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Title: Modelling and optimisation of downhole drilling actuator
Author: Ajala, Olamide Sherifah
ISNI:       0000 0004 7972 5476
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2019
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This thesis has been motivated by the need to develop a reliable design tool employing mathematical models for design optimisation of a new drilling mechanical actuator, which captures the dynamics and kinematics of the actuator under high frequency dynamic oscillations, preload and rotary action. This is achieved using nonlinear Finite Element numerical models and low dimensional models to capture the system characteristics. The developed models capture complex contact interactions and the onset of rolling and sliding motions in assembly. The models presented are suited to any rolling element-race contact, operating condition and behaviour of interest, mainly from the standpoint of rolling and sliding motions of the interacting surface. As a first step, a new analytical model for a three dimensional Rolling Element Bearing (REB) contact interaction is developed, which considers Hertz contact, centrifugal force and elastohydrodynamic lubrication model. The model provides a generalised analysis of the REB motion, contact interaction and energy losses in the assembly. The model utilizes Newton Raphson method of iteration in evaluating the actuator trajectory, and the fatigue life of the REB and wear of the assembly were also estimated. Here, optimum speed and preload of the assembly was established and an optimised exciter geometry is proposed. The analytical model is validated using a nonlinear Finite Element numerical model and commercial available software ADORE [2], in which a quantitative agreement with developed model is achieved. The nonlinear FE model is created to study the dynamics of the actuators, validate the analytical model and capture important characteristics of the actuator such as the stress field, axial oscillations, reactive torque and force on the actuator. The actuator is modelled in commercially available software, Abaqus, as a solid continuum body employing 3-D finite-element mesh, with encastre and displacement boundary conditions on the cage and race respectively to simulate the exciter operation under dynamic load. A second order, 8-node linear brick, hourglass control, reduced integration (C3D8R) and modified second order tetrahedral 3-D stress-displacement elements (C3D10M) were adopted. The model considers both frictionless and isotropic Coulomb friction formulations to simulate the lubricant behaviour of the bearing. Static instabilities in the model, are controlled by adjusting the initial contact states, the excitation sequence and stiffness damping for numerical accuracy. Optimal loading parameters are identified in the study based on the analysis of the system responses and properties. The model is validated with available RED experimental results, where qualitative agreement is obtained and results from analytical model were also validated with simplified numerical models. Energy losses in the assembly were investigated and it was established that the current exciter configuration is dominated by sliding and rolling motions but gross sliding occurs in the system at thrust load greater than 6.4 kN. Finally, the proposed optimised geometry of the assembly with two periodic undulated raceway is analysed. The FEA result shows that the model characteristics of the optimised geometry are better than current design with one wavy race in terms of energy loss, torque and stress fields in the assembly.
Supervisor: Wiercigroch, Marian ; Pavlovskaia, Ekaterina Sponsor: University of Aberdeen
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Actuators ; Mechanical engineering ; Drilling and boring machinery