Note: Descriptions are shown in the official language in which they were submitted.
CA 02587538 2007-05-04
SYSTEM AND METHOD FOR MANAGING THE BUOYANCY
OF AN UNDERWATER VEHICLE
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to the mechanical arts and methods that embody
underwater work methods. More particularly, it relates to devices and methods
for
improving the productivity of a remotely operated vehicle (ROV) engaged in
underwater maintenance and construction work.
Description of Related Art
Conventional underwater work techniques often include the use of remotely
operated
vehicles (ROV's). A surface support vessel and its associated personnel
support and
operate the ROV. The ROV may be deployed directly from the support vessel or
from
the surface via a tether management system (cage). When deployed directly from
the
surface, the ROV is connected to its control and powering components on the
support vessel
with an umbilical cable. When deployed from the surface in a cage, the cage
and ROV
are lowered to a location near the worksite on a similar umbilical cable.
Thereafter,
the ROV may be maneuvered from the cage to the worksite while coupled to a
tether
extending between the ROV and the cage.
Regardless of the method employed to deploy the ROV to the worksite, ROV's are
designed so that they are essentially neutrally buoyant (they neither float
nor sink).
Therefore, addition or removal of payloads (weight) to/from the ROV requires
that the
ROV have either excess thrust capacity or the ability to add or remove
buoyancy or
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ballast to compensate for the addition or removal of weight.
ROV operations include use at an underwater worksite to manipulate various
payloads.
Supporting payloads with a specific gravity (SG) greater than unity tends to
make the ROV sink.
Supporting payloads with a SG less than unity tends to make the ROV float.
Because of this, the
ROV must be able to compensate for or manage its buoyancy when on-loading or
off-loading a
payload.
A typical ROV utilizes fixed buoyant volumes such as syntactic foam or fixed
air voids in
combination with its vertical thruster's capacity to manage its buoyancy
relative to the ROV
equipment's weight or negative buoyancy. When large packages are added to the
ROV, the
package's buoyancy is typically compensated for via fixed buoyant volumes or
ballast tanks added to
the package at the surface, thereby enabling the ROV to manage the package's
buoyancy. The ballast
tank may be filled with gas or liquid or a combination of both. Replacing
liquid with gas in the
ballast tank makes the ROV rise while replacing gas with liquid tends to make
the ROV sink.
Typically, the gas is air and the liquid is water.
When on-loading a dense payload (SG > 1) the ROVs buoyancy may be adjusted by
replacing
liquid with gas (deballasting) in the ballast tank. To compensate for
offloading the dense payload, the
ROV's buoyancy may be adjusted by replacing gas with liquid (ballasting) in
the ballast tank.
Conversely, to compensate for on-loading a scant payload (SG < 1); the ROV's
buoyancy may be
adjusted by replacing gas with liquid. The ROV's buoyancy may be adjusted to
compensate for
offloading the scant payload by replacing liquid with gas.
The ROV consumes compressed gas from an integral (onboard) gas storage system
each
time it performs the deballasting operation. When the integral gas storage
supply is depleted, it
must be replenished. The ROV must retum to the surface for gas replenishment.
A remote operator
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maneuvers the vehicle back to the surface, either directly or via the cage,
where surface vessel
resources replenish its integral gas storage system. Redeployment of the ROV
is in cither case
accomplished by reversing the recovery operations.
ROV productivity is significantly reduced when it is employed to repetitively
move
payloads from one location to another. Repeated on-loading and off-loading of
payloads requires
repeated gas recharge operations which deplete the ROV's integral gas storage
supply. The ROV
is therefore required to make frequent trips to the surface to replenish this
supply. Such trips
to the surface consume time and are inefficient, regardless of how the ROV is
deployed.
Accordingly, there has existed a need for improved ROV buoyancy control
systems.
There is a still further need for improved ROV work methods. The present
invention satisfies these
and other needs, and provides further related advantages.
