December
2014
Oilfield Technology
|
61
One of the most significant challenges to making the
platform‑less vision a reality is providing cost‑efficient,
reliable pumping solutions to the seabed. The development of
pumping systems that allow for greater operational flexibility
by manufacturers such as FMC Technologies and Sulzer Pumps
are providing operators with powerful options that ultimately
translate into the ability to produce more hydrocarbons.
Subseapumpingtechnologyconfiguration
Subsea pumps can be deployed as a standalone boosting station
for pressure support, and/or in combination with a separation
system. Subsea separation project examples including pumps
are: Troll and Tordis in Norwegian shelf with both liquid pump
and multiphase pumps, Pazflor in Angola with gas tolerant
hybrid pumps, and Marlim in Brazil with an injection pump.
An operator’s choice of pumping technology depends
on the conditions the pump will endure during operation.
FMC Technologies and Sulzer have developed a range of subsea
pumps designed to provide operators with more options for
increasing recovery, from multiphase to hybrid to single phase.
The amount of gas that goes through the pump is one
important factor for deciding which pump hydraulics technology
to utilise. A single phase pump is the obvious choice for an
application where the pump will see a low gas volume fraction
(GVF), whereas a high GVF will call for a multiphase pump. A
combination of the two technologies, hybrid pump, may also be
used.
Ì
Ì
Single phase: Water injection and oil boosting.
Ì
Ì
Hybrid: High pressure with relatively low GVF.
Ì
Ì
Multiphase: Liquid and gas, relatively medium to high GVF.
A subsea pump differs somewhat from a standard topside
pump as it requires protection from the outside environment
and the hydrostatic column represented by the actual water
depth. The pump housing and the electrical motor must be
encapsulated in a common pressure containing vessel. All
pumps are configured with the following main elements:
pressure housing, pump cartridge (including single phase,
hybrid or multiphase hydraulics); electrical motor cartridge;
high voltage penetrators and the barrier fluid system. This can
either be in a vertical or a horizontal arrangement.
The most commonly used electrical motor for subsea is
an asynchronous type, either with water‑based cooling and
lubrication or oil‑based cooling and lubrication. However,
development of a permanent magnet motor technology
for subsea has resulted in a 3.2 MW motor manufactured
by Direct Drive Systems, part of FMC Technologies that has
been qualified together with Sulzer’s pump technology. The
permanent magnet motor offers several advantages compared
to the induction motors currently used for subsea pumps. The
permanent magnet motor is liquid filled, applicable for motor
sizes up to 6 ‑ 7 MW and speeds up to 6000 rpm, and offers
increased efficiencies due to its large air gap that significantly
reduces the losses to hydraulic drag.
Technologyqualification
In early 2013, a unique combination of multiphase pump
hydraulics and a 3.2 MW, 6000 rpm permanent magnet motor
was qualified for subsea boosting applications through the
collaboration of FMC Technologies and Sulzer. This new system
exemplifies the drive to deliver a versatile pump toolbox that
can be deployed to subsea boosting in a variety of applications
ranging from deepwater to Arctic and throughout the world.
The system uses helico‑axial pump hydraulics based
on significant multiphase pumping experience in topside
applications, boosting production fluids with high gas volume
fractions (GVFs) as much as 100%.
The qualification programme, started in 2008, was a joint
effort between the two companies and run according to the
well‑established Qualification Process DNV‑RP‑A203 and
API 17N. This programme was 100% funded by FMC & Sulzer,
however, major oil companies supported this effort by
participation through all phases.
In addition, a high viscous testing was initiated in order to
verify operation with high viscous fluids. Sulzer, with the support
of Total and FMC Technologies, demonstrated that the start‑up
of an helico‑axial multiphase pump when used in combination
with a permanent magnet motor (with its high start‑up torque
feature) is possible for viscosities up to 10 000 centipoise.
The permanent magnet synchronous motor with its rotor
and large rotor‑stator ‘air’ gap – integral to the qualified pump
design – presents significant improvements over conventional
induction motor‑driven pumps. Combining a stiff rotor design
with a fully contained rotor allows constant operating speeds of
up to 6000 rpm.
In the high GVF areas this enables differential pressure
two to three times higher than would be possible with an
induction motor operating at 4000 rpm and thereby significantly
increases pump performance flexibility. Sealed cable‑wound
stator and permanent magnetised rotor designs allow for the
use of water‑based barrier fluid suitable for Arctic fields, where
heightened environmental consciousness has already added to
the existing production challenges.
The successful qualification work additionally proved
permanent magnet motor operation in long step‑out
applications that require step up–step down transformers, a
likely scenario in Arctic applications. Further, the low energy
losses and high‑speed capability of the motor enable the
extension of this design up to and including a 6 MW, 15 000 psi
power rating for deepwater Paleogene applications.
Advancedpowerandcontrols
More advanced power and controls solutions to ensure that
these increasingly sophisticated subsea processing schemes are
supported by the right infrastructure to ensure their success.
These technical developments are increasing FMC Technologies’
capabilities in the field of subsea processing.
Futureofpumptechnology
The 3.2 MW design has led to the development of new 6 MW,
15 000 psi, 6000 rpm pumps that utilises similar concepts. This new
design addresses inherent challenges in Arctic fields, demanding
the installation of one pump in place of two, simplifying subsea
power distribution, utilising fewer mechanical interfaces and
reducing the umbilical costs.
Finally, this high power pump system promises increased
drawdown for high GVF applications while still delivering
high pressure rise, resulting in longer tie‑back and deeper
water capabilities. These advantages will act as enablers for
subsea‑to‑beach solutions that are also well‑suited to Arctic fields.
