Onset Of Scour Research Articles

On the onset of pipeline scouring: Reconciling waves and currents forcing

The combined action of waves and currents can lead to the generation of freespans that have significant influence on both pipeline on-bottom stability and structural integrity. Several studies have been carried out since the 80’s to analyse the onset of the scour process and the related phenomena in the presence of either waves or currents, while only a few studies regard the combination of waves and currents. Moreover, the main empirical expressions coming from such studies cannot adequately represent the physics of the phenomena and lead to non-conservative results. For that purpose, a reanalysis of existing datasets on the onset of scour due to waves and/or currents has been carried out to obtain a coherent method for the evaluation of the critical embedment associated to such mixed flow conditions. In addition, an analytical study has been used to quantify the Keulegan–Carpenter parameter when waves are combined with currents, for a more correct application of such empirical formulas. These results will be integrated into a probabilistic model for the long-time evolution of freespans for the evaluation of the structural stability of pipelines in the marine environment, although the present approach can be exploited for a broader range of offshore applications that deal with the combined action of waves and currents.

Unresolved CFD and DEM Coupled Simulations on Scour around a Subsea Pipeline

In this paper, numerical studies were carried out on scour around a subsea pipeline. A coupled solver between computational fluid dynamics (CFD) and discrete element method (DEM) was selected to simulate fluid flow and particle interactions. To select and validate the numerical model parameters in the solver, angles of repose and incipient motion were simulated. From the validation studies, the selected coefficient of rolling friction with spherical particles could predict the behavior of non-spherical particles. The fluid flow around the subsea pipeline was simulated, and the motion of individual soil particles was tracked. Particle motions were generated by the drag force, due to a high velocity. Three scour development process, such as onset of scour, tunnel erosion, and lee-wake erosion, were studied and discussed. The scour depth evolution showed good agreement with the experimental data. It was confirmed that the selected solver, with numerical model parameters, predicted the scour process around a subsea pipeline well.

An Eulerian two-phase flow model investigation on scour onset and backfill of a 2D pipeline

The capability of an Eulerian two-phase model, SedFoam, in simulating the onset of scour underneath a pipeline and the backfill process is investigated. When a pipeline is slightly buried in the sediment bed, the scour onset can be caused by the piping process, which is due to seepage flow moving underneath the pipeline driven by the upstream–downstream pressure difference. To directly resolve piping as part of the scour simulation has been a challenge in the single-phase models. Alternatively, the two-phase models may be capable of simulating piping and backfill as it can resolve the interactions between the flow, structure, sediment transport, and seepage flow using a single set of governing equations and closures. To prove this point, SedFoam is validated by two laboratory experiments for the onset of scour underneath a pipeline. For piping driven by a prescribed upstream–downstream pressure difference, the model captures the temporal evolution of the pore-pressure gradient and the resulting fine-scale bathymetric change around the pipe consistent with the measured data. The model further provides insight into the seepage flow and the creeping movement of sediments during the onset of scour. When simulating piping driven by a unidirectional current, SedFoam successfully predicts piping driven by the upstream–downstream pressure gradient due to the incoming flow deceleration by the presence of pipeline and flow separation. As a proof-of-concept application, SedFoam is applied to simulate pipeline scour driven by an oscillatory flow. Although the boundary layer streaming effect is neglected, the model result shows realistic scour onset and the development of a scour hole. To demonstrate the model’s capability to simulate the backfill process, the pipeline is then lowered artificially into the scour hole as an idealized treatment of the complex pipeline sinking process during scour. The resulting burial depth due to backfill is similar to that predicted by the empirical formula.

