Note: Descriptions are shown in the official language in which they were submitted.
CA 02517969 1996-12-05
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A SUB-SEA PUMPING SYSTEM AND AN
ASSOCIATED METHOD
Field of the Invention
The invention relates to a pumping system and an associated method for
the pumping of an effluent from a sub-sea well where the effluent is
transported to a
floating surface platform or to an on shore site for processing. More
particularly, the
invention relates to a sub-sea pumping station which may be a multiphase
pumping
station for pumping a multiphase effluent and which is used in conjunction
with a deep-
sea well head.
Ilask~~ulsI~IoL
LO As shallow offshore oil and gas production well reservoirs arc being
depleted, more nations and/or companies are taking a greater interest in deep-
sea
offshore oil and gas reservoirs in which sub-sea multiphase pumping systems
are used
to extract and pump the oil and/or gas from these reservoirs.
A sub-sea multiphase pumping system transports a multiphase effluent,
IS which generally consists of mi~ctures of oil, gas, and water, from a sub-
sea pumping
station over a long distance through .a pipeline to a remotely located
processing plant
where the multiphase effluent is then separated into individual fluid
components prior
to further processing. This processing plant may be ort a floating surface
platform or
may be on the land.
20 Worldwide, several different types of sub-sea multiphase pumping
systems are currently being developed and each type of multiphase pumping
system
CA 02517969 1996-12-05
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consists of the same basic components which art: a multiphase pump, a drive
for the
multiphase pump, a power supply system, a controt system, a pressure
compensating
and maintenance system, and auxiliary lubricating and cooling circuits for the
multiphase pump/drive unit. A sub-sea multiphase pumping system generally
consists '
S of one or more of these basic components which are mounted on a base and
then
lowered and installed onto sub-sea trees where they are connected to a deep-
sea
wellhead.
The types of pumps in use today in the multiphase pumping system an
either a rotodynamic pump or a positive displacement pump as these types of
pumps
are generally able to handle more than one phase of effluents. In the deeper
sea
depths, preferably, the latter type of pump is used in that it is less
sensitive to density
and, therefore, Icss sensitive to the pressure variations of the multiphase
effluent being
pumped. Nevertheless, the sub-sea multiphase pump is required to maintain or
incnax the production rate of the multiphase effluent regardless of whether
the will
pressure is high or low.
The drive for the multiphase pump may be a hydraulic turbine or a
variable speed electric motor, the latter having been determined to be more
power
efficient, more flexible in operation, and less sensitive to its remoteness
from the
power source.
For a hydraulic turbine, either pressurized water or oil is used to drive
it. The system for the pressurized water or oil is lotted on the floating
surface
platform, and several conduit feed lines are connected from this pressurized
system to
the sub-sea unit. Additionally, a barrier fluid system, which is generally
dirt than
the process and turbine fluids, is provided for cooling and for lubricating
the bearings
in the multiphase pump/drive unit and for compensating for the varying
pressures in
the system. This barrier fluid is routed to the floating surface platform
where it is
cooled and then returned to the sub-sea unit, and is maintained from the
topside
platform at a prrssure greater than that of the process fluid so that any
leakage that
occurs will be of the barrier fluid either into the sea or through the
mechanical seals '
into the process fluid.
a
If pressurized water is used to drive the hydraulic turbine, then the shaft
seals between the turbine and the multiphase pump cats be eliminated allowing
the
water in the turbine to flow through the close-clearance axial gaps in the
shafting
CA 02517969 1996-12-05
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between the turbine and the multiphase pump and into the
production or process fluid
which, as discussed above, is the multipha~ effluent being
pumped. In this
application, the barrier fluid may also be water which
circulates through the multiphase
pump and through the turbine housing. The pressure compensation
oxurs in that the
barrier fluid Iealmge fmm the turbine flows into the multiphase
pump and into the
process or production fluid in the pump and finally into
the seawater. The barrier
fluid, in effect provides a backpressurt to the lubricating
side of the seals to insure that
the linkagt is into the process fluid or into the turbine
fluid side of the seals.
If oil is used to drive the hydraulic turbine, then seals
are used to
separate the compartment for the turbine fluid from that
of the multiphase effluent-
being pumped. Generally, oil is also used as the barrier
fluid for cooling and
lubricating the bearings in the multiphase pumpJdrive
unit and for compensating for the
varying pressures in the inlet of the multiphase pump.
