Thermal elasto-plastic computation model for a buried oil pipeline in frozen ground

The 813-mm-diameter China-Russia Crude Oil Pipeline enters northeastern China at Lianyin, Mo'he County, Heilongjiang Province and crosses 441 km of warm discontinuous, sporadic and isolated patches of permafrost and 512 km of seasonally frozen ground before reaching Daqing, China. It is inevitable that the buried pipeline is subject to frost heave and/or thaw settlement when it passes through regions of permafrost and seasonally frozen ground with available moisture. Therefore, stress and deformation analyses of the pipe subject to frost action or thaw settlement are important for the safety, long-term stability and economic feasibility of the buried oil pipeline system. Based on the (empirical) frost heave and/or thaw settlement coefficients, a simple thermal elasto-plastic finite element computation model is put forward for analyzing the stress and strain state of the pipe. The influences of soil temperatures on the soil deformation were considered, but those of the soil deformation on the soil temperatures were ignored in the modelling. Finally, two examples of the application of the computation model are presented, in which the stress and deformation of a pipe exposed to frost heave or thaw settlement is calculated. The results of the frost action computation show that the effective stress on the pipe increases linearly with the frost heave deformation. The largest pipe deformations and stresses may occur when crossing frost mounds due to differential frost heaving. The stress and deformation smooth out from the frost mound. The results of thaw settlement computation show that the pipe stress changes greatly near the interface of thaw settlement zone and no thaw settlement zone, and the thaw settlement has small effect on the stability of oil-pipeline. The computation results show that the oil pipeline design in the permafrost regions should pay more attention on frost heave hazard than thaw settlement hazard.