Analysis of the critical instability load of freeze pipes in coal mine shaft freezing engineering

Low-carbon steel freeze pipes, essential refrigerant conduits in artificial ground freezing, are thin-walled cylindrical structures susceptible to buckling and weld failure. This study examined their buckling behavior and critical loads under uniform freezing pressure and shaft sinking-induced displacement using numerical simulations and three-point bending tests at - 32 °C. Results showed that low temperature condition increases nonlinear buckling loads but accelerates post-buckling stiffness degradation. Buckling modes depended on joint type. The critical buckling pressure of jointless freeze pipes was at least 16.39 MPa, while the buckling load of welded-joint freeze pipes exceeded that of jointless pipe sections. Under bending loads, the bending instability of socket welded reinforcement joint freeze pipe (SWRJ) commenced predominantly with ovalization development at the external sleeve, progressing until global structural instability occurred. In contrast, ovalization of internal sleeve butt joint freeze pipe (ISBJ) initiated primarily at the weld seam in the mid-span region of the parent pipe. SWRJ and ISBJ specimens exhibited comparable critical bending moments at diameter-to-thickness ratios of 26-28, but SWRJ specimens exceeded ISBJ specimens outside this range. Based on bending instability analysis, a maximum allowable deflection of one-third of the pipe diameter is recommended for engineering safety.