Visualization of the Onset of Scour under a Pipeline in Waves

This paper visualizes the onset of scour under a pipeline in regular waves and studies the mechanism in this process. After the test started, the sand particles near the downstream mudline began to oscillate periodically, probably due to the distribution pattern of the seepage hydraulic gradient under the pipeline as well as asymmetric waves. As the wave height gradually increased, the sand under the pipeline began to oscillate at increasing amplitude and the coverage of oscillation extended upstream. Some sand rushed out from the bottom of the pipeline, and the sand oscillated almost symmetrically around the bottom line of the pipeline thereafter. The oscillation amplitude of the sand particles continued to rise, probably due to a decrease in the seepage path under the pipeline and loosened sediment in the oscillation. The scour onset occurred after more sand rushed out from under the pipeline. The visualization results reproduced the delay in the scour onset reported previously, and related the delay to the aforementioned increase in sediment oscillation amplitude. This visualization can improve the understanding of the scour onset mechanism, and can serve as a guide to further investigate the critical conditions of scour onset and scour control countermeasures for offshore structures.

Scour Protection of Underwater Pipelines

A detailed literature review on the protection of local scour beneath a submarine pipeline is presented. The review covers two basic parts of countermeasures against the pipeline scour, namely preventing the onset of scour and stimulating the self-burial of a pipeline. The research progress on the methods of the two sections is discussed in detail separately. The methods preventing the onset of scour have been extensively studied, but the understanding on their mechanisms is yet to be improved. The progress in stimulating the self-burial of a pipeline mainly focuses on a spoiler attached to a pipeline, which is investigated comprehensively with both experiments and numerical simulations. Both parts of countermeasures have been applied in some practical engineering projects and the protection effects are generally satisfying.

Competitive dynamics of two erosion patterns around a cylinder

We investigate experimentally the local erosion of a granular bed near a fixed vertical cylinder that emerges from the bed. The onset of erosion arising at the base of the cylinder and usually ascribed to the wrapping horseshoe vortex is determined and rationalized by a flow contraction effect. We report a new erosion pattern visible downstream of the cylinder that consists of two side-by-side elongated holes. This pattern is observed for flow regimes close to the horseshoe scour onset, whose growth usually inhibits its spatiotemporal development.

Numerical simulation of scour around a submarine pipeline using computational fluid dynamics and discrete element method

• Numerical simulation of scour around a pipeline using a coupled CFD-DEM model. • Simulated scour evolution is in good agreement with published experimental results. • Detailed information, e.g., particle position, velocity, and force, is obtained to improve the understanding of scour mechanism. • Particle motion and particle–particle interactions are found to be most intense during tunnel erosion stage. • Both pressure gradient and drag forces are important at onset and early stages of scour, but drag is dominant at the last stage. Scour under a submarine pipeline can lead to structural failure; hence, a good understanding of the scour mechanism is paramount. Various numerical methods have been proposed to simulate scour, such as potential flow theory and single-phase and two-phase turbulent models. However, these numerical methods have limitations such as their reliance on calibrated empirical parameters and inability to provide detailed information. This paper investigates the use of a coupled computational fluid dynamics-discrete element method (CFD-DEM) model to simulate scour around a pipeline. The novelty of this work is to use CFD-DEM to extract detailed information, leading to new findings that enhance the current understanding of the underlying mechanisms of the scour process. The simulated scour evolution and bed profile are found to be in good agreement with published experimental results. Detailed results include the contours of the fluid velocity and fluid pressure, particle motion and velocity, fluid forces on the particles, and inter-particle forces. The sediment transport rate is calculated using the velocity of each single particle. The quantitative analysis of the bed load layer is also presented. The numerical results reveal three scour stages: onset of scour, tunnel erosion, and lee-wake erosion. Particle velocity and force distributions show that during the tunnel erosion stage, the particle motion and particle–particle interactive forces are particularly intense, suggesting that single-phase models, which are unable to account for inter-particle interactions, may be inadequate. The fluid pressure contours show a distinct pressure gradient. The pressure gradient force is calculated and found to be comparable with the drag force for the onset of scour and the tunnel erosion. However, for the lee-wake erosion, the drag force is shown to be the dominant mechanism for particle movements.

