Experiencing vibration is inevitable when rotary drilling systems are used to drill deep bore holes. Field observations have been reported in which drillstrings show severe vibrations which become more severe at the bottom hole assembly (BHA). Vibration is recognized as one of the prime causes of deterioration in drilling performance and failure of drillstrings. Vibrations can be a combination of torsional, axial, or lateral vibration. Several efforts have been made to develop reliable laws that will provide accurate estimates of bit–rock interaction BRI forces and torque. Modeling drillstring vibrations for improved drilling practices relies heavily on the accuracy of the BRI models. Some parameters that are varied in these studies include weight on bit (WOB), speed in RPM, mudflow composition and rate, etc.This paper addresses how to model bit-bounce and stick–slip vibrations via the BRI formulae available in the literature. Stick–slip oscillations start with the sticking phase, when the angular velocity of the drill bit equals zero. The bit remains immobile even if the rotary table keeps rotating at the ground surface level. As the drill pipe rotates, the torque applied on the BHA builds up until it exceeds the BRI torque on the drill bit. Once it exceeds this torque on the drill bit, the stuck drill bit is released and starts to slip beyond the prescribed rotary table speed. The drill bit spin rate goes up from zero to two or even three times the prescribed rotary table speed. The stick–slip phenomenon is detrimental to the entire drillstring and to the bit. During some operations, the WOB, which is the normal reaction force acting on the drill bit, can fluctuate, causing severe axial vibration of the BHA. This is known as bit-bounce. Bit-bounce decreases the rate of penetration and damages the drill bits and drill pipe, resulting in fatigue failure. This decreases the efficiency of the drilling process and increases the operational cost. The third type of vibration is lateral vibration. Lateral or bending vibration can create large shocks as the BHA impacts the wellbore wall. Depending on the friction between the pipe and the wellbore wall, destructive forward or backward whirling of the BHA is generated.Modeling a drillstring is a complex task. Numerous issues arise, such as how to model the structure and its boundary conditions at the surface and at the bottom of the hole. The rate-independent BRI laws presented by Detournay and Richard were used to simulate the axial, torsional, and lateral vibration of a drillstring. Their BRI law separates the WOB and the torque on bit (TOB) into two separate processes: one related to drilling or cutting, and the other representing friction. The relationships among WOB, TOB, angular velocity (ω), and the rate of penetration (v) is presented in the work of Detournay and Richard. A Timoshenko finite element beam model was used with lumped inertias at the nodes. A special force model was included that has ubiquitous use and can produce the commonly observed deleterious bit-bounce and stick–slip vibrations. The code has a companion Excel user interface to facilitate use by practicing engineers and also includes stress stiffening effects and an efficient modal based solver.&nbsp;BRI-induced vibrations were predicted by a general order shaft finite element model, which was solved via a transient numerical integration approach, because of the strong localized nonlinear force and torque phenomena at the drill bit. Modulation of the instantaneous depth of cutting through axial vibratory motion causes an effective force time delay, which is destabilizing for vibrations. This leads to destructive and sustained coupled stick–slip and bit-bounce vibrations. This parametric study illustrates the influence of operating parameters and structural features on bit-bounce and stick–slip. Prediction software can help improve drilling practices and can also be incorporated in automated drilling simulation studies to develop effective control strategies.

Drillstring Simulator: A Novel Software Model for Stick–Slip And Bit-Bounce Vibrations

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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