Page 470 - Offshore Electrical Engineering Manual

Drilling Essentials 457

A.2 DRILLING ESSENTIALS

DRAW WORKS
A rotating column of hollow steel pipe made up of sections screwed together, called
a drill ‘string’ fitted at the bottom with a cutting bit, is hung from a ‘swivel’ connec-
tion inside the derrick, a strong steel pylon, usually 45–55 m high. A wire cable runs
through heavy pulleys to the ‘draw works’ which raise and lower the drill string. The
draw works is driven by a direct current (DC) motor variable-speed drive controlled
from the ‘driller’s console’.

ROTARY TABLE
On the floor of the derrick is a rotary table with an opening in the middle through
which slides the top section of the drill string, a square or hexagonal section pipe
called the ‘kelly’. The rotary table is also driven by a DC motor variable-speed drive
controlled from the driller’s console. The rotary table turns the kelly and hence the
whole drill string. As the drill bit cuts deeper, extra sections of drill pipe are added to
the drill string. The whole string will require to be lifted out to replace worn, dam-
aged or inappropriate drill bits.

TOP DRIVE
As an alternative to using a rotary table to drive the rotation of the drill string, a top
drive is utilised. This is composed of one or more electrical or hydraulic motors,
which is connected to the drill string via a short section of pipe known as the ‘quill’.
Back torque from the drill string has to be prevented from twisting the top drive, so
it slides on a rack attached to the derrick. One benefit of using the top drive is that it
allows the drilling rig to drill a longer section or stand of drill pipe in one operation.
A rotary table rig can only drill 30-ft (9.1 m) (single drill pipe) sections of drill pipe,
whereas a top drive can drill 60- to 90-ft (18–27 m) stands (double and triple drill
pipe, respectively, a triple being three joints of drill pipe screwed together), depend-
ing on the drilling rig size. This saves a lot of time and money each time the whole
drill-string needs to be withdrawn when it is necessary to replace the drill bit, for
example.

MUD PUMPS
During drilling operations, a special fluid is continuously pumped through the top of
the kelly and down the hollow drill string to cool and clean the revolving bit below.
The fluid also acts as a conveyor of drill cuttings to the surface and a means of sealing
and supporting the wall of the hole.
The fluid is basically composed of special clay suspended in water or oil and
known as ‘mud’. Its composition must be varied according to conditions and may