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`Ex. PGS 1052
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`EX. PGS 1052
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`

`

`(12) United States Patent
`Hillesund et al.
`
`111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US006932017Bl
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,932,017 Bl
`Aug. 23,2005
`
`(54) CONTROL SYSTEM FOR POSITIONING OF
`MARINE SEISMIC STREAMERS
`
`(75)
`
`Inventors: Oyvind Hillesund, Riston (GB); Simon
`Hastings Bittleston, Bury St Edmunds
`(GB)
`
`(73) Assignee: Westerngeco, L.L.C., Houston, TX
`(US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.:
`
`09/787,723
`
`(22) PCT Filed:
`
`Sep. 28, 1999
`
`(86) PCTNo.:
`
`PCT/IB99/01590
`
`§ 371 (c)(l),
`(2), ( 4) Date:
`
`Jul. 2, 2001
`
`(87) PCT Pub. No.: W000/20895
`
`PCT Pub. Date: Apr. 13, 2000
`
`(30)
`
`Foreign Application Priority Data
`
`Oct. 1, 1998
`
`(GB) ............................................. 9821277
`
`Int. Cl? ....................... ...... ................... B63B 21/66
`(51)
`(52) U.S. Cl. ........................................ 114/244; 114/253
`(58) Field of Search ......................... 114/242, 244-246,
`114/253, 162, 163
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`4,676,183 A * 6/1987 Conboy ...................... 114/245
`
`4,890,568 A * 1!1990 Dolengowski .............. 114/246
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`
`613025 A1 * 8/1994
`
`G01 V/1/38
`
`* cited by examiner
`
`Primary Examiner-Jesus D. Sotelo
`(74) Attorney, Agent, or Firm-Westerngeco, L.L.C.
`
`(57)
`
`ABSTRACT
`
`A method of controlling a streamer positioning device (18)
`configured to be attached to a marine seismic streamer (12)
`and towed by seismic survey vessel (10) and having a wing
`and a wing motor for changing the orientation of the wing.
`The method includes the steps of: obtaining an estimated
`velocity of the streamer positioning device, calculating a
`desired change in the orientation of the wing using the
`estimated velocity of the streamer positioning device, and
`actuating the wing motor to produce the desired change in
`the orientation of the wing. The invention also involves an
`apparatus for controlling a streamer positioning device
`including means for obtaining an estimated velocity of the
`streamer positioning device, means for calculating a desired
`change in the orientation of the wing using the estimated
`velocity of the streamer positioning device, and means for
`actuating the wing motor to produce the desired change in
`the orientation of the wing.
`
`26 Claims, 3 Drawing Sheets
`
`18
`
`/
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`Ex. PGS 1052
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`

`

`U.S. Patent
`
`Aug. 23,2005
`
`Sheet 1 of 3
`
`US 6,932,017 Bl
`
`Fig.1.
`Prior Art
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`20
`
`Ex. PGS 1052
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`

`

`U.S. Patent
`
`Aug. 23,2005
`
`Sheet 2 of 3
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`US 6,932,017 Bl
`
`Fig.2.
`
`18
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`/
`
`Fig.3.
`
`48
`
`40
`
`18
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`/
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`__,44
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`38
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`Ex. PGS 1052
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`

`

`Fig.4.
`
`36
`
`I
`
`RAM
`58
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`+
`..
`
`...
`
`AID
`
`... ~
`
`Q)
`
`12
`....
`~
`jg
`CIJ
`
`Data
`...
`"""'
`
`EEPROM
`56
`
`~
`
`,
`
`Processor
`Unit
`~
`
`A~
`,~
`
`I
`I
`
`... Communication
`...
`RS 485
`I ~24
`76
`
`I
`t
`
`Ace
`Horizontal
`aa
`
`Ace
`Vertical
`70
`
`Temp.
`Sensor
`12
`
`...
`...
`
`...
`-
`
`Motor
`Driver
`§2
`
`Motor
`Driver
`62
`
`Motor
`
`.. Left Wing
`-
`
`~
`
`Motor
`~ .... Right Wing
`~
`
`"""'
`
`~
`
`:=--
`
`66
`
`1/0
`
`60
`
`..
`
`Pressure
`Sensor
`:M
`
`d
`•
`'J'l
`•
`
`Wlng28
`
`~
`
`Wing 28
`
`...
`
`....
`-
`
`~
`
`Position
`Indicator
`.M
`
`i
`
`Position
`Indicator
`64
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`Ex. PGS 1052
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`

