Deepwater pipelines transporting corrosive fluids at high tem- perature and pressure must combine high strength, good strain capacity and corrosion resistance. Corrosion-resistant alloy (CRA) clad and lined pipelines provide an effective solu- tion in these circumstances, and are increasingly used.
The welding challenges associated with these corrosion resistant
systems limit applicability of traditional fracture mechanics tools
and methods, since they fail to take into account weld strength un-dermatch. Moreover, the traditional failure assessment diagram-based methods do not handle the more complex strain calculations
required for multi-material pipes in complex loading regimes.
The goal here is to outline how these challenges can be overcome
through the use of an advanced methodology based on the finite
element method, which has been validated through focused testing.
A recent example where tangible cost and schedule savings were
made using this approach is also presented.
Girth weld integrity
In recent years, CRA-clad and lined pipelines have become an
increasingly considered solution to handling corrosive fluids from
high-pressure, high-temperature (HP/HT) fields. The use of this
option introduces difficulties of demonstrating girth weld integrity
through engineering critical assessment (ECA). The available welding consumables for CRA girth welds do not necessarily overmatch
the parent pipe, especially at high temperature.
On top of that, at the girth weld, there are high strains caused by
installation, especially during reeling, and operation (such as large
lateral buckling). These two factors in the multi-material domain
push the problem beyond the limits of conventional ECA using the
failure assessment diagram (FAD) approach.
As an alternative, the FEA-based crack driving force (CDF) approach has been employed for ECA on many projects. However, the
FEA-based CDF approach is expensive, in terms of execution time
as compared to the conventional FAD approach. As a result, it is
very difficult to perform early sensitivity and optimization studies,
especially on very tight schedules.
Moreover, the use of the CDF approach is still under develop-
ment for CRA pipes, especially for strain-based applications such as
In the absence of a clear codified guidance, installation contrac-
tors and design houses have executed a number of strain-based
ECA for CRA pipes following published knowledge in the public
domain. This practice exposes a pipeline project to risks such as:
• Underestimation of the complexity of the problem
• Disagreement between design houses/installation contractors
and the verification parties on “grey” areas, which are not well
defined in any standard.
Due to underestimation of the problem complexity, design maturity and test data availability, it is common that insufficient ECA
studies (or even no ECA study) are performed during the FEED
stage. This increases the risk of having incorrect weld metal and/
or incorrect installation approaches selected. Due to the variation in
approaches between the design houses/installation contractors and
the verification parties, some basic assumptions or even the ECA
methodology itself may not be approved. These two factors and the
lengthy execution time expose a pipeline project to significant risks
related to pipeline integrity and/or installation schedule.
ECA is usually carried out using the FAD method, which is based
on the interaction between brittle fracture and plastic failure.
While the failure assessment curve depends on material properties, i.e. tensile properties and fracture resistance of the pipe (
assuming that the weld overmatches the parent pipe), the assessment
Thao (Joe) Tran • Ismael Ripoll
Andrew Low • Luca Chinello
Heerema Marine Contractors UK Ltd.
can enhance deepwater
Heerema’s deepwater construction vessel Aegir in reel-lay operation.
(Photo courtesy Heerema Marine Contractors UK Ltd.)