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 systemIn 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 RSS Bias UnitPivot Stabilizer PDC Bit IDS/String Stabilizer Collar 2 SPE/IADC 140011 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. IntroductionBackward whirl is an extremely violent phenomenon affecting rotary drilling assemblies. Traction is generated at contact points between the drilling assembly and the borehole wall, which can force the drilling assembly into rolling (rather than sliding) contact with the hole and a complex motion leading to dramatically increased loading frequency and multiple borehole impacts. In general there is renewed interest in this dynamic phenomenon due to the emergence of Rotary Steerable Sy...

Continuous High-Frequency Measurements of the Drilling Process Provide New Insights Into Drilling-System Response and Transitions Between Vibration Modes

“…2, where X is the axial direction (along the direction of the drillstring) and Y and Z are in the lateral directions. The radial and tangential directions are similar to Y and Z, but offset and rotated about the tool center line (Brazil et al 2002;Sugiura 2005).…”

“…For example, the theoretical frequency for backward whirl is given by (Vandiver et al 1990;Stroud et al 2011):…”

“…Other researchers have used two sets of accelerometers in an RSS to detect and quantify collar/stabilizer whirl (Minett-Smith et al 2010). With the same sensor, Stroud et al (2011) showed the motion of the collar in transition from "rollingsliding" to "backward whirl" in laboratory testing.…”

Continuous High-Frequency Measurements of the Drilling Process Provide New Insights into Drilling System Response and Transitions between Vibration Modes

“…2, where X is the axial direction (along the direction of the drillstring) and Y and Z are in the lateral directions. The radial and tangential directions are similar to Y and Z, but offset and rotated about the tool center line (Brazil et al 2002;Sugiura 2005). In addition to the high-frequency data, statistics and averages of the measurements are also recorded to low-frequency memory at 0.5Hz.…”

“…Other researchers have used two sets of accelerometers in a rotary steerable system (RSS) to detect and quantify collar/stabilizer whirl (Minett-Smith et al 2010). Using the same sensor, Stroud et al (2011) showed the motion of the collar in transition from "rolling-sliding" to "backward whirl" in laboratory testing.…”