Modern, over-the-line survey
technologies are effective in evaluating coating quality and finding coatingholidays for "non-shielding" coatings. Technologies such as pipeline current mapping (PCM) which utilize a carrier
signal transmitted along the pipeline with
a receiver measuring the line attenuation along the pipelinelength accurately pinpoint areas of significant coating degradation even under concrete or asphalt.
As pipeline operators
implement integrity management programs, the effectiveness of existing cathodic
protection systems comes under greater scrutiny. Pipeline owners battle aging systems with deterioratingcoating systems that can have a significant negative
impact on the effectiveness of the cathodic protection systems. With
enough coating deterioration, the mandate to maintain effective
cathodic protection sends owners and operators on a search for solutions.
External corrosion remains a significant threat for pipeline operators. Historically, pipeline owners have employed a two-tiered approach to
mitigate corrosion risks. The first line of defense against corrosion has been a coating system that is
intended to act as a barrier protecting the steel pipe from its environment.
Cathodic protection is used to supplement the coating system by providing protective current to the
holidays or defects within the coating system.
Recently adopted international standards
for pipeline integrity provide procedures that collectively
seek to address pipeline threats and specifically corrosion-related issues. Many practices in place for decades
and the incorporation of advances in indirect survey technologies have ushered
in approaches being adopted bypipeline operators worldwide. With this emphasis on
integrity management, aging pipelines are
receiving an unprecedented level of scrutiny.
What is the best strategy to remediate
an aging pipeline and bring it into compliance with international
standards? Initial options are to improve/upgrade the cathodic protection
system or to recoat the pipeline. With extreme cases where significant corrosion has already occurred, the only option is to
replace thepipeline. In the majority of cases, the burden of weighing the
alternative of upgrading the cathodic protection system vs. incurring the cost
of recoating is left with the pipeline operator.
Ultimately, the goal of integrity management is to provide a structured
approach to this type of pipeline operation decision and keep the focus on regular
assessment and evaluation of the pipeline's
critical areas.
Coating systems have been used on buried pipelines during the last hundred years and the technology
remains the subject of great research and innovation. Manufacturers are
continually searching for better coatings to
meet the varied needs of industry. At first, the coatings were simple mixtures of crude pitches and
solvents. These early bitumastic/asphaltic systems evolved into engineered coal
tar enamel coating systems which were prevalent into the 1960s. The
introduction of fusion bonded epoxies (FBE) in the 1970s quickly captured much
of the pipeline market, although polyethylene, polypropylene and
coal tar enamels are still used as well.
When evaluating aging pipelines, coating condition is one of the critical issues that
must be addressed. The coating provides the primary defense against corrosion but as the coating system ages and
deteriorates, the risks of corrosion increase exponentially. One challenge that must
be addressed by pipelineowners is to properly identify the type and vintage of
thecoatings along a given pipeline.
In many scenarios, different sections
of pipeline may have different coating systems, depending on the age of the pipelineand the standards in place at the time a particular
section of pipe was installed, repaired or replaced. As part of the integrity
management process, during the pre-assessment phase, pipeline attributes are defined for each segment through
a process that includes documenting the type of coating used during the construction along with the
repair methods and history of the line.
One critical consideration when
evaluating aging pipeline coating systems is to identify whether the coatingsystem fails in a shielding or non-shielding
mode. Coating systems that fail in a non-shielding mode do not
inhibit the flow of current to the pipe surface, making upgrading of the
cathodic protection system a viable alternative to recoating when considering
how to remediate these lines. Other coating systems,
principally tape coating systems, can fail in a manner that shields
cathodic protection current and thus make recoating the only viable option.
Modern, over-the-line survey
technologies are effective in evaluating coating quality and
finding coatingholidays for "non-shielding" coatings. Technologies such as pipeline current mapping (PCM) which utilize a carrier
signal transmitted along the pipeline with
a receiver measuring the line attenuation along the pipelinelength accurately pinpoint areas of significant coating degradation even under concrete or asphalt. The
information gathered using PCM in conjunction with pipe to soil close interval
surveys (CIS) and direct current voltage gradient (DCVG) testing form the basis
for identifying critical risk areas along aging pipelines.
For pipelines with
"shielding" coatings, the over-the-line surveys and indirect assessment
technologies have been proven to be less than effective. For these pipelines, soil surveys and an increased frequency of direct
examination (digs) are used to determine coating condition.
Inline inspection technologies using smart pigs also provide valuable data
regarding coating quality and are especially useful for pipelines with "shielding"coating systems where indirect assessment technologies
are not very effective.
Pipeline coating systems are typically augmented by the
application of cathodic protection. With a well-coated pipeline, cathodic protection can be economically applied to
protect coatingholidays and defects by the placement of discrete
anode beds capable of distributing current over long distances.
In many cases, ground beds can be
located several kilometers apart and still provide sufficient current
distribution to protect the entire pipeline.
