Page 47 - 4274

the integral riser system.
Pure electric control or multiplex systems are also being
manufactured where electric signals and electrical power are sent
to the BOP stack through a single, multicore conductor connected
to each control pod. Acoustic control systems generally are not
being considered as a prime operating system but are being
employed as a backup control system for subsea BOPs.
If and when a higher-then-expected well bore pressure is
encountered while drilling, the annular and ram preventers can be
closed to seal around the drill pipe. The pressure buildup is
controlled by circulating heavier drilling mud through the drill
pipe and back through the choke or kill lines. Once the mud
column pressure exceeds the formation pressure, the preventers
can be opened and drilling may continue.
In the event sea conditions make it imperative to disconnect
from the BOP stack, an emergency drill pipe hangoff tool can be
landed in the subsea wellhead housing. The well can then be
sealed by closing the bottom pipe ram around the drillstem of the
emergency drill pipe hangoff tool. The running tool is released
from the hangoff tool, and the remaining drill pipe may be
retrieved. At this point, the blind ram is closed above the hangoff
tool and the hydraulic connector in the lower riser package is
disconnected. The drilling vessel is then in a position to ride out
the emergency, leaving the well in a secure and shutin condition.
During the reconnection and reentry procedure, it is important to
circulate through the choke-and-kill lines so any pressure built up
under the closed rams is bled off before the drill pipe is lowered
and reconnected.
Only under extreme emergency conditions are the blind
shear rams closed on a drillstring. In such case, one of the pipe
rams below the shear rams preferably would be closed first, just
below a drill pipe tool joint. In this way. the well can be circulated
through the choke-and-kill lines before the blind shear rams are
reopened. The cut section of the drill pipe is in the sealed area
between the blind shear ram and the pipe ram. The sheared end of
the drill pipe is open enough to allow mud from the kill line to
enter the drill pipe and be circulated to the surface through the
choke line below the closed pipe ram.
Since the BOPs are the primary means of controlling the
well in the event of a kick, the importance of their proper operation

47