Until now direct-current motors fed by thyristor rectifiers have dominated as variable-speed drive systems for oil-drilling applications.
During drilling of a test well at Rogaland Research Center in Stavanger, Norway, a new drilling system "The Power Swivel" was tested and the conventional DC motor/SCR system was replaced by an asynchronous motor fed by a GTO frequency converter.
Whilst an enclosure of protection class Ex(P) is required for DC motors operating in hazardous area, asynchronous motors can also be built to protection class Ex(d), (e), or (n) for this type of application. protection class Ex(d), (e), or (n) for this type of application.
Drilling equipment used on board drilling rigs is normally powered by speed controlled direct current motors. A typical drilling motor has a power rating in the range 550 to 750 kW, armature voltage 720 to 750 power rating in the range 550 to 750 kW, armature voltage 720 to 750 VDC, is in Ex. (P) design, and equipped with air-water heat exchanger.
Fig. 1 shows a typical layout of the electrical system for a drilling rig. The three main drives are for the following equipment
Drawworks: Used for lifting and handling of the drill string and drilling equipment during drilling operation. Normally the drawwork is powered by two or three motors mechanically connected.
Mud Pumps: Large piston pumps pumping and circulating drilling mud (often called drilling fluid) into the well through the drill string. The drilling mud is used to remove small chips of rocks and debris from the well as it is drilled. It also cools and lubricates the drill bit and applies pressure in the hole to prevent it from caving in and to keep formation pressure from causing a blowout. Each pump is normally driven by two mechanically connected motors.
Rotary Table: The rotary table transmits the rotary movement to the drill string. It is powered by one motor.