SUMMARY OF THE INVENTION
According to the invention, a system and method is provided for managing the
buoyancy of a
ROV working at an underwater site. The ROV includes an integral ballast tank,
a fluid
connector in fluid communication with said ballast tank, and optionally an
integral pressurized
gas storage tank in fluid communication with the fluid connector. A second
mating fluid
connector is located proximate to the underwater site. One or more pressurized
gas storage
tanks are in fluid communication with said second connector. The gas storage
tanks are separate
from said ROV. Interconnection of the first and second mating fluid connectors
provides for gas
transfer to the ROV. The gas transfer may refill said integral gas storage
tank, recharge the
ballast tank, or both.The ROV is adapted to engage and disengage payloads.
Neutral buoyancy
of the ROV is restored in conjunction with following a payload on-load or off-
load by adjusting
the ROV ballast. Gas consumed by the ROV during these buoyancy adjustments is
supplied
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/replenished by the gas transfer operations.
An underwater workstation may be located proximate to the underwater site. The
payload(s),
second fluid eonnector, and one or more of the gas storage tanks may be
mounted on the workstation.
While adjacent to the payload at a first location, the ROV may exchange a
payload, perform gas
transfer, and adjust buoyancy as needed. Subsequently the ROV inay transport
the payload to a
second location where payload exchange and buoyancy adjustment activities may
be repeated.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is described with reference to the accompanying figures.
In the figures,
like reference numbers indicate identical or functionally similar elements.
The accompanying
figures, which are incorporated herein and form a part of this description,
illustrate the present
invention and, together with the description, further serve to explain the
principles of the invention
and enable a person skilled in the relevant art to make and use the invention.
FIG. I is an isometric view of a ROV spread complete with a cage deployed at a
typical
worksite.
FIG. 2 is an isometric view of a ROV spread excluding a cage deployed at a
typical
worksite.
FIG. 3 is a schematic view of the gas supply and refill components located on
the ROV and
workstation.
FIG. 4 is a schematic view of the gas supply, refill, and liquid transfer
components
located on the ROV.
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FIG. 5 is a schematic view of a ROV and a worksite.
FIG. 6 is a tabular description of a ROV project.
FIG. 7 is a schematic showing operations that comprise the ROV work task.
FIG. 8 is a flowchart showing steps that comprise the payload transfer
activity.
FIG. 9 is a flowchart showing steps that comprise the gas transfer activity.
FIG. 10 is a flowchart showing steps that comprise the buoyancy adjustment
activity.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Introduction
The present invention provides time saving work methods and systems applicable
to the
operation of a ROV. ROV systems operated according to the present invention
have specific
features and advantages, including, but not limited to, increased productivity
and reduced
operating risk. These features and advantages are especially evident when the
ROV is repetitively
moving payloads from one location to another.
As noted above, a ROV may advantageously employ the present invention to
support or to
carry out underwater work, including maintenance, repair, and construction
work. The system
and methods described enable a ROV to replenish its gas supply proximate to
the worksite. These and
other features and advantages of the present invention will now be described
in detail with
reference to the accompanying drawings.
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Improved ROV Work Methods
In an embodiment, Fig. 1 shows a ROV spread 100 with a deployed ROV 116
mobilized at an underwater worksite 130. The worksite typically comprises a
ROV 116,
an underwater workstation 120, a stationary structure 108, and a payload
destination area
134. The workstation is lowered to the worksite by the station winch 132 and
boom 104 on
support vessel 102 using a station umbilical means 118. The ROV is coupled by
a ROV
tether 114 to a cage 112 that includes a tether management system. The cage is
suspended
above the worksite using an umbilical means shown as cage umbilical 110. The
cage
umbilical is deployed from a cage winch 1061ocated on the support vessel.
The support vessel 102 provides a deck 128 where the aforementioned winches,
booms,
tethers, and umbilical means are mounted. The support vessel also provides dry
storage locations on the deck for the ROV 116, cage 112, and workstation 120.
