ABSTRACT: A review is given of hypotheses for the mechanism by which waterjets reduce the specific energy of cutting for drag bits. Several of these are shown to be inconsistent with published evidence and new observations. The notion of a limiting cutting speed, above which waterjets would be incapable of rendering assistance, is shown to be improbable. The hypothesis that seems most plausible is that specific energy reductions are mainly due to the erosion of crushed material from in front of the bit. This hypothesis leads to a prediction that, for a given rock type and jet/bit configuration at a constant depth of cut, the reduction in bit forces should be a function of dW/dx, the jet energy spent per unit length of cut. This dependence upon dW/dx is shown to be consistent with other workers' results.
1 INTRODUCTION
The use of waterjets to augment mechanical rock cutting has advanced greatly since the discovery a decade ago (Hood, 1976) that waterjets diminish the forces acting upon drag bits cutting in strong rock. Prototypical systems which use this technology have been tested, and many of the benefits which had been predicted from laboratory studies have been realized. In particular, the use of waterjets has been demonstrated to give faster cutting rates, slower bit wear, and diminished machine vibrations (Morris and Tomlin, 1985; Barham and Tomlin, 1986), as well as dramatic reductions in dust make (Kovscek et al, 1985; Haslett et al, 1986; Kovscek et al, 1986). However, observations of the effects of waterjets on cutting efficiency have been seemingly contradictory, and no mechanistic theory has yet been developed sufficiently to resolve these discrepancies. Commercial development of this technology is greatly hindered by this incomprehension of the waterjet assistance mechanism, not only because design parameters such as jet position and pressure cannot be optimized via a quantitative theory, but also because there is no way of knowing which parameters to consider in empirical optimization studies. Hence if the full benefits of waterjet assistance are to be realized, a better physical understanding of the assistance mechanism is essential. Since the mechanism of waterjet assistance is still poorly understood, the only guide for the commercial development of this technology is a body of laboratory data which is deficient in at least three respects: Firstly, the data have been gathered from cutting tests in an assortment of rock types, using bits of various geometries, and employing a variety of jet/bit configurations. These data are not necessarily useful for predicting waterjet effectiveness in other rocks, using bits and jet/bit configurations other than the ones specifically tested. Secondly, although laboratory studies have measured the effects of individual parameters such as jet pressure and cutting speed, in general these studies have considered only one or two of these parameters at a time, and have ignored interactions which might occur among the parameters. Thirdly, for many of these studies the statistical significance of the results has not been assessed. These deficiencies arise largely because, in the absence of an established mechanistic theory, so many parameters must be considered that a comprehensive, empirical study is not practical.
