Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR DEPLOYING SEAFLOOR
EQUIPMENT
1. Field of the Invention
The present invention relates generally to deploying seafloor equipment, for
example seismic recorders for use in marine seismic surveying. While the
invention
will be described hereinafter with reference to this application, it will be
appreciated
that the invention is not limited to this particular field of use.
2. Description of the Prior Art
l0
Any discussion of the prior art throughout the specification should in no
way be considered as an admission that such prior art is widely known or forms
part
of common general knowledge in the field.
For various applications it may be necessary to deploy equipment to the
seafloor. For example, seafloor recorders are often used for earthquake
monitoring or
marine seismic operations. These devices are typically referred to as "Ocean
Bottom
Seismometers" and various descriptions can be found in U.S. Pat. No. 4,692,906
to
Neeley (1987), U.S. Pat. No. 5,189,642 to Donoho et al. (1993) and U.S. Pat.
No.
5,253,223 to Svenning et al. (1993). Seafloor recorders typically consist of a
pressure
resistant waterproof container housing: a clock, digital data recording
electronics, a
battery, three geophones to sense the seafloor movement in all directions and
a
hydrophone to sense acoustic pressure. They can also be equipped with other
means
such as a chassis for coupling to the ground, a recovery module usually based
on a
weight release mechanism to ascend back to the surface, and secondary sensors
such
as a magnetic heading sensor, a tilt sensor and depth sensor.
Various methods of deploying seafloor recorders have been proposed for
applications such as oil exploration geophysics which require very high
quality
3o geologic images to be obtained from seismic signals acquired at the seabed.
Nevertheless, the imaging requires a reasonable control of the positioning of
the
sensor during deployment, which is a significant issue in deep water, or in
presence of
strong currents. For instance, U.S. Pat. No. 5,253,223 to Svenning et al.
(1993)
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disclosed a submarine vessel to deploy the recorders. U.S. Pat. No. 6,244,375
B1 to
Norns et al. (2001) disclosed a method using recorders travelling autonomously
along
predefined paths, such as tubing, laid at the ocean bottom. Those methods
require a
very significant and expensive infrastructure to be put in place.
U.S. Pat. No. 6,024,344 to Buckley et al. (2000) disclosed a method for
recording seismic data in deep water where a plurality of seismic data
recorders are
attached to a wire stored on a seismic vessel. A free end of the wire is
deployed into
the water, and the recorders are attached at selected positions along the
wire. The
l0 wire and recorders are lowered into the water as the vessel moves to
control the
recorder deployment. The wire controls recorder location and establishes the
recorder
spacing interval. One significant drawback of this method is that for
effective
deployment in presence of currents, the density and mass per unit length of
the cable
and recorders has to be high compared to hydrodynamic drag, which results in a
very
significant overall weight to be carned by the vessel.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome or ameliorate at least
one
of the disadvantages of the prior art, or to provide a useful alternative.
According to a first aspect of the present invention there is provided a
method
for deploying equipment modules to a seafloor of a body of water, said method
including the steps of:
z5 deploying conveying means having a free end reaching, or proximate to, the
seafloor;
dragging said conveying means through said water;
slidably attaching one or more of said equipment modules to said conveying
means;
releasing said equipment modules such that said equipment modules slide
along said conveying means to the seafloor, whereby said equipment modules
engage
said seafloor so as to secure the equipment modules at a fixed position.
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According to a second aspect of the present invention there is provided
a method for deploying and retrieving seafloor equipment including the steps
of
providing a conveying means with a fixed end and a free end;
releasing said conveying means into a body of water from a vessel until said
free end reaches, or is proximate to, a seafloor of said body of water;
dragging said conveying means behind said vessel at a controllable speed;
slidably attaching said equipment including a recovery module and stopping
means to said conveying means, wherein said equipment, said recovery module
and
said stopping means are secured one to another by a connector;
sliding said equipment to the free end of the conveying means, said equipment
being fixed in position on the seafloor by said stopping means once said
stopping
means reaches the seafloor;
activating said recovery module so as to allow said equipment to ascend from
the seafloor to a surface of the water; and
retrieving said equipment from the surface of the water.
Preferably the conveying means is in the form of a cable.
