New pipe-in-pipe design
ensures effective insulation
Close control of bends is key to success in assembly
Derek Bish
Tata Steel
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
Drawing of the geometry of one pipe into another.
|
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.
16-in. clad bends being transferred to the quenching tank
after austenitization at SMGB
pipe bending mill.
|
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.
04/11/2013
Tidak ada komentar:
Posting Komentar