Vibration of Periodic Drillstrings with Local Sources of Resonance

Abstract

The vibration of drillstrings is, in general, detrimental to the drilling process and may induce premature wear and damage of the drilling equipment which eventually results in fatigue failures. In this proposal, a new design of drillstrings is proposed for mitigating such undesirable vibrations in an attempt to avoid wear and premature failures. In the new design, the drillstring will be provided with optimally designed and placed periodic inserts which can be either passive or active. The inserts will make the drillstring act as a mechanical filter for vibration transmission. As a result, vibration can propagate along the periodic drillstring only within specific frequency bands called the ‘pass bands’ and the vibration will be completely blocked within other frequency bands called the ‘stop bands’. The spectral width of these bands can be tuned actively according to the nature of the external excitation which can be either passive or active. The inserts will introduce impedance mismatch zones along the vibration transmission path to impede the propagation of vibration along the string. The design and the location of the inserts will be optimized to confine the dominant modes of vibration of the drillstring within the stop bands generated by the periodic arrangement of the inserts in order to completely block the propagation of the vibration.

The theory governing the operation of this new class of drillstrings will be developed to describe the complex nature of the vibration encountered during drilling operations. The developed model will account for the bending, torsional, and axial vibrations of the drillstring while operating under the influence of “slip-stick” frictional torques between the drill-bit at the rock surface, “bit-bounce” which make the drill-bit to intermittently lose contact with the rock surface, and the motion of the bit in the Bore-Hole Assembly “bit in BHA”.

Experimental prototypes of the periodic drillstrings will be built and tested under various operating conditions to demonstrate the feasibility and effectiveness of the concept of periodic drillstring in mitigating undesirable vibrations. The experimental results will be used to validate the developed theoretical model in order to develop a scalable design tool that can be used to predict the dynamical behavior of this new class of drillstrings.