Analytical and Experimental Backward Whirl Simulations for Rotary Steerable Bottom Hole Assemblies

Abstract A unique analytical model has been developed to predict dynamic behaviour of the drilling assembly and gain a better understanding of the dominant factors that cause backward whirl. A full-scale test rig has been constructed that is capable of initiating and sustaining full rolling contact backward whirl, repeatably, reliably and safely. Excellent correlation between these near real time analytical simulations and full-scale experimental results has been achieved giving confidence in using the model for serious predictive analysis of system response in real down-hole drilling conditions. This uniquely verified analytical model can be used for rapid iterative analysis to help generate and evaluate potential design improvements and whirl mitigation strategies. These simulations have been focused on a latest generation point-the-bit rotary steerable system (Figure 1 shows the general arrangement of the steering system), but the principles can likely be applied to other rotary drilling assemblies. Figure 1 Schematic diagram of a point-the-bit rotary steerable system In the process of validating the analytical model, some interesting observations were made. The first being the appearance of a ‘partial whirl’ or transition phase preceeding full backward whirl in most cases. The large impact loads predicted by the analytical model over the partial whirl phase correlated with those experienced in full-scale testing and could explain the large lateral vibrations experienced in the field during supposed episodes of backward whirl. The parameters affecting both the length of the partial whirl period and the magnitude of the impacts were found, leading the way for preventative measures. The second finding, and possibly the most instantly useful, was that full backwards whirl could not occur in close to gauge hole unless the friction coefficient between the borehole and the drilling assembly was increased dramatically. This would indicate that devices and procedures aimed at maintaining good hole quality could be effective measures to prevent backward whirl. Improvements to the analytical model have been suggested but even in its current form it has proved an extremely versatile and reliable tool. Now that both the simulated motion and predictive capabilities of the analytical model have been proven, future technical papers will explore the effect of drilling parameters in more depth and will attempt to provide whirl mitigation strategies for field use.