Pipeline–Seabed Interaction

A review of the existing research on the interaction between a pipeline and an erodible bed exposed to waves and/or currents is presented. The review covers three topics: scour, liquefaction, and lateral stability of pipelines. The basic mechanism that leads to scour in two-dimensional (2D) and three-dimensional (3D) cases is first described, as deduced from small-scale laboratory experiments. The onset of scour from piping and the developing tunnel erosion are among the processes described. The lateral expansion of the scour hole along the pipe is described, also based primarily on small-scale laboratory experiments. The state of the art of the mathematical/numerical modeling of the scour processes is presented. The associated self-burial of the pipe is described and compared to field observations. In addition to scour, liquefaction may also constitute a risk for pipeline stability. The cause of liquefaction and the resulting consequence for pipeline stability in a natural environment are discussed. Finally, the lateral stability of pipelines placed on an erodible bed and exposed to waves is briefly described.

An Experimental Study for Analysis of the Self-Burial of Pipelines with Spoilers in Current

Abstract This experimental study deals with the mechanism of spoilers that cause scouring around pipelines. The design methodology was applied to spoilers for accelerating pipelines that undergo self-burial into the sand-bed and evaluated using approaching experimental analysis. The experimental study was conducted to investigate the self-burial ability of submarine pipelines with two types of sand-bed models: silt and sand. Spoilers of different heights were attached to the top of the pipeline for each experiment case. The relationship between the relative scour depth and spoiler height was evaluated by applying different flow velocities and sand diameters and com-parative analysis was performed with values obtained from similarity experiments and theory. The experimental study will be useful in establishing a database for the design of spoilers. Keywords: Spoiler, Self Burial, Scour, Scour Depth, Equilibrium Scour Depth, Model similarity 1. Introduction Submarine pipelines are adversely affected by the marine environment. Especially, current velocity is rela-tively high in shallow water where pipeline installation and operation are expected. Flow around submarine pipelines transforms the sand-bed according to local scour induced vortex shedding and results in additional static or dynamic loads on pipelines, with potential structural damage. Consequently, special regard is re-quired in the design and maintenance of pipelines that are fully capable of accomplishing coastal processes during the onset of scour. Spoilers are convenient during pipeline installation due to the self-burial effect. Pipelines attached to spoil-ers experience lower pressure on the bottom, increasing the inlet velocity of the current between the sand-bed and the bottom of the pipe to a velocity faster than normal. After the pipeline descends, the current de-livers soil and sediment cover to the top of the pipeline, which accounts for the self-burial of the pipeline. Flume tests for similarity models have been conducted and compared to experimental and calculated values for scour depth by Kjeldsen et al. (1974) and by Bijker & Leeuwenstein (1984). 2. Similarity Model

Computational fluid dynamics–discrete element method analysis of the onset of scour around subsea pipelines

In this study, we used a computational fluid dynamics (CFD)–discrete element method (DEM) model to analyze the triggering of sand movement below an offshore pipeline. The advantage of this CFD–DEM model is that it considers the motion and force applied to every individual particle, thereby facilitating detailed simulations of seepage flow and the breakdown of the water–sand mixture beneath the pipeline. The simulation results were similar to previous experimental studies, which showed that scouring occurs after the driving pressure beneath the pipe generates a floating gradient in the sand layer near the wake. Furthermore, the computed critical current velocity that leads to the onset of scour was in reasonable agreement with the observational data. Thus, we consider that CFD–DEM is a promising numerical tool for future investigations of the mechanism that underlies the onset of scour.

Analysis of Submarine Pipeline Scour Using Large-Eddy Simulation of Dense Particle-Liquid Flows

Using large-eddy simulation technique for dense particle-fluid flows, the current-induced scour is predicted for both the mono- and bidispersed systems below a horizontal submarine pipeline exposed to unidirectional flow. The simulations are four-way coupled, which implies that both solid-liquid and solid-solid interactions are taken into account. Particles are assumed to behave as viscoelastic solids during interactions with their neighboring particles, and their motion are predicted by a Lagrangian method. The interparticle normal and tangential contact forces between particles are calculated using a generalized Hertzian model. The other forces on a particle that are taken into account include gravitational pressure gradient force accounting for the acceleration of the displaced liquid, the drag force resulting from velocity difference with the surrounding liquid, and the Magnus and Saffman lift forces. The predicted scour profiles for monodispersed system are found to compare favorably with the laboratory observations. For the bidispersed system, a seepage flow underneath the pipe (which is a major factor to cause the onset of scour below the pipeline) is found to be weakened using an appropriate size for the sand bed. This fiffnding highlights the importance of the bed particle size distribution on the onset of scour below the pipelines.