`

`US 6,932,017 Bl
`
`1
`CONTROL SYSTEM FOR POSITIONING OF
`MARINE SEISMIC STREAMERS
`
`BACKGROUND OF THE INVENTION
`
`5
`
`2
`typical estimates range from 5 to 10%. Simulations have
`concluded that properly controlled horizontally steerable
`birds can be expected to reduce these types of costs by
`approximately 30%.
`One system for controlling a horizontally steerable bird,
`as disclosed in UK Patent GB 2093610 B, is to utilize a
`manually-operated central control system to transmit the
`magnitudes and directions of any required wi~g a':lgle
`changes to the birds. While this method greatly simplifies
`10 the circuitry needed within the bird itself, it is virtually
`impossible for this type of system to closely regulate the
`horizontal positions of the birds because it requires manual
`input and supervision. This becomes a particularly signifi(cid:173)
`cant issue when a substantial number of streamers are
`deployed simultaneously and the number of birds that must
`15 be controlled goes up accordingly.
`Another system for controlling a horizontally steerable
`bird is disclosed in our published PCT International Appli(cid:173)
`cation No. WO 98/28636. Using this type of control system,
`the desired horizontal positions and the actual horizontal
`20 positions are received from a remote control system and are
`then used by a local control system within the birds to adjust
`the wing angles. The actual horizontal positions of the birds
`may be determined every 5 to 10 seconds and there may be
`a 5 second delay between the taking of measurements and
`25 the determination of actual streamer positions. While this
`type of system allows for more automatic adjustment of the
`bird wing angles, the delay period and the relatively lon.g
`cycle time between position measurements prevents this
`type of control system from rapidly and efficiently co.n~ro~-
`30 ling the horizontal position of the bird. A more deteri?~m~tlc
`system for controlling this type of streamer positlomng
`device is therefore desired.
`It is therefore an object of the present invention to provide
`for an improved method and apparatus for controlling a
`35 streamer positioning device.
`An advantage of the present invention is that the position
`of the streamer may be better controlled, thereby reducing
`the need for in-fill shooting, reducing the chance of streamer
`tangling, and reducing the time needed to turn the seismic
`40 survey vessel.
`Another advantage of the present invention is that noise in
`marine seismic data associated with streamer position over(cid:173)
`correction and streamer positioning errors can be signifi(cid:173)
`cantly reduced.
`
`This invention relates generally to systems for controlling
`seismic data acquisition equipment and particularly to a
`system for controlling a marine seismic streamer positioning
`device.
`A marine seismic streamer is an elongate cable-like
`structure, typically up to several thousand meters long,
`which contains arrays of seismic sensors, known as
`hydrophones, and associated electronic equipment along its
`length, and which is used in marine seismic surveyi~g. In
`order to perform a 3D marine seismic survey, a plurality of
`such streamers are towed at about 5 knots behind a seismic
`survey vessel, which also tows one or more seismic so~rce~,
`typically air guns. Acoustic signals produced by the seismic
`sources are directed down through the water mto the earth
`beneath, where they are reflected from the various strata.
`The reflected signals are received by the hydrophones, and
`then digitized and processed to build up a representation of
`the subsurface geology.
`The horizontal positions of the streamers are typically
`controlled by a deflector, located at the front end or "head"
`of the streamer, and a tail buoy, located at the back end or
`"tail" of the streamer. These devices create tension forces on
`the streamer which constrain the movement of the streamer
`and cause it to assume a roughly linear shape. Cross currents
`and transient forces cause the streamer to bow and undulate,
`thereby introducing deviations into this desired linear shape.
`The streamers are typically towed at a constant depth of
`approximately ten meters, in order to facilitate the removal
`of undesired "ghost" reflections from the surface of the
`water. To keep the streamers at this constant depth, control
`devices known as "birds", are typically attached at various
`points along each streamer between the deflector and the !ail
`buoy, with the spacing between the birds generally varyi~g
`between 200 and 400 meters. The birds have hydrodynamic
`deflecting surfaces, referred to as wings, that. a~low the
`position of the streamer to be controlled as It IS towed
`through the water. When a bird is used for depth contr.ol
`purposes only, it is possible for the bird to regularly sense Its
`depth using an integrated pressure sensor and for a local
`controller within the bird to adjust the wing angles to
`maintain the streamer near the desired depth using only a 45
`desired depth value received from a central control system.
`While the majority of birds used thus far have only
`controlled the depth of the streamers, additional benefits can
`be obtained by using properly controlled horizontally steer(cid:173)
`able birds, particularly by using the types of horizontally and 50
`vertically steerable birds disclosed in our published PCT
`International Application No. WO 98/28636. The benefits
`that can be obtained by using properly controlled horizon(cid:173)
`tally steerable birds can include reducing horizontal out-of(cid:173)
`position conditions that necessitate reacquiring seismic data
`in a particular area (i.e. in-fill shooting), reduci?g the cha?ce
`of tangling adjacent streamers, and reducmg the tl~e
`required to turn the seismic acquisition ve~sel when e';!di~g
`one pass and beginning another pass dunng a 3D seismic
`survey.
`It is estimated that horizontal out-of-position conditions
`reduce the efficiency of current 3D seismic survey opera(cid:173)
`tions by between 5 and 10%, depending on weather and
`current conditions. While incidents of tangling adjacent
`streamers are relatively rare, when they do occur they
`invariably result in prolonged vessel downtime. The loss of 65
`efficiency associated with turning the seismic survey vessel
`will depend in large part on the seismic survey layout, but
`
`55
`
`60
`
`SUMMARY OF THE INVENTION
`
`The present invention provides methods and apparat~s for
`controlling the positions of marine seismic streamers m an
`array of such streamers being towed by a seismic survey
`vessel, the streamers having respective streamer positioning
`devices disposed therealong and each streamer positioning
`device having a wing and a wing motor for changing the
`orientation of the wing so as to steer the streamer positioning
`device laterally, said methods and apparatus involving (a)
`obtaining an estimated velocity of the streamer pos~t~on~ng
`devices, (b) for at least some of the streamer positlomng
`devices, calculating desired changes in the orientation. of
`their wings using said estimated velocity, and (c) actuatmg
`the wing motors to produce said desired changes in wing
`orientation.
`The invention and its benefits will be better understood
`with reference to the detailed description below and the
`accompanying figures.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic diagram of a seismic survey vessel
`and associated seismic data acquisition equipment;
`
`Ex. PGS 1052
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`

