Torsional Dynamics and Point-the-Bit Rotary-Steerable Systems
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 163428, ’Torsional Resonance: An Understanding Based on Field and Laboratory Tests With Latest-Generation Point-the-Bit Rotary Steerable Systems,’ L.A. Lines, SPE, D.R.H. Stroud, SPE, and V.A. Coveney, Weatherford, prepared for the 2013 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 5-7 March. The paper has not been peer reviewed.
Torsional dynamics can be extremely damaging to downhole drilling tools and can significantly affect drilling efficiency. The effort to prevent this phenomenon has led to the development of various downhole mitigation tools, improved bit technologies and bottomhole-assembly (BHA) designs, and better management of drilling parameters. This paper outlines two lesser-known types of torsional dynamics observed while drilling interbedded carbonate formations in the Middle East with the latest generation in point-the-bit rotary-steerable systems (RSSs).
Introduction
Aspects of dynamic behavior considered to be particularly undesirable include the following:
Large fluctuations of bit-rotation rate (Ω) and the often-associated amplitude modulation of higher-frequency vibrations. [These features are usually ascribed to stick/slip behavior, wherein Ω is periodically zero, then much larger than the rotation rate at surface (Ω). Potentially, stick/slip could also occur at positions in the drillstring other than the bit.]
Backward whirl, where the bit rolls around the (enlarged) borehole in a sense opposite to Ω.
Resonances of the drill string may-drive or be driven by Items 1 and 2; enlarged or irregularly shaped boreholes are likely to be involved.
Background
A major mainland oil and gas field in the Middle East is notorious for difficult and hazardous drilling conditions; high levels of stick/slip are the norm.
The RSS has a number of onboard dynamics sensors. The two discussed in this paper are
RSS whirl-average indicator. This consists of accelerometers, which are mounted radially (to the drillstring centerline) and measure changes in centripetal accelerations. These are used to infer the severity of off-hole centerline rotation (i.e., whirl). This measurement has an effective sample rate of 40 Hz at 100 rev/min. A whirl-severity indication is then transmitted in real time to the surface.
RSS stick/slip average. This sensor monitors changes in the RSS rev/min measurement over a given sample period. The RSS measures rev/min at an equivalent sample rate of 1 Hz. It is therefore suit-able only for detecting torsional dynamics (rev/min changes) that occur at up to 1 Hz.
The conventional representation of stick/slip can be seen in Fig. 1. This is typical of the rev/min traces experienced elsewhere in the world and is of sufficiently low frequency that it can be detected easily by existing measurements. This low-frequency stick/slip is not particularly damaging to drillstring components.
Although reported stick/slip levels in this particular field are usually severe, the amount of drillstring/drilling-tool damage experienced is disproportionately high compared with that experienced in other fields with similar levels of reported stick/slip. The literature suggests that harmonic/sinusoidal torsional dynamics can occur at significantly higher frequencies than the 1 Hz that existing onboard sensors are capable of detecting. It was apparent that the severity of the drilling environment in this field was not reflected accurately in RSS-sensor data.