Even though the barrier fluid
is compatible with both the fluid in the turbine and the
multiphase effluent being
pumped, one of the disadvantages of this system is that
small amounts of oil tend to
leak into the surrounding seawater thereby creating an
environmental problem.
Even though the hydraulic turbine multiphase pumping systems
are
considered by some as being mechanically and hydraulically
simply in design and
simple to maintain, the topside facilities for these types
of pumping systems are
nyquirod to support extensive systems for the power source,
the hydraulic source, and
the barrier fluid system.
The problem with these facilities is that their power
consumption
increases dramatically with increased pressure drop as
the umbilical feed lines
lengthen. That is, as the sub-sea stations go deeper and
are located further from their
floating surface platform, the hydraulic tine losses for
the hydraulically turbine driven
multiphase pump increases. In general, the more removed
the energy source is from
the sub-sea station, the more complex the recirculating
umbilical feed lines and,
therefore, the more costly it is to provide this type of boosting system for
extracting
. a multiphase effluent from the deep-sea well.
Some system designers have recognized that for deeper wells, submerged
motors provide a more economical alternative to the hydraulic turbine drive.
In one
such system, an elactro-submersible pump has its motor, and in some
applications, a
transformer located on the sub-sea station. The motor/pump unit can both be
oil
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cooled, or the motor can be water cooled and the pump cart be oil cooled. In
the ftrst
system where the oil is the sole lubricating and cooling agent, the oil system
also
provides the pressurization of the system to prevent the back leakage of fluid
from the
pumped fluid, and the oil is transported to an sir cooled cooling unit on the
floating
surface platform. Even though this system is the simpler of the electrical
driven
systems, it still requires umbilical fend and return lines which recireulate
the cooling
medium to the cooling unit on the floating surface platform and back to the
sub-sea
station.
In the second system where the motor is water-cooled and the pump is
oil-cooled, there is an oil cooling circuit for the multiphase pump bearings
and seals,
and a water-glycol circuit for the submerged electric motor bearings and
seals. The
shaft seal leakage from each lubricating circuit enters a chamber between the
motor and
pump which houses the coupling for the motor and pump. The oil and water-
glycol
mix is collated in a leak-off tank. The water-glycol and oil solutions are
periodically
pumped to the floating surface platform where they are separated and recycled
back to
their respective sub-sea supply tanks. Each of the supply tanks have a bladder-
type
diaphragm which coramutticates with the oil supply tank, which, in turn, is in
communication with the pump suction and which, therefore, regulates the
pressure in
the other tanks, resulting in all three tank pressures being equalized to the
pump
suction pressure during all modes of operation of the system regardless of the
external
pressure and water depth. A sub-sea heat exchanger for the oil and a sub-sea
heat
exchanger for the water-glycol transfer their heat loads to the surrounding
water, and
auxiliary impellers attached to the main drive train circulate the two coolant
fluids
thnwrgh the motor and the pump whenever the motor is running. The umbilical
connections between the st:b-sea station and the floating s<trfaa platform
comprise a
three-phase electrical feed, a makeup oil Iine to the oil supply tank, a
makeup water-
glycol line to the water-glycol supply tank, and a leak-off line to the
oiI/water-glycol
separator unit resulting in an increase in the size of the umbilical
connections and
therefore, a complex design for this two fluid system. In general, the current
technologies which feature sub-sea motors employ wet winding motors whose
windings
are directly cooled by the hydraulic cooling circuit medium which generally is
oil. A
disadvantage to using a wet winding motor is that the direct contact of the
windings
with the coolant reduces the long-term reliability of the motor even though
special
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insulating materials are being used. Failure of the motor results in a
substantial loss
of production and increased maintenance costs in that in order to resume
operation, the
sub-station must be removed and replaced.
For a deep-sea well there is a continuing interest in submerged electrical
motor driven pumps for the pumping of an effluent, which may be a multiphase
effluent. However, these presatt system designs are costly and complex, and
require
a great degree of maintenance and manned topside support for their operation.
There remains, therefore, a need in the art to simplify the design for a
sub-sea single or multiphase pumping system, to decrease the costs involved in
providing a sub-sea single or multiphase pumping system, and to provide a more
technically superior and a:onomically advantageous single or multiphase
pumping
system.
There is a further need to provide a sub-sea single or multiphase
pumping system which is substantially maintenance-face, requiring very little
or no
human intervention for its aeration, and which has an increased life
expectancy
compared to present-day systems.
