In this paper, non-equilibrium sediment transport processes around marine pipeline are simulated using a Large Eddy Simulation (LES) turbulence flow model coupled with a transport equation of sediment concentration. The non-equilibrium sediment transport is observed around the pipeline as the sediment transport rate varies significantly both in time and space. It is also observed that the equilibrium sediment transport assumption in the conventional equilibrium sediment transport models does not hold for the local scour around pipelines. It will he shown that the present numerical model gives a better scour prediction than those based on the equilibrium sediment transport assumptions.
A non-equilibrium state of sediment transport exists as an unidirectional current flows around a pipeline laid on an erodible seabed. The sediment transport rate varies with time and location due to the complex nature of the flow. The conventional equilibrium sediment transport model is unable to represent this local process convincingly. On the other hand, since the occurrence of the local scour undermines the stability of pipelines, it is important to develop a prediction method that can accurately estimate the location and depth of scour holes. For more than three decades the local scour research has concentrated on linking up the mean flow parameters with the equilibrium scour hole depth. Only recently attentions have been paid to the turbulence motion and the non-equilibrium sediment transport problems. Sumer et al. (1988) demonstrated, in his experimental tests, that the vortex shedding behind the pipeline plays a significant role on scour hole development. Van Beek and Wind (1990) proposed a scour model around pipelines by combining the k-c model with a suspended load sediment transport model. The morphological change of the seabed due to scouring is calculated based on the continuity equation of sediment transport.