Laws_subbed_Outsourcing_book_temp.qxd 02/11/2009 11:48 Page 120
Material Selection Challenges for Deepwater Hydrocarbon Development
of flowlines, as they are able to partition effectively into water and However, there are concerns when aligning long seams due to the
reach the pipe wall in water-wet areas. In three-phase flow, it has risk of cracks occurring and propagating along the long seam of a
been determined that water can stratify below oil even when the oil weld in a material with poor fracture toughness properties. Fracture
is in slug flow. Flowline and well hydraulics must also be understood toughness determines the material’s ability to arrest a crack, thereby
to be able to select and apply inhibitors effectively. The use of pH preventing a running fracture along the weld.
stabilisation in wet gas pipelines using sodium bicarbonate is also
becoming a proven method, except in high volumes of water. Furthermore, the line pipe specification will require high-quality
standards and control on the slab and plate to minimise the likelihood
Deepwater HP/HT developments now require a subsea high-integrity of defects being present in the material, which will reduce the risk of
pressure protection system (HIPPS), which is an instrumented safety fracture initiation sites. This is particularly important for SCRs, as
system that isolates downstream facilities from overpressure. One of aligning the long seam welds will improve fatigue performance by
the benefits of HIPPS is that the technology allows the use of a lower minimising the high/low, which in turn lowers the overall stress
pressure-rated flowline downstream of the HIPPS than for the concentration factor.
wellhead and trees. This allows HP wells to be tied in to existing low-
pressure subsea manifolds, sleds and pipelines. In addition, line pipe (and forging if applicable) manufacturers are
required to provide weldability data detailing the response of line
There have been a significant number of subsea equipment failures pipe and forging material to thermal cycles in the HAZ of welds in
using both 13 chromium (Cr) and duplex stainless steels.
7
All of terms of hardenability, toughness or softening, crack tip opening
these failures can be attributed to either one or a combination of the displacement (CTOD) and charpy testing, in the coarse- and fine-
following factors: grained regions of the HAZ.
use of high-strength materials (specified minimum yield strength Due to the dynamic nature of SCRs, it is necessary to conduct a test
[SMYS] >500MPa); programme to demonstrate the fatigue behaviour of girth welds. Full-
highly stressed components (residual and service); scale fatigue testing of girth welds should be performed to determine
hydrogen charging due to cathodic protection; the number of cycles to failure at various stress ranges in order to
fatigue loading (thermal/vortex-induced vibrations [VIV]); establish the level of weld quality in relation to fatigue performance
varying thermal expansion co-efficients; and and to allow a comparison with the fatigue performance required by
poor toughness in the weld and heat-affected zones [HAZs]). the SCR fatigue design. Welds to be qualified in the fatigue test
programme should constitute each onshore and offshore welding
These failures highlight the importance of fully understanding the procedure. The fatigue samples may comprise pipe-to-pipe and pipe-
operating conditions and how the recommended materials will to-forging welds to represent the onshore and offshore welds as
behave under such conditions. applicable. The installation method should determine the fatigue
sample configuration.
Careful materials selection and design can enable the prevention
of subsea failures, and as operators move into deeper waters Allowable weld defect sizes are becoming more stringent, but can
these matters become even more critical due to the high costs be relaxed by allowing the use of fatigue crack growth data for steel
associated with repairs and the potential detrimental impact on asset in air to be used. This will also reduce some of the concerns the
integrity. Another challenge is the requirement to use non-metallic industry has related to AUT by increasing the probability of detection
materials in critical components that must also resist the same and sizing of the weld defects. This approach helps to minimise the
deepwater subsea conditions, such as elastomers in manifolds and number of unnecessary cut-outs during installation.
subsea trees, etc.
The method of installation is also critical. For example, S-lay is likely to
Weldability is also a major consideration in the selection of any cause more damage to the field joints during installation. However, if
offshore pipeline material. Generally, the automatic welding systems these field joints are allowed to cure (to an agreed level) prior to
used offshore have a small tolerance for out-of-roundness and installation, damage during installation becomes less of a concern.
misalignment. The large-diameter welded pipe (UOE) pipe
manufacturing process can result in a flattened area along the long In summary, future field developments in harsher, deepwater subsea
seam caused by the crimping process prior to U-forming. The environments will require even greater attention to detail to
resulting misalignment from pipes having their long seam offset may safeguard asset integrity. Selection of appropriate, compatible
cause excessive weld root defects. Aligning the long seams will materials for these projects can be extremely challenging, and the
provide the best weld alignment and highest-quality weld possible, critical role of an experienced materials and corrosion engineer
and hence improved fatigue performance. cannot be overstated. n
1. Lange L, Berge S, Material Selection in the Offshore Industry,
Corrosion Protection and Flow Assurance of Offshore Coatings which Affect Corrosion Prevention of Underground
SINTEF report STF24 A04223, 2004.
Pipelines and Risers, ISOPE, Portugal, 2007. Pipelines, Seventh Middle East Corrosion Conference,
2. Laws PA, Price J, Challenging Material and Fabrication
4. Tator KB, Qualification Testing of Fusion Bonded Epoxy Bahrain, February, 1996.
Solutions for Deepwater Hydrocarbon Developments, ISOPE,
Coatings for Pipeline Exteriors, 2006 PACE Conference, 6. Varughese K, Improving Functional Powder Coatings’
Canada, 2008.
Tampa, Florida, January 2006. Corrosion Protection, Prod Finish, January 2000.
3. Laws PA, Price J, Coating and Insulation Selection for
5. Varughese K, Jacob SP, Critical Properties of Protective
EXPLORATION & PRODUCTION – VOLUME 7 ISSUE 2
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