Abstract:
A rotation unit for a torque tong for making and/or breaking threaded connections between pipes and/or spinning pipes during screwing and/or unscrewing of pipes, primarily pipes used in petroleum production. The unit includes a fixed part and a rotary part arranged to grip a pipe to be rotated. The rotary part includes at least one movable gripping jaw arranged to be moved into engagement with the pipe. The fixed part includes at least one gripping cylinder arranged to move the gripping jaw to engage the pipe when the gripping jaw is operatively engaged with the gripping cylinder.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention regards a rotation unit for a torque tong for making and/or breaking threaded connections between pipes and/or spinning pipes during screwing and/or unscrewing of pipes, primarily pipes used in petroleum production. 
   2. Description of Background Art 
   A prior art torque tong is described in NO 163973, which concerns a torque tong arranged both to break and make a threaded connection between two pipes, and also spin one of the pipes relative to the other in order to uncouple the pipes from each other or tighten the connection. 
   In older solutions, a special device was used to make and break the connection, while another special device was used to spin the pipes apart or together. The solution of NO 163973 allowed both making/breaking and spinning to be carried out in the same apparatus. 
   The solution of NO 163973 also entailed the advantage of being able to handle pipes within a wide range of diameters. 
   In order to achieve this, NO 163973 proposes the use of one or more master cylinders which upon rotation of the rotary part of the tong, and as a result of the placement of the cylinders, are pressed together, applying pressure to a number of slave cylinders. The slave cylinders will in turn displace jaws to engage one of the pipes involved, ensuring that these maintain a sufficiently powerful grip on the pipe to break or make the connection to a prescribed torque without the jaws slipping relative to the pipe. 
   A solution similar to that of NO 163973 has been described in NO 306572. Here the jaws are also equipped with respective slave cylinders. These are pressurized by a master cylinder mounted on the rotary part, which master cylinder is then influenced by a piston mounted outside the rotary part. The jaws are brought into engagement with the pipe by increasing the pressure from the master cylinder Valves ensure that the pressure in the slave cylinders is maintained independently of the master cylinder. 
   A considerable disadvantage of the latter of the above solutions is that the jaw travel is restricted by the displacement of the master cylinder. In a subsequent patent application (WO 00/45027) from the same applicant as NO 306572, it is stated that in the solution of the latter patent, the piston must push the master cylinder repeatedly in order to provide a sufficient volume of hydraulic fluid to push the jaws into engagement and also achieve sufficient retaining power. This causes a significant delay in the operation. In WO 00/45027, this problem is apparently solved by means of pressure accumulators. 
   SUMMARY OF THE INVENTION 
   However, the present invention provides a far simpler solution to this problem. This solution is obtained through a fixed part and a rotary part designed to grip a pipe to be rotated, which rotary part comprises at least one movable gripping jaw arranged to be moved into engagement with the pipe wherein the fixed part comprises at least one gripping cylinder arranged to move the gripping jaw into engagement with the pipe, when the gripping jaw is operatively engaged with the gripping cylinder. 
   By the rotary part comprising at least one holding cylinder arranged to maintain the gripping jaw in engagement with the pipe after having been moved to engage the pipe, the gripping jaw will be certain to maintain engagement with the pipe during rotation of the rotary part. 
   Connecting the holding cylinder to a valve arranged to selectively prevent hydraulic fluid from flowing out of the holding cylinder and allow hydraulic fluid to flow out of the holding cylinder, allows selective retaining and releasing of the pipe. 
   Controlling the valve by means of an actuator disposed on the fixed part and designed to control the valve independently of the position of the rotary part, ensures that the pipe may be released independently of the position of the rotary part. 
   The holding cylinder being hydraulically connected to at least one slave cylinder on the rotary part, and a master cylinder on the fixed part being arranged to actuate the slave cylinder upon operative engagement between the master cylinder and the slave cylinder, facilitates expedient pressurizing of the holding cylinder. 
   Connecting the holding cylinder to an accumulator, which is arranged to provide hydraulic pressure for disengaging the holding cylinder from the pipe, ensures expedient retraction of the gripping jaw. 
   By equipping the holding cylinder with a return spring designed to disengage the holding cylinder from the pipe, it is possible to achieve expedient retraction of the gripping jaw. 
