Page 119 - Drilling Technology in Nontechnical Language

110 Drilling Technology in Nontechnical Language Second Edition

In addition, there are some special units available for working on oil
and gas wells that would not be classed as rigs, such as coiled tubing units
(CTU), snubbing units, and workover hoists. Coiled tubing units consist

of a large drum containing a reel of steel pipe that is quite flexible. This
pipe can be lowered into the well to perform various operations, such as

pumping fluids, manipulating downhole tools, running logs in difficult or
high-angle wells, or even drilling. Snubbing units allow pipe to be forced
into the wellhead under pressure, so that jobs can be carried out without
having to kill the well and risk damaging it. Workover hoists are similar to
a very small rig and may be used to pull or run pump rods out or in, or to
perform small jobs that do not require much power.
The maximum load that can be suspended within the derrick or mast
is also a consideration. Deeper wells require a stronger derrick because
the well will require deeper strings of casing to be run, which will
weigh more.
The cost of the rig is an important consideration. A better, more highly
equipped rig will cost more, but the performance gain should offset at least
some, if not all, of the extra overall cost.
The requirement for a rig is dictated by the drilling program, working
environment, rig availability, and cost.

Classi cations of Drilling Rigs—Descriptions

Each classification of rig shown in figure 5–1 is described in the

following sections.

Heavy land rig

A heavy land rig (fig. 5–2) will be suitable for drilling deep or very

deep wells (over 10,000 ft). The maximum load that the derrick is capable
of suspending will equal or exceed 1,000,000 lb. The rig will have two,
possibly three, high-pressure pumps (described later) for circulating the
drilling fluid around the well. The blowout preventers available for use as

drilling progresses will be high pressure (rated to 10,000 psi or higher).

Storage capacity for consumable fluids and powders will be high; it will
need to store and use large volumes of drilling fluid, diesel oil, cement

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