Drillstring Component Mass Imbalance: A Major Source of Downhole Vibrations

Abstract

SPE Members Abstract A study consisting of surface and downhole field experiments, theoretical analysis, and numerical modelling has shown that mass imbalance of drillstring components is a major source of downhole lateral vibrations. Factors which contribute to imbalance include bore misalignment, initial curvature, and gradual wear during service. The field experiments done on the surface were conducted to quantify drillstring component mass imbalance for modelling purposes. Tests have focussed on drill collars thus far. Lateral displacements of collars were measured while each was rotated in the derrick, and results were interpreted using simple models. All collars were unbalanced to some extent, A similar procedure can be used in the field to identify nearly balanced collars for use near the bit. Field experiments conducted downhole utilized a bull nose in place of a bit to evaluate drillstring vibrations without the bit as a source. A pendulum assembly with stabilizers 65 and 95 ft. from the bull nose was used. Simultaneous surface and downhole measurements of accelerations were made. Lateral shocks due to collar/wellbore collisions were measured at various locations in the drillstring at rotary speeds which caused the collars to whirl. Acceleration magnitudes were heavily influenced by local formation strength. Backward whirl of the drillstring was observed while rotating both on and off bottom. It was identified through downhole measurements, and was associated with sudden, dramatic increases in surface torque. Introduction Axial, torsional, and lateral vibrations of the drillstring can all be harmful to downhole equipment when they are severe, but lateral vibrations are most commonly associated with component failures. In particular, the transition from forward whirl of collars to backward whirl leads to strong lateral vibrations, which are often associated with violent shocks as collisions with the wellbore intensify. The combined effects can lead to drillstring fatigue failures (wash outs, twist-offs) and measurement while drilling (MWD) tool failures. The lateral vibrations of the portion of the bottom hole assembly (BHA) near the bit can also affect its performance, resulting in excessive bit wear and lower rates of penetration (ROP). Usually the source of the lateral vibrations has been assumed to be the bit. While it is true that the bit can generate substantial levels of vibration, the centrifugal forces which are generated when unbalanced drillstring components are rotated may be an even more important source. A rotating body is unbalanced when its center of gravity does not coincide with the axis of rotation. Figure 1 shows a cross section of an unbalanced shaft whose center of gravity is displaced from its geometric center by a distance . This distance is known as the eccentricity, and it causes a centrifugal force to act on the center of gravity when the shaft is rotated. The dynamic force (imbalance force) makes the shaft bow Out, as is also shown in the figure. The magnitude of the imbalance force (per unit length of the shaft) depends on its mass (m), its eccentricity (), and the rotary speed (); it is given by m 2. When the shaft is rotated at one of its natural frequencies of lateral vibration, the deflections due to mass imbalance can become very large. P. 171

Experimental Investigation of the Effects of Rotational Speed and Weight on Bit on Drillstring Vibrations, Torque and Rate of Penetration

“…Mass imbalance also intensifies the lateral vibrations (Dykstra et al 1996). BHA interacts with the borehole and generate shocks which damage its components and bottomhole sensors (Mitchell and Allen 1987).…”

Analyses of the robustness of the mathematical model with the laboratory prototype of the drilling process during simulation and vibration control experiments

“…The performance of drill strings and their effect on drilling performance have been investigated and analyzed in a number of researches [1][2][3][4][5][6][7][8]. Many different models were set up to analyze drill string vibrations including lateral vibrations (whirl) and mode coupling [3,9,10].…”

“…Similar models were also studied by other researchers; for example, Melakhessou et al [5] modelled the drill string as an unbalanced rotor supported by two bearings and research is concentrated on a contact zone between the drill string and the borehole wall. Dykstra et al [1] explains that the source of vibration is the bit and hence the centrifugal forces developed when an unbalanced drill string is rotated can be one of the major sources of vibrations.…”

The Use of the Near-bit Vibration Sensor While Drilling Leads to Optimized Rotary-Steerable Drilling in Push- and Point-the-Bit Configurations

“…In the 1980s and 1990s, drillstring dynamics MWD sensors were developed to detect and record harmful downhole vibrations [5][6][7][8][9][10][11][12][13][14][15][16] . Today, it is well known that severe downhole vibrations can damage drilling equipment including the drill bit, drill collars, stabilizers, MWD/LWD, and RSS.…”

“…Despite the extra stability gained at the RSS, it is inevitable that the tool is exposed to high vibrations due to, for example, torsional vibration (stick-slip) induced from the bit-formation interaction 2,11,13 , lateral vibrations caused by the mass imbalance of drill collars and other BHA components 12,22 , and axial vibrations resulting from the high-low pattern of a rock bit in hard formation 1,14 . Moreover, some of these vibrations do not travel to the surface and can only be detectd downhole.…”