   Connecting the holding cylinder to a closed hydraulic system on the rotary part allows a simple hydraulic system to be achieved, which requires little maintenance and is not subjected to any significant external influences. 
   The hydraulic system comprises an accumulator designed to provide pressure in order to return the slave cylinder so as to allow hydraulic fluid to flow from the holding cylinder to the slave cylinder, thus achieving expedient retraction of the gripping jaw. 
   The gripping cylinder acts on a protrusion on the gripping jaw, achieving expedient cooperation between the gripping cylinder and the holding cylinder. 
   The holding cylinder is disposed inside the gripping jaw or a support for this, possibly integrated into this, thus achieving a compact solution. 
   The rotation unit is equipped with from one to six gripping jaws, preferably-three gripping jaws, thus achieving a good grip on the pipe, also in the event of varying dimensions. 
   The gripping cylinder acts on an arm, which in turn is connected to a tappet that is arranged to exert a force against the holding cylinder when the tappet is rotated from a first to a second position, thus achieving an alternative embodiment, in which the hydraulic system on the rotary part is not dependent on any other pressurizing than that provided by the gripping cylinder. 
   It is practical for the rotary part to be driven by chain drive. A chain drive ensures a more robust design and smoother running. Smoother running reduces the risk of “bite marks” from the jaws on the pipe. The chain will engage the rotary part across a significantly longer area than a cogwheel. This will reduce the loading on each tooth on the rotary part, and compared with direct engagement between a cogwheel and the rotary part, the loading on the chain will be more even. Moreover, the chain will be able to engage the rotary part over a section large enough to ensure that even if the rotary part does not have teeth around its entire periphery (e.g. due to an opening for introduction of pipes), the chain will be in engagement with the rotary part at all times. This would not be the case in the event of a direct engagement with cogwheels, where the cogwheels would engage and disengage the rotary part at every rotation. This increases the strain and the risk of damage to both cogwheels and teeth on the rotary part. 
   In the case of direct engagement with a cogwheel, the component most exposed to wear will be precisely the cogwheel. In the case of chain drive, it will be the chain. It is easier to replace a worn or damaged chain than a cogwheel, as a cogwheel inevitably of necessity would have to be securely fixed to the shaft, while the chain is arranged more or less loosely around the cogwheels. In addition, the teeth on the rotary part may be arranged so as to be replaceable, allowing easy replacement of worn or damaged teeth. The tong will be usable even with missing teeth, as the chain will be in engagement with other teeth. Drive systems incorporating a chain will not be as sensitive to dirt as drive systems based on e.g. direct gearing. The noise generated by the system will also be less. 
   Furthermore, the costs of producing such a system could also be lower. 
   Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail by means of an example of an embodiment shown in the accompanying drawings, in which: 
       FIG. 1  shows a rotary torque tong according to the present invention; 
       FIG. 2  shows the rotation unit of the torque tong according to the invention; 
       FIG. 3  mainly shows the rotary part of the rotation unit; 
       FIG. 4  is a sectional view of the rotation unit; 
       FIG. 5  shows a hydraulic connection diagram of the most important components that bring about the gripping of the pipe; 
       FIG. 6  shows an alternative hydraulic connection; 
       FIG. 7  shows alternative gripping and holding means, with
           FIG. 7   a  showing a jaw fully retracted from the pipe;     FIG. 7   b  showing the jaw about to be pushed into engagement with the pipe; and     FIG. 7   c  showing the jaw fully engaged with the pipe; and       
       FIG. 8  illustrates a principle for distribution of teeth on the rotary part. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows the rotary torque tong according to the present invention. The tong has a frame  60  generally consisting of a horizontal part  61  and a vertical part  62 . The frame  60  may be mounted on a guide rail (not shown) to allow it to be displaced horizontally on a drill floor for the tong to engage or disengage a pipe  70  (shown in  FIG. 4 ). 
   On the vertical part  62  of the frame  60  there is disposed, as the lowermost component, a holding unit (back-up)  63 . This comprises gripping jaws  64  arranged to grip a pipe below a pipe joint (not shown) in order to hold this. The construction of the holding unit is, in principle, conventional and will be understood by a person skilled in the art. Thus this will not be explained in any detail herein. 