A numerical model for onset of scour below offshore pipelines

A numerical model is developed to predict the onset of local scour below offshore pipelines in steady currents and waves. The scour is assumed to start when the pressure gradient underneath the pipeline exceeds the floatation gradient of the sediments. In this model, the water flow field above the bed is determined by solving the two-dimensional (2-D) Reynolds-averaged Navier–Stokes equations with a k- ω turbulence closure. The seepage flow below the seabed is calculated by solving the Darcy's law (Laplace's equation) with known pressure distribution along the common boundaries of the flow domains-seabed. The numerical method used for both the turbulent flow around the pipeline and Darcy's flow in the seabed is a fractional finite element method. The average pressure gradient along the buried pipe surface is employed in the evaluation of onset condition with a calibration coefficient. The numerical model is validated against experimental data available in literature. A unified onset condition for steady currents and waves is proposed. Influences of flow parameters, including water depth, embedment depth, boundary layer thickness, Reynolds number ( Re) and Keuleagan–Carpenter ( KC) number, on the pressure drop coefficient over the pipeline are studied systematically.

Onset of scour below pipelines and self-burial

This paper summarizes the results of an experimental study on the onset of scour below and self-burial of pipelines in currents/waves. Pressure was measured on the surface of a slightly buried pipe at two points, one at the upstream side and the other at the downstream side of the pipe, both in the sand bed. The latter enabled the pressure gradient (which drives a seepage flow underneath the pipe) to be calculated. The results indicated that the excessive seepage flow and the resulting piping are the major factor to cause the onset of scour below the pipeline. The onset of scour occurred always locally (but not along the length of the pipeline as a two-dimensional process). The critical condition corresponding to the onset of scour was determined both in the case of currents and in the case of waves. Once the scour breaks out, it will propagate along the length of the pipeline, scour holes being interrupted with stretches of soil (span shoulders) supporting the pipeline. As the span shoulder gets shorter and shorter, more and more weight of the pipeline is exerted on the soil. In this process, a critical point is reached where the bearing capacity of the soil is exceeded (general shear failure). At this point, the pipe begins to sink at the span shoulder (self-burial). It was found that the self-burial depth is governed mainly by the Keulegan–Carpenter number. The time scale of the self-burial process, on the other hand, is governed by the Keulegan–Carpenter number and the Shields parameter. Diagrams are given for the self-burial depth and the time scale of the self-burial process.

Bed Shear Stress and Scour over Bed‐Type River Intake

Laboratory experiments are carried out to determine the increase in bed shear stress and local scour over the zone of suction associated with a bed-type river intake in a channel with a mobile bed. Bed shear stress is estimated first by observing the movement of indicator grains whose threshold shear stress in uniform flow was already known, and second by observing the rate at which local scour takes place. Evaluation of the two sets of results indicates that, at the onset of scour when particles are lifted from the bed by increased bed shear stress, they do not strike the bed again until they are clear of the scour zone. The final scour depth is found to be directly proportional to the difference between the initial bed shear stress because of suction and the threshold bed shear stress for the bed grains in the presence of suction.

Mechanics of Local Scour Around Submarine Pipelines

The primary objective of this study is to improve understanding of the mechanism causing scour in unidirectional current. Experiments have shown how local scour develops around submarine pipelines in noncohesive sediments. The study shows that piping is the dominant cause of the initiation of scour. Piping and the stagnation eddy combine to undermine the pipeline, and mark the onset of scour. The critical hydraulic gradient associated with the initiation of scour is equal to the flotation gradient of the bed sediment. The pressure drop between the stagnation pressure upstream and wake pressure downstream of the pipe induces this hydraulic gradient. When a pipe is just embedded, the onset of scour does not occur if the ratio of the flow depth to pipe diameter exceeds 3.5. Similarly, the onset of scour does not occur for half‐buried pipes. The reduction in pressure gradient across the pipeline for these flow/pipe combinations accounts for the lack of scour. The onset of scour can be prevented by placing an ...