Abstract
Stick-slip is a dysfunction of rotary drilling, characterized by large cyclic variations of the drive torque and the rotational bit speed. It is recognized as a major source of problems, such as excessive bit wear, premature tool failures and poor drilling rate.
This paper presents a new system for curing and preventing stick-slip motion. Like other competitive systems it fights the stick-slip oscillations by smart control of the drive. But in contrast to other active systems it does not use any kind of torque feed-back, not even the motor current. Fundamentally, the system is a PI-type speed controller that is tuned to effectively dampen torsional vibrations at the observed stick-slip frequency. In addition to automatic tuning of the speed controller the system includes a suite of support functions, such as automatic determination of the stick-slip frequency, estimation of the instantaneous bit rotation speed and calculation of the stick-slip severity, defined as the normalized downhole speed amplitude. All software is implemented in a standard programmable logic controller (PLC).
The paper also includes results from a field test and from Hardware-In-the-Loop (HIL) simulation tests. In the latter tests utilizing the HIL, the software runs on a PLC communicating with an advanced real-time computer model for the drive and the drill string. HIL testing has proved to be cost efficient because the PLC software can easily be tested over a wide range of different downhole conditions that rarely occur in the field.