   Above the holding unit  63  there is a rotation unit  65  arranged to grip a pipe above a pipe joint. The rotation unit  65  will be explained in detail in the following. Above the rotation unit  65  there is disposed a spin unit  66 . This unit is arranged to spin the pipe above the pipe joint out of threaded engagement with a pipe below the pipe joint, or spin the pipe into threaded engagement with the pipe below the pipe joint. The spin unit has a lighter construction than the rotation unit  65  and operates at a significantly lower torque than the rotation unit. Thus it is not capable of breaking or making a pipe joint. The spin unit  65  may however rotate pipes at a considerably higher speed than the rotation unit  65 . 
     FIG. 2  shows the rotation unit  65  of the tong according to the invention. It comprises a rotary part  40  and a fixed part  41 . The rotary part  40  is mounted on a plate  42  attached to the fixed part via bolts  54  and brackets  55 . The plate  42  has an opening  49 . The rotary part is generally disk-shaped with a central cavity  44  and an opening  45  extending from the cavity  44  to the periphery of the disk  40 . Toothing  43  is provided around the periphery of the rotary part  40 . This toothing may consist of single teeth fixed, e.g. screwed, to the disk  40 . The toothing  43  engages two chains  46 ,  47 , each of which extends across two cogwheels  48 ,  50 . One of the cogwheels  50  is power-coupled to a motor  51 , preferably a hydraulic motor. Alternatively, one chain may be used, which extends across a sector of a circle greater than either of the chains  46 ,  47 . When one chain  46 ,  47  passes over the opening  45 , it is important for the chain to land on the first tooth after the opening as accurately as possible, to avoid wear on the tooth and chain to the greatest possible extent, and to avoid jerky movements. Consequently, the distance over which the chain extends between the teeth on either side of the opening  45  is matched so as to be equivalent to a whole number of teeth. It has been found that this may be achieved by satisfying the following two equations: 
                     t   2     ·     1     sin   ⁢           ⁢     (         2   ·   π     -   α       2   ·     N   2         )           =   0           (   1   )                     2   ·   α     ⁢           ⁢   sin   ⁢           ⁢     (         N   1     ·   t       2   ·   r       )       -   α     =   0           (   2   )               
in which:
     t is the chain pitch, in mm,   N 1  is the number of teeth that will fit over the opening  45 , between the two teeth nearest the opening,   N 2  is the number of teeth along the curved section of the rotary part  40 ,   α is the angle (in radians) between the teeth nearest the opening, and   r is the radius of the rotary part  40  at the chain, i.e. the distance from the centre of the rotary part  40  to the centre of the chain rollers.   
   In  FIG. 8 , the relationship defined above through equations (1) and (2) has been illustrated by an example of an embodiment. The figure shows a schematic plan view of the rotary part  40 . Also shown is one chain  46  extending across the two cogwheels  48 ,  50 . A number of teeth  43  are shown around the periphery of the rotary part  40 . In the example shown, it has been decided that there should be room for 67 teeth along the curved section of the rotary part  40 . However, there is no requirement for such a high density of teeth, and so only every third tooth has been installed, except on either side of the opening  45 , where two teeth have been placed close to each other in order to provide greater strength at this location, and diametrically opposite of the opening, where three teeth in a row are missing, in order to achieve symmetry. Using a smaller number of teeth than the maximum possible allows a reduction in costs and makes it easier to mount the teeth. 
   The rectilinear distance L r  between the two teeth  43   a  and  43   b  closest to the opening  45  on either side of this, is shorter than the curved distance L b , that follows the curve of the rotary part  40 . If the chain had followed the curved distance L b  the positioning of the teeth would be given unequivocally by the total number of teeth and the radius r of the rotary part at the chain. The chain will however follow the rectilinear distance L r . Consequently, this distance L r  must provide room for a whole number of teeth. In the example shown, it has been decided that there should be room for 8 teeth along the rectilinear distance L r  between the two teeth  43   a  and  43   b.    
   Also, the chain has been chosen to have a pitch, i.e. a distance t between the centres of each of the chain&#39;s  46  rollers, of 76.2 mm. 
   Inserting these figures into the equations (1) and (2) will make it possible to calculate the angle α and the radius r. This gives the radius as 911.7119 mm and the angle α as 0.68176 rad, which is equivalent to 39.06°. If the stretching of the chain  46  between the teeth  43   a  and  43   b  had not been taken into account, the chain would have missed the tooth by 12 mm. This would have resulted in a great strain on this tooth and jerky movements. 
