Page 34 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

The swab valve. The swab valve is used to gain access to the well for
wireline operations, intervention and other workover procedures (see below).
On top of it is a tree adapter and cap that will mate with a range of
equipment.

The variable flow choke valve. The variable flow choke valve is typically a
large needle valve. Its calibrated opening is adjustable in 1/64 inch
increments (called beans). High-quality steel is used in order to withstand
the high-speed flow of abrasive materials that pass through the choke,
usually over many years, with little damage except to the dart or seat. If a
variable choke is not required, a less expensive positive choke is normally
installed on smaller wells. This has a built-in restriction that limits flow when
the wing valve is fully open.

This is a vertical tree. Christmas trees can also be horizontal, where the
master, wing and choke are on a horizontal axis. This reduces the height
and may allow easier intervention. Horizontal trees are especially used on
subsea wells.

3.5.1 Subsea wells
Subsea wells are essentially the
same as dry completion wells.
Mechanically however, they are
placed in a subsea structure
(template) that allows the wells to
be drilled and serviced remotely
from the surface, and protected
from damage e.g. from trawlers.
The wellhead is placed in a slot in
the template where it mates to the
outgoing pipeline as well as
hydraulic and electric
control signals. Ill:
StatoilHydro

Control is from the
surface where a hydraulic
power unit (HPU)
provides power to the
subsea installation via an
umbilical. The umbilical
is a composite cable
containing tension wires,

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