DRILLSTRING SIMULATOR: A NOVEL SOFTWARE MODEL FOR STICK-SLIP AND BIT-BOUNCE VIBRATIONS
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AbstractDrillstring vibration can be highly detrimental to its mechanical integrity, and significantly reduce overall operational efficiency. Vibrations often arise due to the contact force and moment arising at the bit and formation (rock). These occur due to cutting and friction related actions. The literature has various bit-rock interaction BRI models, which may have time delay and nonlinear terms. The time delay term arises from modulation of the depth of cut per revolution by the vertical vibrations. A major inertia participating in the vibrations is the bottom hole assembly BHA, consisting of the bit, instrumentation and power subs, drill collar and stabilizers. Control of the BHA vibrations is imperative to prevent destructive vibration that may break the pipe, dull the bit and diminish hole trajectory and rate of penetration. The most severe vibration types include stick-slip, bit-bounce and lateral whirl. Stick-slip is caused by the alternate stopping of the bit due to friction and its release when the drillpipes produces a sufficient torque as it winds up. Bit-bounce occurs due to time delay in the torque and axial force due to modulation of the cutting force and torque by axial vibration. Finally, lateral whirl results from friction occurring at lateral contact points of the BHA and wellbore. Modelling of these complex, nonlinear, self-excited vibrations is a challenge given the large order models involved and nature of the BRI forces and moments. The paper provides a systematic means to accurately simulate drillstring vibrations with high fidelity and efficiency.This is achieved using a Timoshenko beam based finite element model FEM, and is illustrated with an example containing the Detournay BRI model. High accuracy codes need user friendly interfaces to be effective for field and design use. The paper also provides algorithms and methods for programming the solution-modelling component of the code, and the user interface component.
