Note: Descriptions are shown in the official language in which they were submitted.
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ACTIVE DEPLOYMENT SYSTEM AND METHOD
BACKGROUND OF THE INVENTION
Related Applications
This Application claims the benefit under Title 35 of
United States Code ~ 119(e) of U.S. Provisional Application
No. 60/194,822, filed April 5, 2000.
Technical Field
This invention relates to the field of
lifting/deployment devices capable of maintaining tension
on a cable or wire rope line. ° More specifically, the
invention relates to a system and method for maintaining
controlled lifting conditions as applied to a mass or load
handled from a moving lifting platform, such as an offshore
drilling rig:
History of the Related Art
Under moving sea conditions, a majority of floating
marine vessels, such as offshore drilling rigs, find
difficulty in deploying sub-sea machinery due to the
problems imposed by rig heave. The oscillations of the
vessel make it difficult to land heavy equipment using
ordinary deployment systems due to the movement imposed on
the deployment system by changing wave conditions. Thus,
under heaving vessel conditions, the equipment or machinery
being deployed may encounter a sudden drop to the ocean
floor on the downward heave cycle. Due to this rislc, most
deployment operations occur in calm seas, and are delayed
whenever wave conditions become unfavorable.
Some passive methods of deployment operate to
compensate for the motion of the vessel. However, such
passive compensators typically operate to absorb only a
small portion of the vessel heave distance. Even with the
ability to absorb vessel heave, however, it is still held
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to be good practice by those experienced in the art to wait
for favorable sea conditions (i.e., calm seas).
Therefore, what is needed is an active deployment
system for regulating a distance between deployed equipment
and a destination delivery point (e. g., the ocean floor)
wherein the equipment is suspended above the destination
point from a cable attached to a moving deployment
platform. Such a system and method would be even more
valuable if the length of the cable could be adjusted in
proportion to the movement direction and velocity.
SUMMARY OF THE INVENTION
The active deployment system and method of the
invention allow equipment deployment operations to proceed
during a wide variety of deployment platform movement. For
example, the active deployment system and method can be
used to assist in deployment of sensitive scientific
instruments to the ocean floor from a heaving vessel.
In essence, the active deployment system comprises a
passive cylinder system coupled with an active cylinder
system. The passive system, including a pre-charged
accumulator, provides load balance for the equipment or
other masslload being deployed. Pressure provided to the
blind end of the passive cylinder provides the majority of
support for the load, as commonly occurs in conventional
pneumatic cylinder arrangements. In the static condition,
the passive cylinder is balanced at mid-stroke with the
load hanging from a cable. A sheave arrangement, placed at
opposite ends of the passive cylinder-active cylinder
combination, is used to introduce a cable travel multiplier
for cylinder displacement. Thus, when the platform moves,
for example, a distance of 16 meters with respect,to the
delivery point, the cylinder combination needs only to move
a distance of 4 meters to compensate so as to allow the
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load to remain stationary in space (with respect to the
delivery point).
The active cylinder is controlled hydraulically to add
or remove loading to/from the passive cylinder system in
order to provide a compensating balance. Accelerometers
measure platform movement (velocity and direction), sending
a signal to a hydraulic power unit which provides hydraulic
flow and pressure to extend or retract the system in phase
with (and opposite in direction to) the platform movement.
Thus, the active deployment system includes a cable
tensioner supporting 'a cable in reeved engagement with a
cable suspending the load, and in mechanical engagement
with a supporting platform. The cable tensioner includes a
rod assembly adapted t~ apply tension force to the cable
under heaving conditions. The deployment system also
includes an accelerometer (to measure platform heave
movement) and a stroke sensor for measuring movement of the
rod assembly within the Cable tensioner. The accelerometer
provides a heave movement signal, and the stroke sensor
provides a feedback signal in response to movement by the
rod assembly. Finally, the deployment system includes a
processing means in electrical communication with the cable
tensioner, accelerometer, and stroke sensor which monitors
the heave movement and feedback signals, and regulates the
distance between the load and the destination point by
adjusting the tension force applied to the cable.