The words "umbilical means," as used herein, refers to one or more lines
and/or
conductors that may be grouped into one or more bundles. The umbilical means
is generally
flexible and may be spooled on a winch or otherwise coiled. The umbilical
means may
include load bearing line(s) (a metallic cable is typical), electrical
cables(s), fiber optic
cable(s), and fluid transport line(s). The word "tether" as used herein,
refers to an umbilical
means extending from the ROV to the cage for the potential supply of load
bearing,
electrical, fiber optic, and fluid transport connectivity.
With continued reference to Fig. 1, the underwater worksite includes an
offshore
structure 108. Submerged metallic portions 124 of offshore structures are
frequently
protected from corrosion by cathodic protection systems such as anodes 122.
One object of the
present invention is to provide an improved ROV work-method for servicing
anodes.
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Referring now to Fig. 2, a second embodiment of the present invention is
illustrated that excludes the cage 112. In this embodiment, a cageless ROV
spread 200 with a
deployed ROV 116 is shown. The ROV is coupled by an umbilical 114 to the
support vessel
102. The umbilical is deployed from a winch 106. With the cageless ROV spread,
the
ROV is deployed directly from the support vessel 102 rather than from an
underwater
cage. The cageless ROV spread may be used for worksites in relatively shallow
water
(generally less than 150 feet in depth).
Referring now to Fig. 3 and the features of the present invention adapted to
gas supply
and replenishment, a gas refill system 300 is shown. Selected gas refill
system equipment is
integrated with the ROV 116 and with the workstation 120. Gas transfer between
the ROV
and the workstation is enabled when a ROV connector 318 and a workstation
connector
350 are mated. In an embodiment, the ROV and the workstation each have one or
more connectors 318 and 350 respectively (one of each is shown). At least one
ROV
connector 318 and one mating workstation connector 350 are capable of
exchanging fluids.
Any of these connectors may also be capable of exchanging one or more of
electrical signals,
optical signals, and mechanical loads with mating connectors.
Still referring to Fig. 3, the ROV equipment comprising a part of the gas
refill system
includes a ROV gas supply system 302 and a ROV connector 318. The gas supply
system includes a ROV gas transfer module 306 and a ROV gas storage tank 304
(optional). The gas transfer module may include valve(s) and control(s) for
managing gas flow and may be partially or wholly integrated with the
connector. An optional
ROV gas transfer line 322 fluidly connects the gas transfer module and the gas
storage tank.
Gas flow in this transfer line is bi-directional as shown by the flow arrows
to 308 and from
310 the gas transfer module.
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Still referring to Fig. 3, the ROV gas inlet line 316 fluidly connects the gas
transfer module and the connector. Gas flow in the inlet line is toward the
gas transfer
module as shown by a flow arrow 330. A gas recharge line 314 fluidly connects
the gas
transfer module 306 and the ballast tank 312. Gas flow in this recharge line
is bidirectional
as shown by the flow arrows to 328 and from 326 the gas transfer module.
Finally, a vent
line 324 fluidly connects the gas transfer module and the underwater
environment 333.
Flow in the vent line is toward the underwater environment as indicated by a
flow arrow
332.
In an embodiment of Fig. 3, the ROV equipment comprising a part of the gas
refill
system 300 excludes the optional ROV gas storage tank 304 and the ROV gas
transfer line
322. This embodiment relies on gas storage means separate from the ROV 116 to
supply
gas to the ballast tank 312 for gas recharge.
In another embodiment of Fig. 3, the ROV equipment comprising a part of the
gas
refill system 300 includes the optional ROV gas storage tank 304 and the ROV
gas transfer
line 322. This embodiment may rely on gas storage means integral to and/or
separate from
the ROV 116 for gas recharge.
With continued reference to Fig. 3, workstation equipment comprising a part of
the gas
refill system 300 includes a station gas supply system 354 and a station
connector(s) 350
designed to mate with the ROV connector(s) 318. The gas supply system includes
a
station gas transfer module 358 and an optional station gas storage tank 356.
