Design Of The System For Thermal Ice Coring From Borehole Wall And Study Of Heat Transfer Mechanism Of The Drill Bit

The ice core samples preserved in the deep layers of polar ice sheets contain a plethora of valuable information,thus possessing significant scientific research value.Numerous ice coring projects have been conducted by countries worldwide to explore these resources.However,as ice core research progresses,the quantity of ice cores obtained from individual boreholes is insufficient to meet the demand.Therefore,the acquisition of additional ice cores at specific depth of the borehole during drilling holds great significance.Additional ice core samples not only contribute to the increased quantity of ice cores available for scientific research but also serve as timely supplements in case of ice core lacking,contamination,or damage during drilling,transportation,storage,or analysis processes.Additionally,the extra ice cores can validate existing research conclusions.However,research efforts addressing ice coring techniques for repeated sampling of ice layers remain limited among nations,resulting in issues such as low coring efficiency,long auxiliary operation time,and the need for depth correction of obtained ice cores.Based on a comprehensive analysis of the advantages and disadvantages of current techniques for replicate ice coring drilling and thermal coring drilling,this paper proposes a new technology that utilizes a thermal coring drill bit for replicated ice cores from borehole walls,enabling coring at any depth and in any orientation within the borehole.This technique does not require the use of whipstock,correction of ice core depth,or change of drill bit.The system for thermal coring from borehole wall is a crucial component in determining the safety and reliability of accessing ice cores.The thermal coring drill bit serves as the direct tool for drilling into the ice and achieving drilling progress,and its ability to efficiently and reliably complete coring operations is the key to the entire technology.This study begins with the conceptual design and feasibility analysis of various components of the system for thermal coring from borehole wall based on the principles and requirements of replicate ice cores from the borehole wall.Subsequently,theoretical calculations,numerical simulations,and experimental tests are employed to conduct detailed design and analysis of the scissor deployable mechanism and thermal coring drill bit.Finally,coring tests in the borehole are conducted on the entire coring system.The main research contents and conclusions of this paper are as follows:(1)Based on the design requirements,working principle,mechanical design theory,and mechanism motion principles of the system for thermal coring from borehole wall,the conceptual design of the system was conducted.The main components include actuator mechanisms,a scissor deployable mechanism,a drive system and a thermal coring drill bit.The feasibility of the designed actuator mechanism with an L-shaped bracket and the scissor extension mechanism is verified through the analysis of mechanism degrees of freedom.The drive system consists of servo motors that is compact in size and can precisely control the output speed and torque,as well as a transmission part composed of a universal joint,gear set,and hexagonal prism.The relationship between the motor output parameters and the motion of the slider was then calculated to establish.(2)The motion characteristics and dynamics of the scissor deployable mechanism were analyzed through theoretical calculations and utilizing the Static Structural and Transient Structural modules of ANSYS Workbench software.The analysis yielded the motion patterns and stress-strain distribution of the mechanism under horizontal drilling,vertical drilling,and ice core retrieval conditions.These results were then employed to determine the structural dimensions of the shear mechanism,and were validated through drill bit motion experiments,with a maximum error of 10%.The stress analysis indicated that the main stress during the operation of the scissor deployable mechanism is concentrated on the scissor rod,with the maximum stress primarily situated at the midpoint of the rod,while other parts can be neglected.Based on the research findings,the ultimate load-carrying capacity of the mechanism under different operating conditions can be determined,and the discovered motion patterns of the drill bit can provide theoretical support for the drill control system.(3)Based on heat transfer theories including heat conduction,convective heat transfer,and latent heat of phase change,the heat transfer characteristics in the process of coring from the borehole wall by thermal drill bit are analyzed theoretically,and the heat balance equation is established.The theoretical model of ice melting performance of the drill bit is established respectively from horizontal and vertical coring drilling,and the penetration rate under different parameters is obtained.A simulation software that can simulate the process of thermal coring from the borehole wall in ice sheet was written using MATLAB software.The software allows for the setting of physical properties and heat transfer parameters of different materials during the simulation process,and it can output physical states,temperature distributions,and other results at any time and at any coordinate through post-processing programs.(4)A thermal coring drill bit experimental testing system was designed and established,and the experiments were conducted within a temperature-controlled lowtemperature container.Using the single-variable method,experiments were performed to test the horizontal and vertical penetration rates of the thermal coring drill bit under different heating powers,drill bit pressures,and ambient temperatures.The maximum and minimum error among experimental results,theoretical analysis,and numerical simulation results were found to be 19.5% and 0.1% respectively.This validates the accuracy of the aforementioned theory and numerical simulation.The analysis results indicate that power has a significant influence on penetration rate and ice core diameter.Higher power can enhance penetration rate while reducing the impact of heat on the temperature of the ice core.When the power is higher than 1500 W,the effect of heat on the internal temperature of the ice core is smaller.The higher the ice temperature,the faster the penetration rate and the smaller the effect on the ice core temperature.Drill bit pressure exhibits a minor influence on penetration rate and temperature of the ice core.(5)The system for thermal coring from borehole wall was tested by real drilling in the ice well of the cryogenic laboratory.The core recovery efficiency,ice core quality,and the reliability of replicate ice core were evaluated through monitoring penetration rate and measuring ice core dimensions.Ultimately,the thermal coring methodology and its associated parameters for replicating ice cores were determined.The results show that the system can safely and reliably obtain the additional ice core from the borehole wall,the highest horizontal and vertical penetration rate is 1.55 m/h and 3.19m/h,respectively.The ice core quality is good with a maximum diameter of 66 mm,and an average ice core take rate of 85.4% was derived from the ratio of core length to vertical drilling depth,and an average coring rate of 72.5% was derived from the ratio of core diameter to the inner diameter of the drill bit.