Page 273 - Drilling Technology in Nontechnical Language

264 Drilling Technology in Nontechnical Language Second Edition

upstream of the choke. How this is used during killing the well is described
later in this chapter.
It would be possible to use a normal valve as a choke. However, mud
that contains solids (barite, bentonite, sand, or other drilled particles)
is quite abrasive when flowing through a restriction at high pressure. A

normal valve would soon erode and fail to hold pressure if it were used to

exert pressure on the flowing mud. A choke valve is designed to handle this
operation with minimum erosion, by its design and by the use of tungsten
carbide internal components. It is still possible for the choke to become
eroded during a well killing operation, and so the rig must carry spares of
these parts. There must also be valves positioned upstream of the choke
that can be closed to allow the choke to be repaired.

BOP control systems

BOP units (bag and ram preventers and some valves) are moved using

hydraulic fluid under pressure (fig. 11–7). To provide this pressure, a
hydraulic control system is used that contains several elements:

1. A reserve hydraulic fluid tank holding fluid at atmospheric
pressure.
2. A set of bottles holding fluid under high pressure (usually

3,000 psi) with pressurized nitrogen.
3. A high-pressure manifold connected to the bottle system.

4. A low-pressure manifold that contains fluid at the working
pressure of the ram preventers (usually 1,500 psi).

5. A pressure regulator that feeds fluid from the high-pressure
manifold to the low-pressure manifold and that reduces the
pressure to the working pressure.
6. A set of valves attached to the low-pressure manifold that can

direct working pressure fluid to the rams (to open or to close

them) and that directs exhaust fluid back to the reserve hydraulic
fluid tank.

7. A valve that controls the opening and closing of the
bag preventer.

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