Drill string valve and method

A drill string valve, comprising:

• a tubular housing having an axis and threaded upper and lower ends for connection into a drill string;
• an axially movable valve element carried in the housing, the valve element having a closed position and an open position that allows drilling fluid to be pumped downward through the housing;
• a spring having an upper end in cooperative engagement with the valve element for biasing the valve element to the closed position;
• a preload piston carried in the housing, defining a preload fluid chamber;
• a preload port for injecting a preload fluid into the preload fluid chamber prior to lowering the housing into the well at a pressure sufficient to cause the preload piston to move a first end of the spring to a selected preload position relative to a second end of the spring;
• and
• a threaded valve stop that is axially movable relative to the housing in response to rotation into engagement with the first end of the spring while the first end spring is held in the preload position by the preload piston, to enable the pressure applied to the preload fluid to be removed without the first end of the spring moving back away from the second end of the spring.

A well drilling operation comprising downward pumping drilling fluid through a drill string into the well, discharging the drilling fluid out a drill bit and returning the drilling fluid up an annulus in the well surrounding the drill string, a method of preventing the drilling fluid from continuing to flow downward in the drill string in the event the downward pumping ceases, comprising:

• mounting in the drill string a drill string valve having a movable valve element with a closed position and an open position, and a spring that is set to exert a bias force to the valve element toward the closed position;
• lowering the drill string into the well and exerting an annulus force against the valve element toward the closed position corresponding to an annulus pressure of the drilling fluid in an annulus immediately surrounding the drill string valve;
• applying sufficient downward pumping pressure to exceed the bias force plus the annulus force to cause the valve element to open;
• and
• wherein the spring is set such that if the downward pumping ceases and a downward hydrostatic force on the valve element due to the column of drilling fluid in the drill string above the valve element exceeds the annulus force, the upward bias force plus the upward annulus force will close the spring, and wherein the spring is set prior to lowering the drill string into the well by forcing a first end of the spring toward a second end of the spring to a preload position, and preventing the first end from moving back away from the preload position.