Page 68 - Hydrocarbon Exploration and Production Second Edition

Drilling Engineering 55

swivel

kelly

kelly-saver sub

drillpipe

drill string stabilizer
drill collars

bit
Figure 4.8 The drill string (schematic).

the hole to the bit. It may take a number of turns of the rotary table to initially turn
the bit thousands of meters down the hole.
The kelly is hung from the travelling block. Since the latter does not rotate, a
bearing is required between the block and the kelly. This bearing is called a swivel.
Turning the drill string in a deep reservoir would be the dimensional equivalent to
transmitting torque through an everyday drinking straw dangling from the edge of a
75-storey high-rise building! As a result, all components of the drill string are made
of high-quality steels.
After the drilling has progressed for some time, a new piece of drill pipe will
have to be added to the drill string (see below). Alternatively, the bit may need to be
replaced or the drill string has to be removed for logging. In order to ‘pull out of
hole’, hoisting equipment is required. On a basic rotary rig this consists of the hook
which is connected to the travelling block. The latter is moved up and down via a
steel cable (block line) which is spooled through the crown block onto a drum
(draw works). The draw works, fitted with large brakes, move the whole drill string
up and down as needed. The derrick or mast provides the overall structural support
to the operations described.
For various reasons, such as to change the bit or drilling assembly, the drill string
may have to be brought to surface. It is normal practice to pull ‘stands’ consisting of
90 ft sections of drill string and rack them in the mast rather than disconnecting all
the segments. The procedure of pulling out of hole (POOH) and running in again
is called a round trip.