Circulation subassembly

A valve assembly for controlling flow of fluid through a bore of a piston, the assembly comprising a valve having a first condition and a second condition, an actuation assembly for changing the valve between the first condition and the second condition, the piston, and a sleeve in which the piston is located,

• wherein:
• the piston is movable to a first axial position, a second axial position and a third axial position relative to the sleeve, the second axial position being between the first and third axial positions;
• when the piston is moved from the second axial position to the third axial position the actuation assembly causes the valve to change from the first condition to the second condition;
• when the piston is moved from the third axial position to the second axial position the actuation assembly causes the valve to change from the second condition to the first condition;
• and
• the actuation assembly does not change the condition of the valve when piston is moved from the second axial position to the first axial position or from the first axial position to the second axial position, wherein the first condition is an open condition in which the valve allows fluid to flow through the bore of the piston and the second condition is a closed condition in which the valve does not allow fluid to flow through the bore of the piston.

A circulation subassembly (circsub) for incorporation in a drill pipe, the circsub comprising

• a piston movable within a bore of the circsub in a first axial direction by pressure of fluid within said bore, the piston having:
• a first axial position corresponding to a flow through condition of the circsub in which the circsub allows fluid to flow through a bore of the circsub and does not allow fluid flow from the bore of the circsub through bypass orifices in the circsub into an annulus located outside the drill pipe;
• a second axial position corresponding to a partial bypass condition of the circsub in which the circsub allows fluid to flow through the bore of the circsub and allows fluid communication through said bypass orifices from the bore of the circsub into the annulus;
• and
• a third axial position corresponding to a full bypass condition of the circsub in which the circsub does not allow fluid to flow through the bore of the circsub and allows fluid communication through said bypass orifices from the bore of the circsub into the annulus,
• wherein the circsub further comprises biasing means for biasing the piston in a second axial direction opposite to the first axial direction, and a selectively adjustable abutment for limiting the movement of the piston in said first axial direction to each of said first, second and third axial positions;
• wherein the circsub comprises a valve having an open condition in which flow through the bore of the circsub is allowed and a closed condition in which flow through the bore of the circsub is substantially prevented, wherein the valve is configured to assume the open condition when the circsub is in the flow through condition or the partial bypass condition and to assume the closed condition when the circsub is in the full bypass condition;
• and
• wherein the valve is located within the circsub by a frangible abutment, wherein the frangiable abutment is configured to break when the valve is in the closed condition and the pressure in the bore of the circsub is greater than a second threshold value, the valve being configured to move within the circsub to a position at which drilling fluid may flow around the valve when the frangible abutment breaks.