| Hot water drilling is the fastest ice drilling methods in the world.It plays an important role in polar scientific research.However,when the hot water drilling works in snow-firn,the hot water will leak through the space of the snow particles,causing hole wall damage and energy waste.Therefore,it is necessary to use other drilling methods to drill a pilot hole for hot water drilling firstly.This paper summarized the status of large diameter drilling rigs at home and abroad,and introduced two kinds of heat exchange modes of thermal drill which used for the pilot hole to the hot water drilling(by thermal fluid and electric).Combining with its technical characteristics,aiming at the current research situation of large diameter thermal drilling tools in the world which are scarce and lack of pertinent theoretical analysis,theoretical and experimental studies on the drilling mechanism of thermal drilling in snow-firn are carried out to fill the theoretical gap of thermal1 drilling in snow-firn.Through consulting literature,theoretical analysis and numerical simulation were carried out for designing a large diameter thermal drill bit.A complete large diameter snow-firn electro-thermal drilling system was developed.The main achievements of this paper are as follows:(1)It is founded that the traditional theory and mechanism of the thermal drilling in ice can not apply to the process of thermal drilling in snow-firn by setting a theory model of thermal drill working in the snow-firn.It must be concerned the influence of the melting water when it is drilling in snow-firn.In the thermal drilling process,the melting water will transfer its heat to the outside repeatedly which will add the drilling velocity.In order to obtain the specific effect of melting water on thermal drilling in snow-firn,it is quantified that the infiltration energy utilization rate is4%-8%,the heat loss rate is 2%-4%by theoretical calculation and experiments.The influence of the snow layer temperature on drilling speed is about 11%-13%.At the same time,considering the relationship between the snow density and depth in the Antarctic ice sheet,the relationship formula of thermal drilling period,hole depth and power was established which will provide reliable theoretical guidance and technical support for the future research field of Antarctic thermal drilling in China.(2)Through literature research and theoretical analysis,the body shape of the thermal drill is determined.Combining with the relevant performance parameters of the electrothermal components,the temperature field distribution of the thermal drill with various heat source structures is simulated by COMSOL Multiphysics software.According to the numerical simulation results,the selection and structure design of the thermal drill are guided.The simulation results show that the spiral coil has the most uniform heat distribution in the three types of thermal source distribution of conical thermal drill,but there is a certain cold zone at the bottom.In order to eliminate the cold zone,the design of the heating bit at the bottom of the thermal drill is carried out.The cone,parabola,truncated cone,combination curve and non-standard curve are used as the five initial structural forms of the copper-based heater at the bottom.The numerical simulation research is carried out and the results show that the non-standard curve structure is most suitable for thermal drilling.Combining with theoretical calculation and numerical simulation results,the optimal power distribution mode of large diameter thermal drill is determined as follows:the upper coil is powered by 25kW,and the lower copper-based heater is powered by5kW.(3)A complete set of large-diameter snow-firn thermal drilling system was designed and developed,which includes five subsystems:drilling,pumping,power supply,detection and control,and water removal.The subsystems are controlled by a mobile control box,which can realize the coordinated control of drilling,power supply and drainage operations.The whole system is designed to installed on the surface of the polar sled to facilitate installation and transportation in the polar field.The maximum power of the thermal t drilling system is designed as 30 kW and the safe power of dry burning is designed as 8 kW.The maximum diameter of the thermal drill is 500 mm and the taper angle of the drill is-15°.The maximum load capacity of the system is 2 ton,and the steel cable has the ultimate lifting force of 8 t.The detection and control system can monitor and record the drilling parameters and operation parameters of the whole system in real time,and display them in the LabView software of the host computer in real time.The dewatering system adopts a pump to pumping water to the surface in real time,and the maximum drainage depth is designed as 117m in the hole.The air compressor was used for dewatering to the pipeline,and the flow rate of the air compressor is 1 m~3/min,which ensures the long-term storage and use of the system under polar low temperature conditions.(4)The theoretical prediction and experimental study on the drilling ability of the large diameter snow-firn thermal drilling system are carried out.According to the different drilling conditions and formation parameters,the drilling rules under the corresponding conditions are obtained.A large diameter snow-firn thermal drilling test platform was constructed and 25 groups of drilling experiments were carried out.The maximum drilling depth was 1.8m,the maximum drilling speed was 3.2m/h,and the maximum drilling diameter was 510mm.At the same time,the feasibility of the drainage system is verified by experiment:the drainage system can realize 117 m of pump lift to meet the need of pilot hole for hot water drill;91.6%of the water in the pipeline can be pumped out by the water removal scheme,which verifies the feasibility of the air compressor water removal scheme;Antifreeze fluid pumping experiment was carried out on the pipeline of the dewatering system which verified that the system can meet the needs of use and storage in polar environment.(5)The experimental results of large diameter snow-firn thermal drilling are compared with the theoretical analysis.The experimental data distribution is consistent with the theoretical calculation results,which verifies the accuracy of the theoretical analysis and simulation results. |
PDF Full Text Request