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Equipment in Underbalanced Drilling Systems 169

Figure 8.3 A cutoff view of a rotating blowout preventer.

In foam UBD operations, the foam returned from the hole can be
either destroyed and discarded or recycled. Figure 8.4 shows a foam
separation system for disposal. The returned foam is destroyed by inject-
ing foam killer (acids) into the blooey line and is then separated by a
cyclone. Figure 8.5 shows a foam recycling system. The returned foam is
destroyed by feeding it with foam killer (acids) in the blooey line. After
separation, about 95% of foaming agents remain in the liquid phase. By
adding alkali to raise its pH value, the liquid will form foam with gas
again.
Snubbing units are employed in some underbalanced drilling operations.
They are similar in principle to those used in well workover operations
(Rehm, 2002).