Investigation of coupled axial-lateral-torsional drillstring vibrations using a multi-body dynamics approach.

Abstract

Drillstring vibration is a very crucial phenomenon which has a great effect on the drilling process. The drillstring is responsible to transfer rotary motion and energy to the drill bit. Unwanted vibration causes reduction in the rate of penetration (ROP), bit wear and connection failure between drillstring parts. In this thesis, a model of a realistic drill collar is generated which can predict any possible motion of the drill collar. This study contains several analyses about drillstring vibration in three main modes. The focus is on the drill collar section of the drillstring because of the importance of this section in vibration generation of the whole drillstring. This study attempts to determine the vibration behavior of the drill collar in axial, lateral and torsional directions in the presence of vibration-assisted rotary drilling (VARD) and unbalanced rotation. The model includes self weight, hydraulic forces due to drilling mud circulation and most realistic boundary condition for each particular scenario. This model also can be used for coupled vibration states and determination of vibration behavior of the drillstring in three coupled modes, simultaneously. Simulation results show the vibration behavior of the drillstring due to several cases and boundary conditions. The time response to each single mode is expressed and will be validated by finite element method (FEM). The vibration behavior of the rotating drillstring in three coupled modes will be studied using unbalanced rotation of the drillstring. The rock-bit interaction will also be applied to the model. Finally, the effect of the VARD tool will be examined.