# Multi-Objective Optimization Study of Annular Fluid Flow Structure in Cordless Core Drilling Tools
## 2 Structural Design and Principles of Hydraulic Lifting Cordless Coring Drilling Tool
### Principles of the Improved NSGA-II Algorithm
The results are shown in Figure 4:4 The experimental results show that the improved NSGA-II algorithm is better than the NSGA-II algorithm in terms of convergence and distribution diversity. Using 500 iterations of the improved NSGA-II algorithm program, the solution set of the Pareto optimal solutions of the five structural parameters of the hydraulic lift cordless coring tool with respect to the drilling fluid flow rate, the fluid back pressure, and the spring force pressure ratio in the parameter range, the Pareto front, is obtained (Figure 6). As shown in Figure 6, it is known from the concept of the Pareto solution set that any point of the blue color in the figure satisfies the optimal solution of the objective function. The difference lies in the different weights of the two response objectives, determined by the positions of the different points. The five structural parameters of the hydraulic lift cordless core drilling tool corresponding to the optimal solution set can be obtained by inverse normalization of the obtained points. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test Function** & **Indicator** & **Improved NSGA-II** & **Normal NSGA-II** \\ \hline ZDT3 & mean (\(IGD\)) & \(3.13\times 10^{-3}\) & \(5.84\times 10^{-3}\) \\ std (\(IGD\)) & \(1.79\times 10^{-4}\) & \(1.07\times 10^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mean and variance of \(IGD\) indexes of two algorithms. Figure 5: Comparison of ZDT3 test function algorithms. (**a**) Improved NSGA-II Algorithm Pareto Frontier. (**b**) Ordinary NSGA-II algorithm Pareto Frontier. Figure 6: The Pareto frontier graph shows the optimal two key objectives. ### Numerical Simulation Verification of the Optimised Annular Fluid Flow Structure

A set of optimized structural parameters is selected, which is suitable for hard rock strata to be set in the numerical simulation model of the spearhead and spool. Tests are carried out to obtain the simulation results, as shown in Figures 7 and 8. The number of spearhead inlets corresponding to the optimization target is 3, the diameter of the spearhead inlet is 16.92 mm, the number of spool outlet ports is 2, the diameter of the outlet port is 16.69 mm, the center distance between the two circles of outlet port oblong holes is 14.79 mm, the outlet flow rate is 3.286 m/s, and the ratio of fluid backpressure to spring force pressure is 1.454. It can be concluded that the flow rate of drilling fluid after optimization is 33.56% higher than that before optimization, and the force ratio is 5.825% lower. ## 5 Field Trial Study Conclusion

According to the optimized dimensions of the designed annular fluid flow structure, the processing of the \(\varphi\)96 cordless coring drilling tool is completed, and trial production is carried out in terms of material selection, processing, assembly, and debugging. After the assembly is completed, the valve body spool, popping card mechanism, suspension mechanism, and single-action mechanism are functionally debugged and adapted to the outer pipe assembly. The gap between the card spring seat and the inner step of the drill bit is adjusted. The gap of the \(\varphi\)96 drilling tool is in the range of 8-10 mm, and the physical drawing of the \(\varphi\)96 hydraulic lift cordless coring drilling tool is shown in Figure 9.

A total of 12 well-entry tests were carried out successively. According to the test conditions, six coring tests with \(\varphi\)96 drilling tools (divided into two tests) and two hydraulic salvage tests with \(\varphi\)96 drilling tools were carried out. In order to simulate the rock stratum coring, the test holes were fully drilled, filled with high-grade cement, and hardened three times. The test was carried out with an XY-5 vertical shaft core drilling rig, and a 250 triplex single-acting horizontal mud pump was selected. The drilling fluid consisted of clean water due to the wall protection of the borehole pipe and cement hardening.