Heave compensation system

Abstract

The invention pertains to a heave compensation system for continuous operation on a floating vessel ( 1 ), the system comprising at least one buoyant body ( 11 ), at least one lever arm ( 10 ) for arrangement in a generally horizontal plane, pivotal means capable of limited rotation about a horizontal axis and arranged to connect one end of the lever arm to the buoyant body ( 11 ), at least one hinge ( 13 ) connected to an intermediate point on the lever arm ( 10 ) and capable of allowing limited rotational movement of the lever arm ( 10 ) in a vertical plane, means to attach the hinge ( 13 ) to the floating vessel ( 1 ), and at least one counterbalance weight ( 12 ) connected to the lever arm ( 10 ) on the side of the hinge ( 13 ) away from the buoyant body ( 11 ), whereby the inertia of the counterbalance weight ( 12 ) resists heave movements of the buoyant body ( 11 ) relative to the floating vessel.

1 . A heave compensation system for continuous operation on a floating vessel, the system comprising at least one buoyant body, at least one lever arm for arrangement in a generally horizontal plane, pivotal means capable of limited rotation about a horizontal axis and arranged to connect one end of the lever arm to the buoyant body, at least one hinge connected to an intermediate point on the lever arm and capable of allowing limited rotational movement of the lever arm in a vertical plane, means to attach the hinge to the floating vessel, and at least one counterbalance weight connected to the lever arm on the side of the hinge away from the buoyant body, whereby the inertia of the counterbalance weight resists heave movements of the buoyant body relative to the floating vessel.  
   
   
       2 . A heave compensation system as claimed in  claim 1 , wherein the hinge for the lever arm is arranged so that forces applied by the tendency of the buoyant body to move vertically with respect to the vessel are balanced by restraining forces arising from the counterbalance weight acting on the lever arm at a distance from the hinge, such that only minimal forces external forces are required to damp heave motions of the buoyant body.  
   
   
       3 . A heave compensation system as claimed in  claim 1 , wherein two of the lever arms are aligned with and diametrically opposed to each other, and wherein the lever arms are attached to the buoyant body at their proximate ends, the hinges on the respective lever arms have means for attachment to the floating vessel, and respective counterbalance weights are connected to respective lever arms on the sides of the hinges away from the buoyant body.  
   
   
       4 . A heave compensation system as claimed in  claim 3 , wherein two pairs of similar lever arms are aligned with and diametrically opposed to each other.  
   
   
       5 . A heave compensation system as claimed in  claim 4 , wherein the two pairs of similar lever arms are at right angles to each other and/or several pairs of similar lever arms are positioned in equal distance to each other around in a circle.  
   
   
       6 . A heave compensation system as claimed in  claim 1 , further comprising means to constrain the buoyant body for movement generally vertically with respect to the floating vessel, in which the means for instance are guides for association with the vessel to restrain the buoyant body for generally vertical movement.  
   
   
       7 . A heave compensation system as claimed in  claim 1 , comprising mooring lines between low points on the buoyant body and the seabed to restrain the buoyant body for generally vertical movement.  
   
   
       8 . A heave compensation system as claimed in  claim 1 , wherein the buoyant body is adjustably ballastable.  
   
   
       9 . A heave compensation system as claimed in  claim 8 , wherein the buoyant body is ballastable with water.  
   
   
       10 . A heave compensation system as claimed in  claim 9 , wherein the water may be expelled from the buoyant body by use of compressed air and/or water pumps.  
   
   
       11 . A heave compensation system as claimed in  claim 1  wherein the counterbalance weight is arranged to be above the wave effected zone in respect of the limiting wave height in which the system is intended to operate.  
   
   
       12 . A heave compensation system as claimed in  claim 1 , wherein the counterbalance weight is adjustably ballastable.  
   
   
       13 . A heave compensation system as claimed in  claim 12 , wherein the counterbalance weight is ballastable with water.  
   
   
       14 . A heave compensation system as claimed in  claim 1 , wherein the effect of the counterbalance weight is supplemented by a hydraulic piston cylinder arrangement disposed between the lever arm and an attachment to the floating vessel.  
   
   
       15 . A heave compensation system as claimed in  claim 1 , wherein the buoyant body has a throughway (moon pool) extending between upper and lower parts of the body.  
   
   
       16 . A heave compensation system as claimed in  claim 15 , wherein the buoyant body is a drilling module.  
   
   
       17 . A vessel when fitted with a heave compensation system as claimed in  claim 1 .  
   
   
       18 . A method of continuously compensating heave between a floating vessel and a buoyant body comprising the steps of constraining the buoyant body for movement generally vertically with respect to the floating vessel, disposing at least one lever arm in a generally horizontal plane on the floating vessel with a hinge at an intermediate point on the lever arm which connects the lever arm to the floating vessel, so that the lever arm is capable of limited rotational movement in a vertical plane, connecting one end of the lever arm to the buoyant body to allow angular movement there between about a horizontal axis, and connecting a counterbalance weight to the lever arm on the side of the hinge away from the buoyant body, whereby vertical movement of the counterbalance weight resists heave movements of the buoyant body relative to the floating vessel.