This work describes an alternative formulation of a system of nonlinear state-dependent delay differential equations (SDDDEs), which governs the coupled axial-torsional vibrations of a 2 DOF drillstring model with a realistic representation of polycrystalline diamond compact (PDC) bits. The regenerative effect associated with the complex cutter layout for such bits can introduce up to 100 state-dependent delays in the equations of motion. This large number of state-dependent delays renders the computational efficiency of conventional solution strategies unacceptable. The regeneration of the bottom-hole surface can alternatively be described by the bit trajectory function, whose evolution is governed by a partial differential equation (PDE). Thus the original system of SDDDEs can be replaced by a nonlinear coupled system of a PDE and ordinary differential equations (ODEs). Via the application of the Galerkin Method, this system of PDE-ODEs is transformed into a system of coupled ODEs, which can readily be solved. The algorithm is further extended to perform a linear stability analysis of the bit motion. The resulting stability boundaries are verified with time-domain simulations. The reported algorithm could, in principle, be applied to a more realistic drillstring model, which may lead to an in-depth understanding of the mitigation of self-excited vibrations through PDC bit designs.
|Original language||English (US)|
|Journal||Journal of Computational and Nonlinear Dynamics|
|State||Published - May 1 2022|
Bibliographical noteFunding Information:
The work presented in this paper was supported by the Theodore W. Bennett Chair in Mining Engineering and Rock Mechanics. This support is gratefully acknowledged.
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