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Empirical formulations for the engineering prediction of equilibrium scour depths, as well as time scales (for both scour and backfilling), are re-visited for monopiles. Following prior work, it is argued on physical grounds that dimensionless time scales ought to scale proportionally to the Shields parameter raised to the − 3 / 2 power, based on scaling common to numerous sediment transport rate formulae. This is in contrast to stronger power dependence found in many existing formulations based on laboratory experiments, which may become unreliable when extrapolated to (storm/strong) field scales. In light of this, existing data sets for scour and backfilling time scales are re-analyzed. Novel parameterizations are developed for their prediction which are in line with the argument above, while maintaining similar coherency with existing data as in previous parameterizations. Re-analysis of existing data sets for equilibrium scour depths is likewise performed. An existing framework valid for slender monopiles is extended to incorporate the so-called large monopile regime (non-negligible diffraction), especially important e.g. for modern wind turbine installations. • Existing parameterizations of scour and backfilling time scales are reparametrized. • Suggested time scales scale according to the Shields parameter to the −3/2 power. • Existing framework for predicting equilibrium scour depth is extended to include large monopiles. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of the tsunami-induced scour process around a monopile foundation, representative of those commonly used for offshore wind farms. The scour process is studied by subjecting the monopile to a time varying current, which enables a properly down-scaled experiment from the boundary layer and scour perspective. It is shown how the scaled experiments corresponds to real life idealized tsunami cases with periods ranging from 10 to 40 min. It is then shown that the boundary layers of the model tsunami are well described by recently developed empirical relations for tsunami boundary layers. By subjecting the monopile to several successive tsunami waves the scour process is shown to occur in a stepwise cumulative fashion, with the final equilibrium scour depth tending to the depth limited steady current limit. It is shown that the entire scour development can reasonably be predicted by a recently developed simple engineering model. Finally, the experimental results are compared to a fully coupled hydrodynamic and morphologic CFD model and a good correspondence is obtained. [ABSTRACT FROM AUTHOR]

A fully-coupled (hydrodynamic and morphologic) numerical model is presented, and utilized for the simulation of tsunami-induced scour around a monopile structure, representative of those commonly utilized as offshore wind turbine foundations at moderate depths i.e. for depths less than 30 m. The model is based on solutions to Reynolds-averaged Navier-Stokes equations, coupled with two-equation k − ω turbulence closure, with additional bed and suspended load descriptions forming the basis for sea bed morphology. The model is first validated for flow, bed shear stresses, and scour within a steady current, where a generally excellent match with experimentally-based results is found. A methodology for maintaining and assessing hydrodynamic and morphologic similarity between field and (laboratory) model-scale tsunami events is then presented, combining diameter-based Froude number similarity with that based on the dimensionless wave boundary layer thickness-to-monopile diameter ratio. This methodology is utilized directly in the selection of governing tsunami wave parameters (i.e. velocity magnitude and period) used for subsequent simulation within the numerical model, with the tsunami-induced flow modelled as a long sinusoidally-varying current. The flow, sediment transport, and scour processes beneath up to ten tsunami waves are simulated in succession. These illustrate a generally accumulative scour process i.e. a relatively rapid scour induced by the leading wave, with an additional build-up of the scour depth during additional trailing waves. The resulting scour seems to approach an equilibrium value after sufficient time duration, which corresponds reasonably to that predicted by existing steady-current scour depth expressions, after accounting for the finite boundary layer thickness induced by the unsteady tsunami wave, i.e. it is important to incorporate both current-like, as well as wave-like aspects of the long tsunami event. Based on the simulated results, a simple methodology for predicting the tsunami-induced scour depth in engineering practice is finally developed. This methodology is demonstrated to match the predicted scour development for all of the simulated flows considered, ranging from the series of transient tsunami waves to the steady-current limit. [ABSTRACT FROM AUTHOR]

