Numerical modeling of sediment dumping in deep water through a rock-fall pipe for subsea pipeline burial

This paper presents a numerical study of sediment dumping in deep water through a rock-fall pipe for the burial of a subsea pipeline. A coupled CFD-DEM model is employed to model the interaction among sediments, ambient flow, pipeline, and seabed. Unlike sediment dumping in shallow water, in deep water sediment particles inside the rock-fall pipe may be accelerated to dangerous speed at the outlet and thus require a sufficient distance between the rock-pipe outlet and the subsea pipeline so that sediments can be slowed down before making impact on the subsea pipeline. The purpose of the study is to find the optimal distance of the fall pipe outlet above the subsea pipeline with different suspension heights above seabed. The numerical model is first validated against available experimental data in terms of flow and sediment simulations. It is then used to investigate the detailed hydrodynamic characteristics and particle motions during sediment dumping in deep water, which can be divided into particle acceleration inside the rock-fall pipe and particle deceleration and diffusion out of the rock-fall pipe. While insufficient distance of the rock-fall pipe outlet above the subsea pipeline may lead to excessive impact force, too large distance may result in over-spreading of sediments and thus less efficiency of subsequent pipeline burial. Using a real project in the Liwan 3-1 region, China, as the case study, the numerical modeling results with prototype scale are used in the analysis to provide the optimal burying solution that uses the minimal amount of sediment to reach the protection criterion of the pipeline burial, when different pipeline suspensions above seabed are considered.