Fig. 1. Resonance hammer drilling.
Luiz Fernando P. Franca, Hans Ingo Weber
Mechanical Engineering Department, Pontifical Catholic University of Rio de Janeiro, Rua Marqu^es de S~ao Vicente 225, Rio de Janeiro,
RJ 22453-900, Brazil
Accepted 11 December 2003
Источник: www.elsevier.com/locate/chaos
New drilling techniques have been studied to increase the penetration in hard rock formations. These techniques use harmonic loads and, in some cases, also impacts to generate a greater penetration rate. Analyzing only a percussive penetration phenomenon, the new model presented in this paper allows the forward motion (with a drift) in stick-slip condition with and without impact. Numerical and experimental investigations are presented and are qualitatively and quantitatively compared. Otherwise looking for other parameter ranges in the simulation, it is shown that the behavior may vary from periodic to chaotic motion. From the engineering side, the main interest in a study like that in this paper is if the rate of penetration can be improved. The simulation may help a lot in that, and in a short conclusion one should look for regions with period-1 behavior.
There has been an increasing effort looking for oil resources in deep seas water. Special drilling techniques have been developed due to the large amount of specific wells that are being done. Drilling may be considerably delayed if there is need to change the drillbits or to stop the procedure, to attend project conditions and improve drilling efficiency. Looking for an increase in productivity, recently, attention has been paid in improving drilling efficiency by imposing dynamic loading at the bit–rock interface [1, 3, 4, 9]. To date, this has been applied only in the restrictive circumstances of shallowness.
The various modes of the drillstrings vibrations (axial, torsional, and bending which in their most severe forms lead, respectively, to bit bouncing, stick–slip oscillations, and bit whirling) are generally regarded as detrimental. However, as shown in this work, it appears possible to control some of these vibrations modes in such a way as to enhance drilling performance.
A new drilling technique called resonance hammer drilling has been studied by us, as an alternative to increase the rate of penetration (ROP) in hard rocks drilling, Fig. 1. This technique has as premise to use the already existent vibrations in the drillstring, in fact the axial vibration due to the cutting process, to generate a harmonic load on the bit and an excitation in a steel mass (hammer). When this excitation frequency is near to the steel mass resonance frequency and, since the steel mass displacement is limited in positive direction by the gap, impacts on the bit may occur.
Therefore, besides the rotative penetration, where the teeth of the bit penetrate in the rock when the drillstring rotates, a percussive penetration happens due to the harmonical load or due the impact, increasing the ROP. Although,
Corresponding author. Tel.: 55-21-3114-1167; fax: +55-21-3114-1165. E-mail address: hans@mec.puc-rio.br (H.I. Weber).
0960-0779/$ - see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.chaos.2003.12.064
790 L.F.P. Franca, H.I. Weber / Chaos, Solitons and Fractals 21 (2004) 789–801
the harmonic force should never be larger than the preload (WOB––weight on bit), due to the possibility of the bit bounce effect, and also the hammer resonance frequency should not coincide with any drillstring natural frequency. Actually, when the exciting frequency, which equals the frequency of displacement at the bit, due to the bit rolling over the lobed bottom hole, corresponds to an axial natural frequency of the drillstring then, resonance occurs and the drillstring can bounce. Consequently, one way to minimize the response to vibrations is to avoid drillstring rotations that induce natural frequency vibrations [2].
The proposition of the present investigation is restricted to the percussive penetration phenomenon. A simple model for the longitudinal behavior of the bit-rock interface is proposed and the drilling resistance is modeled by a dry friction element. Moreover, the model presented in this work moves forward (a drift) in stick-slip phase with or without impact.
Since impact and dry friction are present, usually these systems are nonsmooth. During the past decade, new analytical and numerical tools have been developed for the study of nonsmooth systems [6, 11, 14]. The dynamics of physical systems, whose components can suffer impact or present dry friction, is very important in practical applications. Nevertheless, very few works have considered systems, which associate these conditions with progressive drift [3, 10, 12].
In this context, this work has the objective to investigate the dynamics of the proposed nonsmooth percussive drilling model numerically and validate by experiments the numerical model. For that, some nonlinear tools are used like: phase space, Poincare maps and bifurcation diagrams.
The experimental data related to the new percussive response is obtained from the apparatus depicted in Fig. 2a. Two elements (two cars, A and B) are connected by two springs (2), and can move almost without friction on linear guides (4). The movement of the system occurs along the drift (5) due to the existence of the preload (6) and of the harmonic force generated in the shaker (1). The drift represents the penetration depth of the bit. The preload and the shaker are fixed at the first car. The dry friction (3) is produced by friction shoes on springs, fastened with a screw. This device is able to change the friction force. The data acquisition is done through encoders (7). The impact device (8) has
