Motion compensation system for watercraft connected to subsea conduit

Abstract

A safety device 30 for compensating for ocean heave motion of a vessel or watercraft connected to a conduit 12 or drill string extending to the subsea. The safety device comprises at least one vertical piston and cylinder assembly 40,41 , the piston rod 40 a of which connects to an upper beam 35 of the device 30 and the cylinder 40 b of which connects to a horizontal beam 42 below the upper beam 35 and supported by vertical tension legs 51,52 , each including a gas expansion chamber 51 a, 52 a . A valve controlled conduit system 61,62 connects the chambers 51 a, 52 a and cylinders 40 b, 41 b to a source of pressurized gas whereby a fixed volume of gas and gas pressure positions the pistons at the mid-lengths of the cylinders 40 b, 41 b . The safety device 30 is mounted on a frame or rig 11 mounted on the watercraft and the base 33 of the safety device 30 is connected to the bottoms of the tension legs 51,52 . A connecting means 34 connects the safety device 30 in coaxial alignment with the conduit or drill string 12 which extends through an opening 13 in the base 14 of the frame 11 . A second connector 24 connects the top of the safety device 30 to the vessel or watercraft or to another motion compensator used in series therewith. When the conduit or drill string 12 stress is acceptable, the length of the safety device 30 remains constant. However, should the stress exceed acceptable levels, the pistons 40 a, 41 a move into the cylinders 40 b, 41 b to shorten its length and thereby reduce tension stress to an acceptable level or the pistons 40 a, 41 a move outwardly of the cylinders to increase the safety device 30 length and thereby reduce the compression stress to an acceptable level.

1 . A safety device for compensating for the ocean heave motion of a floating watercraft connected to a conduit extending downwardly to beneath the ocean floor in order to maintain the stress imposed on the conduit by the watercraft to within acceptable levels, said safety device comprising: 
 at least one piston and cylinder assembly disposed vertically with the end of the piston rod connected to an upper beam of the device and the cylinder connected to a horizontal support beam below said upper beam, said horizontal support beam supported by a pair of vertically extending tension leg members, each of which includes a gas expansion chamber therein;    a valve-controlled conduit system connecting the gas expansion chambers and said cylinder to a source of pressurized gas whereby a fixed volume of gas and gas pressure may be supplied to said cylinder and piston to position the piston substantially at the mid-length of the cylinder,    said safety device having a base member connected to the bottom ends of said tension leg members and said safety device being mounted in a frame mounted on said watercraft and having a frame base provided with an opening therein through which said conduit extends;    a first connecting means for connecting said base member of said safety device to the upper end of said conduit which extends through said opening in said frame base;    a second connecting means for connecting said upper beam of the safety device to a connection member provided on said floating watercraft,    said safety device being adapted to respond to stress on said conduit which exceeds a predetermined level by movement of said piston relative to said cylinder to alter the length of said safety device in a direction to relieve said stress and to maintain the stress within an acceptable limit.    
   
   
       2 . A safety device as set forth in  claim 1  wherein said safety device is provided with more than one piston and cylinder assembly with the piston rods thereof connected to said upper beam and the cylinders of said assemblies connected to said horizontal support beam and to said conduit system whereby a fixed volume of gas and gas pressure may be supplied to said cylinders to position the pistons substantially at the mid-lengths of said cylinders.  
   
   
       3 . A safety device as set forth in  claim 1  wherein said safety device is adapted to respond to a level of tension stress on said conduit which exceeds a predetermined level by movement of said pistons outwardly of said cylinders to thereby increase the length of said safety device and thereby relieve and maintain said tension stress within an acceptable level.  
   
   
       4 . A safety device as set forth in  claim 2  wherein said safety device is adapted to respond to a level of tension stress on said conduit which exceeds a predetermined level by movement of said pistons outwardly of said cylinders to thereby increase the length of said safety device and thereby relieve and maintain said tension stress within an acceptable level.  
   
   
       5 . A safety device as set forth in  claim 1  wherein said safety device is adapted to respond to a level of compression stress on said conduit which exceeds a predetermined level by movement of the pistons inwardly of said cylinders to thereby decrease the length of said safety device and thereby relieve and maintain said compression stress within an acceptable limit.  
   
   
       6 . A safety device as set forth in  claim 2  wherein said safety device is adapted to respond to a level of compression stress on said conduit which exceeds a predetermined level by movement of the pistons inwardly of said cylinders to thereby decrease the length of said safety device and thereby relieve and maintain said compression stress within an acceptable limit.  
   
   
       7 . A safety device as set forth in  claim 1  wherein said watercraft and said safety device are connected to a drilling string and said safety device is connected thereto in order to maintain the stress levels imposed on said drilling string by movement of said watercraft by the ocean heave movements to within acceptable limits.  
   
   
       8 . A safety device as set forth in  claim 2  wherein said watercraft and said safety device are connected to a drilling string and said safety device is connected thereto in order to maintain the stress levels imposed on said drilling string by movement of said watercraft by the ocean heave movements to within acceptable limits.  
   
   
       9 . A safety device as set forth in  claim 1  wherein said second connecting means connects said upper beam of the safety device to a traditional motion compensator provided on said watercraft and whereby said safety device becomes operative to relieve and maintain stress imposed on said conduit within acceptable limits in the event of failure of the traditional motion compensator.