The extremely harsh conditions encountered during the downhole drilling process, cost, and environmental issues all put severe demands on the effectiveness, reliability, and safety of the drilling method. Consequently every increase in cutting rate and decrease in tool wear or catastrophic breakage is very welcome.
The basic philosophy behind the proposed approach is to provide the conditions which optimise the fundamental material removal process. This can be achieved by generating high amplitude impact forces at the appropriate ultrasonic frequency, which both decrease drastically the cutting resistance and enhance the propagation of cracks. As the cutting conditions vary from layer to layer within the formation, it is proposed to implement a system which first will estimate a natural frequency of the media and then choose the frequencies, the amplitudes and the shapes of the forcing function. This can be accomplished by a careful modelling of the 'cutting process-drill string' interactions, monitoring the status of the rock layer (via the dynamic response of the ultrasonic drill head assembly), and implementing a suitable control technique, which can maintain the optimal cutting conditions.
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