| A study consisting of theoretical analysis, numerical modeling, and field experiments has been conducted to characterize drill string vibration. Particular attention was focused on lateral vibrations which result from unbalanced drill string components and their frictional interaction with the wellbore constraint. This type of vibration is likely responsible for most drill string component damage, yet has received relatively little attention in the literature.; The theoretical portion of the work comprised formulation of equations of motion which adequately represent the behavior of the drill string, including important nonlinear effects due to axial force-flexure coupling, torsion-flexure coupling, and frictional interaction with the wellbore. The equations were obtained using an energy approach, and were compared with those derived from Newton's second law. The comparison pointed to advantages and disadvantages of both approaches. The equations were numerically solved using the finite element method. A static solution was first obtained using an iterative procedure. Steady state and transient dynamic solutions were then obtained using modal and Newmark numerical integration methods, respectively. A dedicated computer program was developed to provide these solutions. Comparison of dynamic response predictions from the modal and transient routines allowed uses and limitations of each to be identified.; The experimental portion of the study comprised two series of tests conducted on a specially instrumented, full scale drilling rig. The first series characterized the unbalance of typical drill string components by measuring their deflections as they were rotated in the derrick. Reduction of the data provided the eccentricities of the centers of gravity, and these results were used as input data for the computer program. The second series of tests were conducted in the downhole environment. A bull nose was used in place of a bit to evaluate drill string vibrations without the bit as a source. A variety of drill string configurations were used and simultaneous surface and downhole measurements of accelerations were made. The results allowed the effects of operating conditions and formation strength on the dynamic behavior of the drill string to be characterized.; Results from the numerical simulator were consistent with the full scale experimental data. This verified that the program could be used to evaluate drill string design alternatives that would minimize damage to components caused by shock and vibration. |