S~yf~ARY Or 1'tll~. llV~F.N'I ION
The present invention has met the above needs. The pre~t invention
provides a sub-sea pumping system which may be multiphase or single and an
associated method for pumping an effluent which may be multiphase or single.
The
system employs a single medium fluid as a coolant and lubricator and comprises
a
canned electrical motor, a single or multiphase pump connected to the canned
electrical
motor, and a combination heat exchanger and pressure compeasator located on a
sub-
sea module. Tha pressure compensator, preferably, is a bellows device which is
ZS responsive W the pump pressure and which keeps the single fluid medium
travelling
through the motor/pump unit for cooling and lubricating the bearings and seals
in the
motor/pump unit at a pressure generally greater than the suction pressure. A
topside
module vn a floating surface platform has a power supply source and a single
medium
' source. A first umbilical connection consists of a set of threo-phase leads
and connects
the power source to the electrical canned motor, and a second umbilical
connection
consists of an hydraulic Line and connects the single medium source to the
canned
electrical motor. The single medium fluid is compatible with the effluent
being
pumped from the d~p~-sra well and, preferably, this single medium fluid is oil
if the
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multiphase efflua~t being pumped is' multiphase and a mixture of oil, gas, and
water.
Preferably, this single medium fluid is also used in the pressure compensator.
The
pressure compauator autonomously operareS the pumping system under water by
adjusting the pressure of the single medium fluid circulating through the
system to the
suction pressure of the pump and maintains the pressure of the single medium
fluid
circulating through the system at an amount greater than the suction pressure
of the
pump. The internal cooling and lubricating of the bearings and seals of the
motor
pump unit are generated by auxiliary pumps which operate directly off of the
main
motor drive, thereby featuring passively operating auxiliary hydraulic
circuits which
result in a maintenance-free sub-sea pumping station module. The topside
module on _
the floating surface platform is unmanned which not only meets, but exceeds
the
service-free life expectancy of a deep-sea production well pumping station.
The single
medium fluid flows into the effluent in the pump and the amount consumed by
the
system is very minimal requiring the single medium fluid to be replaced every
year or
so depending on the opacity of the supply sourcx on the topside module.
It is, therefore, an object of the present invention to provide a sub-sea
pumping system and associated method for the pumping of a effluent from a deep-
sea
well which is an hydraulically solid pumping system which uses a single medium
fluid
which is pressurized to the pump inlet by a pressure compensator which
maintains a
completely filled hydraulic cooling and lubricating circuit within the motor
and the
pump unit.
Morc particularly, the system of the present invention employs a single
medium fluid as a coolant, a lubricator, and as the fluid in the pressure
compensator,
and is compatible with the effluent being pumpod.
It is a further object of the present invention to provide a sub-sea
pumping system which is simple in design with a limited number of components;
which
is less costly, smaller, more compact, and more efficient than present-day
designs; and
which operates unmanned and autonomously at any sea depth for an extended
period
of time. '
It is still a further object of the present invention to provide a pressure
compensator which employs a bellows assembly.
CA 02517969 1996-12-05
_7_
These and other objects of the present invention will be fully understood
and better appreciated from the following description of the invention on
reference w
the illustrations appended hereto.
' ~$IEE DESCRIPTION OF TIiE~~ON
Figure 1 is a perspective view illustrating the sub-sea pumping system
of the present invention;
Figure 2 is a schematic, cross-sectional view of the main components of
the sub-sea module of the pre-sent invention of Figure 1;
Figure 3 is a schematic showing a part of the hydraulic circuit for the
hydraulic fluid between the pump and the pressure compensawr means for the sub-
sea
module of Figure 1; and
Figure 4 is a schematic illustrating the components of the topside module
and the connection of the topside module to the sub-sea module of the pumping
system
of Figure 1.
j~.~~N OF THE PREFERRED EMBODIMENT
Referring first to Figure 1, there is shown a sub-sea pumping system 1
of the present invention which comprises a topside module generally indicated
at 3, a
sub-sea module genesaIly indi~ at 5, and an umbilical connection, generally
indicted at 7, which hydraulically and electrically connects the several
components of
the topside and sub-sea modules 3, 5, respectively. The topside module 3 may
be
supported on a floating surface platform (not shown) which may also support
the
production station. For the pumping system 1 of Figure 1, the operating and
maintenance personnel are generally located on shore or on a host platform,
and the
pumping system 1 is designed to operate in an unmanned mode.
