Abstract:
Release device for a cable used during seismic surveys at sea and towed through the water behind a vessel so that in the event of the occurrence of obstacles or other occasions when it is desirable to release the cable (1) from the towing vessel, a controlled release of the cable is possible so that it is not severed, but can be recovered for further use. At one or more locations in such cables release sections (2) are inserted which contain sensors which sense loading in the section and in the event of overloading or other occasions when release is desirable, an element (5) is activated which cause a release mechanism (6) to be activated and thereby causes a split in the cable (1) at a predetermined location.

Description:
The invention relates to a method and an apparatus for release of an array from a towing vessel in the event of overloading or obstacles which affect the array or other occasions when it is desirable to release the array, for use especially in seismic surveys at sea and particularly for use in connection with seismic receiver cables such as streamers. 
     BACKGROUND OF THE INVENTION 
     During seismic surveys at sea long arrays are towed behind vessels through the water, comprising both energy sources, i.e. transmitters for seismic signals, as well as arrays consisting of the seismic recording equipment, which is placed in so-called streamer cables. Arrays of both this type and other types of array towed through the water can become overloaded from time to time, obstacles can get in the way of the array or quite simply there can be an urgent need to release the array for one reason or another. In such situations there is a risk that the array can be lost, in that e.g. it can take in water and sink. There is a special problem in connection with seismic arrays and particularly arrays of streamer cables, where electronic conductors and other equipment wiring are threaded through the whole cable up to the towing vessel. If such cables are severed due to overloading because an obstacle gets in the way, extremely valuable equipment can be lost, because, amongst other reasons, such cables are heavier than water and will therefore sink. 
     Several methods have therefore been developed in order to recover such severed streamer cables and the related towed equipment. Examples of recognized techniques can be found, e.g., in U.S. Pat. Nos. 4,541,079 and 4,823,325.In these recognized devices the principle has been applied that when a cable is severed, a supply of compressed gas is automatically released which will inflate the cable thus enabling it to float up to the surface and be recovered. This kind of activation of a gas supply can be released by radio signals and the cable can also be equipped with radio beacons, so that it can be recovered. 
     Such devices, however, constitute a complication of the cable and mean that extra equipment has to be installed which can complicate the construction of an already complicated cable. Furthermore, it has the significant disadvantage that the cable itself will be destroyed, thus preventing it from being used again. 
     BRIEF SUMMARY OF THE INVENTION 
     Thus the basic object of the present invention is to provide a releasing device together with a method for release particularly for a towed cable, where in the event of overloading or when obstacles are encountered, a release of the cable will be initiated at a predetermined location, in such a way that a deliberate division of the cable is performed without tearing it apart. 
     A further object of the invention is to provide a method and an apparatus for releasing a cable in a simple way while it is in the water, either in order to avoid danger or because it is desirable, e.g., to transfer the cable to another towing vessel or transfer it to another location. 
     These objects are achieved by a method and an apparatus which are characterized by the features described below. 
     According to the invention, there are inserted in the cable, preferably at regular intervals, so-called release sections, which either automatically or manually, in the event of an overload or when required, can be activated in such a manner that the rear section of the cable is disengaged and remains lying in the water. Since this release is performed at a defined location, it is also possible to equip the cable with buoyancy chambers or with simple buoyancy devices, chambers which will not be destroyed if the cable is severed, thus ensuring that the cable remains afloat and can be recovered. 
