Abstract
A numerical model of scour beneath subsea structures considering the effect of upward seepage in the seabed is proposed. A small seepage can cause significant changes to the hydrodynamic force on the bed surface and stability of bed particles, which can further affect the sediment transport processes and scour patterns around subsea structures. The present model is developed based on a fully-coupled hydrodynamic and morphologic sediment transport model. The unsteady Reynolds-averaged Navier-Stokes (URANS) equations are solved together with the k−ω turbulence closure. In the presence of seepage, the bed friction velocity, the bed shear stress, and the bed load transport rate will be changed as compared to conditions without seepage. The sediment particle stability is also affected and the critical Shields parameter is changed. In the presence of upward seepage forces, the repose angle of the sediment is also reduced. The present model is validated against existing experiments in terms of streamwise flow velocity distribution subjected to upward seepage. The fully-coupled hydrodynamic and morphologic model is validated against existing experiments of scour beneath a pipeline in the live-bed regime and clear-water regime, respectively. The validated model is then applied to investigate the scour development beneath a submarine pipeline subjected to different upward hydraulic gradients. It is found that the equilibrium scour width is increased with a large upward hydraulic gradient. The equilibrium scour depth stays in the range of 0.6–0.8 of the pipeline diameter for the live-bed cases. For the clear-water case, with a large upward hydraulic gradient, the equilibrium scour depth slightly decreases.
Highlights
Scour beneath a submarine pipeline has been investigated inten sively by experiments, such as Mao (1986), Sumer and Fredsøe (1990) and Sumer and Fredsøe (1996), and numerical simulations such as Chao and Hennessy (1972), Chiew (1991), Brørs (1999), Liang et al (2005), Fuhrman et al (2014) and Larsen et al (2016)
The present study has proposed a numerical model of scour beneath subsea structures considering the upward seepage effect in the seabed
The fully-coupled hydrodynamic and morphologic scour model has been validated against the live-bed scour and clear-water scour ex periments of Mao (1986)
Summary
Scour beneath a submarine pipeline has been investigated inten sively by experiments, such as Mao (1986), Sumer and Fredsøe (1990) and Sumer and Fredsøe (1996), and numerical simulations such as Chao and Hennessy (1972), Chiew (1991), Brørs (1999), Liang et al (2005), Fuhrman et al (2014) and Larsen et al (2016). Cheng and Chiew (1999) derived equations for modifying the critical shear velocity of the sand bed particles in the presence of upward seepage. The present modified model is applied to investigate the scour pattern around a submarine pipeline in the presence of upward seepage forces. Hydraulic gradient ratios i=ic ranging from 0 to 0.9 are modelled in the present numerical investigations
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