   The above way of spacing the teeth on a rotary part, and the condition of equations (1) and (2), may also be used in other contexts than that which has been described, where for various reasons, one may wish to have access to an area inside the toothing of the rotary part. 
   The fixed part  41  comprises a frame  52  that supports the plate  42 , the cogwheels  48 ,  50  and the motors  51 . The frame  52  is mounted so as to float in a joint  53 . Through this mounting, the rotation unit  65  can automatically orient itself relative to the pipe to be gripped. 
   The fixed part  41  has gripping cylinders  4 ,  5 ,  6  mounted on it. These use their piston rod to push against a protrusion  1   c ,  2   c ,  3   c  on each of three gripping jaws  1 ,  2 ,  3 . However, the piston rod is not attached to the protrusion. The holding cylinders  1   a ,  2   a ,  3   a  are located inside the gripping jaws  1 ,  2 ,  3  and so are not visible in  FIG. 2 , but one of them may be seen in  FIG. 4 . Three displaceable gripping jaws may be used, as shown, but it is also possible to use more or fewer gripping jaws. When using fewer gripping jaws, one or more fixed gripping jaws may also be used, which are rigidly mounted to the rotary part. This will depend on how much of the pipe dimension the tong is to be used on. 
   When the rotary part is to be rotated, the motors  51  are actuated, causing the chains  46 ,  47  to move in the same direction. Thus the chains  46 ,  47  rotate the rotary part  40 , which slides on slide bearings (not shown) on the plate  42 . 
   In  FIG. 3  the fixed part of the rotation unit has been removed. Thus in this figure, two slave cylinders  18  and two master cylinders  19  become visible. Preferably, these are positioned so as to act against each other and synchronously, so that the master cylinder  19  does not contribute to the rotation of the rotary part  40 . 
   The rotation unit  65  is equipped with sensors (not shown) to detect the position of the rotary part  40 , to allow the rotary part to be carefully positioned with the opening  45  in line with the opening  49 , so that the tong may be pushed onto pipes to be screwed by guiding the openings  45 ,  49  onto the pipe. The jaws  1  and  3  closest to the opening  45  have been retracted to make room for the pipe to pass. Therefore these jaws  1  and  3  must be moved over a greater distance than jaw  2  before engaging the pipe. 
   Description will now be given of a relief mechanism for the holding cylinders. This comprises two plates  57  and  58  which, apart from an opening  45   a  and  45   b , are annular. The lower plate  58  lies on the rotary part  40  and is operationally connected to three relief valves  10   b ,  11   b ,  12   b  (see  FIG. 5 ). The upper plate  57  is connected to the fixed part  41  via actuators  56 . The valves  10   b ,  11   b ,  12 , which relieve the pressure from the holding cylinders  1   a ,  2   a ,  3   a  (see  FIG. 5 ), are operated by actuating the actuators  56 . The upper plate  57  is forced down against the lower plate  58 , which in turn displaces the valves  10   b ,  11   b ,  12   b  from a first position to a second position. The upper plate  57  will be able to force the lower plate down regardless of the position of the rotary part  40  relative to the fixed part  41 . 
     FIG. 4  is a sectional view of part of the rotation unit showing, among other things, one of the motors  51 , one of the chains  46 , the rotary part  40 , the plate  42 , one of the gripping cylinders  5 , which pushes against the protrusion  2   c  with its piston rod, and one of the gripping jaws  2 . One of the holding cylinders  1   a  may be seen inside the gripping jaw  2 . Also illustrated is a pipe  70 , which has just been gripped by the gripping jaw  2  after the gripping cylinder  5  has advanced this towards the pipe  70 . 
     FIG. 5  shows a possible example of an embodiment of the hydraulic connection for the gripping function of the rotation unit, and also shows a connection for the rotational function. In the figure, components located on the rotary part  40  of the rotation unit  65  are drawn within a line  30 . Components outside this are located on the fixed part  41 . 
   On the rotary part  40  are jaws  1 ,  2 ,  3 , which are designed to grip and hold a pipe  70 , as described above. 
