Page 571 - Air and Gas Drilling Manual

Chapter 11: Specialized Drilling Equipment 11-9
tubulars are attached to the top of the working string and the process repeated until
the entire working string is inserted.
Figure 11-8 shows a BOP stack with the snubbing unit equipment attached.
This combination is called the snubbing stack. Because of safety considerations, the
BOP stack for any snubbing operation must have at the minimum a double BOP
configuration. This has a safety pipe ram on top (to fit the pipe being inserted) and
a blind ram on the bottom to secure the well when no pipe is in the well. The
example shown in Figure 11-8 shows an annular BOP on the top of the BOP stack
above the double BOP configuration. The annular BOP working together with the
pipe ram allows a working string with tool joints to be safely inserted during the
snubbing operation. The pipe ram BOP and the annular BOP seal gas from escaping
around the working string outside surface.

Figure 11-8: Snubbing stack rig up. The arrows show where the double ram type BOPs
are located on the stack (courtesy of Cudd Pressure Control, Incorporated).