The cable tensioner typically comprises a passive
cylinder system and an active cylinder system in
combination, The passive system is pre-charged to support
the load, while the active system moves in phase with the
support platform heave. The rod assembly within the cable
tensioner is moved in proportion to the heave velocity and
direction.
The method for regulating the distance between the
mass and the destination point makes use of the active
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deployment system. Essentially, the method comprises the
steps of measuring a heave movement (velocity and
direction), and adjusting the tension force applied to the
cable upon determining that the heave velocity exceeds a
preselected critical velocity. Typically, the tension
force is adjusted by adjusting the rate of at least one
fluid flow within the cable tensioner, such as the
hydraulic fluid flow applied to the active cylinder system.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the structure and
operation of the present invention may be had by reference
to the following detailed description when taken in
conjunction with the accompanying drawings, wherein:
FIGURES 1A and 1B illustrate perspective views of two
different embodiments of the system of the present
invention;
FIGURE 2 is an operational flow chart for the system
of the present invention;
FIGURE 3 is a logical block diagram of the system of
the present invention;
FIGURES 4A and 4B illustrate perspective, and side,
cut-away views of the deployment system cylinder of the
present invention, respectively;
FIGURES 4C and 4D illustrate section views of the
Deployment System Cylinder of the present invention; and
FTGURE 5 is a flow chart illustrating the method of
the present invention.
DETAILED DESCRIPTION
Referring now to Figures 1A and 1B, two different
embodiments of the system 10, 10' of the present invention
can be seen. Numeric designators are used to refer to
specific elements of the structure. Those corresponding
designators including a prime (') symbol refer to elements
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having a similar or identical structure and function to
those elements having numeric designators without the prime
symbol.
Figure 1A illustrates the active deployment system 10
5 of the present invention as it might be mounted to, or
mechanically 'engaged with, the rig structure 80, of an
offshore drilling rig. Of course, the rig structure 80 may
also be described as a "heaving vessel", a "platform", or
any other base, framework, or structure to which the system
10 may be fixedly attached. It is assumed throughout this
description that the rig structure 80 moves in accord with
the wave motion of the sea (or a corresponding medium
supporting/surrounding the structure 80).
The active deployment system 10 operates to regulate
the distance LD between the mass or load 60 and a
destination point 195 toward which the load 60 is being
deployed. Typically, the load 60 is suspended above the
destination point 195 from the distal end 37 of a cable 30.
The proximal end 33 of the cable 30 is in reeved engagement
with the cable tensioner or Deployment System Cylinder
(DSC) 20, using the rod end sheaves 120 and the blind end
sheaves 130.
The system 10 comprises the Deployment System Cylinder
(DSC) or cable tensioner 20, an accelerometer 32, a stroke
sensor 40, and a processing means 150, such as a central
control unit 150. The accelerometer 32.may be embodied as
a single, multi-axis accelerometer, which gives directional
velocity signal information along the traditional X, Y, and
Z axes (commonly known as a "six" axis accelerometer to
those skilled in the art, for +X, -X, +Y, -Y, +Z, and -Z
acceleration axes). The accelerometer 32 may also be made
up of several unitary accelerometers which provide velocity
and/or directional heave movement information for the
structure 80. The stroke sensor 40 is typically embodied
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in the form of a potentiometer, but may also be embodied in
an encoder (linear or rotary), or other position
measurement devices well known to those skilled in the art.
The cable tensioner or DSC 20 is in reeved engagement with
the proximal end 33 of the cable 30, and in mechanical
engagement, typically fixedly attached, to the heaving
vessel structure 80, or other structure, as described
above.
The accelerometer 32 provides a vessel heave movemebt
signal in proportion to the velocity and direction (heave
velocity and heave direction) of the heaving vessel
structure 80. The stroke sensor 40 is used for measuring
movement of a rod assembly within the DSC 20, providing a
feedback motion signal in proportion to the piston rod
movement distance within the rod assembly. As can be seen
in Figure 1B, the Heave Velocity (HV) and Heave Direction
(HD) are a measure of the heave movement by the base 160 of
. the system 10' illustrated in Figure 1B as it moves in
response to wave motion, for. example, as the wave surface
190 moves from a first location L1 to a second location Z2.
