3D fnite element fracture model for external faw in a clad pipe: (a) general model , and (b) example for reeling ECA.
Improved CDF Approach
INTECSEA has been working to improve
the ECA approach for clad/lined pipelines.
This methodology has also been independently verified by DNV GL, and has been
successfully deployed on recent offshore
projects with challenging schedules.
Two enhancements are proposed: 1) automating the finite element model generation
and post-processing; 2) extending the application of the FEA-based CDF methodology
to the early stages of a CRA pipeline project.
The common approach to addressing the
issue of lengthy execution times has been to
optimize the mesh in a finite element model
or to use more CPUs in parallel with high-performance computers. Nevertheless,
these measures help reduce the model solution time only, and do not necessarily decrease the total execution time, which also
includes pre-processing (model generation)
and post-processing (result extraction).
An actual executed project of a CRA pipe
by reel-lay installation using Heerema Marine Contractor’s (HMC) deepwater construction vessel, Aegir, and subject to large
strains during operation caused by lateral
buckling, is provided for demonstration and
discussion purposes. Coupled with the high
operating temperature, the weld metal partially overmatches the parent pipe material.
Due to the weld mismatching, the large applied strains and the multi-material domain
at the girth weld, the FEA-based CDF is
the most suitable ECA approach for this example.
The example demonstrates that an invest-
ment in computing power does not necessar-
ily decrease the total execution time for the
FEA-based CDF approach. This is because
of the time gaps caused by working hour
constraints. Overtime could be considered,
however this will result in cost increases.
The most efficient solution is to eliminate
these time gaps and use an automated tool
which shortens the time associated with the
pre-processing and post-processing tasks.
INTECSEA has developed a software tool
that is able to perform the entire workflow
automatically. All essential input data such
as geometrical properties, material properties, loadings and mesh density can be input
through the program graphical user interface (GUI), or through a template text file.
The tool can be used for C-Mn pipe or CRA
clad/lined pipe under installation conditions
(S-Lay, J-lay and reel-lay) and operating conditions.
The application of this tool helped HMC
to apply more rigorous Level 3C or FEA-based ECA within the schedules available
during project execution, so that undue
conservatism associated with CRACKWISE
methods is avoided.
Commonly, the ECA study is executed
near the end of the detailed design phase or
at the beginning of the construction phase.
This is a common practice from past industry experience with stress-based ECA for
C-Mn pipes, in which the FAD-based ECA
runs can be performed very quickly for
sensitivity studies or optimization studies.
Additionally, the weld metals as well as the
welding procedures for C-Mn pipes have
been well developed and understood over
decades of practicing.
Unfortunately, this is not the case for CRA
clad/lined pipes. Firstly, the choice of weld
metal for the girth welds in a CRA pipeline
is limited to the anti-corrosion weld metals,
such as Inconel-based or Duplex Stainless
Steel (DSS). The tensile properties and fracture toughness of the Inconel-based weld
metals are not as high as those of the DSS.
Importantly, the material strength degradation of Inconel-based weld metals at high
temperature may be high leading to the possibility of an undermatching or partially undermatching weld in operation. On the other
hand, Duplex has very good tensile properties and fracture resistance. However, the
risk for Hydrogen Induced Stress Cracking
(HISC) of DSS for pipeline girth weld has
not been well understood and codified yet,
therefore rigorous qualification should be
undertaken if DSS is considered as the weld
Secondly, there is no detailed guideline
from any standard in the oil and gas industry on the FEA-based CDF approach, although the methodology has been used on
many different projects by various design
houses and installation contractors. As such,
there is a possibility that disagreement on
basic assumptions or even on the methodology itself might occur between the design
houses/installation contractors and the verification parties.
If the detailed ECA for CRA pipes is car-
ried out near the end of the pipeline detailed
design phase or at the beginning of the con-
struction phase, coupled with insufficient
ECA study during FEED, the project may
face the following risks:
• Incorrect weld metal is selected
• Incorrect installation method is select-
• Final ECA report is not accepted by op-
erators and verification parties on time.
Since the CDF approach is time consuming, any late change in the ECA input parameters, including materials, loadings and
basic assumptions, may result in an inadequate study, resulting in a very small tolerable defect size, potentially leading to higher
construction and installation costs.
To mitigate the above mentioned risks, it