The gas
transfer module includes valve(s) and control(s) for managing the gas flow and
may be
partially or wholly integrated with the connector. A station gas transfer line
360 fluidly
connects the gas transfer module and the optional gas storage tank. Gas flow
in this transfer
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line is bi-directional as shown by the flow arrows to 364 and from 362 the gas
transfer
module.
Still referring to Fig. 3, an optional station gas supply line 366 fluidly
connects the
station gas transfer module 358 and an optional auxiliary gas storage tank 372
that is
separate from the ROV 116. Gas flow in this supply line is toward the gas
transfer
module as shown by a flow arrow 370. A station discharge line 352 fluidly
connects the
station gas transfer module and the station connector 350. Gas flow in the
discharge line
is toward the station connector as shown by a flow arrow 368. Either one of or
both of
the optional gas storage tanks 356 (including transfer line 360) and 372
(including supply
line 366) are included in the workstation equipment comprising a part of the
gas refill
system 300.
In still another embodiment of Fig. 3, the workstation equipment comprising a
part of
the gas refill system 300 includes the optional auxiliary gas storage tank 372
and the station
gas supply line 366; it excludes the optional station gas storage tank 356 and
the station
gas transfer line 360. This embodiment provides a gas storage means that may
be either
integral or external to the workstation 120.
In another embodiment of Fig. 3, the workstation equipment comprising a part
of the
gas refill system 300 includes the optional station gas storage tank 356 and
the station gas
transfer line 360; it excludes the auxiliary station gas storage tank 372 and
the station gas
supply line 366. This embodiment provides a gas storage means that is integral
to the
workstation 120.
In yet another embodiment of Fig. 3, the workstation equipment comprising a
part of
the gas refill system 300 includes the optional auxiliary gas storage tank
372, the station gas
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supply line 366, the station gas storage tank 356, and the station gas
transfer line 360. This
embodiment provides a gas storage means that is integral to the workstation
120 and that
may also be external to the workstation.
Referring again to Fig. 3, workstation features adaptcd for manipulating
payloads 374 are shown. Workstation 120 may provide one or more storage racks
378 for
holding one or more payloads 374. The payload(s) 374 is fitted with a handle
376 suitable
for engagement with the ROV manipulator 320.
In Fig. 4, a ROV buoyancy management system 400 is shown. Selected
buoyancy management equipment is integrated onto the ROV 116. The buoyancy
management equipment functions include varying the liquid fraction in the
ballast tank
312. Minimum ROV buoyancy is achieved when the tank is full of liquid, the
liquid
fraction is 100%, and the gas fraction is 0%. Maximum ROV buoyancy is achieved
when the tank is full of gas, the liquid fraction is 0 %, and the gas fraction
is 100 %.
Still referring to Fig. 4, the ROV 116 has a ballast tank 312. In an
embodiment, the
ROV gas supply system 302 is fluidly connected to the ballast tank via a gas
recharge
line 314. The ballast tank is also in fluid communication with the ROV
underwater
environment 333 via a liquid transfer line 410. When gas is exchanged for
liquid in the
ballast tank (increases ROV buoyancy), a gas flow arrow 326 indicates gas flow
into the
ballast tank while a liquid flow arrow 406 indicates the corresponding liquid
flow from
the ballast tank. When liquid is exchanged for gas in the ballast tank
(decreases ROV
buoyancy), liquid flow arrow 404 indicates liquid flow into the tank while gas
flow arrow
328 indicates the corresponding gas flow from the tank. Ballast tank 312 may
include
pumps, valves, and controls that facilitate ballast exchange operations.
CA 02587538 2007-05-04
Fig. 5 is a view 500 of the ROV 116 adjacent to a payload transfer structure
502. In the present
example, the payload transfer structure is a portion of an underwater
structure and the payload is an
anode. The transfer structure provides one or more interfaces 506 for
receiving the payload. The
transfer structure also provides one or more optional connectors 504 for
mating with the ROV
connector(s) 318. Connector(s) 504 may exchange electrical signals, optical
signals, or mechanical
loads with ROV connector(s) 318. Those skilled in the art will recognize that
the features and
benefits of the present invention are adaptable to many underwater worksites
and to payloads
associated with those worksites.
