A Study of Excitation Mechanisms and Resonances Inducing Bottomhole-Assembly Vibrations

Abstract

Summary. This paper presents a study of important excitation mechanisms and resonances that cause bottomhole assembly (BHA) vibrations during drilling operations. The study is based on dynamic vibration data gathered in a test well with our Advanced Drillstring Analysis and Measurement System (ADAMS) surface measurement system. The data were gathered while three different BHA's were used to drill portions of this well to 800 ft [244 m]. A case study for a large-diameter hole in an actual field well is also presented. The data showed many velocity-dependent excitation mechanisms, including 1 × omega, 2 × omega, 3 × omega, a drillstring walk mechanism, and drillstring whipping mechanisms that were dependent on the drillstring and hole geometries. The data also show important resonances that may be excited, including resonances with strong local contributions, and axial and torsional resonances. When the excitation mechanisms match these resonances, damaging vibrations can be induced. This study identifies the presence of more fundamental excitation mechanisms than the 1 × omega and 3 × omega mechanisms. These should be used in models that predict drillstring and BHA vibrations. The study also shows important resonances that should be considered when BHA vibrations are predicted. Introduction The BHA is a key component in rotary drilling. Not only does the BHA provide the required weight on bit (WOB) to drill various formations, it also provides directional control by variation of its component geometry. Many authors have studied BHA response by developing static and dynamic models, both two- and three-dimensional. These models attempt to predict the inclination tendencies of BHA's by calculating the resultant bit forces. Dynamic forces are also believed to control the azimuth responses of BHA's. Rafie et al. also model formation interaction to assist in predicting BHA response. Although there is some degree of confidence in the design and performance of BHA'S, BHA failures are still frequent and the economic incentive to avoid them is substantial. A few authors have concentrated on predicting dynamic instabilities that cause these failures. Early measurements by Deily et al. assisted in setting up linear lumped-parameter vibration models. Pasley also studied drillstring dynamic stability under lateral constraints. More recently, Dunayevsky et al. formulated a model for the onset of instability that assumes full spiral contact of the drillstring with the borehole. This model seems to perform well. It is limited to BHA's without much stabilization, however, because stabilizers will cause intermittent contact with the hole. Recent field measurements of BHA vibrations have also been investigated. A prototypical hardwire measurement-while-drilling system has been developed for downhole vibration measurements. The system used in gathering this paper's data is based on a surface-mounted measurement sub and is described in more detail later. The mechanisms behind BHA failures are still not well understood. This paper analyzes excitation mechanisms and resonances that are important in BHA dynamics by analyzing actual vibration data. These data were gathered at a Rockwall, TX, well. Comparison is also made between BHA dynamics during rotation alone and during drilling. A case where BHA instability was induced is also studied. The data are then compared with analytic models. The first model is a finite-element eigenvalue solution algorithm developed by Pasley and Besaisow. The second model is a general-purpose finite-element code with frequency-response capabilities. The third model uses simplified beam equations and is described in Appendix A. The models showed close mode prediction results. Sensitivity studies were then run to see how the drillstring resonances were affected by various conditions. A case study for actual drilling of a large-diameter hole was also performed. The case study suggested safe operating velocities (revolutions per minute) by looking at excitation mechanisms and resonances of that BHA. Analytic Background BHA vibrations are similar to machinery and structural vibrations. During rotation, such mechanisms as imbalance, misalignment, bent pipe, drillstring walk about inside wellbore diameter, or other geometric phenomena create excitations that are at the rotational frequency or multiples of the rotational frequency. These mechanisms create forces and stresses that oscillate at frequencies of the excitation mechanisms that are multiples of applied omega. When excitation mechanisms' frequencies match one of the BHA natural frequencies, a resonance condition with growing stresses is generated. The speeds at which resonant conditions occur are called critical speeds. Because both drilling and geometric parameters affect excitation mechanisms and natural modes, accurate knowledge of these excitation mechanisms and accurate estimates of the natural modes of vibrations are required. The first section will describe excitation mechanisms that are viable. The second section will present BHA geometries. The third section will present some natural mode calculations from various models for the assemblies studied in this paper. Excitation Mechanisms in BRA Dynamics. Many physical sources induce excitation mechanisms. Of these, an important mechanism is mass imbalance. Mass imbalance causes excitations primarily in the lateral direction that are on the order of the rotational frequency 1 × omega from a fixed reference frame viewpoint. This lateral excitation yields a smaller or secondary axial excitation that is on the order of 2 × omega (known because one lateral cycle causes a fixed end to cycle axially twice). Torsional excitations on the order of the rotational frequency (1 × omega and 2 × omega) are also induced. A bent-pipe mechanism is similar to the mass-imbalance mechanism. Misalignment of the drillstring or buckling of the BHA causes lateral excitations of 1 × omega primarily. Misalignment also causes axial and torsional excitations of 2 × omega. Assuming that the drilled formation is not perfectly flat at the bottom, asymmetric supports also induce the 2 × omega components in lateral excitation. SPEDE P. 93^