Loading of soil-covered oil and gas pipelines due to adverse soil settlements - Protection against thermal dilatation-induced wear, involving geosynthetics

摘要

Oil and gas pipelines covered with a mixture of sandy gravel and square-edged stones are subjected to abrasive shear loading steming from temperature-induced displacements of the pipeline in longitudinal direction. This requires wear protection of the outer anti-corrosion film of such pipes. In order to assess appropriate means for protection, adverse settlements on both sides of the soil-covered pipeline are assumed as relevant loading scenario. A nonlinear, elastoplastic, cap model-based Finite Element model delivers the radial stresses exerted onto the pipeline, due to settlement-induced load re-arrangement within the cohesionless soil body. Given a cover height of 1.5 m, the aforementioned load re-arrangement leads to an increase of force action onto the pipe by up to 200%. Based on the grain size of the filling material, radial stresses are integrated as to estimate the maximum normal forces exerted by single stone corners onto the pipe's anti-corrosion film. These forces, when analyzed through Archard's law, allow for calculation of the service life of the protection layer enveloping the pipe, once the wear resistance of that layer is determined from experiments, as is done for geosynthetics. Since the service life of a geosynthetic protection layer of the anti-corrosion film turns out to be distinctively smaller than the standard service life of a soil-covered steel pipeline, protection through geosynthetic layers is insufficient. Hence, more effective means for protection are suggested.