   The jaws  1 ,  2 ,  3  are connected to the respective holding cylinder  1   a ,  2   a ,  3   a . The piston sides of the cylinders  1   a ,  2   a ,  3   a  are connected to respective valve assemblies  10 ,  11 ,  12  via respective connecting lines  1   b ,  2   b ,  3   b . The valve assemblies  10 ,  11 ,  12  comprise a check valve  10   a ,  11   a ,  12   a , that opens for hydraulic communication with the respective holding cylinder  1   a ,  2   a ,  3   a  when the hydraulic fluid is at a certain pressure and stops communication in the opposite direction, and the two-way relief valve  10   b ,  11   b ,  12   b , which is mentioned in connection with  FIG. 3 , and which in a first position provides communication with the piston side of the respective holding cylinder  1   a ,  2   a ,  3   a  and stops communication in the opposite direction, and in a second position opens for communication both ways. 
   The respective check valve  10   a ,  11   a ,  12   a  communicates with the piston side of a slave cylinder  18  via a respective line  10   c ,  11   c ,  12   c . Preferably, three mechanically connected slave cylinders  18  are provided, but only one is shown in  FIG. 5 . The respective two-way valve  10   b ,  11   b ,  12   b  also communicates with the piston side of the slave cylinder  18 , via a respective line  10   d ,  11   d ,  12   d  and a common check valve  20 , which opens for hydraulic communication with the slave cylinder  18  at a certain hydraulic pressure and stops communication in the opposite direction. The lines  10   d ,  11   d ,  12   d  also communicate with a common hydraulic reservoir  16 . 
   The two-way valves  10   b ,  11   b ,  12   b  are operated by a relief actuator  56  that acts on the valves  10   b ,  11   b ,  12   b  via a first plate  57  on the fixed part and a second plate  58  on the rotary part. As shown in  FIG. 3 , there are preferably at least three relief actuators  56 . 
   The rod side of the slave cylinder  18  communicates with the piston side of the same cylinder  18  via a valve  21 . The valve  21  comprises a check valve  21   a , which opens for communication from the piston side to the rod side and stops communication in the opposite direction, and a choke  21   b  that allows limited hydraulic communication from the rod side to the piston side. The slave cylinder is equipped with a return spring  18   a  that acts to push the piston  18   b  towards the rod side. 
   The rod sides of the holding cylinders  1   a ,  2   a ,  3   a  communicate with respective valves  13 ,  14 ,  15 . Each valve  13 ,  14 ,  15  comprises a check valve  13   a ,  14   a ,  15   a  that opens for communication from the piston side of the respective holding cylinder  1   a ,  2   a ,  3   a  and stops communication in the opposite direction, and a choke  13   b ,  14   b ,  15   b  that allows limited hydraulic communication with the rod side. The valves  13 ,  14 ,  15  further communicate with a common accumulator  17 . 
   On the fixed part  41  is a hydraulic cylinder  19 , which in the following is denoted a master cylinder  19 . The master cylinder will, upon actuation and when the slave cylinder  18  is in the correct position for this, use its piston rod  19   a  to push against the piston rod  18   c  of the slave cylinder  18 . 
   When the rotary part  40  is located in such a position as to leave the master cylinder  19  and the slave cylinder  18  facing each other operationally, a respective gripping cylinder  4 ,  5 ,  6  will also be located operationally straight opposite the protrusion  1   c ,  2   c ,  3   c  (not shown in  FIG. 5 ) on a respective jaw  1 ,  2 ,  3 . The three gripping cylinders  4 ,  5 ,  6  will, upon actuation in this position, move the jaws  1 ,  2 ,  3  to engage the pipe. 
   On the piston side, the gripping cylinders  4 ,  5 ,  6  are hydraulically connected to a respective slave cylinder  31 ,  32 ,  33 . The pipe  70  is closer to the gripping jaw  6 . The slave cylinders  31 ,  32 ,  33  are actuated via a synchronizing element  36  of a synchronizing cylinder  34 , which is connected to a pump (not shown) via a load holding valve assembly  35 . The cylinder  32  is shorter than cylinders  31  and  33 , as the gripping cylinder  5  will displace its gripping jaw  2  over a shorter distance to engage the pipe, as explained in connection with  FIG. 3 . 
   The piston sides of the gripping cylinders are connected to the pump (not shown) via a respective load holding valve assembly  7 ,  8 ,  9 . 
   The hydraulic motors  51  are connected to a pump (not shown) capable of driving the motors  51  in one direction or the other. Each motor  51  is connected to a respective cogwheel  50  via a gear  37 . Also shown is a mechanical brake  38  operable via valve assemblies  39   a ,  39   b.    
