VIBRATION ANALYSIS OF DRILLSTRING WITH MASS IMBALANCE USING FINITE ELEMENT METHOD

Drill string vibrations pose a major challenge to the operational efficiency of the drilling process in the oil and gas industry. The mass imbalance in the drill string caused by MWD tools and drill collar sag disrupts the accuracy of the drilling operation and can give rise to huge vibration thereby damaging the down hole components. Since the surface readings are not reliable measurements of drill string lateral vibration, it becomes necessary to study drill string vibration under Finite Element method. In this project, ANSYS software was used to build the geometry of a drill string model and appropriate boundary conditions were applied to the model such that the drill string experiences lateral vibrations. The component of mass imbalance was introduced to the drill string by adding an equivalent weight eccentrically on one side of the drill string. Through the use of ANSYS simulation modal and harmonic analysis were performed to study the vibration characteristics of the drill string. Modal analysis was performed to determine the mode shapes and the first three critical frequencies of the drill string. The effect of mass imbalance was studied by comparing the first three critical frequencies before and after the addition of the imbalance. The results showed an increase in natural frequency to 2.03Hz, 2.23Hz, and 4.63Hz. The analysis was repeated with different configurations of drill string to understand the effect of length on the natural frequencies. Harmonic analysis was performed on the drill collar with mass imbalance to study the frequency response to lateral displacements. The results of this project can be used to identify the safe operating ranges of rotary speed for the drill string and identify the lateral displacement of a critical component for a range of frequencies, thereby avoiding damage to the drill string.