Study of Lateral Vibration of Drilling String with Mass Imbalance Using Finite Element

Abstract

The objectives of this study were to analyze the first three critical frequency, critical speeds and mode shapes of the drilling string due to lateral vibration. Drilling 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 drilling string caused by Measured While Drilling (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 measured of drilling string lateral vibration, it becomes necessary to study drilling string vibration under Finite Element method. In this project, ANSYS software was used to build the geometry of a drilling string model and appropriate boundary conditions were applied to the model such that the drilling string experiences lateral vibrations. The component of mass imbalance was introduced to the drilling string by adding an equivalent weight eccentrically on one side of the drilling string. Modal analysis was performed to determine the mode shapes and the first three critical frequencies of the drilling 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 were imposed on the deformation of drilling string. Since the normal operating speeds of a drilling string rotary table is around 100-200 RPM, it was clear that the drilling string without mass imbalance, the resonance of the drilling string occurs at 85 and 262 RPM which is clearly out of the operating speeds. But when a mass imbalance is introduced to the drilling string, the resonance occurs at 121, 133 and 277 RPM. The two operating speeds cause resonance which can critically damage the drilling string. The drilling string which consists of the stabilisers and bit are subject to almost no displacements at all. The best practices of this study are very useful for identify the safe operating ranges of rotary speed for the drilling string and identify the lateral displacement of a critical component for a range of frequencies, thereby avoiding damage to the drilling string.