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 What are the two main categories of rigs?

 What are the differences between land rigs and offshore rigs?

 What does the word “portability” mean?

 Are both land and offshore rigs portable?

 What is MODU?

 What types of submersibles do you know?

 Describe the structure of a bottle-type rig.

 Which types of barges are sometimes called "swamp barges"?

ROTARY RIG TYPES
Many kinds of rotary drilling rigs are available, particularly offshore where the marine
environment plays an important role in rig design. Two broad categories of rig are those that work
on land and those that work offshore. Some experts like to create a third category: rigs that work in
inland waters. Inland rigs usually drill in lakes, marshes, and estuaries, places that are neither land
nor offshore, places where, as one wit put it, "it's too wet to plow and too muddy to drink." For our
purposes, though, dividing rotary rigs into land and offshore types is acceptable, because inland rigs
also drill in water, even if it is shallow.
LAND RIGS
Land rigs look much alike, although details vary. A major difference is their size, and size
determines how deep the rig can drill. Well depths range from a few hundred or thousand feet
(metres) to tens of thousands of feet (metres). The depth of the formation that contains, or is
believed to contain, oil and gas controls well depth. Classified by size, land rigs are light duty,
medium duty, heavy duty, 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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