According to another aspect of the present invention there is provided a
2o method for deploying equipment modules to a seafloor of a body of water,
said
method including the steps of:
deploying conveying means having a free end reaching, or proximate to, the
seafloor, said conveying means further having an equipment module release
mechanism disposed at, or adjacent to, said free end;
dragging said conveying means through said water;
slidably attaching an equipment module to said conveying means;
allowing said equipment module to slide along said conveying means to the
equipment module release mechanism;
activating said equipment module release mechanism so as to selectively
3o release said equipment module when said equipment module is at, or close
to, a
predefined seafloor deployment position; and
allowing said equipment module to engage with said seafloor so as to secure
the equipment module at a fixed position.
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BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention shall now be described, by way of
example only, with reference to the accompanying drawings, in which:
FIGS. 1 a to 1 c depict the process flow of one embodiment according to the
present invention in general overview;
FIGS. 2a to 2c depict the process flow of a second embodiment according to
to the present invention in general overview; and
FIGS. 3a to 3c depict the process flow of another embodiment according to the
present invention in general overview.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Some sample embodiments of the present invention will now be described in
greater detail. Nevertheless, it should be recognized that the present
invention can be
practiced in a wide range of other embodiments besides those explicitly
described, and
the scope of the present invention is expressly not limited except as
specified in the
accompanying claims.
Moreover, while the present invention is illustrated by a number of preferred
embodiments directed to ocean bottom systems, it is not intended that these
illustrations be a limitation on the scope or applicability of the present
invention.
Apart from ocean bottom systems, the present invention is also applicable to
other
applications, such as shallow water operations, for example. Further, various
parts of
the present invention have not been drawn to scale. Certain dimensions have
been
exaggerated in relation to other dimensions in order to provide a clearer
illustration
3o and understanding of the present invention.
The preferred embodiment of the present invention provides a method for
deploying and retrieving equipment such as seismic data recorders from a
surface
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vessel. In some embodiments the equipment takes the form of equipment modules
which may include any one or more of the following: seismic sensors and
recorders,
auxiliary sensors such as heading sensors, positioning sensors, acoustic
transponders,
and/or other like equipment. Referring initially to FIGS. 1 a - 1 c,
illustrated is a
process flow of a preferred embodiment according to the present invention in
general
overview. These drawings merely show several key steps in sequential
processes.
Starting from FIG. 1 a, the present embodiment includes the steps of, firstly,
providing conveying means such as a cable 100 which is typically very long,
for
1o example up to a few kilometres. The cable 100 is preferably metallic to
combine
strength and high density and can feature an outer coating to facilitate
sliding.
Secondly, the cable 100 is released into the water 110 from the seismic vessel
120.
Thirdly, the cable 100 is dragged behind the seismic vessel 120 under a
controllable speed such that a free end 101 of the cable 100 reaches, or is
proximate to
the seafloor 130. As used in this document, the term "seafloor" refers to the
bottom
130 of any body of water 110.
Fourthly, a plurality of equipment modules 140, 141 and 142 are slidably
attached to the cable 100, for example by using clips 143 which preferably
include a
snap-link. The equipment modules may include any one or more of:
one or more seismic data recording units 140, each having sensors, clocks and
associated electronics;
a recovery module 141, for example including buoyancy means; and/or
stopping means 142, such as an anchor, used to maintain the equipment
modules at fixed positions once they reach the seafloor 130.
3o Any two or more of the above mentioned equipment modules 140, 141 and
142 may be bound together by a connector 144 which provides a mechanical link
144,
such as high tensile strength fibre, eg Kevlar or Vectran. However, before
attaching
the recording units 140 to the cable 100, the recorders should be initialized
and the
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clocks synchronized. The purpose of the mechanical link 144 relates solely to
recovery of the modules 140, 141 and 142. During decent and whilst on the
seafloor,
the mechanical link 144 is slack. This helps to avoid vibrational coupling
between
adjacent recorders which could result in false readings.
Fifth, the equipment modules are deployed by allowing the clips 143 to slide
along the cable 100, thereby dropping from the vessel 120 down to the seafloor
130.
Once released, the equipment modules 140, 141 and 142 are forced downwardly by
the combined action of the profile of the cable 100 and the hydrodynamic drag
on the
to equipment modules caused by the dragging of the cable 100 through the
water. Once
the equipment modules 140, 141 and 142 reach the seafloor 130, the measurement
and
recording of seismic data may commence.
