# 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 mean and variance of the _IGD_ indexes have been reduced by 46.4% and 83.2%, respectively, compared to that of the ordinary NSGA-II algorithm [42; 43]. Based on the multi-objective optimization results, the solution sets of the Pareto optimal solutions for the five structural parameters of the hydraulic lift cordless core drilling tool with respect to the drilling fluid flow rate, fluid backpressure, and spring force pressure ratio in the parameter range were obtained. The results of the multi-objective optimization were simulated and verified. It was concluded that the drilling fluid flow rate increased by 33.56% and the force ratio was reduced by 5.825% compared to before the optimization. (3) According to the design and multi-objective optimization of the annular fluid flow structure of the cordless coring tool, the machining trial production of the \(\varphi\)96 cordless coring tool was completed. The coring test of the \(\varphi\)96 drilling tool was conducted six times (divided into two tests) to achieve complete coring. The hydraulic salvage test of the \(\varphi\)96 drilling tool was conducted two times, and the assembly of the drilling tool was pushed to the hole in 40-50 s at the 20 m test well depth. The designed and developed hydraulic lifting cordless coring tool can achieve effective coring and hydraulic lifting after the field test, which can improve the coring efficiency and quality. At the same time, it can reduce the operation cost and energy consumption, providing a new technical solution for drilling operations under complex geological conditions. Conceptualization, Z.Z. ; methodology, F.H., Y.Z. and H.W. ; software, F.H. and C.L. ; validation, Y.S. ; formal analysis, M.J.; investigation, G.L., Y.S. and M.J.; data curation, Y.Z. and G.L. ; writing--original draft preparation, Z.Z. and C.L. ; writing--review and editing, H.W. ; project administration, Z.Z. and H.W. All authors have read and agreed to the published version of the manuscript. This study was funded by the open project of the Key Laboratory of Drilling and Mining Technology for Complex Conditions of the Ministry of Natural Resources and Jinshi Drilling (Tangshan) Company Limited (FZJS230203); the National Key Research and Development Program of China (2023 YFC2813102; 2023 YFC3007003) and Geological Survey Project of China Geological Survey (DD20242850). Not applicable. The raw data supporting the conclusions of this article will be made available by the authors on request. Authors thanks to [PERSON] and Jinshi Drilltech Co., Ltd. for the project funding and the provision of the test site. Author [PERSON] was employed by the company Changjiang Geotechnical Engineering Corporation. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The authors declare that this study received funding from Jinshi Drilling (Tangshan) Company Limited. The funder had the following involvement with the study: methodology. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Serial** & **Number of Water** & **Inlet Diameter** & **Number of Water** & **Outlet Diameter** & \multicolumn{2}{c}{**Distance between the Centres**} & \multicolumn{1}{c}{**Ratio of Fluid Back Pressure**} & \multicolumn{1}{c}{**Drilling Fluid Flow**} \\
**Number** & **Inlets (pcs)** & **(mm)** & **Outlets (pcs)** & **(mm)** & **Outlet (mm)** & **Outlet (mm)** & **Ratio of Spring Force Pressure** & **Rate (m/s)** \\ \hline



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