Numerical simulation of scour around fixed and sagging pipelines using a two-phase model

A key aspect of design and the maintenance of underwater pipelines is the assessment of local scour and its propagation. Scouring around objects placed on a sandy bottom is very complex because it involves two-phase turbulent flows and a myriad of sediment transport modes. This dissertation addresses two principal configurations of scour around pipelines in two parts. First, clear-water scour around a long fixed pipeline placed just above a non-cohesive sandy bed is numerically simulated. Second, live-bed scour around a fixed pipeline and scour below a sagging pipeline are investigated. These two simulations are conducted by using an Eulerian two-phase model that implements Euler-Euler coupled governing equations for fluid and solid phases and a modified k - epsilon turbulence closure for the fluid phase, the modeling system being a part of software FLUENT. Both flow-particle and particle-particle interactions are considered in the model. During the simulations, the interface between sand and water is specified using a threshold volume fraction of sand, and the evolution of the bedforms is studied in detail.For clear-water scour around a fixed pipeline, the predictions of bedform evolution are in agreement with previous laboratory measurements. Investigations into the mechanisms of scour reveal that three sediment transport modes (bed-load, suspended-load and laminated-load) are associated with the scour development. While some previously proposed scour development formulae for cylindrical objects are in good agreement with the simulations, scour predictions based on some operational mine-burial models show disparities with present simulations.For investigations of live-bed scour and scour under a sagging pipeline, the flow and pipeline evolve in two steps: (1) the local live-bed scour around the pipeline developed around a fixed pipeline and (2) the pipeline is lowered to the scour hole in controlled fashion until it reaches the bottom of the scour hole. Three sagging velocities are simulated, and predicted scour profiles agree well with the laboratory data. General characteristics of flow fields, including turbulence, suspension of particles and sediment transport, are described paying attention to their dependence on pipeline sagging. Scour profiles simulated are also in agreement with a LES-based numerical study reported earlier.