Elastodynamic and finite element vibration analysis of a drillstring with a downhole vibration generator tool and a shock sub

Abstract

Applying high-frequency axial oscillation into an oilwell drillstring in the “bottom-hole assembly” (BHA) has the potential to enhance drilling efficiency in extended reach wells. Downhole vibration generator tools such as agitators reduce the drillstring–wellbore friction and enhance the rate of penetration. However, introducing controlled vibrations into the drillstring can result in undesired vibration waves propagating along the drillstring, leading to inefficient drilling and catastrophic fatigue failure of the BHA components, “measurement-while-drilling” tools, and mud motors. A dynamic model of the entire drillstring, including vibration generators and shock subs, is required to study the effect of vibration generators on the complex nonlinear coupled axial-lateral dynamics of a drillstring inside a wellbore, to study the effect of vibration tools on the developed cutting force at the bit, and to facilitate simulation-based design of shock subs. A dynamic finite element model (FEM) and an analytical elastodynamic model, both including the vibration generator tool and a shock sub, have been developed. The “Bypassing PDEs” method was implemented on the Lagrangian of the system to develop the analytical equations. A multi-mode expanded Galerkin’s approximation, in conjunction with a multi-span BHA and Hertzian contact assumption, allowed analysis of multiple BHA contact points and, thus, more realistic estimates of drilling rotary speeds that can cause excessive vibration. The models also include torque, mud damping, spatially varying axial force, geometric nonlinearity, and axial stiffening. While the analytical model has fast running time and symbolic solution, the FEM model enables easy reconfiguration and future extensions of model geometry, interactions, and modified BHA configurations. There is agreement between the analytical and FEM simulation results for the vibration suppression ability of the shock sub, dynamic amplification of the vibrating tool force, critical rotary speeds, axial force along the drillstring, axial and lateral displacements, and the contact locations and severity.