Dynamic Control Research On The Offset Steering Mechanism For Point-the-Bit Rotary Steerable System

Rotary steerable drilling technology is a new drilling technology of mechanical, electrical and hydraulic integration. The prominent advantages of the rotary steerable drilling technology include low drilling resistance and flexible adjustment of drilling direction, et al. And the rotary steerable drilling technology has been widely used in complex boreholes, such as the horizontal wells and 3-D multiple targets boreholes. In consideration of complicated geological conditions, traditional control methods of the rotary steering drilling tools are poor in aspects of dynamic performances, such as the stability and rapidity. Therefore, it is very significant for extending its service life and improving the working reliability to put forward reasonable control schemes for the steering mechanism. This thesis aims at a steering mechanism for static point-the-bit rotary steering system, and some key technologies focus on the mathematical models of formation force and spindle stress, simulation model of steering system, dynamic control simulation analysis and stress testing experiments of the spindle. The main achievements in this paper are as follows:Mathematical models of formation force and spindle stress are established first. On basis of analyzing the working state of spindle systematically, the mathematical model of formation force is established. And then based on the fourth strength theory, mathematical model of code stress of spindle including the axial stress, bending stress, torsion stress, radial stress and circumferential stress is established.And then simulation model of the steering mechanism is established. Based on the Kirchhoff's law, the mathematic model of servo motor system is established. On basis of the Newton's equation, mathematic model of the offset system is established. At last, according to the input-output relationship between the servo motor system and the offset system, the simulation model of steering mechanism is established based on Simulink.Dynamic control simulation analysis of the steering mechanism in Simulink is performed. Parameters of the PID controller are optimizated based on the embedded tool in Simulink. And the simulation results show that optimal parameters of the PID controller can significantly shorten the setting time of the servo motor system and decrease the fluctuation of spindle stress.At last, stress testing experiments of spindle are carried out. The stress testing experimental platform of rotary steering mechanism spindle is established based on the strain gauge, current collector and resistance strain gauge. Then, experiments of the spindle stress under the constant excitation are performed. Experimental results achieve basic level of compliance with theoretical simulation results, which verifies the correctness of the theoretical model and provides theory foundation for actual well drilling.