The processing means, such as the central control unit 150,
is in electrical communication with the cable tensioner 20
(using the Hydraulic Power Unit (HPU) 50 and the stroke
sensor 40). The processing means 150 monitors the heave
movement signal produced by the accelerometer and the
feedback signal provided by the stroke sensor, and
regulates the LD by adjusting the tension force F applied
to the cable 30.
As can be seen in Figure 1A, the HPU 50 may also
include a HPU control unit 110, for manual control or fine
adjustment of the position of the active cylinder assembly
400. There is also a high pressure air supply 100 which is
regulated by an air control 90 by way of an air line 180,
as is well known to those skilled in the art. The air
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control 90 can be used to pressurize the passive cylinder
system to balance the load 60.
The system 10, 10' operates by making use of a passive
cylinder system 410, including a pre-charged accumulator,
to provide balance for the load 60 to be deployed to the
destination point 195. The pressure within the accumulator
of the passive cylinder system 410 provides force on the
blind end of the cylinder 410 to support the load 60, as
occurs in conventional pneumatic cylinder arrangements.
The accumulator of the passive cylinder system 410 is sized
to manipulate the mechanical stiffness of the system 10,
10'. In the static condition, the cylinder 410 is balanced
at mid-stroke with the load 60 hanging from the cable 30
30', which is in turn reeved one or more times around a
sheave arrangement placed on the opposite ends of the
deployment cylinders, viz, the rod end sheaves 120, 120'
and the blind end sheaves 130, 130'. Typically, the
reeving arrangement provides about a 4:1 cable line travel
ratio with regard to the DSC 20 displacement. Thus, as the
load 60 moves 4 meters, the DSC 20 moves only one meter.
However, if the passive cylinder system 410 were used
alone, acceleration of the load 60 caused by vessel heave
would create an unbalanced system. Thus, a single passive
cylinder system 410 would be ineffective to provide
fully-compensated load 60 movement, as the pressure within
the passive cylinder system 410 varies with the stroke of
the cylinder, which in turn provides a varying reaction
force to the tension on the cable 30 produced by the load
60:
The system 10, 10' of the invention solves the problem
of varying pressure within the passive cylinder system 410
by incorporating an active system or active cylinder
assembly 400, which is controlled hydraulically (using the
HPU 50, 50' and central control unit 150, 150'), to add or
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remove tension on the cable 30 in order to provide a
fully-compensating balance for vessel heave and the weight
of the load 60.
Accelerometers, such as the accelerometer 32, measure
the HV and HD, providing signals proportional thereto, and
send signals to the HPU 50 (and/or central control unit
150), which controls the hydraulic pump and motor system
supplying hydraulic fluid through the hydraulic lines 170
to the active cylinder assembly 400. The stroke sensor 40
provides the feedback motion signal to the HPU 50 (and/or
central control unit 150) in a similar fashion, so that the
actual motion of the cylinder rod can be monitored as it
responds to the hydraulic pump and load 60. Thus, the
active system 400 is provided with hydraulic flow and
pressure to extend or retract the piston rod within the
cylinder 410 as necessary to maintain deployment of the
load 60 in phase with the heaving vessel or structure 80,
as the winch 140 lowers or raises the load 60 by means of
the rod end sheaves 120, blind end sheaves 130, and turn
down sheaves 70.
Thus, the Active Deployment System (ADS) 10, 10~
maintains nearly constant motion control of the load or
mass 60 by controlling the length of cable 30 attached to
the load 60 relative to the floating vessel or structure
80, 160. Attaching the cable at the distal end 37 to the
load 60, and reeving the proximal end 33 several times
around the rod and blind end sheaves 120, 130, affording a
travel distance advantage to the DSC 20, allows the ADS 10
to act as a powerful hydro-pneumatic spring. The cable 30
length, measured from the winch 140 to the load 60, is
controlled in direct response to the heaving motion of the
vessel or structure 80, 160. High pressure air is utilized
on the blind end of the passive cylinder system 410 as a
load-supporting medium. No adjustment of the air volume on
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the blind end side of the cylinder 410 is necessary, once
the weight of the load 60 has been established and
balanced.