`

`US 6,932,017 Bl
`
`3
`FIG. 2 is a schematic horizontal cross-sectional view
`through a marine seismic streamer and an attached streamer
`positioning device;
`FIG. 3 is a schematic vertical cross-sectional view
`through the streamer positioning device from FIG. 2; and
`FIG. 4 is a schematic diagram of the local control system
`architecture of the streamer positioning device from FIG. 2.
`
`DETAILED DESCRIPTION OF 1HE
`INVENTION
`
`30
`
`35
`
`4
`or in our PCT International Patent Application No. WO
`98/21163. Alternatively, or additionally, satellite-based glo(cid:173)
`bal positioning system equipment can be used to determine
`the positions of the equipment. The vertical positions of the
`5 birds 18 are typically monitored using pressure sensors
`attached to the birds, as discussed below.
`The global control system 22 preferably maintains a
`dynamic model of each of the seismic streamers 12 and
`utilizes the desired and actual positions of the birds 18 to
`10 regularly calculate updated desired vertical and horizontal
`forces the birds should impart on the seismic streamers 12 to
`move them from their actual positions to their desired
`positions. Because the movement of the seismic streamer 12
`causes acoustic noise (both from seawater flow past the bird
`wing structures as well as cross current flow across the
`15 streamer skin itself), it is important that the streamer move(cid:173)
`ments be restrained and kept to the minimum correction
`required to properly position the streamers. Any streamer
`positioning device control system that consistently overes(cid:173)
`timates the type of correction required and causes the bird to
`20 overshoot its intended position introduces undesirable noise
`into the seismic data being acquired by the streamer. In
`current systems, this type of over-correction noise is often
`balanced against the "noise" or "smearing" caused when the
`seismic sensors in the streamers 12 are displaced from their
`25 desired positions.
`The global control system 22 preferably calculates the
`desired vertical and horizontal forces based on the behavior
`of each streamer and also takes into account the behavior of
`the complete streamer array. Due to the relatively low
`sample rate and time delay associated with the horizontal
`position determination system, the global control system 22
`runs position predictor software to estimate the actualloca(cid:173)
`tions of each of the birds 18. The global control system 22
`also checks the data received from the vessel's navigation
`system and the data will be filled in if it is missing. The
`interface between the global control system 22 and the local
`control system will typically operate with a sampling fre(cid:173)
`quency of at least 0.1 Hz. The global control system 22 will
`typically acquire the following parameters from the vessel's
`navigation system: vessel speed (m/s), vessel heading
`(degrees), current speed (m/s), current heading (degrees),
`and the location of each of the birds in the horizontal plane
`in a vessel fixed coordinate system. Current speed and
`heading can also be estimated based on the average forces
`acting on the streamers 12 by the birds 18. The global
`control system 22 will preferably send the following values
`to the local bird controller: demanded vertical force,
`demanded horizontal force, towing velocity, and crosscur(cid:173)
`rent velocity.
`The towing velocity and crosscurrent velocity are prefer(cid:173)
`ably "water-referenced" values that are calculated from the
`vessel speed and heading values and the current speed and
`heading values, as well as any relative movement between
`the seismic survey vessel10 and the bird 18 (such as while
`the vessel is turning), to produce relative velocities of the
`bird 18 with respect to the water in both the "in-line" and the
`"cross-line" directions. Alternatively, the global control sys-
`tem 22 could provide the local control system with the
`horizontal velocity and water in-flow angle. The force and
`velocity values are delivered by the global control system 22
