United States Patent [19]
`Elholm
`
`[54] DEVICE AND METHOD FOR POSITIONING
`OF TOWING SYSTEMS FOR USE IN
`MARINE SEISMIC SURVEYS
`
`[75]
`
`Inventor: Tor Elholm, Hosie, Norway
`
`[73] Assignee: Geco A.S., Stavanger, Norway
`
`[21] Appl. No.: 194,377
`
`[22] Filed:
`
`Feb.8, 1994
`
`[30]
`
`Foreign Application Priority Data
`
`Feb. 23, 1993
`
`[NO] Norway .................................... 930641
`
`Int. Cl.6
`.............................. G01V 1/38; B63B 21/66
`[51]
`[52] U.S. Cl. ................................ 367/16; 114/244; 367/20
`[58] Field of Search .................................. 367116,17,18,
`367/20, 106; 114/244, 245, 246
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,434,451
`3,605,674
`3,774,570
`4,063,213
`4,087,780
`4,227,479
`4,323,989
`4,404,664
`4,463,701
`4,484,534
`4,723,501
`4,729,333
`4,890,568
`5,052,814
`5,357,892
`5,402,745
`
`3/1969 Brainard, II .............................. 367/18
`9/1969 Weese ................................. 114/235 B
`1111973 Pearson ............................... 114/235 B
`12/1977 Itria eta!. ................................. 367/17
`5/1978 !tria et al. ................................. 367117
`I 0/1980 Gertler et al.
`.......................... 114/244
`4/1982 Huckabee et a!. ........................ 367/17
`9/1983 Zachariadis ............................... 367/19
`8/1984 Pickett et al. ........................... 114/245
`11/1984 Thillaye duBoullay ................ 114/244
`2/1988 Hovden eta!. ........................... 367/16
`3/1988 Kirby et al ............................. 114/244
`111990 Dolengowski .......................... 114/246
`10/1991 Stubblefield .............................. 367/16
`10/1994 Vatne et al. ............................. 114/244
`4/1995 Wood ...................................... 114/244
`
`111111111111111111111111111111111111111111111111111111111111111111111111111
`US005532975A
`[11] Patent Number:
`[ 45] Date of Patent:
`
`5,532,975
`Jul. 2, 1996
`
`5,443,027
`
`811995 Owsley et al. .......................... 114/244
`
`FOREIGN PATENT DOCUMENTS
`
`0018053
`0154968
`2223798
`3737490
`151735
`158970
`1104454
`1551635
`2193476
`2248587
`
`1011980
`911985
`911973
`411992
`211985
`811988
`7/1984
`811979
`2/1988
`411992
`
`European Pat. Off ..
`European Pat. Off ..
`Germany.
`Germany.
`Norway.
`Norway.
`U.S.S.R ................................... 367116
`United Kingdom .
`United Kingdom .
`United Kingdom .
`
`Primary Examiner-Charles T. Jordan
`Assistant Examiner-Theresa M. Wesson
`Attorney, Agent, or Firm-Jacobson, Price, Holman & Stem
`
`[57]
`
`ABSTRACT
`
`A positioning device for seismic equipment which is towed
`by a seismic vessel is designed with a body part which is
`equipped with wings (7) and rudders (8, 6, 9, 5). For the
`control of wings (7) and rudders (8, 6, 9, 5) control means,
`preferably hydraulic or electrical means are used. The device
`further comprises a control unit for processing of signals
`which preferably operate exclusively on the basis of infor(cid:173)
`mation from the vessel or the ship, instruments for use in the
`positioning of the device and a communication system for
`the communication between the vessel and the device and
`vice versa, preferably electrical, acoustic or optical. It is
`further equipped with attachment devices for one or more
`cables (26) and floats, which are preferably provided at the
`front of the device, and preferably in the vicinity of the
`wing's (7) attachment point to the device's body part as well
`as a power supply system.
