The most effective way to mitigate torsional stick-slip oscillations is to apply smart control of the top drive so that it effectively absorbs energy from vibration components near the stick-slip frequency. This paper is a comparative study of several smart control systems available in the market.
The comparison is based mainly on analysis of the top drive mobility over a wide range of frequencies. Torsional mobility is defined as the complex ratio of speed to torque variations at the top drive output shaft and it carries information on both amplitude and phase relations between the two variables. The real part of normalized mobility, defined as the non-dimensional product of this mobility and the characteristic impedance for the upper drill string section, has proved to be a very useful tool for analyzing damping characteristics and for calculating the risk of instabilities.
Four different variants of smart control methods have been analyzed, both with perfect tuning and no signal delays and when signal delays are present. The systems have not been rated against each other, but they all show a relatively high sensitivity to signal delays, even small delays in the order of 50 milliseconds.
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