A fully coupled hydrodynamic and morphologic numerical model was utilized for the simulation of wave-plus-current scour beneath submarine pipelines. The model was based on incompressible Reynolds-averaged Navier-Stokes equations, coupled with k-ω turbulence closure, with additional bed and suspended load descriptions forming the basis for seabed morphology. The model was successfully validated against experimental measurements involving scour development and eventual equilibrium in pure-current flows over a range of Shields parameters characteristic of both clear-water and live-bed regimes. This validation complements previously demonstrated accuracy for the same model in simulating pipeline scour processes in pure-wave environments. The model was subsequently utilized to simulate combined wave-plus-current scour over a wide range of combined Keulegan--Carpenter numbers and relative current strengths. The resulting equilibrium scour depths and trends were shown to be in accordance with existing experimentally based expressions from the literature. The variety of scour profile types emerging under various flow conditions is detailed and reconciled with experimental observations. The resulting matrix of scour depth time series was systematically analyzed, resulting in a new generalized expression for the scour time scale in combined waveplus-current flow environments. This expression is fully consistent with existing experimentally based relations at both pure-current and purewave limits and is appropriate for engineering use. [ABSTRACT FROM AUTHOR]

This paper presents results of complementary experimental and numerical studies involving wave-induced backfilling of current-generated scour holes beneath submarine pipelines. The laboratory experiments are conducted in a wave-plus-current flume, utilizing Laser Doppler Anemometry to measure velocities, synchronized flow visualizations using digital image technology, along with live-bed scour and backfilling measurements. Each experiment is based on a two-stage process: (1) initial scour induced by a pure current, followed by: (2) backfilling induced by pure waves (either regular or irregular). The time series of scour depths are closely monitored through video recordings. Systematic analysis of these has resulted in a closed form expression for the backfilling time scale, which is demonstrated to be a full order of magnitude greater than the well-known time scale of scour (with both governed primarily by the Shields parameter). The developed expression is strictly valid for the current-to-wave backfilling scenarios considered, while likely serving as an upper limit for more general wave-induced backfilling circumstances. The experiments are complemented by similar backfilling simulations utilizing a fully-coupled hydrodynamic and morphodynamic CFD model. The numerical simulations demonstrate the ability of the model to predict backfilling towards expected equilibrium scour depths based on the new wave climate, with time scales reasonably in line with experimental expectations. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) local scour beneath two submarine pipelines in tandem under wave-plus-current conditions is investigated numerically. A fully-coupled hydrodynamic and morphologic model based on unsteady Reynolds-averaged Navier-Stokes (URANS) equations with the k − ω turbulence closure is applied. The model is validated against existing experimental measurements involving live-bed scour beneath a single pipeline and beneath two pipelines in tandem, respectively. The model is then employed to simulate scour beneath two tandem pipelines under wave-plus-current conditions for a variety of Keulegan-Carpenter (K C) numbers and relative current strengths. Horizontal gap ratios (the horizontal gap distance between two pipelines divided by the pipeline diameter) ranging from 1 to 4 are modelled. It is found that for conditions involving waves plus a low-strength current, the scour pattern beneath two pipelines behaves like that in the pure-wave condition. Conversely, when the current has equal strength to the wave-induced flow, the scour pattern beneath two pipelines resembles that in the pure-current condition. It is also observed that in the pure-wave condition the equilibrium scour depth beneath each pipeline is affected by both K C and the horizontal gap ratio, except for K C = 5.6. For such a small K C , the horizontal gap ratio has insignificant influence on the equilibrium scour depth, since vortex shedding does not occur. When the current strength relative to the waves is low, the scour development beneath the upstream and the downstream pipelines are similar. However, when the current has equal strength to the waves, the scour development beneath the downstream pipeline has a different pattern to that upstream. Namely, smaller horizontal gap ratios result in delayed scour beneath the downstream pipeline. [ABSTRACT FROM AUTHOR]