By Derek Bish, Tata Steel
Increasing demand for energy, matched with high
commodity prices and advances in technology, are driving operators to extract
whatever reserves remain in the challenging UK continental shelf. Therefore,
the requirement to transfer these multi-phase products from often
high-pressure/high-temperature (HP/HT) wells back onshore is an even more
demanding prospect.
Up until now, the common belief in the industry was that
pipe-in-pipe systems able to withstand environmental challenges such as
corrosion, structural integrity, and thermal management, would be too costly
and complex to apply to riser systems.
Tata Steel worked closely with supply partners to
engineer, procure, and construct these assemblies to further develop this
innovative technology as a cost-effective solution to flow assurance issues.
Need for insulation
HP/HT fields are technically more complex to develop
because of the inherently higher energy in the well fluid and its multi-phase
composition. Managing the extreme pressure and operating temperature must be
based and evaluated on criteria such as corrosion, maintaining structural
integrity, and thermal management.
One particular challenge is the management of pipeline
shutdown. Less expensive solutions for managing the insulation of bends such as
wet coatings, compromise overall shutdown times due to reduced thermal
efficiency. Solutions, such as "self-draining" spools, present a
significant design challenge that can be mitigated by the inclusion of
pipe-in-pipe bends, enabling the same thermal integrity to be maintained in the
whole line.
Tata Steel has previously implemented a solution for
pipe-in-pipe bends for a North Sea development. Since then, new insulation techniques
have been developed that give far superior insulation properties.
Risers, spools, and bends
The main challenge with the construction of pipe-in-pipe
bends is how to pass the inner flowline bend into the outer casing pipe. It is important
that pipe bends have a straight portion on the end to enable efficient welding
to the next pipe section and this can present the insertion of one bend into
the other.
The second construction challenge is efficient
insulation. Wrapping or sheathing is simply not practical here as the
insulation would occupy the annulus of the assembly and prevent the
integration.
New insulation system
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Drawing of the geometry of one pipe into another.
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The system developed by Tata Steel overcomes these
problems by deploying granular Nanogel insulation into the annulus of the
pipe-in-pipe system. Nanogel is made by first forming a silica gel, then
expelling the water from the silica matrix. The resulting material is granular
with trapped nanopores of air, inhibiting heat transfer by conduction,
convection, and radiation (with the inclusion of an opacifier).
The deployment of a novel polymeric bulkhead, cast
directly into the annulus, provides a solid barrier to retain the insulation,
which allows for the relative movement of the inner and outer bends. The
polymer is a "syntactic" material, silicone rubber with glass
microspheres dispersed through the matrix with high strength, flexibility, and
thermal efficiency. The tangent ends of the inner and outer bends are held
rigidly to ensure that the assembly tolerances achieved at manufacture are
retained when the unit is transferred to the welding contractor for
incorporation into the pipeline spool or riser.
In order for the insulation to be effectively deployed
and provide the consistent thermal performance, the annular gap throughout the
assembly must be uniform. It is important the manufacturing tolerances of the
pipe and bends are closely controlled.
Steel pipe and bend manufacture
Together with Tata Steel, Eisenbau Krämer (EBK) and the
pipe bending plant of Salzgitter Mannesmann Grobblech (SMGB) have developed a
series of controls, including a process and measurement system, to ensure all
bend dimensions are closely controlled and mating bends can be produced,
matched, and paired to ensure the most accurate assembly is produced.
In respect to the process-related thinning in the
extrados of the hot induction bends, the wall thickness for the inner and outer
mother pipes was increased accordingly. To match precisely, the mother pipes
have been manufactured with the same ID as the riser pipes.
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16-in. clad bends being transferred to the quenching tank after
austenitization at SMGB pipe bending mill.
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EBK supplied Tata Steel with the mother pipe, which has
material properties that allow formation through hot induction bending. The
main material challenges are to ensure the mechanical properties are suitable
after bending. Therefore, SMGB is taking responsibility for the chemical design
of the pre-material. This also involves the consideration of a series of heat
treatment and forming processes. EBK uses a multi-pass welding process and
steel plate from premium mills in Europe. The manufacturing process at EBK
generates pipe of the closest dimensional control through a series of cold
forming and sizing operations such as external calibration.
At the SMGB pipe bending plant, the special mother pipes
are bent by hot induction bending. Heat is applied through electrical induction
to the mother pipe materials and the pipe is slowly formed to give the correct
geometry. In most pipeline applications the critical dimensions are the
positions and attitudes of the ends of the bends (center-to-end dimension) to
maintain the overall geometry of the pipeline. However, with pipe-in-pipe bends
it is important that the bend radius is also accurately controlled to ensure
the two bends can be integrated. The precise dimensions after bending also need
to be maintained following heat treatment. For the inner clad bends, a
full-body quench and temper heat treatment is applied at the SMGB bending mill
in order to guarantee homogenized material properties for the bends, to fulfill
mechanical and corrosion requirements.
HP/HT material properties
Additional material complexities have to be overcome.
Generally, in HP/HT lines there are challenges because of corrosion, low
temperature toughness, and strength. These parameters require careful material
selection to maintain the balance of properties from the outset through to bend
production. Thermal stresses need to be managed as the loads are shared between
inner and outer pipe. In addition, the insulation can lead to extremes of
temperature being retained in the pipe materials during operation and shutdown
that can form challenging conditions for conventional steel products.
Conclusion
HP/HT well environments present some of the most
challenging and technologically demanding conditions for field developments,
not least because the properties in each reserve offer significant challenges
in terms of material selection and design.
Tata Steel and its supply partners have expanded
capabilities further with the design and creation of cost-effective insulated
pipe-in-pipe bends for risers and spools - an accomplishment previously
considered too difficult.
Pipe-in-pipe bends, while challenging technologically,
can lead to simplification of overall pipeline design and can give better
pipeline performance in times of operation and shutdown.
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