The sub-sea module 5 has a mounting base 9 whictr is su~orted on a
wellhead tree structure (n~ shown) which usually rests on the sea floor.
Mounting
base 9 consists of several funnel-type guide posts, located at its corners,
some of which
are shown at numerals 11, 13, and 15, and which are used to aIiEn the sub-sea
module
5 over the wellhead tree structure. Mounting base 9 provides a stnrctural
framtwork
for the physical protection of the system 1 of Figure 1 during its handling
operations
for its ultimate installation on the welihead tree structure. The structural
framework
is such that it is countec~balancod to insure that the sub-sea module 5 is
capable of
being lowered onto its wetlhead guide posts in a generally horizontal positi~.
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Still referring to Figure 1, mounting base 9 of sub-sea module 5 supports
units 17 and 19. Unit 17 houses a motor 21 and a multiphase pump 23 which are
integrally connected by a transition housing 25, which provides a flexible
coupling
between motor 21 and pump 23. Unit 19 is hydraulically connected to transition
housing 25 by an hydraulic conduit 29.
Umbilical feed line 7 hydraulically and electrically feeds from topside '
module 3 to motor 21 on sub-sea module 5.
Further details with regard to the Scvcral components of sub-sea module
5 will now be given with reference to Figure 2 where it is shown that unit 19
houses
a heat exchanger 27 and a pressure compensator 28, and where these numbers
appear
in Figure 1.
Referring to Figure 2, motor 21 is, preferably, an electrical variable
spoed motor with a high voltage that does not require a submerged transformer
or step
gear. This requirement is realized by a Westinghouse canted motor which is
well-
known in the art and which is further disclosed in U.S. Patent Nos. 5,101,128;
5,183,545; 5,220,231; and 5,252,875. Motor 21 is chosen to operate in a spend
range
of 2596 to 10096 and provides a constant shaft output torque. Motor 21
basically is
comprised of a housing 31, a stator can 33, a canned rotor 35 with a shaft 37,
upper
seal and bearing arrangement 39 and lower seal and bearing arrangement 40.
Shaft 37 of motor 21 is mechanically connected to shaft 41 of multiphase
pump 23 through flexible coupling 43 in transition housing 25 which, in turn,
is bolted
to housing 31 of motor 21 and housing 45 of multiphase pump 23. Multiphase
pump
23 is coupled to motor 21 and is selxtod to be ideally suitod for the
transport of a
multiphase effluent consisting perhaps of mixtures of oil, gas, and water.
Multiphase
pump 23 may be a twin screw type pump, a helico-axial type pump, or any type
of
pump for pumping a multiphase effluent with oil and gas mixtures up to 9596
GVF (gas
volume fra~crion), or higher.
Multiphase pump 23 has an upper seal and bearing arrangement 51 and
a lower seal and bearing arrangement 53.
The types of multiphase pumps which can be uscd in the present
invention are available in the market place and are well-known to those
sldllcd in the
art and, therefore, no further description of multiphase pump 23 is necessary
for a
complete understanding of the invention.
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Still referring to Figure 2, multiphase pump 23 is connected to a valve
of a wellhead treee structure (not shown) supporting mounting base 9 via a
suction inlet
47 which draws the multiphase effluent being pumped out of the wellhead and
discharges the pumped fluid through discharge outlet 49 from where the
multiphase
effluent is transported through a pipeline to a production station in a well-
lawwn
manner.