A high-pressure air supply 100 is connected to the
cylinder 410 to meet the required load, and hydraulic
fluid, carried in the lines 170, is used in the active
cylinder assembly 400 to provide constant motion control,
as well as speed control of the rod in the event of cable
30 breakage. To understand the operation of the speed
l0 control in the active cylinder assembly 400, please refer
to co-pending patent application No. 09/733,227, herein
incorporated by reference in its entirety. This specially
designed and calibrated speed limiting system is provided
as an adjunct to the active cylinder assembly 400 to limit
the terminal velocity of the active cylinder piston (see
piston 420 in Figure 4B).
The ADS 10 is therefore an active device. Signals
from the stroke sensor 40, which allow the processing means
or central control unit 150 to monitor the motion of the
piston 420, and signals from the accelerometer 32, which
allows the central control unit to monitor the reaction of
the structure 80, 160 in response to movement of the wave
surface 190, are processed and sent to the HPU 50 to match
the movement of the vessel or structure 80,, 160, keeping
the load 60 stationary. As the vessel or structure 80, 160
heaves upward or downward on the wave surface 190, the HPU
50, in electronic communication with the central control
unit 150, is signaled to stroke the piston 420 in or out of
the active cylinder assembly 400 to maintain the desired
location of the load 60. Typically, the piston 420 is
commanded to move with the same velocity as the heave
velocity HV of the structure 80, 160, and in a direction
which opposes the heave direction HD (as measured by the
accelerometer 32).
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Turning now to Figure 2, an abbreviated operational
diagram for the system 10 can be seen. After taking the
system out of a standby status in step 700, the system 10
heave sensing and compensation systems are disabled, and
5 the winch cable is prepared for stroking the passive
cylinder 410 to its mid-point. The cylinder 410 is charged
in step 720, and the load 60 is deployed through the splash
zone into the water in step 730. At this point, an auto-
centering program is activated, and the active cylinder 400
10 is centered using the hydraulic pump in steps 740 and 750,
and the system 10 is placed into the active compensation
mode in step 760.
The load 60 is deployed while the active mode is
maintained in step 770. If it is determined that the
structure 80 will remain attached to the wellhead in step
780, and that the operator desires merely to deploy the
load 60 using tension in the cable 30 in step 790, then
active_ compensation, along with auto-centering, and heave
sensing, will. be turned off in step 800. If the operator
decides to deploy the load 60 using the active system in
step 790, then the winch cable 30 is slackened in step 810
and the active compensation mode remains enabled. The load
60 is then retrieved in steps 820 and 830 until it is
safely clear of the wellhead. The active cylinder 400 is
then taken out of the active compensation mode in step 840.
If the operator determines that the structure 80 will
not remain attached to the wellhead in step 780, then the
active compensation mode remains engaged and the structure
80 .is removed from the wellhead. After the removal
operation is completed, the active compensation mode is
terminated in step 840.
Turning now to Figure 3, a logic block diagram of
the system 10 of the present invention may be seen. Each
of the components of the diagram may be embodied as
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electronic and/or mechanical hardware, software, or a
combination of these. Thus, for example, the first adder
module 200 may be an integrated circuit, a software program
module, a series of operational amplifiers, or even a
mechanical assembly capable of summing various mechanical
inputs, as are well known to those skilled in the art of
control system operation. Any of the other modules 210-370
may likewise exist in the form of the various components.
There are two sets of signal/feedback information
which are used to drive the operation of the system 10.
The first is provided by the stroke sensor 40, appearing in
Figure 3 as the cylinder position feedback module 240 for
the active cylinder assembly 400. The second is the heave
movement detection system, which measures the heave
velocity HV and direction HD, represented by the wave
accelerometer module 320. The position of the piston 420 is
determined several times a second, and from this, the
distance, velocity and acceleration of the piston 420 can
be determined.