`as separate values for each bird 18 on each streamer 12
`60 continuously during operation of the control system.
`The "water-referenced" towing velocity and crosscurrent
`velocity could alternatively be determined using flowmeters
`or other types of water velocity sensors attached directly to
`the birds 18. Although these types of sensors are typically
`quite expensive, one advantage of this type of velocity
`determination system is that the sensed in-line and cross-line
`velocities will be inherently compensated for the speed and
`
`In FIG. 1, a seismic survey vessel 10 is shown towing
`eight marine seismic streamers 12 that may, for instance,
`each be 3000 meters in length. The outermost streamers 12
`in the array could be 700 meters apart, resulting in a
`horizontal separation between the streamers of 100 meters in
`the regular horizontal spacing configuration shown. A seis(cid:173)
`mic source 14, typically an airgun or an array of airguns, is
`also shown being towed by the seismic survey vessel10. At
`the front of each streamer 12 is shown a deflector 16 and at
`the rear of every streamer is shown a tail buoy 20. The
`deflector 16 is used to horizontally position the end of the
`streamer nearest the seismic survey vessel 10 and the tail
`buoy 20 creates drag at the end of the streamer farthest from
`the seismic survey vessel 10. The tension created on the
`seismic streamer by the deflector 16 and the tail buoy 20
`results in the roughly linear shape of the seismic streamer 12
`shown in FIG. 1.
`Located between the deflector 16 and the tail buoy 20 are
`a plurality of streamer positioning devices known as birds
`18. Preferably the birds 18 are both vertically and horizon-
`tally steerable. These birds 18 may, for instance, be located
`at regular intervals along the streamer, such as every 200 to
`400 meters. The vertically and horizontally steerable birds
`18 can be used to constrain the shape of the seismic streamer
`12 between the deflector 16 and the tail buoy 20 in both the
`vertical (depth) and horizontal directions.
`In the preferred embodiment of the present invention, the
`control system for the birds 18 is distributed between a
`global control system 22 located on or near the seismic
`survey vessel10 and a local control system located within or
`near the birds 18. The global control system 22 is typically 40
`connected to the seismic survey vessel's navigation system
`and obtains estimates of system wide parameters, such as the
`vessel's towing direction and velocity and current direction
`and velocity, from the vessel's navigation system.
`The most important requirement for the control system is 45
`to prevent the streamers 12 from tangling. This requirement
`becomes more and more important as the complexity and the
`total value of the towed equipment increases. The trend in
`the industry is to put more streamers 12 on each seismic
`survey vessel 10 and to decrease the horizontal separation 50
`between them. To get better control of the streamers 12,
`horizontal steering becomes necessary. If the birds 18 are not
`properly controlled, horizontal steering can increase, rather
`than decrease, the likelihood of tangling adjacent streamers.
`Localized current fluctuations can dramatically influence the
`magnitude of the side control required to property position
`the streamers. To compensate for these localized current
`fluctuations, the inventive control system utilizes a distrib(cid:173)
`uted processing control architecture and behavior-predictive
`model-based control logic to properly control the streamer
`positioning devices.
`In the preferred embodiment of the present invention, the
`global control system 22 monitors the actual positions of
`each of the birds 18 and is programmed with the desired
`positions of or the desired minimum separations between the
`seismic streamers 12. The horizontal positions of the birds 65
`18 can be derived, for instance, using the types of acoustic
`positioning systems described in our U.S. Pat. No. 4,992,990
`
`55
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`

