Burial and scour around short cylinder under progressive shoaling waves

The results of a laboratory experimental program aimed at better understanding the scour around and burial of heavy cylindrical objects under oscillating flow on a sandy bed are described. This study was motivated by its application to the dynamics of isolated cobbles/mines on a sandy floor under nonlinear progressive waves, such as that occur in shallow coastal waters beyond the wave-breaking region. In the experiments, nonlinear progressive waves were generated in a long wave tank of rectangular cross-section with a bottom slope. Model mines (short cylinders) were placed on the sandy bottom and the temporal evolution of the bed profile and the velocity field in the near field of the object were observed. Experiments were conducted at relatively high Reynolds numbers for a range of flow conditions, which can be characterized by the Keulegan–Carpenter number and Shields parameter. Depending on the values of these parameters, four different scour regimes around the cylinder including periodical burial of cylinder under migrating sand ripples were observed; they were classified as: (i) no scour/burial, (ii) initial scour, (iii) expanded scour, and (iv) periodic burial cases. A scour regime diagram was developed and the demarcation criteria between different regimes were deduced. Semi-empirical formulae that permit estimation of the scour depth with time, the equilibrium maximum scour depth and length, and conditions necessary for the burial of the cylinder as a function of main external parameters are also proposed.     

Self-Burial of Short Cylinders Under Oscillatory Flows and Combined Waves Plus Currents

Self-burial processes of finite-length cylinders under oscillatory flows and waves plus currents were examined with the following two different experimental facilities: a large oscillating water-sediment tunnel (LOWST) and a large wave-current tank. More than 130 experiments, with different model cylinders, were conducted within both facilities. The burial mechanisms studied include burial due to local scour and bedform migration. Burial due to fluidization in the tunnel was also explored, but only in a qualitative way. In the case of experiments with LOWST, the equilibrium burial depth was found to be a power function of the shields parameter (thetas). In the wave-current tank, the equilibrium burial depth was also found to be a function of the Shields parameter, albeit with larger scatter. The experimental observations made in both facilities have similar trends but different magnitudes. For equivalent values of the Shields parameter, smaller equilibrium burial depths were observed in the wave flume when compared to the ones in LOWST. After burial induced by local scour takes place, bedform (ripples and sandwaves) formation and evolution play a strong and, in some cases, dominant role on the equilibrium burial depth of the cylinders. Depending on how the vertical dimensions of bedforms compare to the specimen's diameter, cyclical covering and uncovering of the object may take place due to the passage of the migrating sandwaves. In such case, burial depth B_d no longer coincides with the vertical displacement (V_d) of the object as in the case when the burial process is dominated by local scour.     

Characteristics of Parameters for Local Scour Depth around Submarine Pipelines in Waves

The purpose of this study is to investigate the characteristics of parameters for local scour depth around submarine pipes due to waves. Because it is impossible to consider all the factors that influence the scour, it is reasonable to investigate the correlations between parameters and scour. The experimental works about scour were performed for the conditions of 40, 60, 90 m pipe diameter, various wave periods, and wave height in the case of horizontal bed and 1/10 slope bed, respectively. Analyzing the results of experiments, the Reynolds number, Shields parameter, Keulegan-Carpenter number, Ursell number, and Modified Ursell number were estimated. The correlations between the relative scour depth, which is the maximum equilibrium local scour depth divided by the pipe diameter, and 5 parameters were analyzed. It was shown that there was hardly any correlation with the Reynolds number. In the case of a horizontal bed, the Keulegan-Carpent number had the highest correlation, but in the case of the slope bed the correlation was greatly reduced. The modified Ursell number showed a high correlation regardless of the type of bed.     