The heave direction 355 may be fed forward from the
accelerometer 320, The adder 200, in turn, feeds into the
second adder module 210 and creates a hydraulic pump demand
at the adder 210 output, which feeds into the pump motor
220. The pump demand, in turn, causes the swash valve 230
to open, causing the DFC 20 to move. The position
information derived from the movement of the DSC 20, as
tracked by the cylinder position module 240, is fed back,
after combination with a position offset signal 280 if
desired, (such as may be derived from movement by the
passive cylinder 410 due to changes in the load cable 30
weight, for example) using the third adder module 260 as a
DSC position feedback signal 270. The signal 270 is then
multiplied by several feedback signals 290, 300, and 310
for input into the first adder module 200.
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While the wave accelerometer module 320, combined with
the stroke sensor module 240, provides piston position,
vertical velocity, and vertical acceleration information,
the heave velocity feedforward multiplier module 340, and
the heave acceleration feedforward multiplier module 330
are used to tailor the system 10 response with regard to
the velocity, and acceleration signals which are fed
forward into the first adder module 200.
The KVFF 340 is a velocity feedforward multiplier.
The heave velocity information from the heave movement
accelerometer input is multiplied by the KVFF factor to
allow the DSC 20 to change position faster when ocean waves
are in a dynamic state. Thus, the mechanical portions of
the system 10 can react faster as the velocity of the wave
(and therfore, the HV) changes. As the wave activity
subsides, the KVFF plays an increasingly small part in
determining the final DFC 20 position. The units for KVFF
are in terms of second/meter or second/feet.
The KAFF is an acceleration feedforward multiplier.
The heave velocity information is differentiated with
respect to time to give the rate of change of heave
velocity. The acceleration so derived is multiplied by the
KAFF to allow the DSC to react quickly, as soon as heave
movement due to wave motion is detected. It is the KAFF
signal which enables the system 10 to more immediately
respond to the onset of wave activity. The larger the
value of the KAFF, the faster the system 10 reacts.
However, if the KAFF value is too large, then the DSC 20
will react to mere wave noise, rather than actual waves.
The units of operation for the KAFF are second
squared/meter or second squared/feet.
The feedback signals KPFB (position feedback), KVFB
(velocity feedback), and KAFB (acceleration feedback) are
analogs of the feedforward signals. Thus, the DSC position
feedback multiplier module 290, the DSC velocity feedback
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multiplier module 300, and the DSC acceleration feedback
multiplier module 310 act in opposition to the feedforward
signals to stabilize the system 10. Typically, the
absolute values of the KPFB, KVFB, and KAFB multipliers
cannot be determined until the actual system is tested, or
at least simulated, to derive reaction time information for
the mechanical components activated by operation of the
swash valve 230. The swash valve position feedback module
250 provides a simple feedback loop for the swash valve
controls to ensure that the swash valve 230 position
approximately matches the demand. For example, when the
demand is 500, then the swash valve should be open at
approximately 500 of its fully open value. The KPS value
is positional feedback only for the swash valve 230. The
larger the value of KPS, the longer the swash value 230
will take to reach its final position; however, larger
values also contribute to stability. The value of KPS is
dimensionless.
The bias module 370 constitutes a low-priority input
to the adder module 200, and serves to maintain the
position of the load when minimal or no vessel heave is
present. However, when the heave velocity exceeds a
predetermined critical heave velocity, the bias signal
provided by the module 370 is overriden, and the heave
compensation system (actively compensated system) takes
over.
Figures 4A and 4B illustrate perspective and side,
cut-away views of the DSC 20 of the system 10,
respectively. As mentioned above, the DSC 20 includes an
active cylinder assembly 400 and a passive cylinder system
410. The piston 420 which moves in and out of the active
cylinder assembly 400 is fixedly attached to the rod end
sheave support 380, which in turn houses the rod end
sheaves 120. As can be most easily seen in Figure 4A, the
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stroke sensor 40 is typically mounted to the side of the
active cylinder assembly 400, and a stroke wire 390,
typically rotatably attached to the stroke sensor 40 and
fixedly attached to the rod and sheave support 380, is used
to derive the position of the piston 420.
