. Lacking
AC power lines, however, Transwestern turned to distributed-power
generators at each system location.
Transwestern moves 1.2 billion ft.3 of natural gas through
its system every day, so natural gas is the obvious choice
of fuel for electrical generation. Historically, thermoelectric
generators from Global Thermoelectric Inc. have been the power
source of choice for Transwestern. Natural gas drawn from
the pipeline is burned, and the heat moves through a thermoelectric
module containing an array of lead-tin-telluride semiconductor
elements. This module converts the heat directly into electrical
power with no moving parts.
The thermoelectric generators have provided good service
over the years, but Transwestern's pipeline system is now
41 years old. As the pipeline ages, more energy is required
to maintain adequate cathodic protection. "Cathodic protection
requirements were increasing," reports Gormley. "Thermoelectrics
could not give the output needed."
Transwestern was faced with two options: add additional thermoelectric
generators to increase the power supply at each location or
replace worn-out units with a different type of generator.
"The [thermoelectric generators] were wearing out; repairs
would be two-thirds the cost of a new unit," says Gormley.
"Sticking with thermoelectrics would have required the
installation of additional units." Overhead AC power
was considered but ruled out as too expensive; Transwestern
has many remote locations without cost-effective access to
overhead power lines.
While Transwestern was considering its options, they were
offered the use of a Minotaur remote-power system for evaluation.
Now manufactured by Marathon Engine Systems, the 2.5-kW Minotaur
uses a small internal combustion engine driven by natural
gas or propane to deliver 48–100 V of DC power (or 120
V of AC power). The Minotaur offered five times the power
of a thermoelectric generator at roughly the same cost per
unit. Cathodic protection on the Transwestern pipeline currently
requires a 30-V, 35-A output, a fraction of the maximum output
provided by the Minotaur. The Minotaur provides enough reserve
output to allow for future increases in power requirements
as the pipeline ages.
Transwestern's experience with its first Minotaur, however,
wasn't without problems. The system provided by Marathon's
predecessor included a lot of remote-control and remote monitoring
devices that created problems. Gormley also notes that the
unit's plastic fan blades broke almost immediately. After
buying the Minotaur line, Marathon "bulked up the unit,"
which greatly pleased Gormley. Transwestern now has six Minotaur
generator sets in place: three in western Arizona and three
in central New Mexico.
Replacement of the thermoelectric generators with the Minotaur
units, which have a footprint of only 41 in.2, has been very
simple. "Concrete pads and all connections were already
in place," says Gormley, so Transwestern simply could
pull an outdated thermoelectric system and put a Minotaur
in its place with minimal effort.
Operating costs are low for Transwestern. Natural-gas consumption
is specified by Marathon as 40 ft.3/day, which is a negligible
volume compared to the overall volume of gas handled throughout
the pipeline system. Transwestern has never metered the actual
consumption, which shows up as part of the overall system
loss.
Remote Monitoring
Transwestern's Minotaur units are equipped with remote
monitoring capabilities using Bullhorn telemetry devices from
American Innovations. Because it does not require the purchase
or installation of any equipment other than the telemetry
devices on each Minotaur, the wireless Bullhorn system provided
a simple solution for Transwestern. The Bullhorn devices use
cellular telephone technology to report equipment readings
to an American Innovations operating center. The data are
stored in a database that Transwestern operators can access
over the Internet using a Web-based interface.
The remote monitoring devices transmit four channels of data,
including voltage and amperage readings, at a specified time
every day. The primary use of the remote monitoring equipment,
however, is to notify the pipeline operator of any shutdowns
in the cathodic protection equipment. If the daily data readings
show that a unit has shut down, the American Innovations system
automatically notifies a Transwestern technician by e-mail.
Downtime is minimized because a maintenance team can be dispatched
immediately rather than weeks later when crews visit the remote
location on routine checks.
Remote monitoring using cellular telephone technology might
not sound like a viable solution in the sparsely populated
locations where Transwestern's pipelines are located.
The Bullhorn operates at much higher power levels than those
used for voice transmission, however, and high-gain directional
antennas can be installed; American Innovations claims 98%
coverage of the US.
Maintenance
The remote monitoring system is designed to provide Transwestern
with prompt notification of system malfunctions, but those
malfunctions have been rare with the Minotaur. In fact, Gormley
notes that only one malfunction has occurred since Marathon
took over the Minotaur product line two years ago. In that
instance, a circuit card failed, and Marathon sent a replacement
card, which Transwestern personnel easily installed with no
operating downtime.
Transwestern has found that the Minotaur also requires very
little routine maintenance. Marathon designed the Minotaur's
internal combustion engine to provide up to 40,000 hours of
continuous operation, allowing nearly five years of use on
the pipeline before a major engine overhaul is required. In
the remote locations traversed by the Transwestern pipelines,
routine maintenance intervals are specified at 4,000 hours
by the Office of Pipeline Safety, although Transwestern personnel
visit each unit every other month to gather such operating
data as voltage and amperage outputs. These trips are somewhat
redundant, however, as the Minotaurs are equipped with remote
monitoring capabilities. Gormley expects that the frequency
of visits will decrease as Transwestern comes to rely more
on the daily data reports sent by the remote monitoring system.
Often vandalism can become a maintenance issue. Transwestern's
Minotaurs are secured behind chainlink fences, but chainlink
doesn't stop a bullet when a piece of equipment is used
for target practice. Gormley reports Transwestern is lucky
that no one has taken any shots at the Minotaurs yet, so their
resistance to this type of vandalism has not been tested.
The unit enclosures, however, are constructed of 3/16-in.
aluminum structural members with 1/8-in. aluminum skins.
Future Expectations
Gormley expects that the cost effectiveness of the Minotaur
units now in place along the Transwestern pipeline will be
reevaluated in five years. That is when Marathon recommends
an overhaul be completed, and the economics of overhaul versus
replacement with a different unit will be considered.
After installing the Minotaurs at Transwestern, Gormley returned
to his home state of Nebraska and is now a senior corrosion
specialist for Northern Natural Gas. Most of the pipelines
in Northern Natural Gas's system are in populated areas
with easy access to existing overhead AC power lines, so there
is less need for distributed-energy systems, such as the Minotaur.
Gormley is a strong advocate for the Minotaurs, however, and
he looks forward to promoting their use along the southern
end of Northern Natural Gas's pipeline in western Texas,
where overhead power lines are less available.
For
More Information
The following companies were discussed in this article and
can be contacted directly for additional information:
American Innovations
www.aimetering.com/bullhorn.html
12112 Technology Blvd.
Austin, TX 78727
512/249-3400
Global Thermoelectric Inc.
www.globalte.com
#614, 16760 Hedgecroft
Houston, TX 77060
281/445-1515
Marathon Engine Systems
www.marathonengine.com
2050 Energy Dr.
East Troy, WI 53120
262/642-6436
Northern Natural Gas
www.northernnaturalgas.com
1111 S. 103rd St.
Omaha, NE 68124-1000
402/398-7664
Transwestern Pipeline Company
www.crosscountryenergy.com/about/tw.shtml
CrossCountry Energy Corp.
1400 Smith St.
Houston, TX 77002
713/853-6161
Guest author THOMAS M. ROTH, P.E., is a
geological engineer with Parson's Engineering Science
in Atlanta, GA.
DE - Jan/Feb 2004
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