Horizontal directional drilling technology is one of the fastest-developing, most advanced, and most sophisticated non-intrusive construction methods, holding a dominant position in non-intrusive construction. High-density polyethylene (HDPE) pipelines are widely used in horizontal directional drilling due to their unique properties and are the most commonly used piping material in this technique. However, when laying HDPE pipelines using horizontal directional drilling technology, an important issue often arises: during the pullback process, the HDPE pipeline experiences significant cross-sectional deformation, changing from round to flat, which leads to the failure of the pipeline crossing and laying project.
The failure in pipeline laying can be attributed to the limited development history and insufficient foundational research. Existing studies have not specifically addressed the deformation and damage mechanisms of HDPE pipelines during horizontal directional drilling and retrieval. Currently, due to the lack of foundational research, the control of deformation and damage phenomena during the retrieval process of HDPE pipelines is primarily based on experience. In engineering practice, the variability of stratigraphy and environmental conditions, different pipeline materials, diverse drilling trajectory shapes, and varying construction techniques lead to numerous issues when relying solely on empirical data to guide design and construction, resulting in pipeline laying failures. Therefore, understanding the damage mechanisms of HDPE pipelines during horizontal directional drilling retrieval is both an urgent technical issue at the engineering site and a demand for refining the foundational theories and technological processes of non-invasive construction. Addressing this issue holds significant practical importance for enhancing the technical level of non-invasive construction, reducing engineering risks, pioneering new research fields domestically, and improving project economic benefits.
Based on summarizing and absorbing the research achievements of predecessors, a comprehensive analysis was conducted from the perspectives of mechanics and geometry on the force model of horizontal directional drilling for laying HDPE pipelines. The theoretical analysis of the damage mechanism of HDPE pipelines was also explored. The impact of geological factors, construction engineering factors, and material factors on pipeline damage was discussed; a calculation model for external pipeline pressure was established and theoretically analyzed, addressing the deficiency of previous analyses that only considered the axial tensile force of the pipeline.
Utilizing the large-scale ANSYS finite element software for numerical simulation, we compare the results with theoretical calculations to assess the conformity between the numerical simulation and theoretical outcomes. The analysis evaluates the effects of the pipe's elastic modulus, the ratio of pipe standard dimensions, the deformation rate of the pipe diameter, and the axial tension on the pipe's deformation and damage.
By examining specific engineering cases, the study analyzes the magnitude of the axial tensile stress and the safety factor against instability when the pipeline is filled with ballast liquid. Using finite element analysis and theoretical calculations, the relationship between the maximum stress in the pipeline under complex loads and potential pipeline damage is determined. Effective strategies and measures for preventing pipeline deformation and damage are proposed.
Using the theoretical models and finite element software studied, combined with specific engineering examples, the deformation and damage mechanism of pipelines was investigated and the following conclusions were drawn:
1. Geologic factors, construction quality, and pipe material are significant contributors to pipeline damage. Pipelines in unstable strata like sand and cobblestone are prone to deformation and damage.
Established a calculation model for the external pressure of pipelines and conducted theoretical analysis, resolving the previous limitation of only calculating and analyzing the axial tension of pipelines.
3. Conducted a theoretical analysis of pipeline damage mechanisms and, utilizing the third strength theory, determined that pipeline deformation and damage occur when the net external pressure of the pipeline exceeds the unconfined allowable external pressure.
4. Numerical simulations using large finite element software yielded results that closely align with theoretical calculations. Analysis of the results indicates that as the elastic modulus of the pipeline increases, the unrestricted allowable external pressure of the pipeline also increases. Additionally, with the reduction in the pipe's diameter-to-thickness ratio, deformation rate, and axial tensile force, the unrestricted allowable external pressure of the pipeline increases. The minimum reduction coefficient for the deformation rate can be as low as 0.28, and the minimum reduction coefficient for the axial tensile force can reach 0.65.
5. Theoretical analysis and evidence have proven that filling the pipeline with a filling fluid not only reduces the maximum pull-in force of the pipeline but also significantly enhances its stability safety factor, effectively preventing deformation and damage to the pipeline.
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