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does not move.
If, on a rig visit, you look at the crown and traveling blocks, you may not
realize just how large they are because of the distance you see them from. The
sheaves around which the drilling line passes are often 5 feet (1.5 metres) or more in
diameter, and the pins on which the sheaves rotate may be 1 foot (30 centimetres) or
so in diameter. That's big. Incidentally, the number of sheaves on the crown block
always numbers one more than the number of sheaves on the traveling block. For
instance, a ten-line string requires six sheaves in the crown block and five in the
traveling block. The extra sheave in the crown is needed for reeving the deadline.
Attachments to the traveling block include a spring to act as a shock absorber and a
large hook from which crew members suspend the drill string.

Masts and Derricks

Masts and derricks are tall structural towers that support the blocks and drilling
tools. They also provide height to allow the driller to raise the drill string so crew
members can break it out and make it up. As mentioned earlier, a mast is a portable
derrick that crew members can raise and lower as a unit. A standard derrick, on the
other hand, requires that crew members assemble and disassemble it piece by piece;
they cannot erect it or take it down as a single unit. Most rigs today use masts
because they rig up and down much quicker than standard derricks.
Manufacturers have to make masts strong and, at the same time, they have to
make them portable. Consider that on a deep well, the loads that a mast may support
can be as high as 2 million pounds, or 1,000 tons (907 tonnes). Yet, after finishing up
one hole, crew members usually move the rig several miles to begin another. Some of
the specifications manufacturers use to rate masts and derricks are their height, the
vertical load they can carry, and the wind load they can withstand from the side. For
example, a mast may be 136 feet (41.5 metres) tall, be able to support 275 tons (249
tonnes), and be capable of withstanding 100-mile-per-hour (161- kilometre-per-hour)
winds. These specifications are impressive when you consider that such a strong
piece of equipment is also relatively easy to move.

ROTATING SYSTEMS

Rotating equipment turns the bit. Generally, rigs can rotate the bit in one of
three ways. The traditional way, the method that still dominates drilling, especially on
land sites, uses a rotary table and kelly. A second way uses a top-drive system, which
drilling contractors began to employ widely in the 1980s. A third way uses a dowhole
motor, which contractors use in the 1980s. A third way uses a downhole motor, which
contractors use in special cases.
Rotary-Table System

Today, many contractors, especially those offshore, employ top drives on their
rigs. However, many rigs still use the rotary-table system to rotate the drill string and
bit. A rotary-table system consists of five main parts: (1) a rotary table with a
turntable, (2) a master bushing, (3) a kelly drive bushing, (4) a kelly, and (5) a swivel.

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