Referring to Figures 1 and 2, hydraulic feedline 7 of Figure 1 delivers
hydniulic fluid, preferably oil, into the base of motor 21, whereby auxiliary
impellers
(not shown) which ale mounted on shaft 37 of motor 21 in a well-known manner
circulate the hydraulic fluid into lower seal and bearing arrangement 40
between the
cannod stator 33 and cannod rotor 35, through upper seal and bearing
arrangement 39
for their cooling and lubrication, and into transition housing 25 firom where
the
hydraulic fluid is then conveyed by conduit 29 into unit 19 housing the heat
exchanger
27 and pressure compensator means 28. While this hydraulic fluid is being
pumped
through motor 21, it is also being ddivered into the seal and bearing
arrangements S 1
and 53 of multiphase pump 23. Even though not shown in Figure 2, a system of
hydraulic fluid delivery conduits represtnttd by arrows 59, 61, and 63
delivers the
hydraulic fluid being supplied into motor 21 from topside module 3 of Figure 1
into
multiphase pump 23 for delivering the hydraulic fluid to upper and lower seal
and
bearing arrangements 51 and 53 of pump 23 for their cooling and lubrication. A
system of hydraulic fluid discharge conduits represented by arrows 65 and 67
convey
the hydraulic fluid from multiphase pump 23 into unit 19 along with the
hydraulic fluid
passing through motor 21 and being discharged from transition housing 25 via
hydraulic line 29. In unit 19, the hydrautic fluid is cooled by heat exchanger
27 and
is used by pressure compensator 28 to maintain the pressure in pumping system
1,
more about which will be discussed hereinbelow. hydraulic fluid is only pumped
through motor 21 and multiphase pump 23 when the motor 21 is being operated
since
the auxiliary impellers (not shown) are mountod on motor shaft 37, thereby
eliminating
the need for additional pumps and separate power sources.
The present invention preferably uses a single medium fluid, such as oil
as the hydraulic fluid for lubricating and cooling the seal and bearing
arrangements 39,
40, and 51, 53 of motor 2I and pump 23, respectively, since it is compatible
with the
multiphase effluent being pumpod in that the multiphase effluent consists of
an oil
CA 02517969 1996-12-05
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mixture. Any oil leakage through the seal of seal and bearing arrangements 39,
40,
and 51, 53, of motor 21 and pump 23, respoctively during the cooling and
lubricating
process will flow into the multiphase pump 23, thereby eliminating the need
for a
separate leak-off hydraulic circuit or for separators which are required for
pumping
systems which employ combinations of water, water-glycol, oil and gas systems
for
cooling, lubricating, and pressure control.
Still referring to Figure 2, the hydraulic fluid from the inten~al hydraulic
circuits of pump 23 and motor 21 cycles threugh hit exchanger 27 which
transfers the
heat load in the hydraulic fluid generated by motor 21 and pump Z3 into the
surrounding sea water. Heat exchanger 27 may be a single pass or multi-pass
type
the latter requiring a lesser amount of oil, thereby reducing the weight, size
and cost
of the sub-sea module 5 of Figure 1. They type of heat exchanger witl depend
on the
heat load and the space allocation of the design.
Referring now to Figure 3, as discussed thereabove, unit 19 includes a
pressure compensator 28. Pressure compensator 28 is connected to pump 23
through
a suction pressure Sense line 69. Preferably, pressure compensator 28
comprises a
metal bellows assembly indi~ at numeral 71. Bellows assembly 71 comprises a
welded stainless steel diaphragm 73, tension springs 75 and ?7, and a
plurality of
leaves, one of which is indicated at numeral 79. Tension springs 75, 77 are
connected
to diaphragm 73 and are designed such that bellows assembly 71 is able to
displace at
least two gallons per minute of the hydraulic fluid to the seal and bearing
arrangements
39, 40 and 51, 53 of motor 21 and pump 23, respectively. The leaves 79 are
connected to diaphragm 73 on a side opposite to tensi~ springs 75 and 77.
Preferably, leaver 73 are of alternating thick and thin material, are
ZS welded together, and are made of stainless steel. The thin material
provides for good
axial compliance and flexibility of the bellows assembly 71, and the thick
material
provides a degree of stiffness to resist collapse of the bellows assembly 7I
from the
higher external pressure which may be generated by the surrounding environment
and
water depths.
Still referring to Figure 3, the suction presstrme line 69 communicates to
v
bellows assembly 7 the pressure of the multiphase effluent lxing pumped out of
discharge outlet 49 (Figure 2). This suction pressure sense line 69 from pump
23 to
bellows assembly 71 includes a perforated plate 81 which is located out of the
direct
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line of solids passing through the pump suction area 83. Plate 81 is designed
to
minimize the possibility for the solids in the multiphase effluent being
pumped through
unit I7 of Figure 1 to plug up the pressure communication line bctvveut the
pressure
' compensator 28 and the pump suction area 83. The suction pressure sense line
69
S communicates any changes which may occur in the static pressure of the
multiphase
effluent being pumped in sub-sea module 5 of Figure 2 without attenuation to
the
bellows assembly 7I, that is, without loss of pressure in bellows assembly 71.
