Page 269 - Drilling Technology in Nontechnical Language

260 Drilling Technology in Nontechnical Language Second Edition

Apart from sealing on irregular-shaped tubulars, the annular preventer
can also be used to allow pipe movement in or out of the well under
pressure. Moving pipe in a closed well when the well is under pressure is
called stripping. This may be useful, for instance, if a kick occurs when

little or no pipe is in the hole. In order to kill the well, heavy fluid has to
be pumped through the drillstring to the bottom of the hole. If the pipe is
not deep enough, more pipe can be added until it is deep enough to kill
the well by stripping pipe in through the bag preventer. This is possible
because the rubber element can move to accommodate the thicker tool
joints moving down through the element.

To strip in, the hydraulic closing pressure on the piston is reduced
until a slight amount of mud starts to leak through the seal. This provides
some lubrication. Grease is put onto the tool joints, and the pipe is moved
downwards slowly to minimize wear on the rubber seal.

Ram-type preventer

The other type of preventer uses a pair of large steel rams that shut under
high hydraulic pressure, with great force. These rams are interchangeable
and are of different types:
1. Fixed pipe ram. The seal is sized to fit one outside diameter only

(see fig. 11–3).

2. Variable bore pipe ram. The seal element can accommodate a
narrow range of diameters, for example 3½" to 7". It can only seal

on round pipe, not square or hexagonal shapes (see fig. 11–4).
3. Blind rams. Blind rams are designed to seal on the open hole.

4. Blind-shear rams. Blind-shear rams have blades incorporated
that can cut through pipe (though not through drill collars or
casing) and can also form a seal when closed.
5. Casing shear rams. These are heavy-duty shear rams that do
not seal but can cut through heavy pipe, such as casing. These
are found in subsea BOP stacks and are used if the rig needs
to disconnect from the BOP in an emergency. In this case, the
ability to cut whatever pipe is through the BOP is critical. A
subsea BOP will also have a set of blind-shear rams that can be
closed after the casing shear rams have closed.

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