ON DRILL-STRING NATURAL TORSIONAL VIBRATIONS

Abstract

When drilling oil wells, drill-pipe strings are the main components that constrain reliability and efficiency of drilling rigs. The highest number of drill-string failures is caused by pipe parting. It is suggested that pipe-parting failures are preceded by the formation of a portion having a through-thickness radial lengthwise cut or a notched portion with a smaller cross-sectional area. Neither lengthwise cut nor crack in the rod can be determined using natural frequencies of longitudinal or flexural vibrations, because in the zone of the cut there is no reduction of the area and axial moments of inertia in the rod’s cross section. Therefore, this article deals with natural torsional vibrations of a drill string having damages as a through-thickness radial lengthwise cut and as a lateral notch. It has been found that in the case of the cut or the notch located within the vibration node, natural frequencies do not depend on the lengths of these damages. As the length of the cut or the notch increases, the lower frequencies of natural torsional vibrations will decrease. For the drill-string lengthwise through-thickness cut, changes in the natural torsional vibration frequencies are considerable as compared to similar frequencies for the drill string having a lateral notch. Using two natural torsional vibration frequencies, we can determine the initial coordinate and length of the through-thickness radial lengthwise cut on the drill string. Using three natural torsional vibration frequencies, we can determine the coordinate, length and parameter of the notch.