Where
the internal side of bellows assembly 71 sees the pump suction pressure, the
external
side of bellows assembly 7I, as indicated at numeral $5, is open to the
pressure in the
motor 21 and pump 23, and adjusts the internal pressure of the hydraulic fluid
in the
system for cooling and lubricating the seals and bearing arrangements 39, 40
and S1,
53 of motor 21 and pump 23, respectively, to the pump suction pressure in a
manner
which will be discussed hereinbelow.
The pressure compensator 28, preferably, is located as close as possible
to the pump 23 so that the hydraulic connections 61, 63 (Figure 2) to the seal
and
bearing arrangements 51 and S3 are relatively short in order to improve the
response
to the motor pump side pressure of bellows assembly 71 to the transient
changes in the
Bump suction side of assembly 71.
Referring now to Figure 4, there is shown in further detail the
components for the topside module 3 of Figure 1. These components are a makeup
hydraulic supply tank 87, an electrical power source 89, a control system 91,
and a
monitoring system 93. Preferably, systems 91 and 93 are located on-shore or on
the
production platform where control system 91 is optional and maybe tied into
the output
of the system 1 of Figure 1 on the production platform.
Supply tank 87 and electrical power source 89 are physically supported
by the topside module 3 of Figure 1 where, as discussed hereinabove, umbilical
connection 7 delivers both the hydraulic fluid and the power to sub-sea module
5
which, in turn, is oonndcted W welltree 95 only shown in Figure 4. An arrow
indicated at 97 nprGSa~ts the multiphase flow from the well into sub-sea
module 5, and
an arrow indicted at numeral 99 represents the multiphase flow from sub-sea
module
5 to the production platform (not shown).
With regard to component 89 of topside module 3 in Figure 4, the
electrical power sourcx is preferably a three-phase variable frequency drive
mover and
CA 02517969 1996-12-05
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is electrically connected in a well-known manner to the power generating
equipmart
locate on the production platform. The makeup oil system, comprising hydraulic
fluid
supply tank 89, provides a constant pressure to the sub-sea multiphase pumping
system
1, and periodically replenishes the inventory of the hydraulic oil in sub-sea
module S.
In addition tv supply tank 87, the makeup oil system, preferably, includes a
piston
pump (not shown) which is pressurized by the platform maintenance personnel to
maintain a constant pressure on the discharge end of supply tank 87. This
pressure
combined with the static elevation of the platform relative to that of sub-sea
module 5
provides adequate pressure in make-up line 7 to fill the sub-sea module 5.
T'he in-flow
of hydraulic fluid to sub-sea module 5 is, therefore, automatic whenever the
hydraulic
fluid inventory in sub-sea module 5 falls below the pressure regulator setting
of the fill
valve (not shown) to sub-sea module S. The fill cycle is expected to repeat
about every
three days.
Preferably, the makeup hydraulic supply tank 87 holds about I00
gallons, which, at the expected leak rates of the hydraulic fluid from the
pump shaft
seals and the system relief valve into the pump suction arra 83 of 1 figure 3,
would
require a refill about twice a year. This refill operation would have to be
performed
by an operator and would be the only maintenance requiring human intervention
during
the operating life of the system 1 of Figures 1-4. Otherwvise, multiphase
pumping
system 1 of Flgur~e 1 is essentially autonomous.
With referenoa to Figures 1 through 4, bellows assembly 71 acting as
a pressure compensator permits springs 75 and 77 to load the compensator such
as to
provide a positive pressure in pump 23 which is greater than the wellhead
pressure.
This difference in pressure where the pump pressure is greater than the
welIhead
pressure is refecrod to as the "seal pressure bias." The stiffness of springs
75, 77
combined with the displacement of bellows 71 allows for the development of a
system
for supplying hydraulic fluid from makeup hydraulic supply tank 87 of Figure 4
to
motor 21 and pump 23 of sub-sea module 5 based on changes in the "seal
pressure
bias.' When the hydraulic fluid in motor 21 and pump 23 is low relative to a
preset
limit, the suction pressure in sense line 69 causes bellows assembly 71 to be
extended
and the springs 75, 77 to be compressed wherein the seal pressure bias can be
considered as being slack or low. This low seal pressure bias is usod in the
system to
signal the hydraulic system, through a system of valves (Figure 3) w begin to
deliver
CA 02517969 1996-12-05
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hydraulic fluid to the seal and bearing arrangements 39, 40 and S 1, S3 of
motor 21 and
pump 23, respectively, until the preset limit for the hydraulic fluid in sub-
sea module
S is reached. As motor 21 and pump 23 arse being filled to this preset limit
for the
' hydraulic fluid from supply tank 87 of Figure 4, bellows assembly 7I becomes
more
compressed and springs 7S, 77 are stretched. As springs 75 and 77 stretch, the
' pressure in motor 2I and pump 23 rises and the hydraulic fluid continues to
be
delis until tht pressure in motor 21 and pump 23 reaches a set point pressure
above the pump suction ptassure. This stretching of springs 7S, 77 is sensed
in the
hydraulic system and is used as a signal to interrupt the flow of hydraulic
fluid from
makeup supply tank 87 to unit 17 containing motor 21 and pump 23.