     According to the invention there are various elements which can be used to trigger the release. The most likely triggering factor is for the release mechanism to be activated due to overloading or stretch in the cable. Further triggering factors can be emergency situations, the appearance of an obstacle in the way of the equipment, or the necessity of moving the equipment or the cable for purely practical reasons. It is thus a simple matter to disengage the release section and avoid the imminent problems. The third triggering factor can be damage which occurs, e.g. by an obstacle. This could cause stretching leading to release of the cable or an obstacle which is entangled in the cable and which can be seen will be rendered less dangerous by releasing the cable so that the obstacle is no longer in danger of destroying the cable. A fourth triggering factor can be depth considerations. An array will also often be towed at a certain level under the surface in order to maintain a certain depth. For this purpose the towing cable is equipped with a head section, a so-called &#34;lead in&#34; which is a heavy object which can also be equipped with controls to regulate its operating depth. If a fault should occur in this, so that the head section steers the cable down to the bottom, problems can be avoided by activating the release mechanism, thus enabling the streamer cable to remain on the surface. 
     With regard to the actual construction of the apparatus, many possibilities can be envisaged within the scope of the invention. A brief list of alternatives could include a solution with loose screwing, release of a hook, use of magnetic forces and application of a knife, scissors or saw arrangement, e.g. with rotating edges. All these mechanisms can be released both hydraulically, pneumatically and electrically. 
     One of the principal advantages of the invention is also that the release point is situated in the actual cable, i.e. the release is performed within the array itself, either automatically or by remote control from the towing vessel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be now described in more detail with reference to the embodiments illustrated in the accompanying drawings wherein: 
     FIG. 1 is a diagrammatic representation of an embodiment where the release is performed by means of a screw which is driven by a motor; 
     FIG. 2 is a schematic cross-sectional view of another embodiment of a release mechanism of the invention; 
     FIG. 3 is an outline of the gears taken along line III--III of FIG. 2; 
     FIG. 4 is a schematic view of a further embodiment, showing a section of a release mechanism; and 
     FIG. 5 is an enlarged partial slide view of the embodiment of FIG. 4. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a purely diagrammatic illustration of how a release device in accordance with a first embodiment of the invention can be constructed. Here only the general principle of the device is illustrated, in that the actual technical implementation will be carried out by the craftsman. In a streamer cable 1 which is towed behind a vessel a release section has been inserted which is given the general designation 2. This is connected in the usual manner in order to establish the transmission paths for electronic signals to and from the recording equipment and is also equipped with fluid passages of a known type. These connections are illustrated schematically with reference numbers 3. The actual section 2 in the embodiment of FIG. 1 is equipped with an inner screw 6. The housing for section 2 is made watertight at a severable joint 4. Parts 2a and 2b are kept together by screw 6 which passes through retaining fixtures such as plate members 20, 21 and 22 arranged in the section. In a situation where release is required, a motor 5 is activated which is installed in a waterproof chamber in section 2 and which will turn the screw 6 to the left, so that it is screwed out of part 2b. Thereby parts 2a and 2b are severed and all wiring within this section for the aforesaid transmission paths for electronic signals and aforesaid fluid passages within the section will thus also be severed, but this will be the only damage to occur. The portion of streamer which is thereby released may have buoyancy chambers which are either filled by the activity which is initiated or already contain buoyancy material to enable the streamer section to be recovered. An advantageous solution in this connection is to place this buoyancy medium in the release section 2 which can be, e.g., 2 m long. With this length of buoyancy section it will be possible to place the buoyancy medium and also the emergency beacon in the section, so that these will accompany the released cable. 
     The motor 5 can be an electrically-operated motor, but it can also use a hydraulic system for activating the screw. 
     To activate the release, a sensor, e.g., can be placed in the vicinity of the release section, which via a control system can either issue a warning that a release should be considered, or if a certain value is exceeded it can itself activate release. 
     After the cable has been released, another boat, e.g., can bring out a new, ready-calibrated streamer cable to the acquisition vessel, so that a minimum of operational time is lost during the actual seismic survey, in that the cable which has been lost can be quickly replaced while a reserve boat carries out the rescue of the cable. 
     FIG. 2 illustrates another embodiment of the invention. The streamer cable itself is not shown in the drawing, in that this and also the next embodiment are based on a release or division of those stretch cables 8 which are threaded through a streamer cable. Normally there are three such cables, as illustrated in the figures, but a different number could also be envisaged, without affecting the release principle. In the figures, therefore, only these sections of the cable are illustrated. 