`US 6,932,017 Bl
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`5
`heading of marine currents acting on said streamer position(cid:173)
`ing device and for relative movements between the vessel10
`and the bird 18.
`FIG. 2 shows a type of bird 18 that is capable of
`controlling the position of seismic streamers 12 in both the
`vertical and horizontal directions. A bird 18 of this type is
`also disclosed in our PCT International Application No. WO
`98/28636. While a number of alternative designs for the
`vertically and horizontally steerable birds 18 are possible,
`including those utilizing one full-moving wing with
`ailerons, three full-moving wings, and four full-moving
`wings, the independent two-wing principal is, conceptually,
`the simplest and most robust design.
`In FIG. 2, a portion of the seismic streamer 12 is shown
`with an attached bird 18. A communication line 24, which
`may consist of a bundle of fiber optic data transmission
`cables and power transmission wires, passes along the
`length of the seismic streamer 12 and is connected to the
`seismic sensors, hydrophones 26, that are distributed along
`the length of the streamer, and to the bird 18. The bird 18
`preferably has a pair of independently moveable wings 28
`that are connected to rotatable shafts 32 that are rotated by
`wing motors 34 and that allow the orientation of the wings
`28 with respect to the bird body 30 to be changed. When the
`shafts 32 of the bird 18 are not horizontal, this rotation
`causes the horizontal orientation of the wings 28 to change
`and thereby changes the horizontal forces that are applied to
`the streamer 12 by the bird.
`The motors 34 can consist of any type of device that is
`capable of changing the orientation of the wings 28, and they
`are preferably either electric motors or hydraulic actuators. 30
`The local control system 36 controls the movement of the
`wings 28 by calculating a desired change in the angle of the
`wings and then selectively driving the motors 34 to effec(cid:173)
`tuate this change. While the preferred embodiment depicted
`utilizes a separate motor 34 for each wing 28, it would be
`also be possible to independently move the wings 28 using 35
`a single motor 34 and a selectively actuatable transmission
`mechanism.
`When the bird 18 uses two wings 28 to produce the
`horizontal and vertical forces on the streamer 12, the
`required outputs of the local control system 36 are relatively 40
`simple, the directions and magnitudes of the wing move(cid:173)
`ments required for each of the wings 28, or equivalently the
`magnitude and direction the motors 34 need to be driven to
`produce this wing movement. While the required outputs of
`the local control system 36 for such a two full moving wing 45
`design is quite simple, the structure and operation of the
`overall system required to coordinate control of the device
`is relatively complicated.
`FIG. 3 shows a schematic vertical cross-sectional view
`through the streamer positioning device shown in FIG. 2 that 50
`will allow the operation of the inventive control system to be
`described in more detail. The components of the bird 18
`shown in FIG. 3 include the wings 28 and the body 30. Also
`shown in FIG. 3 are a horizontal coordinate axis 38 and a
`vertical coordinate axis 40. During operation of the streamer
`positioning control system, the global control system 22
`preferably transmits, at regular intervals (such as every five
`seconds) a desired horizontal force 42 and a desired vertical
`force 44 to the local control system 36.
`The desired horizontal force 42 and the desired vertical
`force 44 are combined within the local control system 36 to
`calculate the magnitude and direction of the desired total
`force 46 that the global control system 22 has instructed the
`local control system to apply to the streamer 12. The global
`control system 22 could alternatively provide the magnitude
`and direction of the desired total force 46 to the local control
`system 36 instead of the desired horizontal force 42 and the
`desired vertical force 44.
`
`6
`While the desired horizontal force 42 and the desired
