NONLINEAR ROTOR DYNAMICS AS APPLIED TO OILWELL DRILLSTRING VIBRATIONS

The lower part of a drillstring used for the drilling of oil or gas wells is usually composed of drill collaes (thick-walled tubulars) and stabilizers (coarsely grooved cylindrical elements of a larger diameter than the drill collars, that loosely fit in the borehole). The drillstring rotates during drilling, and this may lead to violent lateral vibrations of the drill collars, an effect analogous to the whirling motion of an unbalanced rotor. In this paper the dynamic behaviour of a section of whirling drill collars is analyzed in terms of rotor dynamics, with account taken of the non-linear influences caused by drilling fluid, stabilizer clearance and stabilizer friction. Conditions for the existence and stability of forward whirl caused by mass out-of-balance and backward whirl caused by stabilizer friction are derived and compared with numerical simulations. Fluid damping reduces the whirl amplitude of a forward whirling collar section. Added fluid mass reduces the critical rotary speed: i.e., the rotary speed at which the whirl amplitude is maximum. Stabilizer clearance reduces the critical speed and influences the stability, while stabilizer friction decreases the maximum amplitude, and further reduces the frequency range for stable forward whirl. Stabilizer friction can produce self-excited backward whirl of the drillcollars with a frequency that is usually close to the natural frequency, resulting in large whirl amplitudes. Such large-amplitude backward whirl can lead to wall contact between the collars and the borehole wall and can produce drill collar precession, a backward rolling motion of the drill collars along the borehole wall. Unstable drill collar motion can be periodic or chaotic. Unstable collar movements do not converge to a circular trajectory, but remain within an annular region in the borehole cross-section. © 1991.