With some of today's high technology factory-applied coatings, the coating efficiencies are
exceptionally high and the groundbed output requirements are very low. These
discrete ground bed systems can either be deep anode ground beds or shallow
ground beds located some distance off the pipeline.
Several issues must be considered when
designing a cathodic protection system. These include coatingquality, soil resistivity, available locations for
electrical power, ground bed right-of-way issues, accessibility for
maintenance, and AC and DC stray current interference.
What is critical for aging pipelines is the regular evaluation of the effectiveness
of the CP system. Frequently, as pipelines age
and the coating quality begins to deteriorate, the CP systems
are unable to provide sufficient, properly distributed current to meet
established cathodic protection criteria. Simply ramping up the output of the
existing system or adding additional ground beds does not always sufficiently
address the problem.
Aging Pipeline Systems
Aging pipeline systems with deteriorating coating systems suffer from poor current distribution
and are characterized by areas of low potentials and exceedingly high levels of
applied current density. The challenge with these pipeline systems is controlling current distribution to
achieve the prescribed polarization levels consistent with international
standards for adequate cathodic protection.
The typical response to this problem is
to increase the overall output of the deep well system. This generally does not
alleviate the problems of not meeting the OFF-potential criteria and leads to
over-polarizing the piping (OFF-potentials greater than -1.2 Volts.) This can
result in coating disbondment, further exacerbating the problem.
The higher output current increases the ground bed's consumption rate reducing
operating life while raising operating costs appreciably. All this occurs
without achieving the required levels of polarization to meet cathodic
protection criteria.
The next step that is taken to fix the
cathodic protection current distribution problem is to add additional ground
beds to reduce the distance between point sources. This too, may prove to be
ineffective as the new ground bed provides only limited additional benefit.
Factoring In Cost
Unfortunately, the problem cannot be
economically resolved by the addition of an ever increasing number of ground
beds applying greater amounts of additional current. The pipeline operator is then faced with the original,
limited number of options: recoat the pipeline or
install a linear anode cathodic protection system.
Recoating/replacing is the only viable
alternative for pipeline systems utilizing shielding type coatings such as tape wrap systems. Recoating costs
typically run $125-150/foot in open right-of-way areas and can be significantly
more expensive in congested urban locations (these are typical numbers
applicable to the U.S. and can vary significantly.) Recoating, when properly
performed, can restore the pipeline
coating system to an asnew condition,
greatly extending the service life of the recoated section. The critical issue
is to ensure that the recoating is executed by an experienced coatings contractor with rigorous quality controls in
place.
Linear Anode Alternative
An economically attractive alternative
to recoat is to utilize a linear anode configuration in lieu of discrete systems.
This option is only viable when the coating system is
non-shielding - this would include asphaltic and epoxy-type coating systems. The application of a linear anode
system typically costs between $15-25/foot in open right-of-way (again these
are general price guidelines and can vary significantly.) These systems
eliminate the current distribution problems experienced by point anode systems;
they are, in effect, an infinite series of point anodes which provide an
optimum current distribution.
It is important to work with a corrosion engineering firm with proven experience in
product selection, versatility, and consistent adherence to standards. In
addition to confirming that the pipeline coating system
is appropriate for the application of linear anodes, the linear anode system
design must take into consideration the critical issue of voltage drop and its
effect on current attenuation.
Voltage drop can have a significant
impact on DC power distribution to the linear anode system. Ideally, rectifiers
would be located no further than a half mile to a mile apart; however,
practical considerations including availability of AC power, right-of-way
issues and other factors can force this to be extended further, complicating
the system design and affecting the installed cost.
While the design can be complicated by
voltage drop considerations, one benefit of a linear anode system is that the
power consumption is relatively low. Ground bed resistance, as determined by
Dwight's Equation, is significantly affected by anode length and this results
in very low groundbed resistance values for linear anode systems relative to
conventional ground beds.
This makes the linear anode system much
more suitable for low wattage power sources such as solar arrays and
thermo-electric generators (TEGs) than conventional ground beds whose wattage
could be two or more times that of a linear anode system to achieve the same
current discharge. Industry-leading engineering firms should walk through these
factors and considerations relative to a pipeline operator's
individual circumstance.
Aging pipeline systems with deteriorating coating systems present a difficult challenge and time
can often exacerbate a pre-existing condition. The more the coating deteriorates, the more difficult it is to
distribute current farther away from the ground bed. Ultimately, pipeline operators should think carefully and consult
industry experts to help them face the choice of recoating the pipeline or installing a linear anode system. The
challenge can be an opportunity to not only solve a short-term threat but also
institute a method for proactive prevention down the road.
Sumber : Huck, Ted. "Linear Anodes Target Aging Pipeline Coating Threats". 26 Januari 2014. http://search.proquest.com/docview/197467022?accountid=31562
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