Numerical modeling of stress and strain associated with the bending of an oil pipeline by a migrating pingo in the permafrost region of the northern Tibetan Plateau

摘要

Small seasonal pingos formed in Quaternary deposits along active fault zones in permafrost of the northern Tibetan Plateau exert destructive forces to oil pipelines, bridges, culverts and other engineering facilities along the Golmud-Lhasa railway and highway. The pingos are particularly hazardous as they change position, or migrate, nearly every year. Three-dimensional finite element modeling reveals the enormous force from exerted by a pingo at the 86th station of the highway. A good representation of the stress and strain fields resulting from an expansion of a pingo and bending of an oil pipeline at the station are calculated after due consideration of the interaction between permafrost, pingo and pipeline. This followed establishing an engineering-geologic model from the field data and determining the mechanical properties of the media from field and laboratory tests. The maximum, intermediate and the minimal principal compressive stresses are calculated as well as those for the plastic strain. Concentrations of principal stress and plastic strain occur beneath the pipeline bend and both the principal compressive stress and resulting plastic strain become very small away from the pingo. Also, the bottom of the pingo is dominated by minimal values of principal stress and strain and the potential bending of a buried pipe caused by an expansion of a pingo is indicated to decrease as depth of burial increases. The pingo growth at the 86th station resulted in the bending upward of a 20m section of a buried oil pipeline, but it did not break and spill oil. Analysis of the pipe within the bend found the maximum, intermediate, and minimal principal compressive stress ranges that leads to plastic strain within the bent pipe. Compressive stress and plastic strain concentrations form in the inner sides of inflexions in the pipe bend, and tensional stress and plastic strain concentrations form in their outer sides where stress exceeds the yield limit of the pipe, but many irregularities are present. Such numerical modeling of stress and strain may offer key parameters for designing oil pipelines and engineered facilities to decrease the hazard from migrating pingos in similar geologic settings in the permafrost of the northern Tibetan Plateau.