`
`24 Claims, 5 Drawing Sheets
`
`HYDRAULIC SYSTEM
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`COMPRESSED
`AIR
`RESERVE
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`BATTERIES
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`1
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`ION 1005
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`U.S. Patent
`U-S- Patent
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`Jul. 2, 1996
`Jul. 2, 1996
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`Sheet 1 of 5
`Sheet 1 of 5
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`5,532,975
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`U.S. Patent
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`Jul. 2, 1996
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`Sheet 2 of 5
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`HYDRAULIC SYSTEM
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`COMPRESSED
`AIR
`RESERVE
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`BATTERIES
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`3
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`

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`US. Patent
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`Jul. 2, 1996
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`Sheet 3 of 5
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`5,532,975
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`U.S. Patent
`US. Patent
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`Jul. 2, 1996
`Jul. 2, 1996
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`Sheet 4 of 5
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`U.S. Patent
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`Jul. 2, 1996
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`Sheet 5 of 5
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`5,532,975
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`Fig.7
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`6
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`5,532,975
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`1
`DEVICE AND METHOD FOR POSITIONING
`OF TOWING SYSTEMS FOR USE IN
`MARINE SEISMIC SURVEYS
`
`BACKGROUND OF THE INVENTION
`
`5
`
`15
`
`20
`
`2
`Another method is to use surface bodies or vessels which
`are provided in such a manner that the vessel is towed at an
`angle, thus achieving a lateral force. Examples of this type
`of vessel which can be envisaged are vessels which are
`equipped with a tilted keel, or tilted foils, thus enabling the
`surface vessel to move out to the side.
`The common feature of these methods and the equipment
`for obtaining a certain width for the towing system is that
`they all have a connection with the surface. This makes the
`10 vessels particularly vulnerable to flotsam. In the event of a
`collision between a vessel and some flotsam the towing
`system or parts of it will often be damaged. In a collision
`with flotsam the vessel could change direction, thus causing
`large sections of the towing system to be damaged. The same
`thing will happen if it comes into contact with fishing tackle
`or other equipment which may be in the location of the tow.
`Another problem is that surface vessels or floats cause a
`great deal of friction. Severe friction gives rise to two
`problems
`in particular;
`increased fuel expenses and
`increased difficulty in obtaining tow width. The same will
`apply to the actual connection consisting of wires, rope,
`chains or the like between the gun array and the surface
`vessel or float, and the connection between the cable and the
`surface vessel or the float.
`The fact that the gun arrays and the streamers are con(cid:173)
`nected with the surface will cause waves to be generated.
`This gives rise to unwanted noise and vibrations for the
`towed equipment. In some cases there will also be problems
`30 with maintaining the equipment at a constant depth, espe(cid:173)
`cially when the survey is being conducted in heavy seas. It
`is also a problem to keep the equipment on the correct
`course, with the equipment usually drifting in step with the
`waves.
`It is obvious that the equipment is subject to a consider(cid:173)
`able degree of wear and tear and damage when it is exposed
`to those forces which are associated with a connection with
`the surface.
`
`The present invention is concerned with seismic surveys.
`More specifically it concerns a device for, and a method for
`towing a seismic tow, consisting of a gun array and seismic
`streamers.
`In marine seismic surveys a ship is used which tows
`seismic sources and seismic streamers. The sources produce
`acoustic energy which is propagated in the geological for(cid:173)
`mations below the seabed where the survey is undertaken.
`The sound pulse or shock wave generated by the sources is
`reflected as acoustic waves. The seismic streamer consists of
`hydrophones which detect the reflected acoustic waves,
`convert the signals to electrical pulses, and pass on this
`information to a data acquisition system on board the vessel.
`Further processing and analysis of the information give an
`indication of the structure in the geological formations in
`which the survey has been conducted.
`A seismic source consists of a number of individual sound
`sources, e.g. airguns or waterguns. An arrangement of indi(cid:173)
`vidual guns is usually called a gun array. Apart from the 25
`actual guns a gun array also includes attachment devices, air
`hoses, electrical wires and sensors. A gun array measures
`10-30 meters in length.
`A seismic streamer, often called a hydrophone cable,
`consists main! y of hydrophones for the detection of acoustic
`waves, electronic modules, electrical wires and sensors apart
`from the actual framework. Seismic streamers are divided
`into sections-approximately 100 meters in length, and have
`a total length of 2,000-6,000 meters.
