The Bottom Hole Assembly Dynamics Analysis

Mechanical static analysis of drillstring can describe its deformation, and be a guidance to control the trajectory of wellbore, but it neglects the effect of rotation. The dynamic performance of drillstring can affect the trajectory of bit, and the serious vibration can cause severe accidents. For example, with the excitation of rotation, drillstring is likely to be failed in alternative stress, which will shorten its service life. Once the rotation speed reaches a certain critical value, drillstring will generate large lateral displacement. This will cause complex accidents, such as casing wear or wellbore instability, and do harm to both the ground equipments and the wellbore trajectory control. Thus, in order to know the dynamic state of drillstring better, and to predict the critical rotation speed, it is imperative to make dynamic analysis of drillstring.BHA, the research object of this thesis, is discreted into some units of direct beam, then a three-dimensional finite element mechanical model is established. After the displacement vector is obtained, the kinetic energy and potential energy of unit are educed in the energy theory. Hence, the Stiffness, Mass, Damp, Load and Stress matrixes in local coordinates are calculated. Based on these theories, a program composed of 15 subprograms is written for computing dynamic response of BHA, so the equivalent stiffness matrix is calculated in the Forced Frequency theory. After the boundary conditions of bit and stabilizer are determined, the dynamic array equations are calculated by Gauss algorithm, then the displacement matrix can be obtained. It's necessary to judge the boundary restriction of each node. If the displacement of a node go beyond the wall, it must be handled with restrict-release iteration. After the displacement and stress matrixes are obtained, the bending stress in every section of unit can be computed. This program outputs the maximal lateral deformation and bending stress with the excitation of different rotation speed, then the relevant Amplitude-Frequency Response curve can be plotted, and the critical speed can be gotten, too. Then figures of maximal deformation and maximal bending stress distributions of BHA in critical speed areplotted respectively. Moreover, the dynamic response of a field case (GD-1 well) and 5 BHAs are calculated in this paper.Furthermore, the mechanism of failures in BHA is studied, and two methods of decreasing vibration by use of the hydraulic thruster and non-rotation stabilizer are developed.The results of this paper can be used as a guidance to select the optimum mechanical parameters for the drilling job, which will decrease the vibration failure.