On drilling rigs equipped with an independent and mechanically driven rotary table, rotary speed is often found to be torque sensitive. This paper describes an automatic control system designed for use on these mechanical rigs to compensate for rotary speed variation with load.
Transfer functions for a loaded rotary table were developed using basic closed-loop control theory, and the components of the resulting control system are discussed. An analysis of the proposed system demonstrating optimum response and stability is given. The design objective of maximum dampening of speed variations without oscillation is demonstrated by simulation.
From this development, it has been concluded that a relatively simple control loop is sufficient for automating a rotary drive engine by positioning a pneumatic throttle. Further, when the control loop is used, the average rotary table speed becomes independent of pipe torque. The system has proved to be reliable under field operating conditions.
In applying technology to a drilling operation, rotary speed is of prime importance. For example, the "d" exponent, formation drillability, roller bit bearing and tooth wear as well as many other drilling relationships are functions of rotary speed.l-3 To use rotary speed in any of these equations, it must be measurable and fairly constant over the time interval of interest. The conventional method of controlling rotary speed on a rig, with an independent and mechanically driven rotary table is through the use of a pneumatic regulator at the drawworks. The regulator supplies air pressure to a diaphragm which mechanically positions the engine throttle. This method does not provide a constant rotary speed. Variations in rotary torque present a constantly changing load to the engine. Consequently, if the throttle is held constant, the angular speed of the rotary table will vary considerably. This effect is illustrated in Fig. 1, which is an actual recording of torque and rotary speed on a rig where the rotary speed was controlled in the manner described above. Note that the average value of rotary speed varied inversely with torque until the driller finally changed the throttle setting to bring the rotary speed back to its initial value. In order to keep rotary speed constant, the throttle would have to be continuously repositioned to compensate for the load changes. An automatic closed-loop control system can perform this operation.