As can be more clearly seen in Figure 4B, the passive
cylinder system 410 comprises an annular tube or passive
cylinder accumulator 440 used as an air bank to hold a
volume of air at the pressure necessary to support the load
60 applied to the passive cylinder system 410, which
includes the weight of the piston 420, the load 60, and the
cable 30. The accumulator 440 houses the inner barrel 460
of the passive cylinder system 410, which in turn is ported
to allow air or other fluid media to move from the
accumulator 440 to support the passive cylinder rod 450.
Thus, the annulus 490 (accumulator) can be pre-charged to
the desired pressure for suspending the load 60 such that
the cylinder rod 450 is moved to mid-stroke within the body
of the passive cylinder system 410. The annulus 510 serves
to retain air as pressure builds against the piston 450.
Directly attached to, and in line with, the passive
cylinder system 410 is the active cylinder assembly 400.
The passive cylinder rod 450 extends up through the passive
cylinder inner barrel 460 into the active cylinder assembly
400, where it is threaded into, or otherwise attached, to
the bottom of the active cylinder piston 420. Thus, the
passive cylinder rod 450 and the active cylinder piston 420
form an integral unit, called the rod assembly 540. It is
the position of the rod assembly 540 which is monitored by
'30 the stroke sensor 40 and passed on to the central control
unit 150 to determine the position, distance traveled,
velocity, and acceleration of the DSC 20. To better
understand the operation of the DSC 10 as it operates in an
uncompensated environment, reference may be made to U.S.
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Patent No. 4,638,978, incorporated by reference herein in
its entirety.
When hydraulic fluid is pumped into the active
cylinder housing 430, via hydraulic ports 550 (and
5 hydraulic lines 170), hydraulic force is applied to the
active cylinder piston 420, and thereby, the rod assembly
540, as hydraulic pressure builds within the annulus 520.
The hydraulic force is controlled using the HPU 50, which
monitors the displacement of the DSC 20 (directly, or
l0 indirectly, via the contol unit 150) relative to rig heave
movement HV, HD. The HPU 50, as commanded by the central
control unit 150, provides flow and pressure using
hydraulic fluid, or other fluid media, which may even be
air or gas, to the active cylinder piston 420, pushing up
15 and down on the piston 420 as necessary.
Thus, in operation, as can be clearly seen from
Figures 1A, 1B, and 4A, after the cable 30 is fed off of
the winch 140, it is reeved from the winch 140 through one
of the outer sheaves on the blind end sheaves 130 (possibly
after passing over an alignment sheave), and then up over
one of the rod end sheaves 120 at the top of the active
cylinder assembly 400. The cable 30 is then reeved back
down through a second sheave in the housing for the blind
end sheaves 130, and up again to one of the remaining rod
end sheaves 120. The cable 30 is then reeved through the
last (empty) sheave of the blind end sheaves 130 (assuming
.that there are three blind end sheaves 130 and two rod end
sheaves 120) to one or more turndown sheaves 70 placed
apart from the system 10 on the rig 80 or other structure
80. The load 60 is then supported on the distal end 37,of
the cable 30, and pressure is applied to the accumulator
440 using the air input port 470 until the rod assembly 540
is stroked at mid-position. The stroke sensor 40,
typically mounted at the top portion o,f the active cylinder
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assembly 400, provides position change information for the
rod assembly 540 (i.e., the distance MD shown in Figure
1B). The accel-erometer 32 provides heave movement
information for the structure 80, such as the heave
velocity HV and heave direction HD. These signals from the
accelerometer 32 and the stroke sensor 40 are provided to
the HPU 50 (typically, via the central control unit 150).
From the rod positional information MD, the HV and HD,
velocity and acceleration can be derived for the movement
of the rod assembly 540 and acceleration can be derived for
the heaving movement of the rig structure or base 80, 160.
