most cases. Limited subsea capability presently 
exists to ocean depths of 600 feet. 
Subsea wellhead equipment may be used when 
directional wells drilled from a single, deepwater 
platform cannot reach all parts of a reservoir. In 
this case, satellite underwater completions could 
be used. However, almost all existing underwater 
completions are for wells producing directly to 
shore without a production platform; several such 
installations exist off the coasts of California and 
Peru. 
3. Characteristics of Subsea Production 
The industry already is studying and developing 
methods for sea-floor well completions, for pro- 
duction and collection techniques, and for separa- 
tion, treatment, and storage facilities. The partic- 
ular choices made by the operator depend on the 
size of the field, the location, depth, etc. 
a. Underwater Christmas Tree The heart of the 
well system is the underwater christmas tree—a 
series of pipes and valves at the wellhead used to 
control the well during drilling and production 
phases (Figure 42). It is installed on the subsea 
landing base by a mobile rig during or after 
drilling. Although most underwater trees are de- 
signed for installation by remote control, divers 
often must lend a hand. Recently an experimental 
robot was built to perform limited operations on a 
specially designed christmas tree. 
b. Divers The use of divers is being extensively 
investigated, and studies are under way to deter- 
mine the usefulness of divers working with diver- 
lockout submersibles. Divers for years have had 
the capability to work at moderate depths, but 
only recently they have extended their operations 
considerably below 200 feet. Even so, diving at 
great depths is considerably more costly than in 
shallower water where decompression time is 
much less. 
The choice of whether divers should be an 
element in the system also affects the choice of a 
specific technological system. Now that divers can 
work at 600 feet or more, it is possible to choose 
between diver-assisted completion methods, or 
automated or remote control completion systems. 
These choices also are available for subsea installa- 
tion inspection and maintenance (Figure 43). 
Figure 42. Underwater christmas tree, installed 
after drilling and completion operations, being 
emplaced in Gulf of Mexico. (Shell Oil photo) 
Figure 43 Diving bell and decompression 
chamber. Using saturation diving techniques, 
divers can perform useful work to depths of 
600 feet. (Ocean Systems photo) 
In deeper continental shelf areas, a fixed 
platform can be erected to extend up to shallower 
depths and hence be more readily accessible to 
divers and yet deep enough to escape the major 
effect of surface waves. Controls, instruments, 
power sources, etc. could be located on the 
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