The fixed position 150 of the equipment modules 140, 141 and 142 on the
seafloor 130, as shown in FIG. lc, is dependent upon a number of factors such
as:
the position of the vessel 120 at the time of release of the equipment modules
140, 141 and 142;
the speed at which the equipment modules 140, 141 and 142 descend along the
cable 100 (which, in turn, is dependent upon the speed at which the cable 100
is
2o dragged through the water behind the vessel 120, the density of the
equipment
modules 140, 141 and 142, the profile of the cable 100, any currents that may
exist at
various depths within the body of water 120, etc);
any friction that may exist between the cable 100 and the clip 143; and
the depth of the water 110.
Advantageously the preferred embodiment of the present invention makes use
of known methods for controlling cable deployment to the bottom of the sea,
for
example methods used in the art of laying intercontinental communications
cables.
Such known methods include the use o~
3o Global Positioning Systems (GPS) to determine the position the vessel 120;
Acoustic Doppler Current Profilers (ADCP) to obtain a map of the variation of
current direction and amplitude in the water column under the vessel 120;
and/or
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Ultra-short acoustic base to obtain the position of a transponder relative to
the
ship 130 to within an accuracy of between 1.5% to 0.5% of the water depth.
When the bathymetry (ie geometry of the sea bottom 130) is known, the
profile of the cable 100 can be calculated from a knowledge of hydrodynamic
coefficients of the cable 100 and its mechanical properties combined with data
from
the GPS and ADCP. Acoustic transponders can also be used to check or refine
calculations. This equipment is advantageously employed in conjunction with
software programs which give navigational advice to the ship 120 in order to
optimize
1o the control of the deployment.
Further technical information associated with control of cable deployment is
available from the following Internet site:
www.makai.com.
Hence accurate positioning of the equipment modules 140, 141 and 142 on the
seafloor 110 requires monitoring of the above factors and precise control of
the timing
of the release of each equipment module. Preferably the release of equipment
modules 140, 141 and 142 from the vessel 120 is controlled by a mechanical
latch
2o system. This advantageously allows for accurate control of the moment at
which each
module is released.
Preferably the variables which impact upon the ultimate positioning of the
equipment 140, 141 and 142 upon the seafloor 130 are controlled sufficiently
for
placement of the equipment 140, 141 and 142 in a position 150 on the seafloor
110 to
within an accuracy of approximately 1.5% of the depth of the water. Some
embodiments of the present invention can provide positioning to within an
accuracy of
approximately 0.5% of the depth of the water. Such accuracy compares
favourably to
the majority of the prior art methods for deploying equipment to a seafloor
3o environment at depths of thousands of meters.
The separation distance between the fixed positions 150 of two adjacent
groups of equipment modules 140, 141 and 142 on the seafloor 130 is also
dependent
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upon the above mentioned factors. Hence the separation between adjacent
equipment
modules 141 can also be controlled by precise timing of their release from the
vessel
130, in conjunction with monitoring of the other relevant factors.
The final step in the first preferred method is ascent of the equipment
modules
140, 141 and 142 and their retrieval from the surface of the water 110. Ascent
of a
given equipment module commences upon activation of the recovery module 141
which causes the recovery module 141 to ascend back to the surface of the
water. The
mechanical link 144 ensures that the recording unit 140 and the anchor 142
1o accompany the recovery module 141 in the ascent to the surface. At this
point the
equipment modules 140, 141 and 142 can be collected by the same vessel 120 or
by
another vessel. Upon retrieval, any data recorded and stored by the recording
unit 140
can be downloaded and the battery reloaded if necessary.
For applications such as oil exploration geophysical surveys, an acoustic
source 195, such as air-guns or marine vibrators, can be used for acoustic
illumination.
The acoustic source may be disposed on a vessel, or deployed onto the cable
100 in
accordance with the preferred embodiment. Once all the equipment modules 140,
141
and 142 have been retrieved, the deployment cycle starts again.
Referring now to FIG. 2a - FIG. 2c, illustrated is a process flow of an
alternate
embodiment according to the present invention in general overview. These
drawings
merely show several key steps in sequential processes.
Starting from FIG. 2a, the present embodiment includes the steps of, firstly,
providing conveying means such as a cable 200 with a fixed end and a free end
201.
