Study On The Fatigue Damage And The Coupled Inertial Boundary Dynamic Characteristics Of Sonic Drill String

With the development of drilling technology and the growing emphasis on environmental protection,the requirements for drilling equipment and tools is increasingly higher,sonic drilling has gained rapid development and wide application in recent years because of its advantages of high drilling speed,wider formation suitability and little environmental pollution.Aiming at the problems that the impact of sonic vibrator mass on the dynamic characteristics and vibration response of sonic drill string with inertial boundary,and the fatigue damage of sonic drill string,this paper has carried out the theoretical researches.The specific research work is as follows:Firstly,the dynamic differential equations of the sonic drill string with inertial boundary are established,we solved the natural frequency equation of sonic drill string with inertial boundary through mathematical analysis,the approximate expressions of natural frequencies is given,and the impact of sonic vibrator mass,drilling string length,drill string density and section area on the natural frequency is discussed.Secondly,the dynamic differential equations of the sonic drill string with inertial boundary are solved by the separation of variables,the displacement and stress response of sonic drill string with inertial boundary are obtained.The impact of sonic vibrator mass on the displacement and stress response of sonic drill string with inertial boundary,as well as the bit displacement amplitude and stress amplitude at the top of drill string(where sonic vibrator and drill string are coupled together)are analyzed.Thirdly,sonic drill utilizes high frequency resonance of drill string to achieve rapid drilling,the high frequency alternating stress in drill string is easy to cause fatigue damage of drill string.Based on the analysis of the internal stress of the drill string in the traveling wave and standing wave vibration stage,combined with the cumulative fatigue damage rule,the fatigue damage formula and the fatigue damage section of the drill string are established.The fatigue damage section in the sonic drill string is near the bit and does not exceed the limiting length value,which is determined by the length of drill string when standing wave vibration starts.Based on the above researches,it will help to accurate control the vibration frequency of drill string during the shallow hole drilling,improving the drilling efficiency and the life of sonic drilling equipment as well as drill pipes;it will alsoguide the sonic drilling equipment design and the production practice of sonic drilling;meanwhile,the methods for calculating the fatigue damage of drill string could also provide a theoretical reference for estimating the fatigue life of drill string under symmetric cyclic stress force.