Study On Key Technologies Of Steering Mechanism For Point-the-bit Rotary Steerable Drilling Tool

With the increasing requirements for special wells of horizontal well,directional well and extended reach well in drilling engineering,conventional slid-steering drilling technology is unsatisfactory for the modern drilling processes,and rotary steering drilling system has been the research focus to improve steering drilling technology.However,due to the high requirements of small structure,huge deviating force,reliability and stability for rotary steering drilling system,steering mechanism,as the core of rotary steering drilling system,is the technological difficulty to develop steering drilling technology.Therefore,it is significant to investigate the technological difficulty of rotary steering drilling system for the improvement of steering drilling technology.In this paper,supported by the key technologies R&D program of Tianjin(Grant No.11ZCKFGX03500),a new steering mechanism for static point-the-bit rotary steerable system is developed for directional wells.In consideration of the vibration from bit-rock interaction and the limited well dimension by wellbore specification,the dynamic modeling,dynamic analysis,experimental verification and optimization are conducted.The main contents and conclusions are as follows.Design for a new steering mechanism for static point-the-bit rotary steerable system: based on a biased planetary drive with small teeth number difference(PDSTND)and a extension-type hollow universal,a new steering mechanism is developed.The steering mechanism can be employed to increase,maintain and decrease the hole angle without pulling out the drilling tool,and it can be controlled precisely.Meanwhile,the extension-type hollow universal is helpful to alleviate the alternating stress of mandrel in drilling process,which is beneficial to improve the lifetime and reliability of rotary drilling tool.Dynamic modeling of steering mechanism: In consideration of time-varying meshing stiffness,damping,comprehensive transmission error,bearing stiffness and damping,the coupled translation-torsion dynamic model is established by lumped-parameter method,and the system excitations as time-varying stiffness and bit-rock interaction are analyzed,which provide basis for the dynamic analysis and experiment research.Study of modification coefficient selection based on dynamic characteristics: for the case of modification coefficient selection in PDSTND of steering mechanism,a K-H-V PDSTND is taken as an example to study the effect of modification coefficient on nonlinear dynamic characteristics of PDSTND.Based on the dynamic model of steering mechanism,chaos theories as bifurcation diagrams,poincaré maps,trajectories and frequency spectrums are employed to study the modification coefficient selection in consideration of the dynamic characteristics of the system.The study results reveal the influence rules of backlash and transmission error to the dynamic characteristics of the system with different modification coefficients,which provide new theoretical basis for modification coefficient selection.Construction of vibration-testing platform for steering mechanism: based on the principle scheme of steering mechanism,the prototype of steering mechanism is constructed,the control system and excitation system for alternating load are designed,and the vibration-testing platform is established to verify the steering mechanism and dynamic model,which provide theoretical and experimental basis for the optimization and improvement of steering mechanism.Multi-objective optimization of steering mechanism based on modified NSGA-II and fuzzy set theory: a nonlinear dynamic model is established by taking the structural parameters of teeth,modulus and width as design variables,by taking the requirements of gear manufacturing and drilling process as constraints,and by taking the minimization of maximum acceleration responses and outer diameter of steering mechanism as objectives,and a optimization procedure based on modified NSGA-II and fuzzy set theory is proposed for the optimization model.The comparison between original and optimized designs shows that the maximum dynamic response decreases sharply and the dynamic excitation is reduced effectively,which show that the proposed optimization procedure is feasible to solve the optimization model of steering mechanism.Meanwhile,the optimized model is restructed by Solidworks and simulated by ADAMS,and the dynamic characteristics have the same law and trend to the optimized results,which provide theoretical basis fro the next generation of protype.The above study results can provide theoretical guidance for the design and optimization of steering mechanism for point-the-bit rotary steerable system,which is significant to improve rotary steering drilling technology.