Analysis On Vibrating Properties Of The DTH Drill String

In recent years, with the rapid development of rock and soil drilling engineering,drilling impact problems have attracted more attention. It is important to study DTH drilling system as the statue of DTH in the drilling industry highly improved. When researching DTH drill string, there exist many difficulties because it bears complex force and has varied forms of vibration. In this paper, the longitudinal vibration,transverse vibration, torsion vibration and coupling vibration of DTH drill string is the focus of our research content. The vibration equations are established based on domain decomposition method and then the vibration laws are analyzed. Specific contents are as follows:Firstly, take the case of longitudinal vibration in DTH drill string, the periodic impulses that have the forms of rectangular waves, half sine waves and triangle waves are considered to study the influence of periodic impulse characteristics on DTH drilling systems.The results showed that When the impulse is equal, the drill string vibration response under periodic impulses with different characteristics is the same.Secondly, the potential energy functional of longitudinal vibration, transverse vibration, torsion vibration and coupled vibration with DTH drill string is build based on domain decomposition method. Then kinetic equations are derived and solved using Matlab. On the base of these, the effect of segment number and displacement functions terms on the drill string modal frequency is analyzed, the effect of drill string length and local section size are also discussed. The results showed that the order of exact modal frequency increase when the segment number and displacement functions terms increased, at the same time the computing time is also increased. The modal frequency of drill string decrease with the increase of string length but it is not sensitive to local section size.Finally, the potential energy functional of coupled vibration with DTH drill string is build based on domain decomposition method and the kinetic equations are also derived and solved. The influence of drill string length and thickness on the drillstring modal frequency is studied, that of load form and structure damping is also discussed. It is showed that with the increase of thickness, the modal frequency increase simultaneously, and that is more significant in low order region. Structural damping can significantly affect the high order modal frequency of the drill string.Through the above research, more about the vibration laws of the DTH drill string in drilling process is obtained and it has great reference value for other forms of drilling and designing or optimizing of drill string system. The successful use of domain decomposition method in DTH drill string vibration provides a new way of thinking and methods for drill string dynamics researching.