`A seismic tow or a seismic towing system consists of 35
`seismic streamers and seismic sources. A common feature of
`these units is that they can be positioned astern of and to the
`side of the ship's line of travel. In addition they are sub(cid:173)
`merged in the ship's line of travel. In addition they are
`submerged in the water, the gun arrays at a depth of 5-15 40
`meters below the surface and the streamers at a depth of
`5-40 meters.
`None of the above figures are absolute, and can, of course,
`vary outside those limits given here.
`Seismic towing systems are becoming progressively more
`complex, i.e. they are composed of more sources and
`streamers. Increasing demands are also being made on the
`efficiency of a towing system. The efficiency can usually be
`measured on the basis of the extent of coverage obtained by 50
`a seismic tow. The coverage will be dependent on the width
`of the tow. One of the factors to which particular importance
`is attached is the positioning of the seismic tow. The quality
`of the collected data is dependent on how accurately the
`towing system has been positioned. In this context the term 55
`positioned refers to how each unit in the towing system,
`sources and streamers is positioned in relation to the others
`in the longitudinal and width directions.
`One of the existing methods for positioning of seismic
`equipment which can be mentioned, is the use of otter-like 60
`devices with one or more foils. The otter is connected with
`a float on the surface, and by means of foils will guide a tow
`out to the side. There are several variations of the otter
`device. One known method is to have an individual foil or
`wing which can be mounted directly on to the streamer
`which has to be controlled. This type of otter device is also
`dependent on connection to a float on the surface.
`
`SUMMARY OF THE INVENTION
`
`The object of the present invention is to avoid the above(cid:173)
`mentioned problems, and to employ a positioning unit or
`vessel for gun arrays and streamers and a method for towing
`45 sources and streamers whose purpose is to achieve an
`efficient and correctly-positioned tow.
`In more precise terms one of the objects of the invention
`is to reduce the friction in a seismic tow.
`A second object is to reduce the noise which results from
`the influence of surface and wave noise on the equipment. A
`third object of the invention is to reduce wear and tear and
`damage to the towing system.
`A fourth object is to obtain the greatest possible width or
`separation of the equipment which is being towed in order
`thereby to increase the efficiency.
`A further object of the invention is to protect the equip(cid:173)
`ment from objects floating in the water which can cause
`damage to the towing equipment.
`These objects are achieved by a device, generally called
`a positioning unit or a vessel, which comprises a body part
`to which are attached at least two fairly large wings with a
`balance rudder or a large wing called a main wing. At the
`rear of the body part there is located a rudder or tail rudder.
`65 Both wings and rudder are adjustable and are thus equipped
`with control bodies. On the front part of the body there is
`provided a stabilizer rudder which can also be adjustable.
`
`7
`
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`5,532,975
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`4
`FIG. 7 illustrates a towing system according to the inven(cid:173)
`tion, where the towing of a gun array is illustrated. The
`towing system is viewed from above.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`20
`
`25
`
`3
`The interior of the positioning unit consists among other
`things of a power receiver part and battery. Furthermore
`instruments in the form of an inclinometer, a compass,
`pressure transmitters and pitot pressure transmitters are
`located in a compartment in the body part. Both these 5
`compartments, the compartment for instruments and the
`compartment where batteries are located are watertight.
`Common components for the control bodies, in this case the
`hydraulic system, are located in a compartment in the body
`part. In addition the actual body of the positioning unit can
`have one or more ballast tanks or buoyancy tanks with
`associated air supply system, together with a compressed air
`compartment. The vessel is also equipped with acoustic
`positioning equipment. This equipment consists of hydro(cid:173)
`phones and transducers which transmit and receive sound in
`such a manner that it is possible to calculate the distance
`between the units in the towing system. Electrical cables are
`provided between the ship and the positioning unit. Com(cid:173)
`munication to and from the vessel is conducted by trans(cid:173)
`mitting electrical pulses, acoustic pulses or by another
`appropriate method for the transmission of information.
`The device has a control unit for the control of informa(cid:173)
`tion and the control body.
`In each wing there can be provided buoyancy tanks with
`associated air supply system and valves.