推进波作用下海底管线周围局部冲刷试验研究

考虑行波作用下的海底管线的局部冲刷问题。采用波浪水槽模型试验的方法,研究波浪荷载引起的管线周围局部冲刷机理和冲刷形态,探讨行波作用下管线周围局部冲刷的演化规律,包括冲刷起动、水土界面沙波的形成以及平衡冲刷深度与KC数(keulegan-carpenter number)和相对埋置深度的关系。     

Prediction of wave-induced scour depth under submarine pipelines using machine learning approach

The scour around submarine pipelines may influence their stability; therefore scour prediction is a very important issue in submarine pipeline design. Several investigations have been conducted to develop a relationship between wave-induced scour depth under pipelines and the governing parameters. However, existing formulas do not always yield accurate results due to the complexity of the scour phenomenon. Recently, machine learning approaches such as Artificial Neural Networks (ANNs) have been used to increase the accuracy of the scour depth prediction. Nevertheless, they are not as transparent and easy to use as conventional formulas. In this study, the wave-induced scour was studied in both clear water and live bed conditions using the M5’ model tree as a novel soft computing method. The M5’ model is more transparent and can provide understandable formulas. To develop the models, several dimensionless parameter, such as gap to diameter ratio, Keulegan-Carpenter number and Shields number were used. The results show that the M5’ models increase the accuracy of the scour prediction and that the Shields number is very important in the clear water condition. Overall, the results illustrate that the developed formulas could serve as a valuable tool for the prediction of wave-induced scour depth under both live bed and clear water conditions.     

Burial and scour of short cylinders under combined random waves and currents including effects of second order wave asymmetry

This paper provides an approach by which the burial and scour of short cylinders under combined second order random waves and currents can be derived. Here the formulas for burial and scour for regular waves plus currents presented by Catano-Lopera and Garcia [Catano-Lopera, Y.A. and Garcia, M.H. (2006). Burial of short cylinders induced by scour under combined waves and currents. ASCE J. Waterway, Port, Coastal and Ocean Eng. 132(6), 439–449., Catano-Lopera, Y.A. and Garcia, M.H. (2007). Geometry of scour hole around, and the influence of the angle of attack on the burial of finite cylinders under combined flows. Ocean Eng. 34(5, 6), 856–869.] are used together with Stokes second order wave theory by assuming the basic harmonic wave motion to be a stationary Gaussian narrow-band random process. An example of calculation is also presented.     

Scour at the head of a vertical-wall breakwater

This paper presents the results of an experimental investigation on the near-bed flow patterns, the bed shear stress amplification and scour around the head of a vertical-wall breakwater, using regular waves. The Keulegan-Carpenter number (KC), based on the diameter of the breakwater head, is found to be the major parameter that governs the flow and the equilibrium scour depth. Basic flow structures are identified as function of KC. The scour depth is found to increase with increasing the Keulegan-Carpenter number. The necessary extent of the conventional stone protection is studied. An empirical formula is worked out for the width of the protection layer as function of KC. Also, the effects of head shape, the angle of attack and the presence of a co-directional current are investigated. The results indicate that the scour depth is increased considerably in the presence of a current. Likewise, the scour depth is increased when the head shape is changed from a round shape to a sharp-edged one. It is found that the angle of attack is also an influencing factor as regards the scour depth.     

Numerical Simulations of the Flow and Sediment Transport Regimes Surrounding a Short Cylinder

The 3-D flow field and bed stress surrounding a short cylinder in response to combined wave and mean-flow forcing events is examined. Model simulations are performed with a 3-D nonhydrostatic computational fluid dynamics model, FLOW-3D. The model is forced with a range of characteristic tidal and wave velocities as observed in 12-15 m of water at the Martha's Vineyard Coastal Observatory (MVCO, Edgartown, MA). The 2.4-m-long and 0.5-m diameter cylinder is buried 10% of the diameter on a flat, fixed bed. Regions of incipient motion are identified through local estimates of the Shields parameter exceeding the critical value. Potential areas of sediment deposition are identified with local estimates of the Rouse parameter exceeding ten. The model predictions of sediment response are in general in agreement with field observations of seabed morphology obtained over a one-week period during the 2003-2004 MVCO mine burial experiment. Both observations and simulations show potential transport occurring at the ends of the mine in wave-dominated events. Mean flows greater than 10 cm/s lead to the formation of larger scour pits upstream of the cylinder. Deposition in both cases tends to occur along the sides, near the center of mass of the mine. However, the fixed-bed assumption prohibits the prediction of full perimeter scour as is observed in nature. Predicted scour and burial regimes for a range of wave and mean-flow combinations are established.     

