COUPLED AXIAL AND TRANSVERSE VIBRATIONS OF OILWELL DRILLSTRINGS

Transverse vibrations of drillstrings caused by axial loading and impa ct with the wellbore wall is studied. The drillstring is modeled as a slender beam with a simply-supported lower part. The Assumed Modes Met hod is used to obtain the governing equations of motion. Non-linear co upling terms are retained in the formulation which leads to full coupl ing between the axial and transverse vibration modes. The Hertzian con tact theory is used to model impacts between the drillstring and the w ellbore wall. The governing non-linear equations are solved numericall y to obtain the response. Simulation studies are used to compare the d ynamic response obtained from both fully coupled and uncoupled models. Significant differences between the two models are observed. The coup led model yields an unstable behavior at a lower load than the uncoupl ed model, and the subsequent response is not periodic. (C) 1996 Academ ic Press Limited