Pulsating Rotary Steerable System

Abstract

A drilling assembly disposed on a drillstring can deviate a borehole advanced by a drill bit. A housing coupled between the drillstring and the bit transfers rotation of the drillstring to the bit. A mandrel in the housing's axial bore has a least one inclined surface on at least one side. The mandrel is subject to pressure in the housing and can be displaced longitudinally. A biasing element biases the mandrel toward one position. At least one piston disposed on the housing's side can be moved by the at least one inclined surface and can be displaced transversely in response to the longitudinal displacement of the mandrel. To vary the transverse displacement of the piston and change a trajectory of the borehole, changes to the differential pressure across the mandrel longitudinally displaces the mandrel and moves the piston.

What is claimed is: 
     
         1 . A drilling assembly disposed on a drillstring for deviating a borehole advanced by a drill bit, the assembly comprising:
 a housing coupled between the drillstring and the drill bit and transferring rotation of the drillstring to rotation of the drill bit, the housing having an axial bore along a longitudinal axis communicating fluid flow from the drillstring to the drill bit;   a mandrel disposed in the axial bore of the housing and having a least one inclined surface disposed on at least one side thereof, the mandrel being subject to first pressure in the housing and second pressure in the borehole and being displaceable in the axial bore along the longitudinal axis; and   at least one piston disposed on a side of the housing relative to the at least one inclined surface of the mandrel, the at least one piston being displaceable on the housing transverse to the longitudinal axis in response to the longitudinal displacement of the mandrel,   wherein the longitudinal displacement of the mandrel responds to a pressure differential between the first and second pressures across the mandrel and varies the transverse displacement of the at least one piston.   
     
     
         2 . The assembly of  claim 1 , further comprising a flow control controlling the fluid flow through the housing, the flow control creating a change in the pressure differential across the mandrel and periodically varying the transverse displacement of the at least one piston with the longitudinal displacement of the mandrel. 
     
     
         3 . The assembly of  claim 2 , wherein the flow control bypasses at least a portion of the fluid flow away from the housing. 
     
     
         4 . The assembly of  claim 2 , wherein the flow control comprises a valve disposed within the fluid flow to the housing. 
     
     
         5 . The assembly of  claim 2 , wherein the flow control in a first condition produces a first of the pressure differential across the mandrel; and wherein the flow control in a second condition produces a second of the pressure differential across the mandrel, the second pressure differential being greater than the first differential pressure. 
     
     
         6 . The assembly of  claim 1 , wherein the transverse displacement of the at least one piston displaces the longitudinal axis of the housing relative to the advancing borehole. 
     
     
         7 . The assembly of  claim 1 , wherein the at least one piston disposed on the side of the housing comprises an external portion exposed externally on the housing and comprises an internal portion disposed in the axial bore of the housing, the internal portion of the at least one piston engageable with the at least one inclined surface of the mandrel. 
     
     
         8 . The assembly of  claim 1 , further comprising a biasing element disposed in the housing and biasing the mandrel toward a first position in the housing. 
     
     
         9 . The assembly of  claim 8 , wherein the biasing element comprises one or more springs having one portion fixed in the axial bore of the housing and having another portion engaged with the mandrel. 
     
     
         10 . The assembly of  claim 8 , wherein the biasing element is loaded with an axial force equating to a transverse force of the at least one piston displaced transversely on the housing when the mandrel is longitudinally displaced toward the first position. 
     
     
         11 . The assembly of  claim 1 , wherein the mandrel defines an internal passage communicating the fluid flow therethrough from adjacent the drillstring to adjacent the drill bit. 
     
     
         12 . The assembly of  claim 1 , wherein the mandrel comprises a first portion sealably engaged in the axial bore of the housing and defining a first piston area subject to the first pressure. 
     
     
         13 . The assembly of  claim 12 , wherein the mandrel comprises a second portion sealably engaged in the axial bore of the housing and defining a second piston area subject to the second pressure. 
     
     
         14 . The assembly of  claim 13 , wherein the housing comprises at least one port communicating the second pressure in the borehole with the second piston area of the mandrel. 
     
     
         15 . The assembly of  claim 14 , wherein the at least one port comprises a nozzle restricting communication between the borehole and a chamber in the housing to which the second piston area is exposed. 
     
     
         16 . The assembly of  claim 1 , wherein the mandrel rotates with the housing. 
     
     
         17 . The assembly of  claim 1 , wherein the mandrel comprises first and second seals sealably engaging the axial bore of the housing and sealing a space between the mandrel and the housing adjacent the at least one inclined surface. 
     
     
         18 . The assembly of  claim 1 , wherein the at least one piston is sealed in an external opening in the side of the housing. 
     
     
         19 . The assembly of  claim 1 , wherein the at least one piston is extended from the side of the housing in response to a first of the pressure differential across the mandrel longitudinally displacing the mandrel toward a first position. 
     
     
         20 . The assembly of  claim 19 , wherein the at least one piston is retracted from the side of the housing in response to a second of the pressure differential across the mandrel longitudinally displacing the mandrel toward a second position. 
     
     
         21 . The assembly of  claim 20 , wherein a first state of the fluid flow through the housing produces the first pressure differential, and wherein a second state of the fluid flow through the housing produces the second pressure differential, the first and second states of the fluid flow being produced by at least one of a pump and a valve. 
     
     
         22 . The assembly of  claim 1 , wherein the at least one piston comprises more than one piston disposed along a same side of the housing. 
     
     
         23 . The assembly of  claim 1 , wherein the at least one piston comprises first and second pistons disposed on first and second opposing sides of the housing, the first and second pistons being oppositely retracted and extended relative to one another on the first and second opposing sides of the housing. 
     
     
         24 . The assembly of  claim 23 , wherein the first and second pistons produce two comparable deflections of the longitudinal axis of the housing relative to the borehole within one revolution of the housing. 
     
     
         25 . A drilling system having a drill bit on a drillstring to advance a borehole, the system comprising:
 a drive rotating the drillstring to rotate the drill bit;   a pump pumping fluid flow along the drillstring to the drill bit;   a flow valve operable to control the fluid flow; and   a steering mechanism disposed on the drillstring upstream of the drill bit and downstream of the flow valve, the steering mechanism having an inner mandrel and at least one piston, the inner mandrel being displaceable along a longitudinal axis of the steering mechanism in response to a pressure differential across the mandrel from the controlled fluid flow, the inner manner longitudinally displaced varying a transverse displacement of the at least one piston relative to a side of the advancing borehole.   
     
     
         26 . A drilling method, comprising:
 advancing a borehole with a drill bit on a drilling assembly coupled to a drillstring by rotating the drilling assembly and the drill bit with rotation of the drillstring; and   deviating the drilling assembly in the advancing borehole by:
 controlling fluid flow through the drilling assembly; 
 varying a pressure differential across a mandrel inside the drilling assembly with the controlled fluid flow; 
 displacing the mandrel along a longitudinal axis in the drilling assembly with the varied pressure differential; and 
 alternating, relative to the rotation of the drilling assembly, transverse displacement of at least one piston on at least one side of the drilling assembly with the longitudinal displacement of the mandrel. 
   
     
     
         27 . The method of  26 , wherein controlling the fluid flow through the drilling assembly comprises:
 determining a steering direction for the drilling assembly;   sensing an angular orientation of the drilling assembly; and   varying the fluid flow through the drilling assembly based upon the determined steering direction and the sensed angular orientation.