Roller cone drill bit

Abstract

A roller cone drill bit having a bit body. The bit body has a journal bearing coupled to an end portion thereof. A roller cone is coupled to the journal bearing. A first cutting element is coupled to the roller cone. A ratio of a radius of curvature of a crest portion of the first cutting element to a diameter of the first cutting element is between about 0.3:1 and about 0.8:1. A second cutting element is coupled to the roller cone. A ratio of a radius of curvature of a crest portion of the second cutting element to a diameter of the second cutting element is between about 0.05:1 and about 0.3:1. A height of the second cutting element is greater than the height of the first cutting element h between about 0.1 mm and about 6 mm.

What is claimed is: 
     
         1 . A roller cone drill bit, comprising:
 a bit body;   a roller cone rotationally coupled to the bit body;   a first cutting element coupled to the roller cone, a ratio of a radius of curvature of a crest portion of the first cutting element to a diameter of the first cutting element being between about 0.3:1 and about 0.8:1; and   a second cutting element coupled to the roller cone, a ratio of a radius of curvature of a crest portion of the second cutting element to a diameter of the second cutting element being between about 0.05:1 and about 0.3:1, the second cutting element having a height between about 0.1 mm and about 6 mm greater than a height of the first cutting element, as measured from an outer surface of the roller cone.   
     
     
         2 . The roller cone drill bit of  claim 1 , the ratio of the radius of curvature of the crest portion of t le first cutting element to the diameter of the first cutting element being between about 0.4:1 and about 0.7:1. 
     
     
         3 . The roller cone drill bit  claim 1 , the first cutting element being a semi-round top cutting element. 
     
     
         4 . The roller cone drill bit of  claim 1 , the ratio of the radius of curvature of the crest portion of the second cutting; element to the diameter of the second cutting element being between about 0.14:1 and about 0.25:1. 
     
     
         5 . The roller cone drill bit of  claim 1 , the second cutting element being at least partially conical. 
     
     
         6 . The roller cone drill bit of  claim 1 , the height of the second cutting element being greater than the height of the first cutting element by between about 0.2 mm and about 1 mm. 
     
     
         7 . The roller cone drill bit of  claim 1 , the roller cone being configured to rotate about an axis of a journal bearing coupled to the bit body and the roller cone. 
     
     
         8 . The roller cone drill bit of  claim 7 , the axis through the journal bearing being oriented at an angle between about 20° and about 70° with respect to an axis of the bit body. 
     
     
         9 . The roller cone drill bit of  claim 1 , each of the first and second cutting elements including a diamond layer at least partially disposed on a carbide substrate. 
     
     
         10 . The roller cone drill bit of  claim 1 , the roller cone drill bit having only one roller cone. 
     
     
         11 . A hammer assembly, comprising:
 a housing having a bore therein;   a hammer arm disposed at least partially within the bore;   a bit body movably coupled to the hammer arm, the bit body being configured to move axially with respect to the hammer arm;   a journal bearing coupled to an end portion of the bit body;   a roller cone coupled to the journal bearing;   a first cutting element coupled to the roller cone, a ratio of a radius of curvature of a crest portion of the first cutting element to a diameter of the first cutting element being between about 0.3:1 and about 0.8:1; and   a second cutting element coupled to the roller cone, a ratio of a radius of curvature of a crest portion of the second cutting element to a diameter of the second cutting element being between about 0.05:1 and about 0.3:1, the second cutting element having a height between about 0.1 mm and about 6 mm greater than a height of the first cutting element, as measured from an outer surface of the roller cone.   
     
     
         12 . The hammer assembly of  claim 11 , the roller cone being adapted to rotate about an axis of the journal bearing. 
     
     
         13 . The hammer assembly of claim II, further comprising:
 a first magnet coupled to the housing; and   a second magnet coupled to the hammer arm, the first and second magnets being cone figured to cause the hammer arm to move with respect to the housing.   
     
     
         14 . The hammer assembly of  claim 13 , the hammer arm being configured to move and thereby cause the bit body and the roller cone to move with respect to the housing. 
     
     
         15 . The hammer assembly of  claim 11 , the ratio of the radius of curvature of the crest portion of the first cutting element to the diameter of the first cutting element being between about 0.4:1 and about 0.7:1, and the ratio of the radius of curvature of the crest portion of the second cutting element to the diameter of the second cutting element being between about 0.14:1 and about 0.25:1. 
     
     
         16 . A method for drilling a borehole, comprising:
 running a hammer assembly into a borehole, the hammer assembly including:
 a housing having a bore therein; 
 a hammer arm disposed at least partially within the bore; 
 a bit body movably coupled to the hammer arm and configured to move axially with respect to the hammer arm; 
 a journal bearing coupled to an end portion of the bit body; 
 a roller cone coupled to the journal bearing; and 
 first and second cutting elements coupled to the roller cone, a ratio of a radius of curvature of a crest portion of the first cutting element to a diameter of the first cutting element being between about 0.31:1 and about 0.8:1, and a ratio of a radius of curvature of a crest portion of the second cutting element to a diameter of the second cutting element being between about 0.05:1 and about 0.3:1, the second cutting element having a height of between about 0.1 mm and about 6 mm greater than a height of the first cutting element, as measured from an outer surface of the roller cone; and 
   contacting a subterranean formation with the first and second cutting elements of the hammer assembly to form a borehole.   
     
     
         17 . The method of  claim 16 , further comprising:
 moving the hammer arm with respect to the housing.   
     
     
         18 . The method of  claim 17 , wherein moving the hammer arm includes moving the hammer arm in response to interaction between a first magnet coupled to the housing and a second magnet coupled to the hammer arm. 
     
     
         19 . The method of  claim 17 , further comprising:
 moving the bit body and the roller cone with respect to the hammer arm.   
     
     
         20 . The method of  claim 16 , wherein the ratio of the radius of curvature of the crest portion of the first cutting element to the diameter of the first cutting element is between about 0.4:1 and about 0.7:1, and wherein the ratio of the radius of curvature of the crest portion of the second cutting element to the diameter of the second cutting element is between about 0.14:1 and about 0.25:1.