Drillstring Component Mass Imbalance: A Major Source of Downhole Vibrations
- M.W. Dykstra (Amoco E&P Technology) | D.C-K. Chen (Sperry-Sun Drilling Services) | T.M. Warren (Amoco E&P Technology) | J.J. Azar (U. of Tulsa)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1996
- Document Type
- Journal Paper
- 234 - 241
- 1996. Society of Petroleum Engineers
- 1.6.1 Drilling Operation Management, 1.10 Drilling Equipment, 4.3.4 Scale, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.4.4 Drill string dynamics, 1.12.1 Measurement While Drilling, 1.2.5 Drilling vibration management
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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.
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.
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