     The streamer cable&#39;s stretch cables on the two sides of the release mechanism are designated 8a and 8b respectively. These sections are connected by a shaft 14, on which are fitted, e.g. welded, cogwheels which are designated 9. In gear meshing relationship with these cogwheels 9 in the center of the cable is placed a further cogwheel 13, as best illustrated in FIG. 3. This cogwheel is mounted on the shaft from a motor 5, which serves to operate the release mechanism. The shafts 14 are permanently fitted in the stretch cables 8b on one side and screwed into the ends of the stretch cables 8a on the opposite side of the cogwheels. Between the cogwheels or in grooves in these, lock pins 7 can be inserted which prevent rotation of the cogwheels, as illustrated in FIG. 2. These lock pins are connected to a cylinder 15, which can be connected by a pulling mechanism to the outer cable section. 
     As an alternative solution, (not shown), instead of affecting the cogwheels, the lock pins can be constructed so that they block a rotation of the shafts 14. 
     When the release mechanism is activated, the cylinder 15 is pulled outwards, e.g. under the influence of tensile forces or by other means under the influence of a sensor or a release mechanism. This releases the cogwheel mechanism for rotation and at the same time will by recognized means deliver an impulse to the motor 5 causing this to start and turn the cogwheels in such a direction that the shafts 14 are screwed out of the stretch cable sections 8a, so that the two cable sections 8a and 8b are released from each other. Simultaneously with this process, the creation of, e.g., buoyancy gas can be effected in isolated portions of the streamer cable on both sides of the fracture, so that the ends of the cable are kept afloat. 
     In a simplified version of this solution the lock pins 7 can be omitted, so that when it is not in action, it is only the motor which blocks the mechanism. 
     FIGS. 4 and 5 show another variation, which is also based on division of the stretch cables in the streamer cable. In this example, as illustrated in FIG. 5, the two stretch cable sections 8a and 8b are connected by means of an insertable pin part 12 on section 8b, which is pushed into a corresponding hole at the end of section 8a. The two sections are kept together by means of a lock pin 7, as schematically illustrated in the two figures. In this connection is should be mentioned that this lock pin or lock bolt 7 is illustrated in an exaggeratedly conical shape. It is advantageous for pin 7 to be slightly tapered in order to facilitate release. 
     The lock pin 7 is fitted on to a shaft part with threads at the end, which is screwed into a lock housing or actuator 10, on the end of which is formed a toothed rim part 10a. This toothed rim part 10a is in gear meshing relationship with a cogwheel rim 9 which can be made to rotate by means of a motor (not shown), in a similar way to that described in the previous embodiment in FIGS. 2 and 3. 
     When the release mechanism is activated, the motor will make the cogwheel 9 rotate, and this again will rotate the lock housing 10 via the toothed rim 10a. Thereby the shaft part of lock pin 7 will be screwed into the housing and pull the pin out, thus releasing part 12, and thus releasing stretch wire parts 8a and 8b from each other. 
     The above illustrates a screw mechanism and two different solutions, where cogwheel mechanisms are used. It should be obvious that other mechanical solutions can be modelled on similar principles within the scope of the invention. Such solutions are not described here in detail, but should be able to be implemented and designed by a craftsman. As an example of how such mechanisms can be implemented, mention can be made of a releasable hook, in that two parts of a streamer cable can be joined together by means of a hook, which by means of a pulse or motor drive can be opened for release. 
     Another alternative is the use of magnetic gripping mechanisms, which make an electrical triggering or release possible, in that, e.g., a release pulse can activate a spool which provides a magnetic counterfield which weakens the binding magnetic forces to such an extent that the parts can be pulled apart. Other conceivable possibilities are that a motor operates a knife, scissors or saw device which when activated will cut through retaining or binding wires in the release section. It should therefore be obvious that many modifications will be possible within the scope of the invention.