Page 102 - Well Control for Completions and Interventions

94 Well Control for Completions and Interventions

3.1.4 The Christmas Tree
Not every well is equipped with a Christmas tree. Beam pump wells, for
example, only have a stuffing box. Where a tree is installed its main func-
tions are:
• Enable production (or injection) from the well to be controlled
through opening and closing of pressure containing valves. When
closed and tested, these valves form a mechanical barrier between the
reservoir and surface.
• Provide access for well intervention work.
• Provide a connection point for the flowline (or injection line).
• Provide a connection and seal at the tubing hanger and the wellhead.
• Isolate the well from other adjacent wells.
The configuration of the Christmas tree will vary depending on well
location, flow control requirements, pressure and temperature. Christmas
trees fall into one of two categories: conventional (vertical) trees and hor-
izontal (or spool) trees.

3.1.4.1 Conventional (vertical) Christmas trees
Most trees used on land or offshore platforms are the conventional (verti-
cal) design. On a vertical tree, the valves used to control flow are arranged
vertically through the center-line of the tree body. The bottom flange on
the tree is connected to, and will have the same pressure rating as, the
tubing head flange on the wellhead. The internal profile in the base of
the tree (or in the adapter flange) will mate over the tubing hanger neck
and create an internal seal. Conventional trees will either be composite
(made up from individual valves flanged together), or solid block (some-
times called mono-block) where the valves are housed in a solid casting
(Fig. 3.7). Each valve on a vertical Christmas tree will have a specific
function (Table 3.2).

3.1.4.2 The horizontal or spool tree
Horizontal trees were first used in the early 1990s, and were developed
primarily for subsea use. Unlike the vertical tree, the horizontal tree does
b
not need a bespoke riser system for running and interventions. More
recently, “dry” horizontal trees have been developed for land and plat-
form applications. It is important to understand the distinct differences
between the two tree types, as well intervention pressure control
b
Riser and intervention system for both vertical subsea trees and horizontal subsea trees
are described fully in Chapter 13, Subsea BOP and Marine Riser Systems.