   The principle of operation of the hydraulic connection in  FIG. 5  will now be explained in greater detail. 
   In order to activate the three gripping jaws  1 ,  2 ,  3 , which form part of the rotary part of the tong, use is made of the three gripping cylinder  4 ,  5 ,  6 , which are activated and positioned synchronously via synchronizing cylinder  34  and slave cylinders  31 ,  32 ,  33 . Preferably, the synchronizing cylinder receives hydraulic power from the ring main or a stand-alone hydraulic motor-driven pump, which may be disposed on the tong or near this. The gripping cylinders are controlled by means of the hydraulic load holding valve assemblies  7 ,  8 ,  9  and synchronized by the synchronizing cylinder  34  being driven towards the three slave cylinders  31 ,  32 ,  33 , which are mechanically interconnected via the synchronizing element  36 . The slave cylinders  31 ,  32 ,  33  are connected to the gripping cylinders  4 ,  5 ,  6 , so that when the synchronizing cylinder  34  is driven towards the slave cylinders  31 ,  32 ,  33 , a hydraulic volume flow from the respective slave cylinders  31 , 32 ,  33  will be transferred to the respective gripping cylinders  4 ,  5 ,  6 , achieving a synchronized movement of the gripping cylinders. 
   Movement and positioning of the gripping jaws is performed by running the respective gripping cylinders towards the protrusion  1   c ,  2   c ,  3 , c  on the jaws  1 ,  2 ,  3 , the jaws thus being pulled out towards the centre of the cavity  44  until they meet the pipe  70 . The gripping cylinders will keep the jaws at a standstill, pressing against the pipe  70 . 
   When the jaws are pulled towards the pipe, they also pull three holding cylinders  1   a ,  2   a ,  3   a  with them, sucking hydraulic oil from the open reservoir  16  through the valve assembly  10 ,  11 ,  12  and into the piston side of the holding cylinders  1   a ,  2   a ,  3   a . The valves  10   b ,  11   b ,  12   b  are then in the position shown in  FIG. 1 , in which oil is permitted to flow past in the direction of the holding cylinders  1   a ,  2   a ,  3   a , but is not allowed to flow away from these. The hydraulic oil on the rod side of the holding cylinders  1   a ,  2   a ,  3   a  is evacuated through the valves  13 ,  14 ,  15  to the accumulator  17 . 
   In order to increase the clamping force between the gripping jaws and the pipe a volume of oil is delivered to the piston side of the holding cylinders  1   a ,  2   a ,  3   a . Since the added volume of oil does not generate any movement of the gripping jaws, this added volume of oil will cause the pressure, and consequently the clamping force, to increase. The delivery of this volume of oil is achieved by the master cylinder  19 , which is disposed on the fixed part of the tong, pressing against the slave cylinder  18 , which is disposed on the rotary part of the tong. This volume of oil flows to the holding cylinders  1   a ,  2   a ,  3   a  via the valves  10   a ,  11   a ,  12   a . The pressure in the master cylinder  19  is regulated by means of a pressure transmitter in a closed loop with a proportional directional valve (not shown). Since the gear ratio between the master cylinder  19  and the slave cylinder  18  is constant, the pressure in the holding cylinders  1   a ,  2   a ,  3   a  can easily be controlled. Upon reaching the desired pressure, the master cylinder  19  returns to the initial position. When the cylinder  19  returns, the cylinder  18  will follow, due to the return spring  18   a , and oil will flow from the rod side of the cylinder  18  to the piston side via the valve assembly  21 . At the same time, the cylinder  18  will also be refilled from the reservoir  16  via the check valve  20 . As the valve assemblies  10 ,  11  and  12  stop oil flowing away from the holding cylinders  1   a ,  2   a ,  3   a , these will maintain their clamping force against the pipe. 
   When the gripping cylinders  4 ,  5 ,  6  are also brought back to their initial positions, the tong may rotate freely with the pipe until the desired torque has been obtained. The tong can be rotated as shown by means of hydraulic motors, impellers and chains. The torque is regulated by a closed control loop with torque feed-back from the fixture for the fixed part of the tong and a proportional valve (not shown) connected to the hydraulic motors  51 . 
