7/28/2019 Drilling Driving Mechanisms 01

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Railsback'sPetroleum Geoscience and Subsurface Geology

Driving mechanisms for drilling in the early 2000s

Sources: Schlumberger Oilfield Glossary; Rigzone's How it Works.

Mud motor

Topdrive or Top DriveKelly Bushing

In this mechanism, the drill string of pipedescending from the drilling floor to the bithangs from a four-side or six-sided piece ofpipe or hollow bar called the Kelly. The Kellypasses through the Kelly Bushing, a rotatingbushing with a four-sided or six-sided central

hole. The four-sided or six-sided nature ofboth that hole and the Kelly allows rotation ofthe Kelly Bushing to turn the Kelly and thusto turn the entire drill string. The central holeof the Kelly Bushing meanwhile lets the Kelly,and the entire drill string, slip downward asthe drill bit advances into the Earth.

This was the mechanism used onalmost all petroleum drilling rigs in the 1900s,and the elevation of the Kelly Bushing wasalmost always the datum for well log depths.Rigs built in the 2000s have instead been fittedwith Top Drive, but many rigs using the KellyBushing remain in operation, especially inonshore drilling.

In this mechanism, the drill string of pipedescending from the drilling floor to the bithangs directly from a large motor called theTopdrive. The motor, an electric or hydraulicdevice, hangs above the drilling floor from thehoist, and thus it can be lowered as the drillbit

progresses into the Earth.In this mechanism, the Kelly Bushing andKelly are no longer needed, allowing fasteraddition of new pipe to the drill string asdrilling progresses. Addition of 90-foot standsof pipe, rather than 30' stands, also lessensthe chances of sticking the drill string duringstops to add pipe.

This mechanism was introduced inthe 1990s. It was soon widely used offshore,but older rigs using the Kelly Bushing remainin operation, especially in onshore drilling.

A mud motor is a device in the bottom-hole of assembly of the drill string, just abovethe drill bit. The passage of drilling mudthrough this device turns the motor, and therotation of the motor is transferred downwardto turn the bit. This mechanism is useful fordirectional drilling, in that the bottom-holeassembly can be turned to a new directionindependent of the drillstring above.

The drilling of a

borehole requires(1) turning the drill-bit at the bottom ofthe hole, so that thebit excavates morehole, and(2) letting the bitand long string ofdrilling pipe fromwhich it hangsslip down into thenewly excavatedhole. This pageillustrates threesolutions, two inwhich power at thesurface turns bothpipe and drillbit, anda third in which the

drillbit is turned inde-pendently at the bot-tom of the drillstringof pipe.