EFFECT OF TORSION ON STABILITY, DYNAMIC FORCES, AND VIBRATION CHARACTERISTICS IN DRILLSTRINGS

Traditionally, the analysis of stability in drillstrings is based on l ongitudinal vibrations only. However, field experience indicates the p resence of torsional effects in the form of modulations in longitudina l vibrations. Moreover, torsional dynamic forces are significant and l ead to joint problems during drilling. In this paper, we discuss the e ffect of torsional vibrations: on the stability and both longitudinal and torsional forces in drillstrings. Using a model of a typical drill string that incorporates both longitudinal and torsional multi-degree of freedom, we develop the equations governing displacements and force s at any joint. Using time simulation techniques, we obtain the stabil ity diagram and calculate the dynamic forces at any joint as a functio n of time. We finally show the effect of torsion on the nature of vibr ations through frequency analysis, a technique that is useful in the i nterpretation of field measurements under various drilling conditions.