Numerical Simulation Of Submarine Landslides Process And Their Impact On Pipeline

Debris flows and landslides are commonly observed in mountainous areas after long or intense rainy periods. They are common in both subaerial and subaqueous environments. Different with landslides in subaerial, submarine landslides occur frequently on gentle slopes (0.5to3°). Massive submarine landslide may travel distances as long as hundreds of kilometers. They may seriously damage fixed platforms, submarine pipelines, cables and other subsea facilities and cause severe economic impact. To avoid hazards and reduce economic loss, it is significant to study and predict the characteristics of mud flows and their impact on submarine pipelines.The first part of the thesis is to simulate submarine landslide process. According to the basic equations of fluid mechanics, such as the boundary layer approximation theory, continuity equation, momentum equation, Navier-Stokes equations, etc, the governing equations for Herschel-Bulkley and Bilinear rheological model are derived. Besides, difference schemes for the governing equations are also obtained. These equations prove the program code of one-dimensional submarine landslide simulation software BING is correct. Then two software, ANSYS CFX and BING, are used to simulate submarine landslide. We found that the higher shear strength of the slurry, the closer results both the software simulates. Since the computing speed of BING is fast, it takes only1to2minutes for common computation. Using software BING to simulate the situation that shear strength of the slurry is high will greatly improve efficiency.The second part is to study the impact of submarine landslides on submarine pipelines. Accurate prediction of the drag force is the concern of offshore engineers. The impacted forces of submarine landslides on pipelines are usually expressed in terms of drag factors within a traditional fluid mechanics framework. Zakeri (2009) employed Computational Fluid Dynamics (CFD) analysis to numerically simulate impact of clay-rich submarine debris flows on a pipeline at five angles of attack and proposed a method to estimate the normal and longitudinal drag forces at any attack angle. Randolph and White (2012) proposed a failure envelope following a more geotechnical approach to estimate the impact forces for any attack angle based on a re-analysis of numerical results from Zakeri (2009). In this paper, CFD numerical simulation was performed on the cases studied by Zakeri (2009) as well as much more cases. Based on the simulation results, it is found that the impact angle also has apparent influence on normal drag factor, but has little influence on longitudinal drag factor. Then improved approximate formulae are developed to better estimate normal and axial impact forces on suspended pipelines at any attack angle. These numerical simulation results were also re-analyzed following the geotechnical approach and a new failure envelope was obtained. The predicted forces by geotechnical approach and the CFD-calculated forces compared very well.