RIASSUNTO
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Abstract
High-amplitude, downhole bending vibrations are often responsible for twist-offs and expensive drillstring failures. Yet, it is well known that even large, lateral bending vibrations cannot be detected uphole at the surface and corrected. This paradox is explained using ideas based on new ""group velocity"" interpretations of the conventional beam equation.
The predictive model shows how bending disturbances generated both at the drillbit downhole, and uphole by drillstring and borehole interaction, focus and become ""trapped"" at the neutral point, leading to large local accumulations of vibration energy which are undetectable uphole. Leaky waves that do escape to the surface ""stretch"" beyond recognition and also dissipate. Detailed numerical results for all pertinent vibration parameters, based on the general nondimensionalized pertinent vibration parameters, based on the general nondimensionalized governing equations, are tabulated and provided for immediate practical use. Typical applications, for example, may include bottomhole assembly design and the optimal placement of downhole ""Measurement-While-Drilling"" (MWD) tools. Finally, means to detect violent bending vibrations at the surface, and the use of tapered drillstrings with varying material and geometrical properties, are also suggested.
INTRODUCTION: DRILLSTRING VIBRATION PROBLEMS
The shock and vibration loads generated during drilling are very intense. Typical mile long, million pound drillstrings will often break, the result of large static forces and moments, and high cyclic fatigue loadings.
Different modes of vibration are excited by rock/bit interaction and by drillstring/borehole contact. unfortunately, and exact dynamical analysis is difficult because it is not always possible to characterize these excitation sources accurately.
So much depends on the particular tricone bit configurations used and on their ""wobble signatures"" as they enter unknown rock formations. Transient borehole contacts are equally difficult to describe. The combined pipe motion due to whirling, whipping, axial, torsional and lateral pipe motion due to whirling, whipping, axial, torsional and lateral vibration, strongly dependent on the details of the bottomhole assembly used, is far from simple. This is especially true in the curved holes drilled offshore.
However, the practical consequences of vibration are significant. Many twist-offs occur downhole, particularly near the ""neutral point"", and expensive, time-consuming fishing jobs are needed to keep the drilling program on schedule. This report presents a simple, predictive model that program on schedule. This report presents a simple, predictive model that completely describes the breakdown process.