Page 472 - Offshore Electrical Engineering Manual

Drilling Essentials 459

The casings are inserted in a preplanned sequence as each section of the well is
drilled and then cemented in place. If the producing layer is firm, such as limestone,
the final part of the hole need not be lined and the oil can be produced directly into
the bore well.

PERFORATION
If the producing layer is friable, a final string of casing sections is run right down to
the bottom, cemented in place and holes are shot through it with a perforating gun
to let in the oil.
This last operation usually requires radio silence to be maintained for several
hours on the platform, as operation of transmitters can inadvertently trigger the per-
forating gun before it is in place.
If the producing formation is loose sand, a slotted pipe called a ‘liner’ with a ‘gravel
pack’ or ‘wire filter’ round it can be hung from the casing in the producing rock strata.

CHRISTMAS TREES
At this stage, the well is still full of heavy drilling mud. Because the casing and liner
must remain in the well for a long time and their repair or replacement would be
expensive, another string of small-bore pipe, ‘the tubing’, hung from the wellhead
at the top of the casing, is run down to the foot of the well where it is sealed against
the production casing with a packer. This is the pipe string through which the oil and
gas flow to the surface.
The drilling mud is flushed out of the tubing with water, allowing the oil or gas
to emerge. Because of the high well pressures, often in hundreds of bars, the oil flow
needs careful regulation at the surface, and a cluster of valves and fittings which seal
off the well and control the flow, known as the ‘Christmas tree’, is a familiar sight in
documentaries about oil.

SUBSEA COMPLETIONS
Where other oil or gas fields are discovered adjacent to existing offshore production
facilities, it may not be necessary to install a new platform or floating facility.
Instead, a seabed wellhead facility is installed which, after production drilling,
allows oil and gas to be piped to the adjacent established production facility, either
onshore or offshore, for processing and export. The adjacent facility also acts as a
control point for operation of the subsea wellhead valves and the point of supply for
electrical equipment located on the subsea wellhead manifold.

DEVIATED DRILLING
With present day technology, it is possible to guide the direction of drilling so that
the well bore is deliberately deflected from the vertical in order to achieve good