Geometry of scour hole around, and the influence of the angle of attack on the burial of finite cylinders under combined flows

Experiments were conducted to investigate the geometry of the scour hole and flow structure around short cylinders under the action of waves alone (WA) and combined flows (CF). The study is aimed at better understanding the dynamics of isolated objects on a sandy floor under oscillatory flows as occurs in shallow water regions in coastal areas. Flow velocities within the fluid core were recorded and 3D mapping of the bottom was performed with sub-aquatic acoustic sensors. Experiments were conducted for cylinder Reynolds wave number and Keulegan-Carpenter number within the ranges 10⁴R_e1.7×10⁵ and 2KC71, respectively. The present experimental evidence shows that the geometric characteristics of the scour hole (length and width) depend primarily on the Keulegan-Carpenter number (KC) and the cylinder aspect ratio (a_r=L_c/D). The effect of variation in the angle of attack of the flow with respect to the cylinder main axis was also investigated. Initial orientations of zero and ninety degrees were found to be stable while cylinders with intermediate initial orientations tended to orientate their main axes perpendicular to the flow direction. The final angle of orientation was found to be primarily a function of the Shields parameter, θ, and the initial angle of attack, α_i.     

Predicting Seabed Burial of Cylinders by Wave-Induced Scour: Application to the Sandy Inner Shelf Off Florida and Massachusetts

A simple parameterized model for wave-induced burial of mine-like cylinders as a function of grain-size, time-varying, wave orbital velocity and mine diameter was implemented and assessed against results from inert instrumented mines placed off the Indian Rocks Beach (IRB, FL), and off the Martha's vineyard coastal observatory (MVCO, Edgartown, MA). The steady flow scour parameters provided by Whitehouse (1998) for self-settling cylinders worked well for predicting burial by depth below the ambient seabed for (0.5 m) diameter mines in fine sand at both sites. By including or excluding scour pit infilling, a range of percent burial by surface area was predicted that was also consistent with observations. Rapid scour pit infilling was often seen at MVCO but never at IRB, suggesting that the environmental presence of fine sediment plays a key role in promoting infilling. Overprediction of mine scour in coarse sand was corrected by assuming a mine within a field of large ripples buries only until it generates no more turbulence than that produced by surrounding bedforms. The feasibility of using a regional wave model to predict mine burial in both hindcast and real-time forecast mode was tested using the National Oceanic and Atmospheric Administration (NOAA, Washington, DC) WaveWatch 3 (WW3) model. Hindcast waves were adequate for useful operational forcing of mine burial predictions, but five-day wave forecasts introduced large errors. This investigation was part of a larger effort to develop simple yet reliable predictions of mine burial suitable for addressing the operational needs of the U.S. Navy.     

Experimental and numerical investigation of local scour around a submerged vertical circular cylinder in steady currents

Local scour around a submerged vertical circular cylinder in steady currents was studied both experimentally and numerically. The physical experiments were conducted for two different cylinder diameters with a range of cylinder height-to-diameter ratios. Transient scour depth at the stagnation point (upstream edge) of the cylinder was measured using the so-called conductivity scour probes. Three-dimensional (3D) seabed topography around each model cylinder was measured using a laser profiler. The effect of the height-to-diameter ratio on the scour depth was investigated. The experimental results show that the scour depth at the stagnation point is independent on cylinder height-to-diameter ratio when the later is smaller than 2. The increase rate of equilibrium scour depth with cylinder height increases with an increase in Shields parameter.     