`The objects of the invention are achieved by the employ(cid:173)
`ment of a method which uses the vessel according to the
`invention in order to position a seismic tow consisting of
`sources and streamers. By having the vessel located in the
`water astern of the ship and together with the streamers and
`sources which have to be towed, the vessel will be able to
`position the gun array or cables. Control signals are trans(cid:173)
`mitted to the vessel via a central source or a computer
`programme on board the ship, and information from the
`various instruments is transmitted from the vessel to the
`boat. On the basis of given parameters concerning depth,
`pressure, speed, separation out to the side, etc., a computer
`programme will be able to calculate which control signals
`should be transmitted to the vessel in order to guide it into
`the correct position. The calculation of the optimum position
`for the device can also be performed internally in the vessel.
`The position of the wings, tail rudder, stabilizer rudder and
`ballast tanks will ensure the correct adjustment of the
`positioning unit at all times. The ballast tanks will give the
`vessel a positive static buoyancy. This is essential in order to
`prevent the vessel from sinking in the event of something
`unforseen happening such as the towing system no longer
`moving forwards.
`
`FIG. 1 illustrates a known embodiment of a marine
`seismic survey system. The drawing shows that in this case
`a vessel is used which has a towing system or tow consisting
`10 of two streamers (2) and two sources (1), each consisting of
`3 gun arrays. An otterboard (3) with several foils produces
`the lateral force, thus causing the equipment to be pulled out
`to the side of the ship's (40) line of travel. Due to the weight
`of the towed equipment and the otterboard, a float (4) is
`15 provided which is placed on the sea surface and keeps the
`whole system at a specific depth below the surface of the sea.
`FIG. 2 illustrates the device according to the invention.
`FIG. 2a is a principal/side view of the vessel (4'). It
`comprises a body part (30), wings (7) with balance rudder
`(8) and tail rudder (6). FIG. 2b is a front view of the
`positioning unit, in which the stabilizer rudders (9) are
`provided. In FIG. 2c these rudders (9) are illustrated even
`more clearly. A view from above also provides a picture of
`the two other tail rudders (5). The cable (26) is the cable or
`connection, which the vessel (4')has with the towed equip(cid:173)
`ment or with the ship.
`FIG. 3 shows the design of the actual body of the vessel
`(4') according to the invention. The solution illustrated in the
`30 drawing is an example of how the body of the vessel is
`designed and arranged. The solution is not intended to be
`restricted to this form and layout, thus enabling vessels with
`a different solution to fall within the scope of the invention.
`In the vessel's forward and aft end there is provided
`35 acoustic positioning equipment (25). The positioning equip(cid:173)
`ment comprises transducers and hydrophones, which trans(cid:173)
`mit and receive sound. On this basis distances between the
`units in the towing equipment can be calculated. In addition
`to the positioning equipment, instruments are provided in a
`40 watertight compartment (19) for use in positioning the
`vessel (4'). The instruments involved are an inclinometer, a
`compass, static pressure transmitters, pitot pressure trans(cid:173)
`mitters and other pressure transmitters. In addition there are
`transmitters which can record angle deflection on wings and
`rudders. There is a hydraulic system which operates the
`position of the adjustable parts of the vessel (4'). Common
`components for the hydraulic system such as pumps, oil,
`control valves are located in a compartment (21). In the
`same compartment there are provided cylinders (32) for the
`50 adjustment of the tail rudders (5) and (6). A power supply is
`required for the electrical system. The power supply is partly
`transmitted from the towing cable directly to the electrical
`system, but batteries (20) are also provided in a watertight
`compartment (31). The batteries are located in the vessel (4')
`55 as a reserve and supplementary power supply. There are two
`sizeable ballast tanks or buoyancy tanks (16) in the vessel
`(4'). These will take in water as soon as the vessel (4') is
`placed in the water. If extra buoyancy is required, these tanks
`can be filled with air. The air is transported from the
`60 compressed air reserve in a compartment (17) on board the
`vessel ( 4') via air pipes to either the ballast tanks in the actual
`body part of the vessel (4') or to the ballast or buoyancy
`tanks in the wings. A valve (18) is provided between the air
`supply to the wings and the air supply to the reserve tanks
`in the body part of the vessel (4'). The wings are moved by
`two parallel cylinders (10). These are provided with the
`wing trunnion (12) illustrated in FIG. 4.
