Practicality of Tricone Bit Wear Condition Assessment

Abstract

A practical bit condition monitoring system is a necessary component of autonomous drilling. Tricone bits are widely used in blasthole drilling in mining. Bits experience a variety of wear mechanisms during the operation and rolling element failure is the dominant catastrophic failure mode of tricone bits. Bit lifetime and performance significantly vary based on the working condition and the critical components of the bit i.e. rolling elements, are invisible to the direct condition monitoring systems. At McGill University, extensive research work is conducted to develop an indirect bit condition monitoring and failure prediction approach relying on the vibration signals and the technology is currently patent pending. This article presents real-world experimental evidence to show the unreliability of conservative bit changing strategy based on the bit operation life or drop in the rate of penetration (ROP) and ineffectiveness of direct wear monitoring techniques to cover the dominant failure mode.

Objective

To demonstrate the unreliability of tricone bit replacement relying on bit operation life or ROP measurement and ineffectiveness of vision-based monitoring techniques for autonomous drilling.

Introduction

Blasthole drilling is a preliminary step in surface mining, therefore, delays caused by bit failure could impose significant losses on the production. Tricone bit design is relatively complex (Figure 1). Cones are connected to the bit body by bearings elements. Progressive bit wear results in failure of rolling elements which is known as the dominant catastrophic failure of tricone bits and usually happens as locked or detached cones. Some research works are conducted on wear detection of tricone bits employing a variety of techniques including drilling performance measurement, study of load distribution, vibration analysis and image processing approaches (Cooper, 2002; Ghosh et al., 2016; Naganawa, 2012; Rafezi & Hassani, 2017; 2018; Saeidi et al., 2014; Sheppard & Lesage, 1988). This article discusses the effectiveness of ROP monitoring and image processing methods for the purpose of tricone wear monitoring based on real-world drilling data and feasibility restrictions.

Figure 1. Tricone bit installed on the rig.

Table 1. Operation life of four tricone bits in one mine site

Method

Blasthole drill rigs were instrumented with data acquisition units and accelerometers in two mines in Canada for measurement while drilling (MWD). Drilling vibration, control signals as well as the ROP were recorded during the complete lifecycle of tricone bits. For data labeling, bit wear condition was manually inspected, recorded, and digital images were acquired while operating in various geological formations.

Results

In this study capability of ROP assessment and imaging approaches for tricone bit wear monitoring and failure prediction were investigated. The ROP trend during the bit lifecycle, starting from a brand new bit until the bit is worn out and replaced, has a fluctuating trend (Figure 2). Consequently, as the bits become worn out, a decremental trend in the ROP is not necessarily identified in a dynamic subsurface condition. Direct assessment of bit condition through digital imaging provides an insight into the shape of the bit and the missing or broken teeth are detectable using this approach.

Figure 2. The rate of penetration (ROP) trend during the complete lifecycle of a bit.

Discussion

It was observed that the ROP trend is significantly influenced by geological variations and as presented in the table, the lifetime of tricone bits can significantly vary (in terms of hours of operation and length of drilling) depending on the working condition for the identical bit brand, model, and diameter in the same mine site. For the bits listed in the table, the bit lives are ranging from 21.8 hrs. to 68 hrs. and from 746.1 m to 2805.4 m. On the other hand, processing of digital images for condition monitoring of tricone bits has some essential drawbacks; infeasibility of real-time monitoring while drilling down the hole and inability to assess the bearings condition which leads to the dominant failure mode of tricone bits.

Conclusions

The effectiveness of ROP monitoring and vision-based assessments for automated tricone bit condition monitoring was investigated in fieldworks. It is concluded that the ROP is mainly determined by geological condition and extraction of bit wear status from ROP trend requires a precise knowledge subsurface condition. Analysis of tricone bit wear through visual techniques, could provide an insight about the teeth wear state, however, it is not able to cover the bearings condition. In comparison, bit vibration frequency signatures reveal information about bit rolling elements condition and have the possibility of real-time measurement without interruption of the operation. Therefore, drilling vibration analysis is a promising and practical approach for tricone wear monitoring and failure perdition in industrial applications for fully autonomous drilling.