Operation
Referring to Fig. 6, a typical underwater project 600 utilizing the present
invention is
outlined in tabular format. The project employs a ROV 116 to move multiple
payloads 374
between a first submerged location 136 and a second submerged location 134.
Project tasks include
ROV deployment 604, ROV work 606, and ROV recovery 608. These tasks comprise
operations
that are more fully described below.
Figs. 1 and 2 illustrate alternative operations for deploying the ROV. In Fig.
1, the first
deployment altemative 610 involves the use of a ROV cage 112. The ROV is
lowered in the cage
from the surface vessel 102 to a location 136 near the worksite 130 and is
then maneuvered from the
cage to the worksite. In Fig. 2, a second deployment alternative.612 does not
involve use of a ROV
cage. Here, the ROV is deployed directly from the surface vessel and is
maneuvered from the surface
126 to the worksite 130. Once the ROV deployment task is completed, the ROV
work task
may begin.
Fig. 7 illustrates sequential operations comprising an exemplary ROV work
task. From
the ROV work task start 701, the methodology progresses to the workstation
operation 614
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which includes payload transfer. A tiunsport operation 616 follows where the
payload is moved to a
second location 134. The methodology then progresses to a destination
operation 618
where the ROV off-loads the payload. A decision step 710 follows. If the ROV
work
task cycle is to be repeated, a first cycle alternative 712 is chosen; the
methodology then
proceeds to a return operation 620 where the ROV is maneuvered back to the
first payload
transfer location 136. If the ROV work task operation is to be discontinued, a
second cycle
altemative 714 is chosen and the methodology then proceeds to a ROV work task
end-
step 716. The end-step 716 may be followed by ROV recovery task 608. These
operations
comprise activities that are further described below.
Referring again to Fig. 6, the workstation operation 614 is further described
by
tabulated activities. In particular, the figure shows that the workstation
operation 614
comprises activities including payload transfer 626, gas transfer 628, and
buoyancy
adjustment 630. These activities are described below and illustrated in the
flowchart form of
Fig. 8.
Referring to Fig. 8, the payload transfer activity 626 is further illustrated
by
flowcharted steps. The methodology begins at step 801 and progresses to step
802
where the ROV is located adjacent to a payload transfer location 136, 134.
Step 804 follows
where during on-loading the ROV engages (disengages during off-loading) the
payload
handle(s) 376 with its manipulator 320 to form manipulator connection(s) 704.
The
methodology may also include the optional step 806 where the ROV connector(s)
318 is
engaged with the workstation connector(s) 350 to form a ROV/workstation
connection(s)
702. The manipulator connection provides a mechanical connection between the
ROV and
the payload for transporting the payload. The ROV/workstation connection(s)
may provide
any one or more of fluid, electrical, optical, and mechanical connections
between the ROV
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and the workstation. In an embodiment; mechanical connection(s) provided by
the
ROV/workstation connection(s) stabilize the ROV during payload transfer and or
periods of
non-neutral buoyancy. End-step 808 follows steps 804 and 806. While adjacent
to the
workstation, the ROV may receive buoyancy compensation gas from the
workstation as
described below.
Referring to Fig. 9, examplary gas transfer activity is further illustrated by
flowcharted steps. The methodology begins at step 901 and progresses to step
902
where the ROV is located adjacent to the payload transfer location 136. Step
904
follows where the ROV engages its connector(s) 318 with the workstation
connector(s) 350
forming a ROV/workstation connection(s) 702; at least one of the
ROV/workstation
connection(s) is a fluid connector. Decision step 908 follows to check for the
presence of a
ROV gas storage tank 304. If a tank is present the methodology proceeds along
flowchart
branch 910 to step 912 where gas is selectively transferred to one or both of
the ROV gas
storage tank and the ballast tank 312. If a ROV gas storage tank is not
present then the
methodology proceeds along flowchart branch 914 to step 916 where gas is
selectively
transferred to the ballast tank. Steps 912 and 916 lead to Step 917 where the
ROV/workstation connection(s) is disconnected. End-step 918 follows step 917.
