Vibration Response Analysis Of Drill String In The DTH Hammer Drilling System

As the status of drilling engineering in the construction of economic improved, the DTH hammer with its many advantages brought to the attention of the drilling industry at home and abroad, and gradually become a kind of conventional drilling methods. And in the actual drilling process, owing to the difference of rock properties and the change of the drilling depth and other factors underground, the DTH hammer and surface equipment will vibrate. It will easily cause the drill string fatigue failure and make the drilling efficiency decline. In this paper, the longitudinal vibration response of drill string in the process of the DTH hammer drilling is the focus of our research content. From two aspects of theory analysis and numerical simulation to study, specific research work is as follows:First, based on one-dimensional wave theory, the mathematical model of drill string longitudinal vibration is established. The longitudinal vibration responses of the drill string under the action of arbitrary harmonic exciting force are solved. Through the analysis of "the force of piston" and "the force of bit" in the process of DTH hammer drilling, get the force curve at the bottom of the drill string. With the method of polynomial fitting, the function of the harmonic exciting force can be expressed and the numerical response solutions of the drill string under this force are solved. It has revealed the theoretical foundation of the drill string longitudinal vibration response in the DTH hammer drill string coupling system.Second, using LS-DYNA explicit dynamic finite element method(FEM) the coupled multi-body dynamics model of the DTH hammer and drill string is established. How to implement the impact force of the piston in LS-DYNA cyclically and how to make the right initial conditions and boundary conditions of the rock, bit and drill string components are discussed in this paper. With the analysis method in time domain and frequency domain, the longitudinal vibration response of drill string are analyzed when the DTH hammer impact to the rigid body. The results show that the low-order natural modal response of the drill string system was aroused in different parts of the drill string.Finally, the impact of different material rock properties and different length of drill string on the vibration characteristics of the DTH hammer drill string system in the process of drilling are analyzed by LS-DYNA software. The results show: for the same length of drill string, the harder the rock is, the easier the low order natural modal was caused in vibration response of the drill string, that is, the easier to induce the drill string resonance phenomenon. For the different length of drill strings, the low order natural modal was caused when impacting the hard rock and it will cause resonance, however, when impacting the soft rock, with the increase of drill string’s length, the more easy to arouse the low-order modal vibration responses, but their amplitudes are small, and it does not produce resonance phenomenon.By all the works mentioned above, the vibration response of the DTH hammer drill string has thoroughly researched and the mathematical model is established. The impact of different material rock properties and different drill string’s length on the vibration of the drill string are analyzed, which provide a reliable theoretical basis and research methods for the study of the dynamic response of DTH hammer drill string.