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and very heavy duty. Table 1 arranges them according to this scheme
and shows the depths to which they can drill.
Keep in mind, though, that a rig can drill holes shallower than
its maximum rated depth. For example, a medium-duty rig could drill
a 2,500-foot (750-metre) hole, although a light-duty rig could also
drill it. On the other hand, a rig cannot drill too much beyond its rated
maximum depth, because it cannot handle the heavier weight of the
drilling equipment required for deeper holes.
Another feature land rigs share is portability. A rig can drill a
hole at one site, be disassembled if required, moved to another site,
and be reassembled to drill another hole. Indeed, land rigs are so
mobile that one definition terms them "portable hole factories." The
definition sounds odd, but it is accurate.
MOBILE OFFSHORE RIGS
A widely used offshore drilling rig is a mobile offshore drilling
unit, or MODU, for short (pronounced "mow-du"). Another is a
platform. Although drilling occurs from platforms, companies mainly
employ them on the producing side of the oil and gas business. This
book concentrates on drilling, so it does not cover platforms.
However, more information about platforms is available in the
PETEX publication, A Primer of Offshore Operations.
MODUs are portable; they drill a well at one offshore site and
then move to drill another. MODUs are either floaters or bottom-
supported. When drilling, floaters work on top of, or slightly below,
the water's surface. Floaters include semisubmersibles and drill ships.
They are capable of drilling in waters thousands of feet (metres) deep.
MODUs that contact the ocean bottom and are supported by it are
bottom-supported. Bottom-supported units include submersibles and
jackups. Submersibles are further divided into posted barges, bottle
types, inland barges, and arctic. Generally, bottom-supported rigs drill
in waters shallower than floaters. Table 2 lists MODUs.
Bottom-Supported Units

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