Referring again to Figure 3, there is shown a schematic of the hydraulic
system for the hydraulic fluid for lubricating and cooling seal and bearing
arrangements
39, 40, and 51, S3 of motor 2I and pump 23, respectively, of Figure 2. Valves
for
these hydraulic connections are indicated at numerals 99, 101, 103, 105, and
107 in
1S Figure 3.
The hydraulic system of Figure 3 utilizes a pair of back pressure
regulating valves 109 and 111 in conjunction with orifices 113, I 15 to form a
pair of
pressure sensing relays. Each of the valves 109, 111 is equipped with a
sensing piston
that is referenced on its one side to the pump suction pressure, and on its
opposite side
to the pump/motor internal pressure. Consequently each valve 109, 111
references the
differential pressure between the hydraulic fluid in motor 21 and pump 23 and
the
pump suction pressure. Each valve 109, 111 has a range spring set to establish
a
setpoint. Valve 109 has the charavctcristic that it is closed at pressures
below their set
points and above these set points. Since they are referenced to the suction
pressure, these setpoint pressures are above the pump suction pressure.
Valves I09, III are in series with orificGt 113, 115, respocdvely.
When each valve, 109, 111 is closed, there is essentially no fluid flow, and
the
pressure down stream of each orifice 113, I 1S is equal to the pressure at the
discharge
of a hydraulic pump 117 (400-600 prig higher than the pressure of the
hydraulic fluid
in motor 21 and pump 23). Likewise, when each of the valves 109, 111 opens,
flow
through the orifices 113, 11S decreases the pressure downstream of each
orifice to
essentially the pressure of the fluid in motor 21 and pump 23. Valves 119, 112
are
normally closed valves which roquire a pressure higher than the pressure of
the
CA 02517969 1996-12-05
-14-
hydraulic fluid in motor 21 and pump 23 when applied to their diaphragm or
piston for
their opening. Likewise, valve 123 is a normally open valve requiring a
pressure
higher than the pn~sure of the hydraulic fluid in motor 21 and pump 23 for its
closing.
When pump 117 is operated by main motor 21 there is a high pressure to the
inlet to
valve 123 and to each of the orifices 113, 115 . If the pressure of the
hydraulic fluid
in mover 21 and pump 23 is below 30 psig above pump suction pressure, then
valve
109 will be open, and there will be no pressure on the operator (piston or
diaphragm)
of valve 123. The pressure of the hydraulic fluid in motor 21 and pump 23 will
also
be below the setpoint of valve 111, and valve 111 will be open. In this case
there is
a pressure dtrop across orifice 1 I5. The consequence of tow pressure between
orifice
115 and valve 109 is that the operator of valve I I9 is not pressurized. Valve
119, a
normally closed valve, is closed permitting the operator supply line to waive
121 to be
pressurized. High pressure from pump 117 through valve 123 pressurizes the
operator
(piston or diaphragm) of valve 12 t . This valve 121 opens and hydraulic fluid
flows
from the umbilical supply Line 7 (Figure 4). The umbilical line 7 is
pressurized to
assure that this is sufficient pressure for hydraulic fluid to flow into the
motor 2 t and
pump 23. As hydraulic fluid flows into the motor 21 and pump 23 the bellows
assembly 71 is compressed. The action of compressing the bellows assembly 71
stretches the bias springs 75, 77 and the pressure of the hydraulic fluid in
motor 21 and
pumg 23 rises relative to the suction pump pressure. As the pressures of the
hydraulic
fluid in motor 21 and pump 23 rises above the setpoint of valve I09, this
valve 109
closes. Closing valve 109 shuts off the flow through orifice 113 and valve 123
closes.