`
`45
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The positioning unit or vessel according to the invention
`is illustrated in the drawings, where
`FIG. 1 shows a marine seismic survey system,
`FIG. 2a is a principle or side view of the device,
`FIG. 2b is a front view of the device,
`FIG. 2c illustrates the vessel viewed from above,
`FIG. 3 illustrates the vessel viewed from above, and with
`an view of how the vessel looks inside,
`FIG. 4 is a side view of the vessel, and particularly
`illustrates how the wings are arranged,
`FIG. 5 illustrates a tow of a streamer, where the tow is
`viewed from above,
`FIG. 6 illustrates a tow of a gun array or a streamer
`viewed from the side according to the invention, and
`
`65
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`FIG. 4 also illustrates how each of the wings is provided
`with a buoyancy tank (11). An air pipe (13) is provided in
`order to supply air to the tanks, which are otherwise filled
`with water through open inlets (15). In order to be able to
`regulate the amount of air or water in the wing tanks, a valve 5
`(14) has been located on the outside edge of the wings. The
`tip of the wing which has to regulate the buoyancy will
`always point upwards. The air, of course, will always be at
`the top of the tank. Thus no valve is required in order to close
`off the inflow of water. The water will flow in and out of the
`holes (15), depending on how much air is blown into or
`released from the tank in the wing. The balance rudders (8)
`are regulated via the cylinders (23). The cylinders are
`located inside the wings, closest to the body. A shaft (24)
`ensures that the force is transferred to the balance rudders. 15
`The vessel (4') will be "open" to water, thus permitting the
`free flow of water in and out of the vessel, apart from the two
`watertight compartments which are designed for battery or
`power supply, and the compartment in which the instruments
`are located. The advantage of having a through-flow of 20
`water in the vessel is that the vessel thereby becomes heavy
`in water and light in air, a feature which is advantageous
`when the vessel has to be deployed into the water and
`recovered on to the ship's (40) deck.
`Hydraulic control is used for control of the movable parts 25
`of the vessel (4'), wings and rudders. The pump which
`generates the oil pressure is electrically operated. A com(cid:173)
`pletely electrically operated control system can be used. The
`power supply is by means of electrical cables from the ship
`(40). This is already a tried and tested technique. Batteries 30
`are located in the vessel (4') in order to cover an unexpected
`power requirement for a limited period. The batteries will be
`charged from the power supply from the ship (40) to the
`vessel ( 4').
`Even though the vessel (4') will be steered mainly from
`the ship's (40) central control system, the vessel (4') itself
`will be able to adjust its course by means of its own control
`loops, on the basis of information from angle indicators,
`pressure transmitters, speed transmitters, depth gauges, posi(cid:173)
`tioning instruments and other instruments installed in the
`vessel (4') in order to provide as accurate positioning as
`possible.
`A method for positioning of the vessel (4') will be either
`by directly mounting it on gun arrays or streamers, or via a
`float which has slightly less buoyancy than the weight of that
`which is to be towed from the float. It will therefore be
`submerged in the water, but still take most of the weight
`from the towed equipment. An important purpose of this
`float is that it prevents vibrations and noise from the towing
`cables from propagating further to the streamers or gun
`arrays. The reason for this is that the underwater float will
`provide a change in impedance, thus causing the vibrations
`to be reflected instead of being passed on to the towing
`equipment.
`FIGS. 5 and 6 are a drawing of a tow viewed from above
`in FIG. 5 and from the side in FIG. 6. The figures illustrate
`how one possible application of the positioning unit is
`envisaged. In this case an underwater float (27) is used as a
`junction for the towing cable to the ship (40), the towing 60
`cable to the actual hydrophone cable, and the towing point
`to the vessel (4') which pulls the whole system out to the
`side.
`FIG. 7 is an example of a tow viewed from above, where
`an underwater float is not used, but the vessel is connected 65
`directly to the equipment which has to be towed and pulled
`out to the side.
`
`6
`The figures and their descriptions illustrate only one
`embodiment of the invention. There will be many possible
`modifications and variations within the scope of the inven(cid:173)
`tion and the patent claims.
