DEEP7 2017 - Subsea 7 - page 13

approaching ambient sea-water
temperature.
Without active heating or conditioning of the
fluid, this temperature reduction may lead to
wax deposits on the pipe wall and hydrate
formations during operating or shutdown
conditions.
In terms of the thermal challenge, there are
two alternative approaches for transporting
hydrocarbons over long distances.
The traditional method is based on
maintaining the temperature of the carried
fluid by using passive materials such as wet
coating, double-wall “Pipe-in-Pipe” (PIP)
with an insulated dry annulus, or high-
performance PIP using field-proven Izoflex™
insulation augmented by reduced internal
pressure.
When the step-out distance increases, even
high-performance passive insulation systems
may no longer be capable of maintaining the
required temperatures and need auxiliary
heating to be applied through either electrical
induction or hot water circulation systems.
Instead of maintaining the heat performance
along the flowline, an alternative for very
long tie-back distances is to cool down the
fluid and thereby force wax deposition at
the pipe wall in a controlled section of the
flowline. Thereafter, wax is removed by a
continous local pigging operation inside
the cooler. When temperature is reduced to
ambient sea water temperature, and wax has
been removed, no further wax deposition
will occur and the fluid can be transported
through simple uninsulated flow lines for
export. AWax Control Unit (WCU) based on
our bundle and pipe-in-pipe technology is
designed to perform this function.
This Cold Flow Technology (CFT) approach
requires Subsea Processing (SSP) facilities
near the field and is especially valuable for
Arctic subsea developments.
Reservoirs may also have insufficient
pressure to ensure production, in which
case additional pumping becomes necessary.
Through our Subsea Integration Alliance we
have access to market-leading capabilities in
boosting low-pressure of subsea production
and processing systems.
Our alliance partner OneSubsea’s fully
integrated processing, boosting and
multiphase metering systems accelerate and
enhance oil recovery, even in long-offset
developments. Their multiphase pumps
have achieved production rate increases of
30%–100%.
Current developments
Subsea 7 is developing technologies in
all these areas to meet the challenges of
long-distance tie-backs. Subsea 7 is the
only subsea contractor with extensive
experience in three different active heating
technologies: Direct Electrical heating (DEH),
hot water circulation within a PIP Bundle and
Electrically Heat Traced Flowline (EHTF).
Active heating technologies are currently
based on topsides power distribution, which
by default limits the length of the flowline
that can be heated. We are combining subsea
electrical power distribution with our highly
efficient EHTF technology to greatly extend
the range of active heating technologies.
Subsea 7’s EHTF technology is already
recognised as one of the most efficient
active heating system in the current market
thanks to its unique combination of vacuum
in the PIP annulus and electrical wires.
Through our technology development
programme, we have acquired valuable
expertise in managing cold spots at every
location in a flowline along sizeable
distances.
Applying thermal insulation or auxiliary
heating to long-distance tie-back systems
has mechanical design implications. We
are currently examining the effect of
uncertainties on such elements as in-place
buckling behaviour, the spans of different
modes of transportation, the effects of
prolonged shutdown and the technical
challenges of installation of long-distance
tie-backs.
Subsea 7 is currently developing and
qualifying the newWCU as part of a larger
development programme supported by the
Norwegian Research Council.
Extending the reach of EHTF
The Electrically Heat Traced Flowline,
which has been developed by Subsea 7 in
collaboration with manufacturer ITP Interpipe
delivers the world’s leading flowline insulation
performance.
EHTF is based on a thermally-enhanced Pipe-
in-Pipe (PIP) design which requires lower
power requirements than Direct Electrical
Heating (DEH).
To maintain a constant fluid temperature
at every location along a flowline requires
a remediation of potential “cold spots”.
Given the higher U-values of connections,
structures or line-ends, when compared with
the PIP flowline, this can result in overheating
which compromises the power efficiency of
the system.
With the advent of long-distance tie-backs,
we have addressed the issue of cold-spot
management through added insulation, local
increases of heating power and the innovative
re-design of key components, including the
“Thermally Efficient Ring Arrangement”
(TERA), which complies mechanically with
reeling conditions while mitigating heat
losses
.
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