Elastoplastic stress analysis of multi-refrigerant combined heterogeneous artificial frozen wall

Objective Affected by the temperature difference of the refrigerant and the distance difference between the frozen formation and the freezing pipes, the heterogeneity of the multi-refrigerant combined double-row-pipe frozen wall is more significant. In order to evaluate the safety of such frozen wall reasonably, the elastic plastic stress analysis should be carried out for the artificial frozen wall with multi-refrigerant considering the heterogeneity.

Methods The frozen wall at 1/4 pipe distance was selected as the characteristic section. Then, the temperature distribution curve on this section was equivalently converted to three linear functions, and the frozen wall was regarded as a heterogeneous material that changes linearly with temperature. On this basis, the analytical expression of plastic-elastic stress of multi-refrigerant combined double-row-pipe heterogeneous frozen wall was derived according to four kinds of frozen soil yield criteria. Based on the analytical expression, the mechanical characteristics of the multi-refrigerant frozen wall were calculated, and the calculated results were compared with those of the homogeneous frozen wall.

Results and Conclusions  The study indicates that: (1) The radial stress increases with the increase of the relative radius r, and the circumferential stress shows different trends in different frozen intervals (Ⅰ, Ⅱ, Ⅲ) in the brine-CO₂ combined double-row-pipe frozen wall. (2) Based on the calculation theory of the homogeneous frozen wall, the maximum circumferential stress appears on the inner side of the frozen wall in the elastic limit state, at the elastic-plastic interface of the frozen wall in the elastic-plastic state, and at the outermost side of the frozen wall in the plastic limit state . However, based on the calculation theory of heterogeneous frozen wall, the maximum circumferential stress always occurs at the partition boundary of frozen wall (r=2). (3) With consideration to the heterogeneity, the elastic ultimate bearing capacity of the frozen wall decreases by 1.8%, while the plastic ultimate bearing capacity increases by 8.1%. In the plastic-elastic state, the heterogeneous frozen wall has higher bearing capacity compared with that in the same relative radius of plastic zone, and this phenomenon becomes more obvious with the increase of the relative radius of the plastic zone. The research results have important reference value for the design of multi-refrigerant combined freezing curtain in water-rich strata.