`I claim:
`1. A device for positioning of seismic equipment in a
`desired underwater position when towed by a seismic vessel,
`said device including a mechanism for placement underwa(cid:173)
`ter and for maintaining said desired underwater position,
`said mechanism comprising:
`a body part equipped with wings and rudders,
`a control body for controlling the wings and rudders so as
`to achieve said desired underwater position,
`instruments for use in positioning the device in said
`desired underwater position,
`a communication system for communicating between the
`seismic vessel and the device,
`a control unit for processing signals which operate on the
`basis of information from at least one of two locations,
`said two locations consisting of the seismic vessel and
`said instruments
`an attachment device for at least one cable and at least one
`float, and
`a power supply system.
`2. The device according to claim 1, wherein said device
`is perforated to enable water to flow in and out of the device,
`and
`wherein said device further comprises watertight com(cid:173)
`partments, which are not perforated, for holding said
`instruments.
`3. The device according to claim 1, wherein said wings
`·have at least one opening for the flow of water, said at least
`one opening being located immediately adjacent to said
`35 body part.
`4. The device according to claim 1, wherein said wings
`are located on a front part of the device.
`5. The device according to claim 1, and further compris(cid:173)
`ing buoyancy tanks with an associated air supply system and
`40 valves, including a source of compressed air in the device
`for supplying air to said buoyancy tanks via a pipe system
`of said associated air supply system.
`6. The device according to claim 1, characterized in that
`at least one of said wings has a balance rudder.
`7. The device according to claim 1, wherein at least one
`of said wing has at least one buoyancy tank with a valve
`located in an outer edge of said at least one of said wings.
`8. The device according to claim 1, wherein the control
`unit governs the control body and transmits and receives
`information between the seismic vessel and the device.
`9. A method for positioning of seismic equipment in a
`desired underwater position when towed by a seismic vessel,
`said method comprising the steps of:
`attaching a positioning device to a front part of a towing
`cable, said positioning device including a mechanism
`for placement underwater and for maintaining said
`desired underwater position, said mechanism compris(cid:173)
`ing:
`a body part equipped with wings and rudders,
`a control body for controlling the wings and rudders so
`as to achieve said desired underwater position,
`instruments for use in positioning the device in said
`desired underwater position,
`a communication system for communicating between
`the seismic vessel and the device,
`a control unit for processing signals which operate on
`the basis of information from at least one of two
`
`50
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`locations, said two locations consisting of the seis(cid:173)
`mic vessel and said instruments,
`an attachment device for at least one cable and at least
`one float, and
`a power supply system;
`placing said positioning device underwater; and
`controlling said positioning device using control signals
`which are calculated in at least one of said two loca(cid:173)
`tions, said controlling of said positioning device being
`performed so that said positioning device achieves the
`desired underwater position.
`10. The method according to claim 9, and further com(cid:173)
`prising the step of attaching said device directly to towing
`equipment.
`11. The device according to claim 1, wherein said control 15
`body is hydraulic.
`12. The device according to claim 1, wherein said control
`body is electrical.
`13. The device according to claim 1, wherein said com-
`munication system is electrical.
`14. The device according to claim 1, wherein said com(cid:173)
`munication system is acoustic.
`15. The device according to claim 1, wherein said com(cid:173)
`munication system is optical.
`16. The device according to claim 1, wherein said at least 25
`one cable and said at least one float are provided at a front
`portion of the device.
`
`8
`17. The device according to claim 1, wherein said at least
`one cable and said at least one float are provided in the
`vicinity of points of said body part where said wings are
`attached.
`18. The device according to claim 1, wherein at least one
`of said rudders is arranged in a vertical plane and at least one
`of said rudders is arranged in a horizontal plane.
`19. The device according to claim 1, wherein said rudders
`10 are located at at least one of two longitudinal end portions
`of said body part.
`20. The device according to claim 1, wherein at least one
`of said wings has a balance rudder in an outer edge of said
`at least one wing.
`21. The method according to claim 9, and further com(cid:173)
`prising the step of attaching said device to towing equipment
`via a junction.
`22. The method according to claim 21, wherein said
`junction is an underwater float.
`23. The method according to claim 9, and further com(cid:173)
`prising the step of attaching said device to a surface float.
`24. The method according to claim 9, and further com(cid:173)
`prising the step of attaching said device to an underwater
`float.
`
`20
`
`* * * * *
`
`10
`
`