To maintain a riser under tension, the riser can be connected to a gripper ( 5, 12 ) which is connected to vertical toothed racks ( 4 ) that are in constant engagement with gear wheels ( 22 ) which are driven with constant torque, preferably by hydraulic motors ( 9 ) at constant pressure. An apparatus is also described for putting the riser under tension. The apparatus includes vertical guides ( 7 ) in a derrick, toothed racks ( 4 ) in the vertical guides ( 7 ), a gripper ( 5, 12 ) designed for interaction with a riser connected to the racks ( 4 ), and drive units ( 9 ) arranged in the derrick for drive-actuation of the racks ( 4 ) in the vertical guides ( 7 ).

BACKGROUND OF THE INVENTION 
     The present invention relates to apparatuses for putting risers under tension, heave-compensating rig floors and for guiding equipment through a moonpool. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be explained in more detail with reference to the attached figures, wherein: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 FIG. 1 
                 is a side view of a derrick; 
               
               
                   
                 FIG. 2 
                 is a front view of a derrick; 
               
               
                   
                 FIG. 3 
                 is a side view of an apparatus according to the invention; 
               
               
                   
                 FIG. 4 
                 is a front view of an apparatus according to the invention; 
               
               
                   
                 FIG. 5 
                 is a more detailed side view of an apparatus according to 
               
               
                   
                   
                 the invention; 
               
               
                   
                 FIG. 6 
                 is a more detailed front view of an apparatus according to 
               
               
                   
                   
                 the invention; 
               
               
                   
                 FIG. 7 
                 is a top view of an apparatus according to the invention; 
               
               
                   
                 FIG. 8 
                 is a top view of a derrick looking towards the rig floor; 
               
               
                   
                 FIG. 9 
                 shows a detail of the drive gear of the derrick and the 
               
               
                   
                   
                 apparatus according to the invention; 
               
               
                   
                 FIG. 10 
                 is a sectional view of the apparatus according to the 
               
               
                   
                   
                 invention with drive gear and heave-compensated 
               
               
                   
                   
                 rig floor; 
               
               
                   
                 FIG. 11 
                 shows a detail of the drive gear of the apparatus 
               
               
                   
                   
                 according to the invention; 
               
               
                   
                 FIG. 12 
                 is a sectional side view of the apparatus according to 
               
               
                   
                   
                 the invention with drive gear and heave-compensated 
               
               
                   
                   
                 rig floor; 
               
               
                   
                 FIG. 13 
                 is a top view of a riser gripper platform according to 
               
               
                   
                   
                 the present invention; 
               
               
                   
                 FIG. 14 
                 is a front view of the riser gripper platform; 
               
               
                   
                 FIG. 15 
                 is a side view of the riser gripper platform. 
               
               
                   
                   
               
             
          
         
       
