The Oil and Gas Industry, like many is highly reliant on the continued and prolonged structural corrosion of pipelines both externally and internally has always been a problem that materials scientists, engineers and plant operators have had to battle with.
Not only do
pipelines form an integral part of many process systems in the oil and gas
industry but one only needs to think about the potential value of the contents
flowing through those pipes to understand just how important structural
integrity and mitigation of loss is to an oil and gas company.
Failure of spirally-welded
pipeline from Stress Orientated Hydrogen Induced Cracking.(Courtesy M Hay Shell Canada – replicated
with Permission from Exova)
For a
plethora of reasons including strict regulatory requirements, oil and gas
pipelines are subjected to a comprehensive test regime. However, pipeline
corrosion really isn't a straight forward subject to study, test or rectify.
There are a vast range of variables to consider:
·
The environment -
external and internally
·
The pipe
materials
·
The age of the
pipework and how long it has been immersed in the environment
·
Coatings used
·
The product
carried by the pipework
·
Conditions of use
(thermal cycling etc)
With such a
diverse range of corrosion factors it is not surprising that pipelines have
been found to fail for a variety of reasons. In light of this, engineers need
to carefully test both the materials used and pipeline fabrication methods to
ensure the most efficient and reliable pipework is installed for each specific
purpose.
To test the
base materials and weld properties of pipelines, there are a number of
materials
standards
and test qualification programmes available. For most offshore oil and gas
pipelines NACE MR0175/ISO 15156 is used. This materials recommendation standard
provides the environmental limits for a range of typical pipeline materials; it
also covers hardness limits, load levels and test limits etc. to help engineers
understand the likely performance of their pipeline within certain situations.
Covering
most of the common issues associated with Wet Hydrogen Sulfide (Wet Sour)
corrosion in oil and gas pipelines, this standard focuses largely on cracking
as a result of the presence of Hydrogen Sulfide. In particular:
·
Sulfide stress
Cracking (SSC)
·
Hydrogen Induced
Cracking (HIC)
However, one
particularly problematic failure mechanism; Stress Orientated Hydrogen Induced
Cracking (SOHIC) does not have a dedicated test regime.
Dr. Chris
Fowler a corrosion expert at Exova and
his team have recently completed an extensive research and development project
into 'Corrosion Testing of Pipework'. The culmination of which is a
ground-breaking new approach to the way the offshore oil and gas industry will
test corrosion of critical pipework. In particular, the testing of pipework
with respect to Stress Orientated Hydrogen Induced Cracking (SOHIC).
A completely
new test method has been developed to provide greater insight and a more robust
test regime and associated set of results for the specific issues of SOHIC.
This is not only of great interest to pipeline engineers but, the same issue
has been found to affect pressure vessels as well - so the new test method
could potentially offer a solution to all manner of industries.
SOHIC has
the potential to cause catastrophic damage to a pipeline and Exova's research
has found that at least 9 critical pipelines in the oil and gas industry have
failed due to this type of crack mechanism over the past 20 years.
Dr Chris
Fowler, the lead scientist in developing Exova's new test method explains the
rationale behind the project:
“As a business we look to continually innovate and develop
new testing methods that will safeguard our customers operations. In this case
we have developed the SOHIC testing system to help ensure pipeline integrity in
the oil and gas industry, particularly as exploration is now entering harsher
environments.
“This new testing method will give companies working in the oil
& gas sector the reassurance they require in relation to pipeline
management. It’s a significant step towards minimizing the environment’s impact
on this important infrastructure and help safeguard the provision of energy.”
The initial
objective of the project (which started in 2005) was to find a solution to the
SOHIC problem, one that arises from a diverse range of conditions and prior to
the completion of this project had been extremely difficult to accurately
replicate within a controlled test environment.
The team at Exova, led by Dr Chris Fowler have
successfully designed a new test method and rig which will enable test
pipelines to be subjected to the bending and twisting forces associated with
this type of residual stress adjacent to welds. Thus, offering engineers the
ability to accurately control load levels to effectively replicate the 'live'
environment.
The “new” test rig which can
impart controlled bending and twist.
Now, for the
first time, a dedicated test regime has been designed, we are at the point
where engineers and scientists can work to accurately identify and quantify the
specific variables responsible for the development / onset of SOHIC.
One key
stage of development has been the ability to prove that materials which are
known to be susceptible to SOHIC do crack under the conditions the Exova team
have managed to replicate and, that a range of materials that are known to be
resistant to SOHIC didn’t experience the same cracking issues. This effectively
offers engineers a simple yes or no approach to materials to be used in these
specific environments.
The next
step is for researchers to delve deeper into the specific material properties
that make a steel more susceptible to SOHIC than others:
·
Material
microstructure
·
Alloying elements
/ Chemical Composition
·
Steel Chemistry
·
Hardness
·
Fabrication
techniques etc.
However, it
is clear that SOHIC is not just a material specification issue, the research
team are now looking at other recent technological developments in pipeline
fabrication that may also have an impact. For example new welding techniques
such as Girth welding.
The team at
Exova is working with both NACE and EFC to try and better understand a range of
other variables that make pipeline more susceptible to SOHIC. Their findings
are due to be published at the end of 2014.
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