Biogas is a renewable fuel generated through the anaerobic fermentation of
organic matter. Biogas is a mixture of gases, mainly methane and carbon
dioxide, accompanied by trace elements dependent on the feedstock and
production method.
The refining process can eliminate carbon
dioxide, water vapor and other trace gases, transforming biogas into renewable
natural gas (RNG) or biomethane, aligning it with natural gas industry
standards. RNG can be injected into the existing natural gas grid and used
interchangeably with conventional natural gas.
The principal challenges associated with biogas
transportation involve the consistent control of non-methane constituents, notably
water content and the maintenance of continuous flow. Instances of process
upsets pose the risk of introducing water and corrosive constituents. During
periods of intermittent or low flow rates, condensation or stagnation may
occur, leading to internal corrosion.
Many times, RNG laterals connecting a biogas
producer to injection points to a transmission pipeline are smaller diameter,
relatively short lengths. These lines lack design features such as internal
inspection launchers and receivers for inline inspection. Furthermore, their
construction may involve 90-degree fittings, telescoping diameters, and other
characteristics limiting the feasibility of routine internal cleaning.
Despite these challenges, methodologies exist to
enable operators to anticipate locations prone to internal corrosion.
Identifying such sites facilitates the implementation of corrosion monitoring
programs, thereby addressing concerns related to internal corrosion in biogas
transport systems.
Gascade
partnered with Dynamic Risk to investigate the internal corrosion
susceptibility of a biogas line in advance of a larger project targeting the
prioritization of difficult-to-inspect pipelines.
About Gascade
Gascade Gastransport GmbH operates a gas pipeline
network throughout Germany. The Kassel-based transmission
system operator offers its customers modern and competitive transport services
for natural gas and, in future, other gases in the heart of Europe via its own
high-pressure pipeline network, which is about 2,300 miles (3,700 km) long.
Gascade, which
wants to convert its pipeline network to the transport of hydrogen, is active
in several onshore and offshore hydrogen projects, and has undertaken a
comprehensive system-wide initiative to enact integrity management principles
within its operations.
Recognizing the
significance of evaluating challenging-to-assess pipelines, the organization
has prioritized this aspect. In the year 2023, a notable project was initiated,
marking the inaugural endeavor to scrutinize the threat posed by internal
corrosion in a non-piggable biogas lateral through the application of modeling
techniques.
Solution
A study leveraged the principles in NACE standard
SP0206-2016, Dry Gas Internal Corrosion Direct Assessment (DG-ICDA), to
identify susceptible locations along the pipeline where water might collect
that could lead to internal metal loss.
The process to identify susceptible internal metal loss locations involved developing the centerline inclination profile, then based on operating gas quality characteristics and flowrate properties, calculate a critical inclination angle beyond which liquid would become stagnant in the line (i.e., gas momentum could no longer push the fluid along the profile). Since the line historically has operated at several combinations of pressure and flowrate, multiple calculations and evaluations were performed.
The anticipation of critical inclination angles
in this line is influenced by the comparatively low gas flow rates, yielding
predictions with values below 2 degrees. During periods of zero biogas flow,
the critical inclination angle is projected to be zero.
The elevation profile of the line exhibits a relatively flat trajectory in the initial few kilometers, subsequently descending toward the outlet. A subtle undulation, characterized by a slight rise and fall, is observed before reaching the termination point of the line.
The first critical inclination angle site occurred
within the first 130 feet (40 meters) of the pipeline length, immediately
downstream of the biogas production facility. Furthermore, since the line
frequently has no flow, water normally restrained at the first critical
inclination angle site will gravitate upstream back toward the pipe inlet
during periods of no flow.
The analysis predicting internal corrosion
indicates a substantial probability of water ingress becoming entrapped within
the initial part of the system. Operational circumstances are conducive to
internal metal loss occurring predominantly at the site characterized by the
first critical angle.
However, during intervals of zero flow, the
preferential metal loss site transitions to the low point of the pipeline.
Given the rate-dependent nature of corrosion, both sites underwent thorough
investigation.
Recommended corrective actions to reduce the
potential for internal corrosion and further enhance pipeline reliability
included:
“We are planning to
inspect the pipeline at the suggested two to three locations by robotic
internal MFL and visual inspection while the line is taken out of service due
to re-routing activities in 2024,” said Stefan Klein, asset manager at Gascade. “I look forward to reviewing the results of the inspections.”
PROJECT OUTCOME
The 2023 Internal Corrosion on Predictive
Analysis project was completed just a few weeks after receiving the operational
data. Subsequent discussions revealed a historical pattern of water
accumulation and flow restriction, attributed to ice formation in proximity to
the line's commencement.
Biogas can have unique gas qualities with the
potential to increase the threat of internal corrosion. Monitoring and
controlling inlet gas characteristics are engineering controls used to minimize
this threat. However, inspection is needed to establish the condition of an
asset per integrity management regulations. When complete asset inspection is
not possible or practical, corrosion modeling with targeted inspections can be an
effective alternative.
Benefits
This approach can be applied to a single pipeline
or larger networks. P&GJ