Bending Rigidity and Flexuosity Stress of Pipeline for Underwater Cross-ing

River crossing pipelines and subsea pipelines usually involve composed structure in order to ensure safe operation. The former is often protected with packaged concrete weights or casing pipes, while the latter with continuous concrete coating, altogether forming composite structure. For the strength calculation with such structure in the design stage, however, the interaction between concrete (cement) and pipeline generally have not been taken into account due to the complexity and technical-economic factor. For this reason, taking the structure of crossing pipeline as a composed beam consisting of different materials, supposing no interaction exists between concrete and steel pipes, and ignoring the influence of insulation coating, the equations calculating the bending rigidity and flexousity stress have been derived. Besides the strength of underwater crossing pipeline, these equations can also be directly used to calculate the curvity radius of elastic laying for crossing pipeline, as well as the resonant suspension length. Also, the equations can be used to study the longitudinal stability of the crossing pipelines.