Page 173 - 4637

casing head, a drilling spool above the ram preventer and the two
ram preventers in series above the drilling and the annular
preventer above the ram preventer.
In some cases, it may be desirable to conduct drilling
operations with a slight surface pressure on the annulus. A
rotating head, which seals around the kelly at the top of the BOP
stack, must be used when this is done. A rotating-type BOP is
shown in Fig. 12.3. Rotating heads most commonly are
employed when air or gas is used as a drilling fluid. They also
can be used when formation fluids are entering the well very
slowly from low-permeability formations. However, this
practice is dangerous unless the formation being drilled has a
very low permeability. This must be established from experience
gained in drilling in the local area. For example, this practice is
known to be safe in the Ellenberger formation in some areas of
west Texas.

Figure 12.3 – Rotating heads

172