Page 10 - 4925

The operator may make additional preparations before moving in the rig. The
terrain, the well’s depth, the underground pressures expected, and the operator's and
contractor's preferences determine how they start the well. At land sites where the
operator has ordered a deep, high-pressure well, for example, a work crew, using dirt
moving equipment, may dig a rectangular pit, or cellar. Sizes vary, but a typical cellar
is about 10 feet (3 metres) on a side and perhaps 10 feet (3 metres) deep. The exact
size and depth depend on the characteristics of the well and the rig's configuration.

Sometimes, the workers line the cellar with boards or pour concrete walls to
keep it from caving in. The cellar accommodates a tall stack of high-pressure control
valves under the rig. The bottom of the stack will sit in the cellar, below ground level.
Since the crew installs the stack below ground level, the rig’s substructure – the base of
the rig – does not have to be as tall to allow the rig floor to clear the stack. In short, a
cellar provides more working room under the rig.

Rathole

Some rigs use a special pipe called the “kelly," which is part of the drill string.
The kelly is part of the system that rotates the bit. Rigs with kellys require a rathole –
a shallow hole drilled off to the side of the main borehole. On land, the operator
sometimes hires a special truck-mounted, light-duty unit called a “rathole rig” to drill
the rathole. Or, after the rig is set up (rigged up), the drilling crew may drill the rathole
with special equipment. Offshore, if the rig needs a rathole, it is a large-diameter
length of pipe that extends below the rig floor. In the case of drilled ratholes, the crew
extends pipe from the drilled part of the rathole up to the rig floor. The rathole goes
through the rig floor and protrudes a few feet, or a half metre or so, above it.
During drilling, the crew uses the rathole to store the kelly temporarily. A kelly
can be up to 54 feet, or 17 metres, long. Even the tallest land rig substructures are
only about 40 feet(12 metres) high and most are even shorter. The contractor there-
fore has to drill part of the rathole; otherwise, the rathole would extend too high
above the rig floor to be accessible.

Mousehole
The rathole rig or the main rig itself may also drill a mouseholeon land sites. A
mousehole, like a rathole, is also a shallow hole lined with pipe that extends to the rig
floor. The mousehole is a lined hole into which the crew puts a length, or joint, of drill
pipe during drilling operations. When crew members are ready to add the joint to the
drill string as the hole deepens, they add it from the mousehole. A joint of drill pipe is
around 3o feet (9 metres) long. If the regular rig’ssubstructure is appreciably shorter
than this height, then the rathole crew also drills a mousehole.

Conductor Hole

The rathole crew may also drill the first, or top, part of the main borehole. The
operator can, in some cases, save time and money by having the rathole rig actually
start, or spud, the main hole before moving in the regular rig. The rathole crew backs
the rathole rig to the cellar. A special bit starts the main hole in the middle of the
cellar. This hole is shallow in depth but large in diameter. Termed conductor hole, it

9