Pipeline Corrosion
05.10
In the United States, the annual cost
associated with corrosion damage of structural components is greater than the
combined annual cost of natural disasters, including hurricanes, storms,
floods, fires and earthquakes(1). Similar findings have been made by studies conducted
in the United Kingdom, Germany, and Japan.
According to the U.S. Department of
Transportation Office of Pipeline Safety, internal corrosion caused
approximately 15% of all reportable incidents affecting gas transmission
pipelines over the past several years, leading to an average cost of $3 million
annually in property damage, as well as several fatalities. The need to manage
and mitigate corrosion damage has rapidly increased as materials are placed in
more extreme environments and pushed beyond their original design life.
Typical corrosion mechanisms include uniform
corrosion, stress corrosion cracking, and pitting corrosion. Corrosion damage
and failure are not always considered in the design and construction of many
engineered systems. Even if corrosion is considered, unanticipated changes in
the environment in which the structure operates can result in unexpected
corrosion damage. Moreover, combined effects of corrosion and mechanical
damage, and environmentally assisted material damage can result in unexpected
failures due to the reduced load carrying capacity of the structure.
Ensuring long-term, cost-effective system
integrity requires an integrated approach based on the use of inspection,
monitoring, mitigation, forensic evaluation, and prediction. Inspections and
monitoring using sensors can provide valuable information regarding past and
present exposure conditions but, in general, they do not directly predict
remaining life. Carefully validated computer models, on the other hand, can predict
remaining life; however, their accuracy is highly dependent on the quality of
the computer model and associated inputs. Mitigation (corrosion prevention)
methods and forensic evaluations play a key role in materials selection,
assessment and design.
Pipeline Inspection
A significant portion of many pipeline systems
cannot be inspected through traditional methods. Nondestructive evaluation
(NDE) and inspection tools are critical to assessing the integrity of
pipelines. Traditional NDE methods involve the use of pipeline inspection
gauges (PIGs), which travel through the inside of a pipe and detect the
presence of mechanical damage or corrosion.
Researchers at SwRI have developed an
inspection system for inspecting pipelines that cannot accommodate traditional
PIGs. This system uses remote field eddy current (RFEC), and was designed for
use with the Carnegie Mellon Explorer II Robot. However, this technology can be
adapted to other transport mechanisms. The system can expand to inspect 6-8
inch (150-200 mm) diameter lines. The sensor arms retract to accommodate line
restrictions, such as elbows, tees and gate valves.
SwRI has also developed a guided-wave
inspection technology that can be used to inspect pipelines and other
structural components such as tubes, rods, cables and plates. The
Magnetostrictive Sensor (MsS) inspection system uses inexpensive ribbon cables
and thin magnetostrictive strips that are bonded to the component for
inspection. The sensors attached to the pipe can accommodate a range of pipeline
diameters, which is a significant advantage of guided-wave inspection systems
that use an array of piezoelectric sensors. Because the sensors are low profile
and relatively low cost, permanent installation of the sensors to perform
structural health monitoring is a practical option.
Corrosion Fatigue
Corrosion can degrade the mechanical integrity
of a material through chemical attack. For example, the presence of hydrogen
sulfide (H2S) has been found to reduce the fatigue life of offshore riser
materials by approximately a factor of 10, and in the presence of a notch (that
acts as an initiation point for corrosion fatigue) the fatigue performance can
be decreased by a factor of 100. SwRI has developed customized test facilities
for characterizing the performance of pipeline materials in corrosive
environments. Servohydraulic load frame setup with a custom-designed test cell
and redundant H2S containment systems. Full-thickness fatigue specimens are
machined from riser pipes to preserve through-thickness residual stresses and
to capture welds in joined pipe.
SwRI recently developed a high pressure, high
temperature (HPHT) corrosion fatigue test facility. In this facility the
underlying fatigue crack growth behavior of riser materials subject to HPHT H2S
(and other aggressive) environments can be quantified . This unique test
facility provides the capability to quantify inter-related corrosion-fatigue
mechanisms, and provide data for calibrating and validating corrosion-fatigue
computer models.
Corrosion Exposure Testing
As new materials are developed and
environmental conditions change, assessing material performance due to
corrosion and stress corrosion cracking is of increasing importance. SwRI has a
well-established corrosion testing facility to perform HPHT testing in
extremely aggressive environments. In most cases, the testing environment
consists of a simulated process or reasonable worst-case scenario. These
include determining the effects of H2S, CO2, oxygen, and microbiological
organisms on corrosion/cracking of pipeline materials. Testing conforms to
NACE, ASTM, API, or ISO standards and test materials are analyzed for mass
loss, localized corrosion or stress corrosion cracking (SSC)/sulfide stress
cracking (SSC).
SwRI staffers are highly experienced in
designing, constructing and operating specialty tests to mimic a specific
operation that does not conform to standardized methods. One such capability is
performing the environmental exposure on the API 16C – Flexible Choke and Kill
Systems, which evaluates the effects of gas permeation, gas decompression and
test fluid exposure at the rated temperature.
Corrosion Prediction
Computer modeling is useful to help understand
the mechanisms of internal corrosion, external corrosion and stress corrosion
cracking, and to predict corrosion damage, failure and the most likely location
of corrosion in oil and gas pipelines. These predictions can help support the
development of practical guidelines to assist the pipeline industry in
mitigating existing, or preventing future, corrosion failures.
Forensic Evaluations
Although a comprehensive corrosion-control
program based on inspection, monitoring and model predictions can be an
effective means for controlling pipeline corrosion, unexpected events or
undocumented changes in operating conditions can still lead to premature
pipeline failure. When these occur, it is essential to perform a thorough
forensic evaluation of the failure to determine the failure mechanism and its
root cause. By identifying the root cause of the failure, the pipeline operator
will know if this resulted from an event or operating condition outside of the
general conditions included in the corrosion-control program.
Steps can then be identified to mitigate
future failures by eliminating recurrence of the event. If such an event is not
identified as the root cause of failure, the results of the evaluation can be
instrumental in identifying necessary changes to the corrosion-control program.
Additionally, destructive evaluations, which are a routine part of a forensic
evaluation, can be a valuable tool for validating the effectiveness of a
corrosion-control program.
Sumber:
http://pgjonline.com/2010/03/05/corrosion-control-in-oil-and-gas-pipelines/
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