Heave Compensator for Constant Force Application to a Borehole Tool

Abstract

The compensating device has a through passage that goes to the borehole tool. There is a lateral passage to a piston housing. Through the use of a differential piston area on the outer housing, a net uphole force results from backpressure as a result of flow pumped through a section mill that mills in an uphole direction. If the vessel goes down the mill is just pushed away from the tubular being cut. If wave action takes the vessel up fluid is displaced back into the mandrel but the constant force up that is dependent on the existing backpressure in the tubing keeps a steady uphole force on the mill. The tool can be reversed for applications that require a net down force during milling. Rotational locking between the mandrel and the outer housing can be used. Ports are sized to prevent damping responses.

We claim: 
     
         1 . An apparatus for applying force to a borehole tool from a moving source, comprising:
 a mandrel having a passage therethrough and an upper and lower ends;   an outer housing slidably mounted to said mandrel for predetermined relative movement in opposed directions, said outer housing comprising an outer housing passage which continues said mandrel passage;   said mandrel and outer housing defining a first variable volume chamber which communicates with said passage such the pressure in said first variable volume chamber results in a force applied to the borehole tool that is insensitive to said predetermined relative movement.   
     
     
         2 . The apparatus of  claim 1 , wherein:
 pressure in said chamber arises from flow resistance in the borehole tool.   
     
     
         3 . The apparatus of  claim 1 , further comprising:
 a second variable volume chamber communicating to outside said outer housing;   
       said variable volume chambers comprise opposed piston faces to create a net force on the borehole tool. 
     
     
         4 . The apparatus of  claim 1 , wherein:
 said mandrel and said outer housing are rotationally locked.   
     
     
         5 . The apparatus of  claim 4 , wherein:
 said rotational locking comprises a keyway on one of said mandrel and said outer housing engaged to a tab on the other of said mandrel and said housing.   
     
     
         6 . The apparatus of  claim 3 , wherein:
 said first variable volume chamber is closer to said uphole end of said mandrel than said second variable volume chamber.   
     
     
         7 . The apparatus of  claim 3 , wherein:
 said first variable volume chamber is further from said uphole end of said mandrel than said second variable volume chamber.   
     
     
         8 . The apparatus of  claim 6 , wherein:
 pressure in said first variable volume chamber creates a tensile force on the borehole tool.   
     
     
         9 . The apparatus of  claim 7 , wherein:
 pressure in said first variable volume chamber creates a compressive force on the borehole tool.   
     
     
         10 . The apparatus of  claim 6 , wherein:
 one of said mandrel and said outer housing comprising opposed surfaces that define said predetermined relative movement.   
     
     
         11 . The apparatus of  claim 7 , wherein:
 one of said mandrel and said outer housing comprising opposed surfaces that define said predetermined relative movement.   
     
     
         12 . The apparatus of  claim 3 , wherein:
 said first and second variable volume chambers defined by an upper seal, a middle seal and a lower seal.   
     
     
         13 . The apparatus of  claim 12 , wherein:
 said first variable volume chamber communicating with said passage between said upper and middle seals and said second variable volume chamber communicating with outside of said outer housing between said middle and lower seals.   
     
     
         14 . The apparatus of  claim 12 , wherein:
 said first variable volume chamber communicating with said passage between said middle and lower seals and said second variable volume chamber communicating with outside of said outer housing between said upper and said middle seals.   
     
     
         15 . The apparatus of  claim 10 , wherein:
 said mandrel and said outer housing move in tandem after one of said opposed surfaces are engaged.   
     
     
         16 . The apparatus of  claim 11 , wherein:
 said mandrel and said outer housing move in tandem after one of said opposed surfaces are engaged.   
     
     
         17 . The apparatus of  claim 10 , wherein:
 said opposed surfaces are located in said first variable volume chamber.   
     
     
         18 . The apparatus of  claim 11 , wherein:
 said opposed surfaces are located in said first variable volume chamber.   
     
     
         19 . The apparatus of  claim 1 , wherein:
 said mandrel is connected to the borehole tool and said outer housing is connected to the moving source.   
     
     
         20 . The apparatus of  claim 1 , wherein:
 said first variable volume chamber is located between said outer housing passage and said mandrel passage.   
     
     
         21 . The apparatus of  claim 20 , wherein:
 said mandrel comprising a seal to said outer housing to allow said mandrel to act as a piston exerting a force on the borehole tool.   
     
     
         22 . The apparatus of  claim 21 , wherein:
 said outer housing comprising opposed surfaces that act as travel stops in opposed directions for said mandrel.   
     
     
         23 . The apparatus of  claim 1 , wherein:
 pressure in said passages is created by flow restriction through the borehole tool which communicates with said first variable volume chamber to put a force on said mandrel in the direction of the borehole tool.