Modelling of drill string buckling and tool indentation in rock drilling and fragmentation

Sammanfattning: In rock drilling, quality of drill hole and efficiency of drilling are two important aspects of great concern to drill operators and manufacturers. The first part of the thesis deals with the deviation of blast holes. One of the criteria for the quality of the drill hole is hole precision, specified by an allowance of hole deviation from the designed path. Drill hole deviation causes significant economic loss due to the need for rectification work, increased equipment consumption and delay to following procedures. On the other hand, improvement of hole precision makes it possible to increase the scale of mine production and bring about a large saving in operating cost. The objectives of the work presented in this thesis are to enhance the understanding of hole deviation phenomena and hence to improve the technique of precision drilling. Hole deviation due to string buckling are dealt with through a helical buckling model for a drill string subjected to an axial thrust and gravity force in an inclined hole. Laboratory experiments were performed to obtain an understanding of string buckling and to verify the theory developed. The major parameters which influence drill string buckling, and consequently hole straightness, are drilling thrust, gravity force, string stiffness, hole clearance and hole inclination. The string can be in a state of either transition buckling or steady buckling. The later dominates the loading process. String buckling leads to an uneven distribution of load on bit cutters which results in hole deviation. The theoretical prediction of the string buckling is in fairly good agreement with those from the laboratory tests. The second part of the thesis deals with rock breakage by mechanical tools. A deeper understanding of the mechanisms of rock breakage and their influencing factors will assist in the improvement of drilling efficiency. In the thesis, rock indentation, a simplified method of modelling one or a few cutting tools in a drill bit, is considered. Previous indentation experiments carried Out in three typical rocks, namely sandstone, marble and granite have been analysed. Three different types of indenter, hemispherical, truncated and cylindrical, were used in the tests. The rock damage consists of a crater, crushed zone and major and minor cracks outside this zone. In the analysis, the indentation field is divided into a crushed zone beneath the indenter and an elastic zone outside the crushed zone. An improved cavity model has been applied to specify' the post-failure behaviour of rock in the crushed zone and the stresses in the elastic zone. Indentation fractures in the elastic zone are simulated both numerically and analytically using two different fracture models for the side and subsurface cracks respectively. The numerical simulation models the progressive developments of side cracks and the formation of chips. The analytical fracture model predicts a fan-like pattern of subsurface cracks under the indenter. The formation of conical cracks in rock using a rock splitter was also investigated using the same fracture model as in the side crack simulation. Good agreement was obtained between the experiments and the simulations. The load pressure obtained at peak indentation force is a measure of the resistance of the rock to tool penetration. Its value is dependent upon rock properties and indenter features. The cylindrical indenter has the highest value and the hemispherical the lowest. The side cracks are driven either by tensile or by shear stress, or by combinations of these. The cracks can propagate to the free surface or terminate inside the rock. Some of them are unstable. The subsurface cracks propagate along the trajectories of the major compressive principal stress. Long subsurface cracks are formed along a semi circular band a short distance from the indenter. Conical fractures are formed in tension. To drive such a fracture and form a fragment, the proper ratio of lateral expansion force and axial dragging force is needed.