Patent ID: 6679341
Filing Date: 2004-01-20
Classification: E21B

Abstract:
A tractor assembly, comprising a tractor for moving within a borehole, said tractor comprising:an elongated body having first and second pistons longitudinally fixed with respect to said body, each of said pistons having aft and forward surfaces configured to receive longitudinal thrust forces from fluid from a pressurized source, said body having a flow passage; a first gripper assembly longitudinally movably engaged with said body, said first gripper assembly having an actuated position in which said first gripper assembly limits relative movement between said first gripper assembly and an inner surface of said borehole, and a retracted position in which said first gripper assembly permits substantially free relative movement between said first gripper assembly and said inner surface, said first gripper assembly configured to be actuated by fluid; a second gripper assembly longitudinally movably engaged with said body, said second gripper assembly having an actuated position in which said second gripper assembly limits relative movement between said second gripper assembly and an inner surface of said borehole, and a retracted position in which said second gripper assembly permits substantially free relative movement between said second gripper assembly and said inner surface, said second gripper assembly configured to be actuated by fluid; an elongated first propulsion cylinder longitudinally slidably engaged with respect to said body, said first cylinder having an elongated internal propulsion chamber enclosing said first piston, said first piston slidable within and fluidly dividing said internal propulsion chamber of said first cylinder into an aft chamber and a forward chamber; an elongated second propulsion cylinder longitudinally slidably engaged with respect to said body, said second cylinder having an elongated internal propulsion chamber enclosing said second piston, said second piston slidable within and fluidly dividing said internal propulsion chamber of said second cylinder into an aft chamber and a forward chamber; and a valve system comprising: a propulsion control valve having a first position in which said propulsion control valve provides a flow path for the flow of fluid to said aft chamber of said first cylinder, said propulsion control valve having a second position in which said propulsion control valve provides a flow path for the flow of fluid to said aft chamber of said second cylinder, said propulsion control valve being movable from said first position to said second position in response to fluid pressure increases; and a gripper control valve having a first position in which said gripper control valve provides a flow path for the flow of fluid to said first gripper assembly, said gripper control valve having a second position in which said gripper control valve provides a flow path for the flow of fluid to said second gripper assembly; wherein in said first position said gripper control valve prevents fluid pressure from moving said propulsion control valve from said first position to said second position, and in said second position said gripper control valve permits fluid pressure to move said propulsion valve from said first position to said second position.