The Parameters Optimization Of The Reverse Circulation DTH Hammer And Studies On Its Drilling Technique

Down-the-hole impactor drove by compressed air has been catching technicists'attention in drilling realm for its higher rate of penetration and percussive method of fracturing rock since its appearance. With the development of science and technology and need of engineering, the DTH drilling technique has been developing continually, many new drilling tools and drilling technique methods were invented one after the other. From general DTH hammer positive circulation drilling technique to CSR local reverse circulation drilling technique, hollow-through DTH hammer reverse circulation drilling technique appeared ultimately. The multi-techniques drilling technique was developed at the basis of single drilling technique. The appearance of every new technique and method resulted in large advance of the drilling technique and enhancement of penetration rate and drilling quality, at the same time, good economic and social benefits were produced.The technique of coring continually by reverse circulation with hollow-through DTH hammer is regarded as the latest DTH drilling method, which integrated three advanced drilling methods of DTH hammer fracturing rock, fluent removal by reverse circulation and continually obtaining core and sample during drilling. This kind of technique has been applied successfully in practice after several years'study and test. However, in the face of complex geology and construction conditions, there are also many problems needed to be solved. Therefore, the CGS's projects"the research and application of hollow-through DTH hammer drilling technique in the exploration of solid mine"and"serialization research and application of hollow-through DTH hammer"were set up to support the research. At present, the projects have been accomplished successfully, experts thought highly of the achievements. The theory and test studies on the problems existed in hollow-through DTH hammer reverse circulation drilling technique were done in this paper.The performance of DTH hammer is the key to the drilling effect. The mathematical model of movement discipline of piston in cylinder and state change of compressed air in fore-and-after gas chambers was built to improve the performance and structural parameters of DTH hammer. Furthermore, differential equations of piston motion and gas pressure and temperature fluctuation in fore-and-after gas chamber were set up. The special software for power process simulation of DTH hammer was further improved. At the base of a great deal of emulational calculation and weighing analysis, the performance and structural parameters of a series of hollow-through DTH hammers were optimized and redesigned. We have achieved physical design, drawing and manufacture of many different specification hollow-through DTH hammers which were debugged indoor. The redesigned DTH hammer has many specialties such as lower air consumption, lower loss of pressure and good performance. Design based on finite element analysis and structural optimizations were done to key parts, so the reliability and service life of DTH hammer were enhanced.The special reverse circulation bit is the key part of the reverse circulation drilling system. In order to perfect the structure of the bit, the theoretic and practical studies on the bit have been done. Using the CFD technique, the computational model of the special reverse circulation bit's hydro-channels detail was set up with the software—FLUENT. Logical initial and boundary conditions for calculation were defined, then, analog simulations of bottom flow field of different structure bits were done. By which we got the regularities of distribution of pressure field, velocity field and thermal field. At the same time, the influence of the size and position of injector hole on distribution of pressure and velocity field was simulated. The area proportional relation of injector hole and bottom jet orifice was analysed. We can get following conclusions: 1.Using the reverse circulation bit with bottom jet orifices, the reverve circulation can form successfully for the pumping and jetting function of bottom jet orifices in integrity formation. However,the effect of reverse circulation will be influenced seriously by air leaking at the bottom. 2.The problem of air leaking in broken formation can be solved by using reverse circulation bit with inner jet orifices. Low pressure zone is formed above the inner jet orifices for its pumping function, which can reform the shape of flow field at the hole bottom and strengthen the reverse circulation. The reverse circulation can form successfully even if the formation is complex and leaking seriously. 3.Smaller value ofαangle, the inclination of inner jet orifices, is good for formation of reverse circulation. But 25°is a ciritical point. When the value ofαis between 20°and 30°,the reverse circulation can form successfully. 4. When the acreage ratio of inner jet orifices and bottom jet orifices is larger than 1,the effect of reverse circulation is better than that smaller than 1.The results of analog simulation play an important part in optimum design of structure and size of reverse circulation bit and curtailing the spending of time and money in researching Which was good for further test research of hollow-through DTH hammer. Piston is a moving part of DTH hammer, which produces impact force and realizes fracture of rock. Because of its complex state of loading, it is necessary to do further study on the force state of piston. Firstly, using stress wave theory, analysis and count of the compressive and tension stress wave produced by the impact of piston during the moving process were done. The position of maximum crushing stress and tension stress was found out. Then analysis of stress and strain was done with finite element software, the results of which are useful as the basis of optimization design. The core tube is other very important part of DTH hammer. Its interior passageway is the essential way for cuttings discharged out of hole by reverse circulation, and its external construction assorts with piston to achieve distribution of compressed air. Core tube does not bear any impact force directly, but fracture failure once appeared during using process, at the base of analysis of stress and strain, the fracture failure of core tube was avoided after improvement. .Luanchuan molybdenum-tungsten mine located in Henan province is the biggest molybdenum mine in china, the formation of the mine zone belongs to classical complex strata. The diamond and general DTH drilling methods were once used, but the drilling couldn't be proceeded because of drilling trouble caused by serious leaking of stratum. Aiming at the existed technique problems, further study on the application of hollow-through DTH hammer in practice and optimum design of special reverse circulation bit and proper choice of drilling rig and air compressor were done. The technique parameters and operating methods of the technique of coring continually by reverse circulation with hollow-through DTH hammer were generalized. Thousands of meters footage was done, which not only brought a huge amount of economic benefits for the enterprise but also accumulated a mass of practice experience of hollow-through DTH hammers'application.The natural environment of Baiganhu tungsten-tin mine in Xinjiang is bad, and geological conditions are complicated. The formations is composed mainly of quartz schist, quartz sandstone, marble, granite and relax deposit. The stability of strata is bad because of fracture. Leaking of the strata is serious. Permafrost exists beneath the surface. Because of lack of water, the drilling cost of general drilling methods which uses water-based fluid as circulation medium is high and the rate of penetration is very low, sometimes it can't be used at all. The drilling technique problem under complicated conditions was solved by using hollow-though DTH hammer reverse circulation drilling technique with proper drilling parameters. The rate of penetration of hollow-through DTH hammer drilling method is more than six times higher than that of general diamond drilling. The applications in practice show that the technique of coring continually by reverse circulation with hollow-through DTH hammer is of abroad using field and can be used widely in solid mineral and resources exploration. The DTH drilling technique has many advantages, such as higher rate of penetration, high percentage recovery of core and sample, lower cost of drilling, the ability to adapt to complicated strata and so on. At present, the hollow-through DTH drilling technique as an advanced drilling technique has been perfected relatively through a series of studies on the theories and experiments, and it is of the advanced level in international domain.