Preferably the cable 200 is comparatively heavy to ensure that it adopts a
reliable
profile when hanging from the vessel 220.
3o Secondly, releasing the cable 200 into water 210 from a vessel 220 until
the
free end 201 of the cable 200 reaches, or is proximate to, the bottom 230 of
the water
210.
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Thirdly, dragging the cable 200 behind the vessel 220 under a controllable
speed. Ideally the length of the cable 200 is just sufficient for the cable to
touch the
bottom 230 and perhaps drag over a few tens of meters.
Fourth, slidably attaching equipment, including a plurality of recorder units
240, a recovery module 241, and stopping means 242 such as an anchor, to the
cable
200. The ratio of recorder units 240 to ancillary equipment (such as recovery
modules
241 and anchors 242) in the preferred embodiment illustrated in figures 2a to
2c is
higher than that shown in figures 1 a to 1 c. This generally allows for more
economic
1o deployment of a large number of recorder units 241 as it is not necessary
to provide a
recovery module 241 and an anchor 242 for each recorder unit 204.
The slidable attachment may be provided by clips 243 which are designed to
slide along the cable 200, and can, for instance, consist in a snap-link and a
rope. The
equipment 240, including the recovery module 241 and the stopping means 242,
are
tethered together by a connector 244, such as a rope. Preferably the rope 244
is made
from a light material, such as a high tensile strength low-density fibre, for
example
Kevlar or Vectran. Use of a connector 244 allows deployment and retrieval of a
plurality of equipment modules, for example many seismic recording units 240,
at the
same time. However, before slidably attaching the seismic recording units 240
to the
cable 200, the recording units 240 are preferably initialized and their clocks
synchronized.
The recovery module 241 may take the form of a pop-up buoy, or a
combination of a buoy, a weight and a weight release mechanism. Some
embodiments of the recovery module are automatically activatable, for example
by a
timer. Alternatively, the recovery module may be remotely activatable, for
example
upon detection of a signal, such as an acoustic signal.
Fifth, deploying the equipment 240, including the recovery module 241 and
the stopping means 242, by allowing the equipment 240 to slide along the cable
200
and thereby drop from the vessel 220 down to the bottom 230 of the water 210.
In
this case, the equipment 240 is forced to the bottom 230 by the combined
action of the
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shape of the heavy cable 200, the drag of the water and the weight of the
equipment.
With reference to FIG. 2b, once the equipment 240 reaches the bottom 230, the
equipment 240 is maintained in contact with the seafloor 230 at a fixed
position 250
by the stopping means 242 and due to the weight of the equipment. Whilst
deployed
on the seafloor, the rope 244 between equipment modules remains slack. This
assists
to avoid undesirable vibrational coupling between adjacent recording units
241.
Precise timing of the dropping of the equipment is provided by a mechanical
latch system. The positioning of the equipment on the seafloor, and the
separation
distance between adjacent equipment, is dependent upon the same factors as
outlined
above in relation to the first embodiment.
Sixth, causing the equipment to ascend and retrieving the equipment. This is
best illustrated in FIG. 2c. When the time comes for the equipment to ascend,
the
recovery module 241 is activated. In one embodiment activation of the recovery
module 241 occurs once a timer indicates that a predetermined length of time
has
elapsed. In another embodiment the recovery module 241 is adapted to activate
upon
detection of a signal. Upon activation, one embodiment of the recovery module
241
activates a weight release mechanism. In another embodiment, activation of the
recovery module causes inflation of a membrane. In any event, upon activation
the
recovery module 241 assumes a positive buoyancy sufficient for the ascension
of the
equipment 240 from the seafloor 230 to the surface of the water. Finally, the
equipment is retrieved from the surface of the water.
Immediately after deployment, the same vessel 220 can be used to illuminate
the area with an acoustic source and then to collect the data recorders 240
once the
recovery modules 241 have been triggered. Alternatively, another vessel can be
used
for acoustic illumination and/or recovery.
Advantageously the preferred embodiments provide very good control of the
positioning of comparatively light equipment due to the use of a heavy cable
200.
In preferred embodiments, positioning means, such as acoustic transponders
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and auxiliary sensors, can be deployed by being tethered to the equipment
modules
240, 241 and/or 242. In another embodiment such positioning means are slidably
attached to the cable and then deployed and retrieved using the same method as
for the
equipment modules.
