Lockdown system for use in a wellhead assembly

Abstract

A wellhead assembly having an outer tubular, an inner tubular inserted into the outer tubular, an annular space between the inner and outer tubulars, a lock ring in the annular space, and an activation ring that axially strokes between the lock ring and one of the tubulars. The lock ring selectively locks together the inner and outer tubulars when the activation ring slides between the lock ring and the one of the tubulars. The surface of the activation ring that contacts lock ring is contoured so that an interface surface between the activation ring and lock ring when the lock ring is in its locked position, is offset an angle from an axis of the wellhead that is less than an offset between the axis of the wellhead and an interface surface between the activation ring and lock ring when the activation ring is stroking downward.

What is claimed is: 
     
         1 . A wellhead assembly comprising:
 an axis;   an outer tubular;   an inner tubular inserted into the outer tubular and defining an annular space between the inner tubular and outer tubular;   a lock ring in the annular space that is moveable from an unlocked position into a locked position;   an activation ring having a profiled surface that is axially slidable against and in contact with the lock ring to define a contact interface that is offset an angle from the axis when the lock ring is in the locked position, and to define a contact interface that is offset an angle from the axis when the lock ring is in the unlocked position, so that the angle when the lock ring is in the locked position is less than the angle when the lock ring is in the unlocked position.   
     
     
         2 . The wellhead assembly of  claim 1 , wherein a portion of the profiled surface is curved that is in contact with the lock ring when the lock ring is in the unlocked position, and wherein a portion of the profiled surface that is in contact with the lock ring when the lock ring is in the locked position is substantially linear. 
     
     
         3 . The wellhead assembly of  claim 1 , wherein the lock ring is set radially inward from the outer tubular when in the unlocked position, and wherein the lock ring comprises a protrusion that engages a depression formed in an inner radial surface of the outer tubular. 
     
     
         4 . The wellhead assembly of  claim 1 , wherein the lock ring is set radially outward from the inner tubular when in the unlocked position, and wherein the lock ring comprises a protrusion that engages a depression formed in an outer radial surface of the inner tubular. 
     
     
         5 . The wellhead assembly of  claim 1 , further comprising a seal assembly that transfers a downward axial force to an upper end of the activation ring and that is energized by an energizing force. 
     
     
         6 . The wellhead assembly of  claim 5 , wherein a force applied to the activation ring that slides the activation ring along the lock ring and urges the lock ring from the unlocked position to the locked position, and wherein the energizing force is greater than the force applied to the activation ring. 
     
     
         7 . The wellhead assembly of  claim 1 , wherein the outer tubular comprises a wellhead housing and the inner tubular comprises a tubing hanger. 
     
     
         8 . A system for locking together tubulars that are disposed in a wellhead assembly comprising:
 a lock ring that axially rests on one of the tubulars and selectively engages an adjacent tubular thereby axially locking together the one of the tubulars and the adjacent tubular;   an activation ring axially moveable to between the lock ring and the one of the tubulars and having a surface in sliding contact with the lock ring that transitions from a curved profile to a linear profile as the activation ring moves to between the lock ring and the one of the tubulars.   
     
     
         9 . The system of  claim 8 , wherein the activation ring contacts the lock ring along an interface that is offset from an axis of the wellhead assembly by an angle up to about 5 degrees when the lock ring is engagement with the adjacent tubular. 
     
     
         10 . The system of  claim 8 , wherein the activation ring contacts the lock ring along an interface that is offset from an axis of the wellhead assembly by an angle that ranges from at least about 5 degrees to about 30 degrees when the curved profile is in contact with the lock ring. 
     
     
         11 . The system of  claim 8 , wherein the one of the tubulars is a tubing hanger and the adjacent tubular is a wellhead housing. 
     
     
         12 . The system of  claim 8 , wherein the one of the tubulars is wellhead housing and the adjacent tubular is a tubing hanger. 
     
     
         13 . The system of  claim 8 , wherein an upper end of the activation ring is in axial contact with a seal assembly, where the seal assembly is energized with an axial force that exceeds a force applied to slide the activation ring from a position above the lock ring to a position adjacent the lock ring. 
     
     
         14 . A method of locking together tubulars in a wellhead assembly comprising:
 providing a lock ring on a lateral surface of one of the tubulars;   applying a force onto the lock ring in a direction oblique with an axis of the wellhead assembly to radially urge the lock ring towards an adjacent tubular;   changing the direction of the force to be substantially perpendicular to the axis and engaging the lock ring with the adjacent tubular; and   retaining the lock ring in engagement with the adjacent tubular by continuing to apply the force in a direction substantially perpendicular to the axis.   
     
     
         15 . The method of  claim 14 , further comprising providing an activation ring having a contact surface that transitions from a curved portion to a linear portion. 
     
     
         16 . The method of  claim 15 , wherein the step of applying a force onto the lock ring in a direction oblique with an axis of the wellhead assembly comprises axially urging the activation ring so the curved portion slides against a side of the lock ring. 
     
     
         17 . The method of  claim 15 , wherein the step of applying a force onto the lock ring in a direction perpendicular to an axis of the wellhead assembly comprises axially urging the activation ring so the linear portion slides against a side of the lock ring. 
     
     
         18 . The method of  claim 15 , wherein the step of continuing to apply the force in a direction substantially perpendicular to the axis comprises retaining the activation ring adjacent the lock ring so the linear portion is in contact with a side of the lock ring.