Study Recognizes and Assists in Mitigation of Bottomhole Assembly Lateral Vibration Chatter

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 197231, “Recognition and Mitigation of the Bottomhole Assembly Lateral Vibration Chatter Mode,” by Jeffrey R. Bailey, SPE, and Harshit Lathi, ExxonMobil, and Matthew T. Prim, SPE, ADNOC, et al., prepared for the 2019 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11-14 November. The paper has not been peer reviewed. Lateral vibration modeling of certain bottomhole assembly (BHA) designs has shown great sensitivity to the proximity of stabilizer blades. This paper explores the nature of the vibrational dysfunction known as BHA chatter. A field-proven frequency-domain model illustrates the cause of the dysfunction, its rotary-speed dependence, and mitigation methods and results. The complete paper provides three case studies exploring this phenomenon, one of which is included in this synopsis. Introduction The authors first describe the similarities and differences of a BHA and a stringed instrument. The string of a violin, for example, typically has two fixed nodal points: the first at the bridge, which does not change, and the second at the position of the musician’s finger, which is moved along the fingerboard in order to play notes of different frequencies. The finger pressing on the string causes it to have zero displacement at that location, which defines a nodal point. Additional nodal points may occur in the motion of the string as harmonics of the fundamental mode, but these are not considered to be fixed nodes because the amplitudes of the harmonics vary. The string is relatively flexible, so it can be described adequately with a second-order differential equation. Significantly, a BHA typically has more than two nodes. A lateral wave propagating along the BHA must satisfy the nodal point constraint of zero lateral deflection at all these locations. These nodes typically are placed without regard to the frequency of the wave traveling along the string, which is governed by the rotary speed and the type of lateral excitation. The geometric compatibility requirement that the pipe has zero displacement at the fixed nodes has ramifications. The nodal point constraints force the pipe to adapt to the locations of these nodes through contact forces that literally push the pipe back into position to honor the constraints. In some scenarios, this process requires large forces. One consequence of large forces pushing the pipe to maintain geometric compatibility is that these forces are applied to the outer diameter of a body that is rotating, so this response may also generate torque and associated wear of the contacting surfaces. This observation applies to both static and dynamic forces but most commonly is recognized in the static domain. It is not typically recognized in dynamics as applied to BHA design.