Effects of axial forces on drillstring lateral vibrations

Lateral vibrations of stabilized bottomhole assemblies (BHA) of drill-strings often exhibit quite complicated patterns. Previous work has taken into account nonlinearities due to fluid damping, wall contact, eccentric collars, and initial curvature of the collar. This thesis addresses the effect of these forces, along with linear and parametric excitation of the lateral motion due to an applied static or dynamic weight on bit (WOB).; The model is described by four state-space nonlinear ordinary differential equations in two-dimensional space. A static value of WOB along with a time variant zero-mean axial force are considered. The force is described either as a periodic deterministic function or as a stationary random process. Random and chaotic whirling characteristics of drill-strings are investigated using the minimum phase-volume deconvolution technique.