Clear water scour around a finite array of cylinders

This experimental study presents clear-water scour and deposition patterns around hexagonal arrays of circular cylinders in steady flow conditions. Understanding the scour processes around such configurations could facilitate the design of several hydraulic and marine engineering structures, such as bridge piers and piles. The flow alteration caused by the examined porous obstacles depends on the solid volume fraction of the obstacles and on the angle of attack of the incoming flow, due to the limited number of cylinders constituting the array. Flume experiments with erodible bed were carried out for four array densities (solid volume fractions: 0.14, 0.20, 0.32 and 0.56) under three different orientations (regular, angled and staggered configurations). The scour/deposition characteristics were obtained by means of laser scanner and the results were compared to solid cylinders of equal circumambient diameter. Different angles of attack of the incoming flow lead to different blockage ratios, which have direct impact on the scour characteristics and deposition patterns. The arrays with the higher solid volume fraction generated scour/deposition patterns similar to solid cylinder, while in the arrays with the lower solid volume fractions, local scour around the individual small cylinders became evident. Finally, considering that the load bearing capacity of a pier basically depends on the area of its cross-section, a comparison of the maximum induced scour depth and volume by the cylinder arrays and the solid cylinder with equal solid cross-sectional area is presented, in order to introduce an alternative pier configuration that induces less scour. The results showed that the array of cylinders could generate 27% less scour volume and 22% less scour depth compared to its single solid cylinder counterpart.     

泥沙突变理论在海底管线冲刷中应用

基于泥沙突变理论,针对海底管线冲刷下泥沙的运动特征,建立恒定流作用下的泥沙起动模式,确定希尔兹数、无量纲参数、冲刷坑深度之间相互作用的非线性方程,推导了恒定流作用下海底管线冲刷坑深度的预测公式。将相同条件下该公式的计算结果与前人的试验资料进行了对比,可发现尽管计算结果存在一定的误差,但也基本能满足对冲刷坑深度的预测和判断,从而证明了泥沙突变模型在预测海底管线冲刷坑深度中的适用性。     

Estimation of pile group scour using adaptive neuro-fuzzy approach

An accurate estimation of scour depth around piles is important for coastal and ocean engineers involved in the design of marine structures. Owing to the complexity of the problem, most conventional approaches are often unable to provide sufficiently accurate results. In this paper, an alternative attempt is made herein to develop adaptive neuro-fuzzy inference system (ANFIS) models for predicting scour depth as well as scour width for a group of piles supporting a pier. The ANFIS model provides the system identification and interpretability of the fuzzy models and the learning capability of neural networks in a single system. Two combinations of input data were used in the analyses to predict scour depth: the first input combination involves dimensional parameters such as wave height, wave period, and water depth, while the second combination contains nondimensional numbers including the Reynolds number, the Keulegan–Carpenter number, the Shields parameter and the sediment number. The test results show that ANFIS performs better than the existing empirical formulae. The ANFIS predicts scour depth better when it is trained with the original (dimensional) rather than the nondimensional data. The depth of scour was predicted more accurately than its width. A sensitivity analysis showed that scour depth is governed mainly by the Keulegan–Carpenter number, and wave height has a greater influence on scour depth than the other independent parameters.     

Numerical study of the Reynolds-number dependence of two-dimensional scour beneath offshore pipelines in steady currents

A numerical model for local scour below pipelines is employed to investigate the Reynolds-number dependence of the two-dimensional scour beneath offshore pipelines in steady currents. A novel wall function is proposed in calculating the suspended sediment transport rate in the model. Scour developments beneath a model pipeline and the corresponding prototype pipeline are simulated under the same undisturbed Shields parameter but different values of Reynolds number. The effects of the Reynolds number difference on the scour profile development are investigated. It is found that scour depths for prototype pipelines are about 10–15% smaller than those for model pipelines. The flow phenomena that causes this difference are discussed by means of flow visualization. The normalized time scales are found to be approximately the same. The simulated scour profiles for the model pipelines agree well with the experimental results from an independent study.     