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a derrick  1  comprising a drive mechanism for a top drive rotary machine as described in PCT/NO/98/00/30. This drive mechanism will not be explained in more detail here; instead reference is made to the aforementioned document in which there is a comprehensive explanation thereof. FIG. 2 is a side view of the derrick  1 , and here the drive mechanism  2  with toothed rack  3  is seen more clearly. FIG. 3 is a side view of the apparatus according to the invention, whilst FIG. 4 is a front view of the same apparatus. The apparatus comprises preferably four, but at least two, toothed racks  4 , which are arranged substantially parallel to one another. The racks are connected to a gripper platform  5 , preferably via a relatively flexible articulation  6 . 
     FIGS. 5-7 show the apparatus according to the invention in more detail. The toothed racks are arranged to be passed down into a cavity in a part of the derrick structure. In the illustrated case each rack is accommodated in a respective hollow leg  7 , which constitutes a part of the lattice work of the derrick. A drive gear  8 , which in the illustrated case comprises eight hydraulic motors  9 , arranged in pairs on each side of each rack  4 , is adapted to move the racks  4  in and out of the legs  7 . To obtain synchronisation of the eight hydraulic motors  9 , these may optionally be connected to each other via shafts  10  and  11 . Of course, the synchronisation of the eight motors can also be achieved by other means, e.g., hydraulically or electronically. 
     The four toothed racks  4  are connected to the gripper platform  5  via an articulation or flexible connection  6 . The articulation or flexible connection  6  is capable of neutralising forces that arise due to any minor imprecision in the parallel movement of the racks  4  in and out of the legs  7 . The gripper platform  5  comprises two flaps  12 , which are adapted to grip below a collar on a non-illustrated riser. 
     The drive gear  8  is preferably located beneath the rig floor  13 . The rig floor  13  itself may optionally also be physically connected to the racks  4 , e.g., by means of hydraulic keys  14 . In this way, the rig floor  13  is able to follow the movement of the gripper platform  5 , and can thus be automatically heave-compensated. In order to adjust the height of the rig floor, the rig floor may be provided with hydraulic cylinders  15 , thereby eliminating the need to disconnect the hydraulic locks  14  from and reconnect them to the racks  4  when making minor height adjustments of the rig floor  13 . 
     FIG. 8 is a top view of the rig floor  13 , and also shows a section through the derrick structure  1 . This figure also indicates the legs  7 , into which the racks  4  are passed. When the racks  4  have been passed into the legs  7 , the racks  4  will not be visible above the rig floor, and nor will they represent any danger to personnel or take up space. FIG. 8 also shows a pipe rack  16  and a pipe handling means  17 . On the rig floor  13  there are also provided rails  18  for a dolly  19 , which is designed to carry equipment into position above the moonpool. 
     FIG. 9 is a sectional view of the drive gear  2  for the rotary machine  20  (FIG.  2 ). This figure also shows a hydraulic motor  9  for actuating the rack  4 . 
     FIG. 10 is a more detailed sectional view of the drive gear  8  for the racks  4 , and shows two of the machinery rooms  21  in which the hydraulic motors  9  are arranged. The figure also shows the shaft  10  and the end of the shafts  11 , which connect the hydraulic motors in each machinery room  21  to one another to allow them to be run synchronously. Also shown in this figure is the rig floor  13 , which is equipped with four hydraulic locks  14  and four hydraulic height-adjusting cylinders  15 . 
     FIG. 11 shows a hydraulic motor  9 , which via a gear wheel  22  is in toothed engagement with a toothed portion (not shown) of the rack  4 . As can be seen, the shafts  10  and  11  are equipped with gear wheels  23  and  24  at their ends, which gear wheels are in engagement with toothed portions (not shown) of the racks  4 . In the illustrated case the shafts  10  and  11  are not in direct operative connection with the hydraulic motor  9 , but are in engagement with the racks  4 . However, a direct operative connection between the hydraulic motors  9  is also conceivable. 
     FIG. 12 is a sectional side view of the apparatus according to the present invention, in which the gear wheels  22  on the hydraulic motors  9  can be seen, as can a shaft  11  and the gear wheels  23  and  24 . The rig floor  13  is also indicated. 
     FIG. 13 is a top view of the gripper platform  5 , whereas FIG. 14 is a front view of the gripper platform  5 , and FIG. 15 a side view of the gripper platform  5 . The gripper flaps  12  are equipped with a respective hydraulic cylinder  25 , which is adapted to swing the gripper flaps  12  between a horizontal and a vertical position. In the vertical position, the gripper flaps allow the riser and other equipment clear passage between them. In the horizontal position, the gripper flaps  12  engage with a riser  30 , which comes to rest in the recesses  26  in the gripper flaps  12  (FIG.  13 ). The recesses  26  together form a substantially circular hole, which has a smaller diameter than a collar on the riser, which collar comes to rest on the top of the gripper flaps. The gripper flaps  12  are also equipped with stops  27 , which come to bear against the gripper platform  5 , so that load taken up by the cylinders  25  is not particularly large. 
     In addition to functioning as a tension device for a riser and as a heave compensator for the rig floor, the apparatus according to the invention is also capable of functioning as a hoist for equipment that is to be passed through the moonpool on a drill and production ship. For this function, the gripper platform  5  may be equipped with a roller or the like, which can be brought into contact with the moonpool walls, to prevent any risk of the gripper platform striking against the moonpool walls, and causing damage. In this way, e.g., a BOP can be moved safely and securely through the moonpool, it being secured by the gripper platform throughout the lowering operation and until it is clear of the bottom of the ship. 
     The hydraulic motors  9  are of a conventional type and equipped with a gear box. By operating these at constant pressure when the riser is to be held under tension, and connecting the pressure via an accumulator, constant tension in the riser may easily be obtained. Owing to the constant tension, the racks  4  will always follow the wave lo motion, and a derrick that is connected to the racks will therefore also follow the wave motion. In particular when lowering coiled tubing, this will be advantageous, as the coil can be placed on the rig floor, and the coiled tubing will not be subjected to large loads whilst being lowered. 
     The toothed racks are preferably designed to be rigid, but for certain purposes they may conceivably be constructed so as to be hinged. 
     Motions have shown that the racks under tension will withstand loads of as much as 300 tonnes without any difficulty.