Hole deviations in percussion drilling and control measures : theoretical and field studies

Abstract: In this thesis, we study the problem of hole deviations in percussive drilling and present a solution to minimize the deviations. Field tests are performed to study the interactive behaviour of drilling parameters. Standard theories in beam bending are used to study the effect of thrust, drill string weight and drill string inclination on drill string deflection, an important source of hole deviations in percussive drilling. The effect of drill string torsion on penetration rate is also studied. A simple model is developed to estimate the minimum thrust required for bit-rock contact before succeeding stress wave. The drill string behaviour predicted by the models is validated with the results obtained from field tests. The effect of the bit-rock contact on percussive drilling, also an important source of hole deviation, is also studied. A new bit-rock contact model is developed to estimate the minimum torque required to maintain constant bit rotation. Results obtained from this model agree very satisfactorily with those obtained from further field tests. The understanding from results of our theoretical and field studies of drill string deflection and bit-rock contact is then used to formulate a solution to the problem of hole deviations in percussive drilling. In the solution, torque is used as a key parameter for automatic control of the drilling process. The solution is tested in the field and its performance is considered to be satisfactory.