Sediment transport and trench development beneath a cylinder oscillating normal to a sandy seabed

Abstract

The purpose of this study is to explore the conditions in which trenches form beneath oscillating cylinderssuch as pipelines, cables or idealised chains-close to the seabed. Experiments are conducted by oscillating a circular cylinder in a direction normal to an initially flat sandy bed. Across a relatively wide parameter space, the transport patterns and trench geometries reveal three transport regimes that are linked to vortex dynamics and depend primarily on the ratio of oscillation amplitude to cylinder diameter (KC number). For KC 4 sediment motion results in bedload transport that is symmetric about the cylinder centreline. This leads to the formation of two parallel trenches with a prominent ridge forming directly beneath the cylinder. For 4 KC 9 sediment motion occurs via localised transport events, which are associated with the motion of vortices shed from the cylinder. These transport events are irregular but occur on both sides of the cylinder and lead to the formation of a symmetric trench geometry. For 9 KC 12 the sediment motion is characterised by localised transport events and asymmetric bedload transport driven by overall vortex dynamics. In terms of trench size, the maximum (equilibrium) depth is found to increase with KC and a mobility number (ψ) defined in terms of the maximum cylinder velocity. The initial rate of trench development also increases with KC number and ψ, with an additional dependency on the cylinder Reynolds number. The cylinder motions required to initiate trenching are predicted well using continuity arguments and an oscillatory boundary layer assumption, provided the KC number and minimum gap between the cylinder and the bed are relatively small. The findings in this study provide insight into the mechanisms and prediction of trench formation. In particular, this study reveals that significant trenches can form in sandy seabeds solely due to fluid flow induced by pipeline/cable/chain motion without direct seabed contact, which has implications for structural fatigue.

Observations of pumping and vortex dynamics due to a cylinder oscillating normal to a plane wall

“…Nevertheless, the general observations are also consistent with experimental observations of sediment transport on a movable sand bed beneath an oscillating cylinder in otherwise still fluid at similar (Tom et al. 2018). In Tom et al.…”

“…In Tom et al. (2018) tests with similar conditions, sediment movement was qualitatively observed to primarily occur sporadically in space along the cylinder and in time, for any given cycle of motion. However, a symmetric trench about the cylinder centreline eventuated at equilibrium, implying a long-term averaging of the asymmetry – a feature also present in the experimental study of Guan et al.…”

“…These features characterise the near-wall velocity response and in turn affect the formation of seabed trenches (Tom et al. 2018). Important physical insight may be attained by quantitatively exploring how the velocity response varies due to these observed flow features.…”

“…The experiments consider , which is representative of typical motions experienced by submarine risers, or cables and pipelines during lay (e.g. Tom, Draper & White 2018). A single (=500) is adopted on the basis that the vortex flow regimes are expected to be relatively insensitive to if it is large, at least for (Sumer et al.…”

Wave‐influenced deposition of carbonate‐rich sediment on the insular shelf of Santa Maria Island, Azores

“…If that stress depends on roughness due to bedforms and biogenic features, this adds significant prediction uncertainty. Effects of bed roughness will need to be considered during the modelling of seabed sediment recovery after sand dredging (Gonçalves et al ., 2014) and assessing risks of subsea pipelines or cables from scouring (Mohr et al ., 2016; Tom et al ., 2018).…”