FREE TORSIONAL VIBRATIONS OF BIG DIAMETER DRILL STRING AND 103/4" DRILL PIPE BREAKING

The purpose of the researches presented in this paper is to make evident the cracking and breaking causes of the 103/4" drill pipes, used for the drilling of the mining shafts which have a diameter of 3.62 m. In this way, the free torsional vibrations of the 103/4" drill string are studied by using the drill string model with non-uniform distributed mass and mass moments of inertia concentrated at the ends. Then, the electro-hydrostatic driving group inertia and the compressibility of the hydraulic medium in the hydrostatic transmission lines are taken into account. It is ascertained that the influence of the hydraulic medium compressibility is unfavorable from dynamic point of view because it decreases the natural angular frequency of torsional vibration in the working speed area of the drill string. The appearance of the resonance phenomenon in this working area is made evident by experimental researches and can be a reason regarding the breaking of the drill pipes assembled in the lower third of the drill string.