When valve I23 closes, pressure is trapped in the operator of valve 121 and
hydraulic
fluid continues to flow from the umbilical line 7 into the motor 21 and pump
23.
The hydraulic fluid continues to flow into the motor 21 and pump 23
until the pressure exooeds the setpoint of valve 111. At that point valve 111
closes and
the flow through the orifice 111 stops. The pressure down stream of orifice I
1S rises
to the level of the output of pump I I7. Valve I 19, connected to the
downstream of
orifice 115, opens. This vents the pressure from valve 121 and permits it to
close
stopping the supply flow of hydraulic fluid down the umbilical lint 7.
A relief valve 125 is provided to cope with chin operational situations.
Typically, when managing an oil field, portions of the field are shutdown
while
draining other arias. When ttu: sub-sea system 1 Wig. 1) is restarted after a
tong idle
CA 02517969 1996-12-05
- IS -
period, it is likely that hydraulic fluid leaking out of the arrangements 39,
41, 51, 53
will reduce the precharge to essentially zero. At that time, as soon as motor
21 has
sufficient spend to pressurize valve 121, the system will fill. The system
fills in a
fairly short period of time, say, roughly 1 to 2 minutes. While filling,
subsequently
the temperature of the motor 21 and pump 23 will increase until the heat
exchanger 27
' reaches equilibrium with the surrounding ocean and the remaining hydraulic
fluid in
the motor 21 and pump 23 expands.
As the system 1 of Figures 1-4 operates, hydraulic fluid leakage thrrough
the seal and bearing arrangemaits 39, 40 and 51, 53, respectively, of motor 21
and
pump 23 will flow into the multiphase effluent being pumped in pump 23,
resulting in
the pressure in motor 21 and pump 23 dropping, bellows assembly 71 expanding,
and
springs 75 and 77 contracting. Since the extension of springs 7~, 77 is
reduced, the
pressure of the hydraulic fluid in motor 21 and pump 23 dect~ relative to the
pump
suction pressure. When the pressure in motor 21 and pump 23 drops below the
minimum setting, which generally will be above the suction pump pressure, the
hydraulic system of Figure 3 begins the cycle again, whereby the hydraulic
fluid is
delivered from makeup supply tank 87 to the seal and bearing arcartgements 39,
40 and
51, 53, respectively, of motor 21 and pump 23. Relief valve 125 (Figure 3) in
the
hydraulic system of Figures 3 avoids over-pressurization and premature vvoar
of the
seals of the seal and bearing arrangements 39, 40 of motor 21 and 51, 53 of
pump 23.
Assuming continuous operation of system 1 of Figure 1, it is expected
that hydraulic fluid from makeup supply tank 87 to sub-sea module 5 will be
lost at an
estimated rate of about 2.5 liters per day. The topside module 3, through the
hydraulic
system, is expected to deliver about 2 gallons of hydraulic fluid to sub-sea
module 5
about every third day. with makeup supply tank 87 being refilled by an
operator about
one, two, or three times a year. It is important to appreciate that the
hydraulic fluid
from supply tank 87 leaks into the process fluid and is reclaimed, and that
the leakage
dons not leak into the environment.
Referring again to Figure 4, the monitoring of the operation of system
1 will be performed from the production platform control room. The power
levels for
motor 21 and the head and flow of the multiphase effluent being pumped will be
monitored during the pumping prod~tion procxss. The ambient water temperatures
will be sufficient to provide adequate cooling of the components of sub-sea
module 5.
CA 02517969 1996-12-05
- 16-
The sub-sea mufti-phase system 1 of the present invention is a
hydraulically solid pumping system in that it is a completely filled or closed
"solid" (no
gas pressurization) hydraulic system. System 1 uses a single medium fluid
which is
pressurized to the pump inlet by pressure compensator 28 which maintains a
completely
filled hydraulic cooling and lubricating circuit within the motor 21 and the
pump 23.
It is to be appreciated that even though the invention has been addressed
herein to a multiphase pumping system, it can be used in a single phase
pumping
system.
While specific embodiments of the invention have been described in
detail, it will be appreciated by those sitilled in the art that various
modifications and
alternatives to those details could be developed in light of the overall
teachings of the
disclosure. Accordingly, the particular arrangements disclosed are meant to be
illustrative only and not limiting as to the scope of invention which is to be
given the
full breadth of the claims appended and any and all equivalents thereof.
