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When the drill string is in the hole, the BOP stack can be
used to stop only the flow from the annulus. Several additional
valves can be used to prevent flow from inside the drill string.
These valves include Kelly cocks – and internal blowout
preventers. Generally, an upper Kelly cock having left-hand
threads is placed above the Kelly and a lower Kelly cock having
right-hand threads is placed blow the Kelly. The lower Kelly
cock also is called a drill stem valve. Two Kelly cocks are
required because the lower position might not be accessible in an
emergency if the drill string is stuck in the hole with the Kelly
down.
An internal BOP is a valve that can be placed in the drill
string if the well begins flowing during tripping operations. Ball
valves can be used as an internal BOP. In addition, dart-type
(check-valve) internal BOP's are available. This type of
internal BOP should be placed in the drill string before drill pipe
is stripped back in the hole because it w il l permit mud to be
pumped down the drill string after reaching the bottom of the
well. Internal BOP’s are installed when needed by screwing into
the top of an open drills ring with the valve or dart in the open
position. Once BOP is installed, the valve can be closed or t he
released.
A high-pressure circulating system is used for well control
operations. The kick normally is circulated from the well
through an adjustable choke. The adjustable choke is
controlled from a remote panel on the rig floor. Sufficient
pressure must be held against the well by the choke so that the
bottom hole pressure in the well is maintained slightly above
the formation pressure. Otherwise, formation fluids would
continue to enter the well.
Mechanical stresses on the emergency high-pressure
flow system can be quite severe when handling a kick. The
rapid pressure release of large volume of fluid through the
surface piping frequently is accompanied by extreme

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