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Cellar

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 therefore 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 mousehole on 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’s substructure 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 may be 36 inches (91 centimetres) or more in diameter. It may be only
tens of feet (or metres) deep or it may be hundreds of feet (or metres) deep, depending on the
surface conditions.
The rathole crew lines the conductor hole in the cellar with conductor pipe. Conductor pipe,
or casing, keeps the hole from caving in. It also conducts drilling mud back to the surface when
regular drilling begins. The crew often secures the conductor pipe in the hole with cement or con-
crete. With the conductor pipe, rathole, and mousehole prepared, the drilling contractor can move in
the rotary rig to drill the rest of the hole.

Other Considerations
On drilling locations where the ground is soft, a rathole rig and crew may not be needed.

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