   The pipe is disengaged from the gripping jaws  1 ,  2 ,  3  by operating the relief actuator  56 , which via plates  57  and  58  displaces the valve  10   b ,  11   b ,  12   b  in the valve assembly  10 ,  11 ,  12  to the position that allows communication in both directions. Thus the pressure will be relieved from the piston side of the holding cylinders  1   a ,  2   a ,  3   a , relieving the pressure of the gripping jaws. The accumulator  17 , which is connected to the rod side of the holding cylinders  1   a ,  2   a ,  3   a , delivers pressure to the rod side of the holding cylinders  1   a ,  2   a ,  3   a  through choke  13   b ,  14   b ,  15   b . This pressure ensures that the holding cylinders are returned to their initial position. The chokes  13   b ,  14   b ,  15   b  will control the speed of this return stroke. 
     FIG. 6  is a simplified view of an alternative hydraulic connection. Here the reservoir  16  has been removed. The accumulator  17  may be a bladder accumulator filled with nitrogen, as shown, or a piston accumulator. Instead of a return spring in the slave cylinder  18 , each holding cylinder  1   a ,  2   a ,  3   a  is equipped with a return spring  1   c ,  2   c ,  3   c . When the two-way valves  10   b ,  11   b ,  12   b  are open, these return springs will push the pistons of the holding cylinders back, thereby forcing the hydraulic fluid back to the slave cylinder  18  and returning this. The accumulator  17  will also contribute to this. Thus there will be no requirement for a return spring in the holding cylinder. 
   An alternative solution for increasing the clamping force between the pipe and the gripping jaws after the gripping cylinders have moved these to engage the pipe, is shown in  FIG. 7 . Instead of using the hydraulic arrangement shown to supply hydraulic power to the holding cylinder, use is here made of the gripping cylinders  4 ,  5 ,  6  ( FIGS. 7   a, b, c  show only one 4 of the cylinders) to push against an arm  80  connected to a tappet  81  on the gripping jaw  1 . In  FIG. 7   a  the jaw  1  is fully retracted, and the gripping cylinder  4  is ready to push on the arm  80 . In a first phase (see  FIG. 7   b ) the gripping cylinder pushes against the arm  80  but without rotating this about the tappet  81 . This will move the jaw  1  towards the pipe  70  to engage this. At the same time, the holding cylinder  1   a  is pulled along. The holding cylinder sucks hydraulic fluid from a reservoir (not shown). After the jaw  1  has engaged the pipe  70  and no further displacement of the jaw  1  is possible, the gripping cylinder will start to rotate the arm  80  about the tappet  81 . This will cause the tappet  81  to attempt to lengthen the gripping jaw  1 . However, this is not possible in the direction of the pipe  70 , and so the piston rod and piston of the holding cylinder  1   a  will be forced into the actual cylinder while the centre line  82  of the holding cylinder and the piston rod is rotated over the centre of rotation  83  of the tappet. This will reduce the available volume for the limited quantity of oil in the holding cylinder  1   a , thus increasing the pressure. The force required by the gripping cylinder  4  to rotate the arm with the tappet  81  and the position of the arm  80  will be related to the pressure in the holding cylinder  1   a , allowing the clamping force between the pipe  70  and the gripping jaws to be determined and controlled. When the force from the gripping cylinders stops acting on the arm  80 , the net force from the pressure against the piston of the holding cylinder  1   a  will attempt to displace the piston forward in the actual cylinder, but as the holding cylinder has rotated about its fixture in the actual cylinder, over the centre of rotation, it will be mechanically locked. The holding cylinder will therefore act as a hydraulic spring. 
   For the embodiment of  FIG. 7 , a simplified hydraulic arrangement may be used, which includes no master and slave cylinders, but which will include valves for relieving hydraulic pressure from the holding cylinders, in accordance with the principles illustrated in  FIGS. 5 and 6 . 
   Return of the jaws can be achieved e.g. by opening a valve (equivalent to valves  10   b ,  11   b ,  12   b ) that relieves the pressure from the holding cylinders. The jaws will be retracted, either by means of a return spring or by hydraulic pressure. The arm  80  with the tappet  81  may be equipped with a return spring (not shown) to bring it back to its initial position. Alternatively, the return of the arm  80  can be brought about through gravity alone. 
   An alternative embodiment for synchronization of the gripping cylinders would be to have position measurement for each gripping cylinder with separate proportional valves, to allow the gripping cylinders to be individually positioned and thereby synchronized. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.