Referring also to Fig. 3, the gas transfer activities of steps 912 and 916
require a
source of gas external to the ROV. External gas supplies include a gas storage
tank integral
to the workstation 356, a gas storage tank separate from the workstation 372,
or a
combination of both.
During the workstation operation 614, the payload transfer activity 626 is
typically associated with a buoyancy adjustment activity 630. Referring to
Fig. 10, the
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buoyancy adjustment activity is illustrated by flowcharted steps. The
methodology
begins at step 1001 and progresses to step 1002 where the ROV either on-loads
or offloads a
payload. A decision block 1004 follows. If the ROV tends to sink as a result
of the payload
transfer, then the methodology proceeds along flowchart branch 1006 to step
1008. In step
1008, gas is exchanged for liquid (deballasting) in the ballast tank to
restore neutral buoyancy
to the ROV. Conversely, if the ROV tends to rise after the payload transfer,
then the methodology
proceeds along flowchart branch 1010 to step 1012. In step 1012, liquid is
exchanged for gas
(ballasting) in the ballast tank to restore neutral buoyancy to the ROV. From
either step 1008 or
1012 the methodology progresses to step 1014 where it ends leaving the ROV in
a neutrally buoyant
state.
Referring also to Figs. 3 and 6, when the ROV 116 has an integral gas storage
tank 304, the
buoyancy adjustment activity 630 and the gas transfer activity 628 need not
occur simultaneously, for
that case the ROV may perfonn one or more gas recharge steps 1008 prior to
carrying out a gas
transfer. Conversely, if the ROV lacks an integral gas storage tank, the gas
recharge step 912
does require a simultaneous gas transfer to the ROV. When the workstation
operation 614 is
completed, the transport operation 616 follows.
Referring to Figs. 6 and 7, the transport operation 616 is further described
by tabulated
activities. In particular, the figure shows that the transport operation
comprises the ROV
moving with the payload 632 from a first location 136 to a second location
134. When the transport
operation is completed, the destination operation 618, which includes off-
loading, follows.
Referring to Figs. 6, 7, and 8, the destination operation 618 is further
described by tabulated
activities. In particular, the figures show that the destination operation
comprises activities
including the payload transfer activity 626 and the buoyancy adjustment
activity 630. The
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payload transfer activity, including on-loading and offloading, has been
described above in
connection with Fig. 8. The buoyancy adjustment activity has also been
described above in connection
with Fig. 9. When the destination operation is completed a continuation
decision 710 elects either a first
cycle altemative 712 or a second cycle alternative 714.Referring again to
Figs. 6 and 7, the
first cycle alternative 712 is followed by the return operation 620 that
continues the ROV
work task 606. In particular, the figures show that the return operation
comprises the ROV
move without the payload 638 from the second payload transfer location 134
back to the first
payload transfer location 718. When the return operation is completed the ROV
is ready to
begin another cycle of the work task.
Referring to Figs. 6 and 7, the second cycle alternative 714 reflects the
choice to
discontinue the current ROV work task 606. The ROV 116 may be recovered 608 at
this
time. Figs. 1 and 2 illustrate alternative operations for recovering the ROV
116 when the
second cycle alternative is chosen. In Fig. 1, a first recovery alternative
622 involves the use
of a ROV cage 112. The ROV is maneuvered from the second payload transfer
location 134
into the cage at location 136 near the worksite. The cage is then recovered to
the surface
vessel 102. In Fig. 2, a second recovery alternative 624 does not involve use
of a ROV
cage. Here, the ROV is recovered directly from the second payload transfer
location to
the surface vessel 102. Those skilled in the art will recognize that in lieu
of
immediate recovery, the ROV might be employed on other similar tasks prior to
being
recovered to the surface.