Referring now to FIG. 3a - FIG. 3c, illustrated is a process flow of a third
embodiment according to the present invention in general overview. These
drawings
merely show several key steps in sequential processes.
to Starting from FIG. 3a, the third embodiment includes the steps of, first,
providing conveying means such as a cable 300 with a fixed end and a free end
equipped with a towed vehicle 301 which includes an equipment release
mechanism,
for example an electromagnetically actuatable latch.
Second, the cable 300 is released into water 310 from a seismic vessel 320
until the towed vehicle 301 approaches the bottom 330 of the water 310. Third,
the
cable 300 is dragged behind the seismic vessel 320 under a controllable speed.
The
length of the cable 300 is controlled to what is needed for the towed vehicle
to
approach the bottom 330 within a few meters. The location of the towed vehicle
301
may be precisely measured by using an ultra short acoustic base located on the
vessel
320 and an acoustic transponder fixed onto the towed body 301.
Fourth, equipment modules, such as a plurality of seismic recording units
340 and a recovery module 341, are attached to the cable 300 by using a
clipping
system 34 which is designed to slide along the cable 300 and can, for
instance, consist
in a snap-link and a rope. Before attaching the seismic recording units 340 to
the
cable 300, the recording units need to be initialized and clocks need to be
synchronized.
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The equipment modules are secured one to another by connecting means
344, such as a rope, which allows retrieval of many seismic recording units
340 with a
single recovery module 341. The rope 344 may be comparatively light weight
through the use of high tensile strength low-density fibre such as Kevlar or
Vectran,
for instance. It is important that the rope length between adjacent equipment
modules
is longer than the intended deployment spacing between adjacent modules on the
seafloor, so that the rope lies slack on the bottom and no vibrational
coupling occurs
between adjacent recording units 340 that might otherwise detrimentally affect
the
quality of the seismic data stored by the recording units
The recovery module 341 can consist in a pop up buoy or can be made of a
buoy, a weight and a weight release mechanism, which can be activated by a
timer or
remotely.
Fifth, the equipment modules 340 and 341 are deployed by allowing the
clippings 343 to slide along the cable 300 and so as to descend from the
seismic vessel
320 down to the towed vehicle 301. In this case, the equipment modules 340 and
341
are forced towards the bottom 330 by the combined action of the shape of the
heavy
cable 300, their hydrodynamic drag in the water and their negative buoyancy.
Once
2o the equipment modules 340 have reached the towed vehicle 301, they are
restrained
at, or adjacent to, the free end of the cable 300 by the electromagnetically
actuatable
latch until the equipment modules 340, 341 are in, or close to, an intended
seafloor
deployment position. At this point in time, as shown in FIG. 3b, one or more
equipment modules 340 and/or 341 are released by the electromagnetically
actuatable
latch and allowed to sink to the bottom 330 at fixed deployment positions 350.
Precise
timing and location of the dropping of each equipment module 340 and/or 341
can be
controlled from the vessel 320 by communication of a signal from the vessel
320 to
the towed vehicle 301, either electrically through a conductor in cable 300 or
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acoustically through the water 310. Upon receipt of the signal, the
electromagnetically actuatable latch releases one or more of the equipment
modules
340 and/or 341.
Sixth, referring now to FIG. 3c, the recovery module 341 is activated by
either a timer, remotely or by any other means for allowing the equipment
modules
340 and 341 to ascend from the bottom 330 to the surface of the water. The
rope 344
ensures that the recorder units 340 ascend with the recovery module 341.
Finally, the
equipment modules 340 and 341 are retrieved from the surface of the water.
The present embodiment allows fulfilment of complex deployment
geometry. Immediately after deployment, the same vessel can be used to
illuminate
the area with an acoustic source and then collect the data recorders 340 once
the
recovery modules 341 have been triggered. Alternatively, another vessel can be
used
for acoustic illumination and/or recovery.
The preferred embodiments advantageously allow for very good control of
the positioning of equipment modules using a considerably lighter system for a
given
positioning performance.
Although specific embodiments have been illustrated and described, it will
be obvious to those skilled in the art that various modifications may be made
without
departing from what is intended to be limited solely by the appended claims.
