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Modelling of vibro-impact penetration of self-exciting

percussive-rotary drill bit A.D. Batako*, V.I. Babitsky, N.A. Halliwell Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK Received 17 September 2002; accepted 24 February 2003

Abstract

The penetration of a drilling tool into a hard medium under periodicimpac t action is analyzed and the simulation model presented. This is further development of previously investigated model of a self-excited percussive-rotary drilling system. The system used the stick–slip phenomenon to generate an impact action superimposed on the drilling process. A phenomenological visco-elasto-plastic model of the media is used and the system response is studied numerically, first as a forced vibration and second as a result of a selfexcited vibro-impact process. Relief of the main drive has been obtained and an increase in the rate of penetration is observed with increased impact intensity and hardening of the medium. Results of the preliminary drilling experiment with superimposed dynamica ction have shown an improvement in the rate of penetration. r 2003 Elsevier Ltd. All rights reserved.

1. Introduction

Improvement is constantly sought when drilling in order to increase the rate of penetration (ROP), the time span between maintenance services, the life of the drilling system and to reduce the overall cost of the drilling process. Current drilling trends tend to increase the ROP by balancing the speed of rotation and the weight on the bit. This often leads to a failure of the drill string, which undergoes complex loading. The load is exerted in such a way that the upper part of the drill string is in extension and the lower part is in compression. The top drive rotates the drill pipe with the bit, which cuts the rock formation at the bottom. Consequently, the entire drill ARTICLE IN PRESS *Corresponding author. Tel.: +44-1512312126; fax: +44-1512312590. E-mail address: a.d.batako@livjm.ac.uk (A.D. Batako). 0022-460X/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-460X(03)00642-4 string works in torsion and experiences axial and torsional vibration. To control the parameters of the drilling process (actual speed of rotation, weight on bit, and torque on bit), complex techniques are used in order to detect and monitor the whirl, stick–slip vibration and the forces and stresses in the drill string. This increases the cost of operation with a reduced outcome.

In Ref. published previously, the authors presented a self-oscillatory system for percussiverotary drilling, which relieves the drill string of torsional vibration and the main drive from overload. In this paper, the authors investigate the relief of the thrust due to the impacts generated by the mechanism. The model presented takes the properties of the media being drilled into consideration in order to estimate the ROP.

2. Early vibro-impact penetration concepts

Vibro-impact mechanisms have been studied and widely used in engineering in the former Soviet Union since the post-war period. The major research work was concerned with finding adequate parameters (maximum impact force) for the development of new machines and intensification of the penetration due to the use of different mechanisms of excitation of vibration and impacts. Basic models of machines were developed including the interaction with the medium during the penetration. This gradually brought about the formation of the general theory of vibro-impact systems.

Early models of vibro-impact penetration of a tool into a medium were introduced by Tsaplin, who assumed an instantaneous impact force and the velocities before and after impact were related through a restitution coefficient. Later, Tsaplin introduced a depth dependency by gradually increasing the mass of the driven element as the pile moves into the medium. The vibration and vibro-impact methods of penetration into different media owed their practical widespread to the works of Barkan, Savinov and Luskin and Tseitlin et al.

The initial theoretical analysis of the vibro-penetration process was made by Neimark and Blekhman. Two models of the indenter–footing resistance were mainly used to reflect the primary experimental data. In the purely plasticmodel, the resistance was presented as weightless plug held in the borehole by the permanent force. The advance of the plug is possible when the sum of the applied forces exceeds the resistance force. Under these circumstances the plug followed to the movement of the indenter.

An elasto-plastic model, known as Prandtl model, took into consideration the elasticity of the medium by adding an ideal spring between the indenter and plug. In this model, the movement of the plug is only possible when the elastic force of the spring exceeds the resistance of the medium. More complex rheological models of the medium were also used.

In percussive-rotary drilling the tool moves into the medium by an impact–scratching action and inherently removes off cuttings. This permits use of a simplified model to estimate the ROP due to impact. The model below takes into consideration the frontal resistance of the medium as an elasto-plastic process and viscous dissipation during the vibration of the bit.

3 Study of the system under impact loading

To study the response of the system under successive impacts, a pulse generator is used and the duration of impact is 0.002 s.

From the UCS test, the stiffness of the rock cores was estimated to be 0.9, 1.6 and 2.11MN/m for the sandstone, limestone and the granite, respectively. The threshold force D was also obtained as 61.9, 140 and 204kN for the respective types of rock. The core samples and the test complied with the requirements of the ASTM and the British Standard BS1610. The diameter ?d? of the cores was 54mm and the height ?L? 108 mm; the ratio L=d is 2 and the standard requires that 2pL=dp4. The UCS test was carried out in the civil engineering laboratory of rock mechanics on the ‘‘Denison’’ Block Testing System. A load was applied to each core at the rate of 0.17 kN/s.

The computer controlling the testing machine produced the plots of the load against the displacement. The UCS values for the limestone and the granite seem rather higher than average because the rock blocks were of a very good quality, fresh and dry. Consequently, the investigation was carried out within a range covering these values and the following settings were used: k3A[1,5] MN/m, DA[50,300]kN with a damping ratio zA[0.5,1.5]. These values of the damping ratio allow the observation of the underdamped, critically damped and overdamped motion of the system under impact excitation.