The Modal Analysis Of Pneumatic DTH Short Drillstring

Owing to the advantages of high efficiency, low consumption and safety, the pneumatic down-the-hole hammer (DTH) is widely used in the realm of water well drilling, mining, etc.The vibrations of DTH quietly differ from those of oil-well drilling technique, as DTH works with short drill-string which has high natural frequency. The DTH itself can generate periodic non-linear impacts with wide spectrums, which may excite resonance of short DTH drillstring. In addition wide impact spectrum may cause drilling system forced vibration. The vibration will accelerate drilling system wear and tear, especially tungsten-carbide inserts in bit, then reduce the penetration rate, consequently increase DHT drilling cost. So it is necessary to analyze and study the dynamic characteristics of short drillstring. After getting the natural frequencies of all kinds of shot drillstring under the boundary conditions which are close to the reality, the natural frequencies can be identified from the simulation of the coupled drilling system of DHT and short drillstring.The characteristic and mechanism of pneumatic DTH are introduced followed by the summary of the present status of dynamic researches on the drillstring home and abroad. Based on the finite-element method (FEM), the 3-dimesion FE model of short drillstring was developed through the finite-element analysis (FEA) software ANSYS. Then the modes of different short drillstrings were analyzed under diverse boundary conditions. After analyzing the result of modal analysis, the law of the natural frequency of short drillstring was discovered. And the effect of different factors on the natural frequency has also been discussed.The optimum design of DTH drilling system and the adoption of drilling technology can be developed based on the data and result.