`vertical force 44 are preferably calculated by the global
`control system 22, it is also possible for the local control
`system 36 in the inventive control system to calculate one or
`5 both of these forces using a localized displacement/force
`conversion program. This type of localized conversion pro(cid:173)
`gram may, for instance, use a look-up table or conversion
`routine that associates certain magnitudes and directions of
`vertical or horizontal displacements with certain magnitudes
`and directions of changes in the vertical or horizontal forces
`required. Using this type of embodiment, the global control
`system 22 can transmit location information to the local
`control system 36 instead of force information. Instead of
`the desired vertical force 44, the global control system 22
`can transmit a desired vertical depth and the local control
`15 system 36 can calculate the magnitude and direction of the
`deviation between the desired depth and the actual depth.
`Similarly, instead of transmitting a desired horizontal force
`42, the global control system 22 can transmit the magnitude
`and direction of the displacement between the actual hori-
`20 zontal position and the desired horizontal position of the bird
`18. One advantage to this alternative type of system is that
`the required vertical force can be rapidly updated as the local
`control system receives updated depth information from the
`integrated pressure sensor. Other advantages of this type of
`25 alternative system include reducing communication traffic
`on the communication line 24 and simplifying the program(cid:173)
`ming needed to convert the measured vertical and/or hori(cid:173)
`zontal displacements into corresponding forces to be applied
`by the birds 18.
`When the local control system 36 has a new desired
`horizontal force 42 and desired vertical force 44 to be
`applied, the wings 28 will typically not be in the proper
`orientation to provide the direction of the desired total force
`46 required. As can be seen in FIG. 3, the wings 28 introduce
`a force into the streamer 12 along an axis perpendicular to
`the rotational axis of the wings 28 and perpendicular to the
`streamer. This force axis 48 is typically not properly aligned
`with the desired total force 46 when new desired horizontal
`and vertical force values are received from the global control
`system 22 or determined by the local control system 36 and
`some rotation of the bird 18 is required before the bird can
`produce this desired total force 46. As can be seen, the force
`axis 48 is directly related to the bird roll angle, designated
`in FIG. 3 as <jl.
`The local control system 36 optimizes the control process
`by projecting the desired total force 46 onto the force axis 48
`(i.e. multiplying the magnitude of the desired total force by
`the cosine of the deviation angle 50) to produce an inter(cid:173)
`mediate desired force 52 and then adjusting the wing com(cid:173)
`mon angle a (the angle of the wings with respect to the bird
`body 30, or the average angle if there is a non-zero splay
`angle) to produce this magnitude of force along the force
`axis. The calculated desired common wing angle is com(cid:173)
`pared to the current common wing angle to calculate a
`desired change in the common wing angle and the wing
`motors 34 are actuated to produce this desired change in the
`orientation of the wings.
`A splay angle is then introduced into the wings 28 to
`produce a rotational movement in the bird body 30 (i.e. to
`rotate the force axis 48 to be aligned with the desired total
`force 46). The splay angle is the difference between the
`60 angles of the wings 28 with respect to the bird body 30. As
`the bird body 30 rotates and the force axis 48 becomes more
`closely aligned with the desired total force 46, the bird roll
`angle and the bird roll angular velocity are monitored, the
`splay angle is incrementally reduced, and the common angle
`65 is incrementally increased until the intermediate desired
`force 52 is in the same direction and of the same magnitude
`as the desired total force. The local control system 36
`
`10
`
`55
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`

