Longitudinal condition assessment of exposed pipelines encased in concrete box in a riverbed

Scour around a river crossing pipeline creates a free span of pipeline and leads to excessive flexural and deformation stresses. Generally, generated stresses on pipelines under high internal pressure are dominated by circumferential stress. Longitudinal stress, however, is a main contributor in the procedure of stress development because a free-span pipeline exhibits unstable behavior as a simple supported beam and results in bending stress. For this study, a specific section of river crossing pipeline was defined as follows: API 5L Gr. X65, Do=762mm (30-inch). Based on a plastic design concept, the interface stress for the steel-concrete composite pipeline and its performance under specific flow velocity were defined using results from FEM analysis. In order to describe the loading condition of the exposed encased pipelines, Prandtl-von Karman universal velocity law is adopted with rough river bed surface, fixed water depth and variable scouring depth. In the general condition of a river, although 20m free-span pipelines have been developed, encased pipelines perform well according to the von-Mises failure criterion.