The central control unit 150, including the control logic
of the system 10 shown in Figure 3, actuates the HPU 50 to
provide the correct flow and pressure of hydraulic fluid
through the hydraulic lines 170 to the hydraulic ports 550
of the active cylinder assembly 400.
Thus, the active deployment system 10 for regulating a
distance LD between a mass/load 60 and a destination point
195, when the mass 60 is suspended above the destination
point 195 from the distal end 37 of the cable 30 attached
to a heaving vessel (or other moving platform structure) 80
at the proximal end 33 of the cable 30 includes a cable
tensioner 20, an accelerometer 32, a stroke sensor 40, and
a processing means in electrical communication with the
cable tensioner 20, the accelerometer 32, and the stroke
sensor 40. The accelerometer 32 and the stroke sensor 40
provide the two signals necessary for system 10 operation:
the position (or movement distance) of the rod assembly MD,
and the heave movement signal, including HV and HD. The
cable tensioner 20, in reeved engagement with the proximal
end 33 of the cable 30, and mechanically engaged to the
heaving vessel (or other support structure) 80, includes a
rod assembly 540 adapted to apply a tension force F to the
cable 30 under heaving conditions. The processing means
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150, such as the central control unit 150 (which may be a
computer or other data processing means well known to those
skilled in the art) is adapted to monitor the MD, HD, and
HV signals, and to regulate the distance LD by adjusting
the tension force F applied to the cable 30.
The cable tensioner or DSC 20 includes a passive
cylinder system 410 including a passive cylinder rod 450
and an active cylinder assembly 400 housing the rod
assembly 540. The rod assembly 540 is formed by fixedly
attaching an active cylinder piston 420 to the passive
cylinder rod 450.
The passive cylinder system 410 is pre-charged with
air (or other fluid media) to a mid-stroke position of the
passive cylinder rod 450 so as to suspend the mass 60 at
the distance LD, and to balance the weight of the mass 60.
The tension force F applied to the cable 30 is adjusted by
moving the rod assembly 540 in proportion to the heave
velocity (HV), and in the opposite direction to the heave
direction (HD). The tension force F on the cable 30 may
also be adjusted by moving the rod assembly 540 in opposite
proportion to the heave velocity and heave acceleration, as
measured by the wave accelerometer module 320.
Figure 5 illustrates the method of the invention. As
mentioned previously, the passive cylinder system 410 is
pre-charged with air (or other fluid media, such as oil) so
that the rod assembly 540 is at a mid-stroke position in
step 600. After this operation, active management of the
load 60 position with respect to the destination/delivery
point 195 begins.
The method for regulating the distance LD between the
mass 60 and the destination point 195, wherein the mass 60
is suspended by the destination point 195 from the distal
end 37 of the cable 30 in reeved engagement with the cable
tensioner 20 includes the steps of: measuring the heave
CA 02403015 2002-09-17
WO 01/77000 PCT/USO1/10701
18
movement (HV and HD) experienced by the heaving vessel (or
other moving platform structure) 80, 160, along with the
rod assembly 540 movement distance MD, and adjusting the
tension force F applied to the cable 30 upon determining
that the heave velocity HV exceeds a preselected critical
velocity. Thus, the heave velocity/direction and rod
assembly movement distance are measured in step 610, the
heave velocity is compared with a predetermined critical
velocity in step 620. Then, if the critical velocity has
been exceeded, the tension force F on the cable 30 is
adjusted in step 630. The tension force F can be adjusted
by changing or adjusting the rate of at least one fluid
flow within the tensioner 20, such as the flow of hydraulic
fluid into and out of the hydraulic ports 550. This occurs
in step 640, the rod assembly can then be moved in opposite
proportion to heave velocity, and/or acceleration, in step
650.
Although the invention has been described with
reference to specific embodiments, this description is
meant to be construed in a limited sense. The various
modifications ~of the disclosed embodiments, as well as
alternative embodiments of the invention, will become
apparent to those persons skilled in the art upon reference
to the description of the invention. It is, therefore,
contemplated that the appended claims will cover such
modifications that fall within the scope of the invention,
or their equivalents.