`

`US 6,932,017 Bl
`
`5
`
`15
`
`7
`carefully regulates the splay angle to ensure that the
`streamer is stable in roll degree of freedom. The calculated
`common wing angle and the splay angle are also regulated
`by the local control system 36 to prevent the wings 28 from
`stalling and to ensure that the splay angle is prioritized.
`When using the type of birds described in our published
`PCT International Application No. WO 98/28636, where the
`bird 18 is rigidly attached, and cannot rotate with respect, to
`the streamer 12, it is important for the control system to take
`the streamer twist into account. If this is not taken into
`account, the bird 18 can use all of its available splay angle
`to counter the twist in the streamer 12. The bird 18 will then
`be unable to reach the demanded roll angle and the generated
`force will decrease. The inventive control system incorpo(cid:173)
`rates two functions for addressing this situation; the anti(cid:173)
`twist function and the untwist function.
`In the anti -twist function, the streamer twist is estimated
`by weightfunction filtering the splay angle measurements
`instead of simply averaging the splay angle measurements to
`improve the bandwidth of the estimation. The anti-twist
`function engages when the estimated twist has reached a
`critical value and it then overrides the normal shortest path
`control of the calculated roll angle. The anti-twist function
`forces the bird 18 to rotate in the opposite direction of the
`twist by adding +/-180 degrees to the demanded roll angle.
`Once the twist has been reduced to an acceptable value, the 25
`anti-twist function disengages and the normal shortest path
`calculation is continued.
`The untwist function is implemented by the global control
`system 22 which monitors the splay angle for all of the birds
`18 in each streamer 12. At regular intervals or when the 30
`splay angle has reached a critical value, the global control
`system 22 instructs each local control system 36 to rotate
`each bird 18 in the opposite direction of the twist. The
`number of revolutions done by each bird 18 is monitored and
`the untwist function is disengaged once the twist has reached
`an acceptable level. FIG. 4 is a schematic diagram of the 35
`architecture of the local control system 36 for the bird 18.
`The local control system 36 consists of a central processor
`unit 54, having EEPROM 56 and RAM 58 memory, an
`input/output subsystem 60 that is connected to a pair of
`motor drivers 62, and an analog to digital conversion unit 66. 40
`The motor drivers 62 are connected to and actuate the wing
`motors 34 to produce the desired change the orientation of
`the wings 28 with respect to the bird body 30.
`The wing motor 34/wing 28 units are also connected to
`wing position indicators 64 that sense the relative positions 45
`of the wings and provide measurements to the analog to
`digital conversion unit 66 which converts the analog wing
`position indicator 64 measurements into digital format and
`conveys these digital values to the central processor unit 54.
`Various types of wing position indicators 64 can be used, 50
`including resistive angle or displacement sensors inductive
`sensors, capacitive sensors, hall sensors, or magneto(cid:173)
`restrictive sensors.
`A horizontal accelerometer 68 and a vertical accelerom(cid:173)
`eter 70, placed at right angles with respect to one another, are
`also connected to the analog to digital conversion unit 66
`and these accelerometers convey measurements that allow
`the central processor unit 54 to determine the roll angle and
`roll rate of the bird 18. An angular velocity vibrating rate
`gyro (rategyro) can also be used to measure the roll rate of
`the bird 18. A temperature sensor 72 is connected to the 60
`analog to digital conversion unit 66 to provide temperature
`measurements that allow the horizontal accelerometer 68
`and the vertical accelerometer 70 to be calibrated.
`A pressure sensor 74 is also connected to the analog to
`digital conversion unit 66 to provide the central processor
`unit 54 with measurements of the water pressure at the bird
`18. To calculate an appropriate depth value, the measured
`
`8
`pressure values must be filtered to limit the disturbance from
`waves. This is done in the inventive control system with a
`weightfunction filter that avoids the large phase displace(cid:173)
`ments caused by mean value filters. Instead of using an
`instantaneous depth value or simply calculating an average
`depth value over a given period of time (and thereby
`incorporating a large phase displacement into the depth
`value), the inventive control system uses a differentially
`weighted pressure filtering scheme. First the pressure values
`are transformed into depth values by dividing the pressure
`10 sensor reading by the seawater density and gravitational
`acceleration. These depth values are then filtered using a
`weight function filter. Typical incremental weighting func(cid:173)
`tions values range from 0.96 to 0.90 (sample weights of 1.0,
`0.9, 0.81, 0.729, etc.) and the filter will typically process
`depth values received over a period of at least 100 seconds.
`The central processor unit 54 is also connected to a RS485
`communications unit 76 that allows information to be
`exchanged between the local control system 36 and the
`global control system 22 over the communication line 24
`that passes through the streakier 12. The RS485 bus may, for
`20 instance, utilize Neuron chips that com