In rotary down-the-hole (DTH) percussion drilling, penetration is achieved by the repeated application of a large impulsive force to a rotating rock drill bit in contact with the rock [1,2,3732]. This impulsive force results from the impact of a pneumatically-operated piston (hammer) on a shank adapter (anvil). The kinetic energy conveyed by the piston is transformed into compressive stress waves upon contact with the adapter, waves that propagate through the drill bit down to the rock, leading to rock destruction by indentation, crushing and chipping . The generated rock debris then are flushed to the surface by the compressed air activating the piston. Fig. 1 provides an illustrative representation of the tools.
This technology mostly is used to drill medium- to hard-rock formations, where it has proven to be most adequate and efficient compared to other drilling techniques.
Research and Objectives
By nature, percussion drilling is multi-physics process, for it involves several aspects of mechanics. Modeling its dynamics — a principal objective of this project — therefore requires the assembly of the process components in a global model. A convenient approach is to see the process as a system consisting of subprocesses that interact together. As such, four main subprocesses can be identified: (1) hammer thermodynamics, (2) rock breakage mechanics or bit/rock interaction, (3) bit and drillstring dynamics, and (4) flushing or debris removal. These are also listed in Fig. 2.
The second principal objective of this project is the identification of the control parameters that maximize the average rate of penetration of the bit — i.e., locate the experimentally observed sweet spot in the space of control parameters. As shown in Fig. 3, the proper adjustment of the control parameters is expected to yield substantial gains in the rate of penetration and, subsequently, significantly decrease the time required to complete a given borehole.
Several experimental studies on the modeling of bit/rock interaction have been conducted at the University of Minnesota (see, for instance, [6,7]. They have shed light on some important aspects of the interaction. First, it is rate-independent in the range of penetration velocities spanned by percussion drilling. Second, as a first approximation, it can be represented by a bilinear relation between the force exerted by the rock on the bit and the bit penetration, the unloading phase being assigned a higher stiffness. We follow this representation in the model currently being developed.
Multiple Timescale Nature
If we restrict our consideration of down-the-hole percussion drilling to the interaction of rock breakage mechanisms and bit/drillstring dynamics (red frame in Fig. 2), essentially the dominant subprocesses of the overall process, two timescales can be identified: (T1 = O(1) ms), a fast one, associated with the wave propagation in the hammer bodies, and (T2 = O(50) ms), a slow one associated with the free motion of the bodies in-between percussive activations, T1≪T2 (see the illustration provided by Fig. 4).
Common sense thus dictates the framework for the analysis of these interactions: wave propagation should be accounted for on timescale T1 using a detailed model of the geometry (e.g., finite elements), while a model of reduced dimension should be on timescale T2 to represent the motion of the DTH system bodies (e.g., rigid body dynamics). The coupling of these two timescales then will enable the simulation of the system long-term response, required to quantify the tool average rate of penetration in the rock.
Most of the research related to this project is conducted by Alexandre Depouhon as part of his Ph.D. thesis research work, conducted at both the University of Minnesota and the Université de Liège (Belgium). Support was provided by Itasca Consulting Group, CSIRO Earth Sciences and Resource Engineering, and the University of Minnesota Doctoral Dissertation Fellowship. Related publications can be found here.
 M. Amjad (1996). Control of ITH percussive longhole drilling in hard rock, data from field experiments at Little Stobie Mine, Canada, 1996.
 G. L. Cavanough, M. Kochanek, J. B. Cunningham, and I. D.Gipps (2008). A self-optimizing control system for hard rock percussive drilling. IEEE-ASME T. Mech., 13(2):153-157.
 L. E. Chiang and D. A. Elias (2000). Modeling impact in down-the-hole rock drilling, Int. J. Rock Mech. Mech. Min. Sci., 37(4):599-613.
 B. Haimson (1965). High velocity, low velocity and static penetration characteristics in tennessee marble. 1965.
 W. A. Hustrulid, and C. Fairhurst (1971). Theoretical and Experimental study of percussive drilling of rock – Part I – Theory of percussive drilling, Int. J. Rock Mech. Min. Sci., 8(4):311-333.
 W. A. Hustrulid and C. Fairhurst (1971). Theoretical and experimental study of percussive drilling of rock – Part II – Force-penetration and specific energy determinations, Int. J. Rock Mech. Min. Sci., 8(4):335-356.
 E. Nordlund (1989). The effect of thrust on the performance of percussive rock drills, Int. J. Rock Mech. Min. Sci. Geomech. Abstr., 26(I):51-59.