Prediction of local scour around circular piles under waves using a novel artificial intelligence approach

Abstract

The scour phenomena around vertical piles in oceans and under waves may influence the structure stability. Therefore, accurately predicting the scour depth is an important task in the design of piles. Empirical approaches often do not provide the required accuracy compared with data mining methods for modeling such complex processes. The main objective of this study is to develop three data-driven methods, locally weighted linear regression (LWLR), support vector machine (SVR), and multivariate linear regression (MLR) to predict the scour depth around vertical piles due to waves in a sand bed. It is the first effort to develop the LWLR to predict scour depth around vertical piles. The models simulate the scour depth mainly based on Shields parameter, pile Reynolds number, grain Reynolds number, Keulegan–Carpenter number, and sediment number. 111 laboratory datasets, derived from several experimental studies, were used for the modeling. The results indicated that the LWLR provided highly accurate predictions of the scour depths around piles (R?=?0.939 and RMSE = 0.075). Overall, this study demonstrated that the LWLR can be used as a valuable tool to predict the wave-induced scour around piles.     

In-line force on a cylinder translating in oscillatory flow

Experiments were conducted with smooth and sand-roughened cylinders moving with constant velocity in a sinusoidally oscillating flow to determine the drag and inertia coefficients and to examine the effect of wake biasing on the modified Morison equation. The various flow parameters such as the relative cylinder velocity. Reynolds number, and the Keulegan-Carpenter number were varied systematically and the in-line force measured simultaneously. The principal results, equally valid for both smooth and rough cylinders, are as follows: the drag coefficient decreases with increasing relative current for a given Reynolds number and Keulegan-Carpenter number; the effect of wake biasing on the drag and inertia coefficients is most pronounced in the drag-inertia dominated regime; and the two-term Morison equation with force coefficients obtained under no-current conditions is not applicable to the prediction of wave and current induced loads on circular cylinders.     

Three-dimensional scour below offshore pipelines in steady currents

This paper presents the results of an experimental investigation on three-dimensional scour below offshore pipelines subject to steady currents. The major emphasis of the investigation is on the scour propagation velocity along the pipeline after the scour initiation. Physical experiments were conducted to quantify the effects of various parameters on scour propagation velocity along the pipeline in a water flume of 4 m wide, 2.5 m deep and 50 m long. Local scour depths directly below the model pipeline were measured using specifically developed conductivity scour probes. Effects of various parameters such as pipeline embedment depth, incoming flow Shields parameter and flow incident angle (relative to the pipeline) on scour propagation velocity along the pipeline were investigated. It was found that scour propagation velocity generally increases with the increase of Shields parameter but decreases with the increase of the pipeline embedment depth. A general predictive formula for scour propagation velocity is proposed and validated against the experimental results.     

Measured and Predicted Burial of Cylinders During the Indian Rocks Beach Experiment

Burial of instrumented mine-like cylinders as a result of wave-induced scour was measured during experiments conducted in shallow water (15-16 m) with fine-sand (133-mum) and coarse-sand (566-mum) sediments off Indian rocks beach (IRB), FL. scour pits developed around the instrumented cylinders in the fine-sand site when significant waveheights exceeded 2 m, causing the cylinders to pitch, then roll into the developing scour pits, often changing heading to align parallel with the wave crest. Final cylinder burial was nearly 40 cm (about 70%-80% mine diameter) relative to the sediment-water interface, but only 20%-50% relative to surface area covered. The difference was caused by the lack of complete infilling of scour pits. Little development of scour pits and burial was noted on the coarse-sand site and the cylinders buried to only 20%-40% of the cylinder diameter below the sediment surface. Burial results, although variable, are in general agreement with the wave-induced scour model developed by Trembanis et al. (2007) for the fine sand, but not for the coarse sand where measured burial was much less than predicted.     

Numerical study on wave-induced filtration flow across the beach face and its effects on swash zone sediment transport

A numerical model, coupling an analysis of beach groundwater flow with an analysis of swash wave motion over a uniform slope, is presented. Model calculations are performed to investigate the variations of swash-induced filtration flows across the beach face for different input parameters. Swash zone sediment transport under the influence of such filtration flow across the beach face is investigated through modification of effective weight of sediment particle and modification of swash boundary layer thickness. These effects are quantified based on a bed load transport model with a modified Shields parameter.