Water powered percussive rock drilling process analysis, modelling and numerical simulation

Sammanfattning: This thesis is devoted to problems, processes and systems related to the recently developed water powered percussive rock drilling method. The technology, which uses ordinary water to drive down-the-hole hammers, has been used to produce more than 6 million meters of blast holes within the mining industry. The method has several advantages such as low energy consumption, dust free environment and the capability to drill to virtually any depth. A natural disadvantage of this method is the need for relatively large amounts of preferably high quality water to drive the hammer tool, occasionally also leading to waste disposal problems. To understand the central hammer tool in the system, the function of a 100 mm diameter hammer has been analysed, modelled and simulated. One- dimensional wave propagation theory was used for modelling impacts and axial motions in a drilling system. The rock was assumed to be an elastic- plastic material, where all absorbed energy was used for crush work. Simulation results showgood agreement between measured and simulated piston blow frequencies (~60 Hz). A disadvantage with the hammer tool’s function is the discontinuous consumption of water, causing large pressure fluctuations in the feed water line. Measurements indicate peak pressures to be ~3 to 4 times larger than the lowest pressure. Since large pressure variations increase the risk of mechanical damages, a flexible element (pulsation dampener) to reduce the variations was developed. Test bench experiments show pressure fluctuations reduced by up to 40% with the prototype dampener. For all drilling methods, an efficient rock penetration process is essential for the methods overall competitiveness. The process is also significant for the dynamic behaviour of the water powered hammer tool, since different rock properties have been shown to cause variations in the piston blow frequency. The general bit-rock impact process is therefore discussed and field measurements of the penetration rate during ~115 mm diameter well drilling are presented. A penetration process was also analysed with the assistance of a non-linear explicit FEM code, where the rock material was represented by an established constitutive model. Results show, e.g., the ratio between the indenter’s rebound- and initial kinetic energy to decrease with increased initial energy, where a small part of this initial energy is transmitted by stress waves into the formation. During the penetration process, crushed rock is flushed away with outlet water from the hammer tool, i.e. used particle contaminated drill water should be recycled when the method is used at locations with limited water access and/or when waste disposal is difficult to accomplish. This has resulted in the development and construction of a prototype mobile cleaning system that makes re-use possible. Hence, this system is described together with measured and simulated results of the unit’s cleaning capacity. Another phenomenon during the drilling process is the dissipation of a large part of the injected borehole energy into heat. The drill water and the formation will therefore be thermally influenced, providing the possibility to evaluate the ground thermal conductivity with the drill work. This new principle is presented in detail, together with an energy balance equation and a heat transfer analysis during drill work.