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ability to close the well immediately, independent of normal rig
power. For safety, stand-by accumulator pumps are maintained
that use of secondary power source. The accumulator fluid is
usually the non-corrosive hydraulic oil with a low freezing point.
The hydraulic oil should also have good lubricating characteristics
and must be compatible with synthetic rubber parts of the well
control system.

Figure 12.2 – Annular preventer

The accumulator is equipped with the pressure regulating
system. The ability to vary the closing pressure on the preventers
is important when it is necessary to strip pipe (the lower pipe with
the preventer closed) into the hole. If a kick is taken during the
trip, it would be better to strip back to the bottom to allow efficient
circulation of the formation fluids from the well.
The application of too much closing pressure to the preventer
during stripping operations causes rapid wear of the sealing element.
The usual procedure is to reduce the hydraulic closing pressure during
stripping operations until there is a slight leakage of well fluid.

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