Experimental Uncertainty Analysis on Drillstring-Borehole Contact

The drillstring vibration response is subject to uncertainty due to the operating speed, manufacturing tolerance and downhole conditions. A drillstring-borehole contact exhibits complicated dynamic behaviour due to its nonlinear nature. The complexity in the drillstring vibration is due to the drillstring-borehole interactions, which induce nonlinearities due to friction and impact. The objective of the study is to analyse the drillstring-borehole interaction experimentally. A conceptual model of a system of the drillstring is developed, consisting of a flexible rotating shaft with two discs supported on roller bearings. The drillstring-borehole interaction is modelled using a localised contact element. A run-up analysis was conducted to determine critical speeds, wherein a high whirling response is produced. An experimental uncertainty study is carried out by varying critical system parameters, such as the phase angles of the unbalanced masses attached to the disc and the coefficient of friction of the contact element. The response is captured using an accelerometer at the bearing location. The maximum response was analysed at the critical speed of the rotor, wherein a high response is expected. A probability density function was obtained for the maximum response at the bearing location. The results from the experimental study indicate that the response is sensitive to the modal interaction of the interacting whirling modes. Future work will include the development of a theoretical model of the system using the stochastic finite element method covering a wider range of parameter space.