Quantitative Composition of Drag Forces on Suspended Pipelines from Submarine Landslides

Abstract

The impact forces of submarine landslides (i.e., non-Newtonian fluids) on oil and gas pipelines, especially the most dangerous drag force, are of great significance in the design of deep-water pipelines. The drag force is composed of two parts: the pressure drag force and the frictional drag force. However, previous studies have not quantified their proportion and magnitude, and thus it is highly difficult to analyze their evolution characteristics and mechanisms in detail. In this paper, a methodology to quantitatively obtain the pressure and frictional drag forces of submarine landslide-ambient water–pipeline interaction using computational fluid dynamics (CFD) is first proposed. Second, under four typical Reynolds number conditions, homogeneous fluidized submarine landslides impacting suspended pipelines applied by two boundary conditions (i.e., free slip and no-slip wall boundary conditions on the pipeline surface) are systematically simulated, respectively. Third, the quantitative relationship between the total, pressure, and frictional drag force coefficients is established, and the variation of their characteristic values with changing Reynolds number is analyzed. Finally, the evolutionary mechanism of the frictional drag force is explained by the change in the tangential stress of the landslide in the boundary layer on the pipeline surface, and the variation mechanism of the pressure drag force with changing Reynolds number is elucidated by the boundary layer separation, streamline evolution, and distributed pressure variation around the pipeline, which provides a theoretical basis for submarine pipeline design.