Numerical Investigation of Scale Effects in Modelling Scour Below Offshore Pipelines Under Steady Currents

2011 ◽  
Author(s):  
Xu Zhao ◽  
Ming Zhao ◽  
Liang Cheng
Keyword(s):  
Scale Effects ◽  
Real Life ◽  
Scale Model ◽  
Navier Stokes ◽  
Small Scale ◽  
Offshore Pipelines ◽  
Bedload Sediment ◽  
Equilibrium Scour Depth ◽  
Parameter Values ◽  
Normalized Equilibrium

Scale effects must be considered when results of small-scale laboratory tests on scour below offshore pipelines are extrapolated to real-life situations. In the present study, a 2D Finite Element Method (FEM) numerical model for scour simulation was employed to evaluate scale effects in modelling scour below a single pipeline subjected to steady currents. In the model, the Reynolds-Averaged Navier-Stokes (RANS) equations with a k-ω turbulence closure are supplemented by a morphological model with both suspended load and bedload sediment transport rates included. Scour simulations were conducted at a number of scales with various Shields parameter values and sediments of two median grain sizes. The numerical results indicate that for the range of parameters investigated in this study, the normalized equilibrium scour depth below the pipe centre decreases in a linear fashion as the pipe diameter increases. Accordingly, an expression delineating scale-effect trends that relate deviations in normalized equilibrium scour depths to those in pipeline diameters is proposed, thereby contributing to an accurate extrapolation of small-scale model test results to prototype scale in modelling scour below pipelines in currents.

Centerwell Water Motions and Hydrodynamic Loading Using Viscous Flow Calculations

2008 ◽  
Author(s):  
Samuel Holmes ◽  
Joseph Gebara ◽  
Allan Magee
Keyword(s):  
Scale Effects ◽  
Vertical Motion ◽  
Scale Model ◽  
Navier Stokes ◽  
Open Area ◽  
Flow Models ◽  
Free Surface Waves ◽  
Natural Response ◽  
Termination Point

Most Spar platforms have a wet centerwell which provides a termination point for umbilicals and risers. The column of water in the centerwell is a dynamic system which can be excited by the wave action around the Spar as well as the platform’s own motions. When the exciting frequencies are close to the natural frequency of the water column, the vertical motion of the water in the centerwell can become large in large seastates. This might damage structures within the centerwell. A natural response to this problem is to restrict the fluid flow at the bottom of the centerwell by adding a plated structure to partially close the opening. The remaining open area in the centerwell determines the amount of damping as well as the loads on the plating which can be quite large in heavy seas. The problem addressed in this paper is the determination of the appropriate open area in the centerwell plate that will control the fluid vertical motion without requiring expensive reinforcements to the plating beyond the riser guide structure already present. Traditional design tools based on potential flow models appear to perform poorly for this problem because they do not model the viscous damping in the flow correctly. In this paper we use a Navier-Stokes solver to study the centerwell motions and centerwell plate loads for three centerwell plate geometries. It is found that the Spar motions and the free surface waves need to be included in these simulations. The centerwell water motions and centerwell plate loads are compared with those measured in a scale model experiment. Full-scale calculations are also carried out to determine the corresponding centerwell plate loads and centerwell water motions to assess scale effects.

Pipeline plough performance in sand waves. Part 1: model testing

2010 ◽  
Vol 47 (1) ◽  
pp. 49-64 ◽  
Author(s):  
Mark Fraser Bransby ◽  
Michael Brown ◽  
Andrew Hatherley ◽  
Keith Lauder
Keyword(s):  
Model Testing ◽  
Model Tests ◽  
Scale Model ◽  
Soil Conditions ◽  
Small Scale ◽  
Offshore Pipelines ◽  
Sand Waves ◽  
Increased Risk ◽  
Small Scale Model ◽  
Spoil Heaps

Offshore pipelines are often buried in the seabed by ploughing a trench, placing the pipe at the base, and then backfilling. The ploughing operation is critical in terms of cost and project time, with increased risk due to uncertain soil conditions or geohazards. One problem that can be encountered is the presence of sand waves or megaripples on the seabed surface. This may affect the progress of the plough, prevent the plough from generating a level trench or modify the size of the spoil heaps for backfilling. These aspects have been investigated by conducting a series of small-scale model tests in the laboratory. These have revealed information about the plough kinematics and the resulting trench conditions when ploughing in sand waves with different wavelengths and amplitudes. It is shown that it may be possible to plough through regions of sand waves and estimate likely plough performance by knowing the sand wavelength and amplitude relative to the plough size.

Experimental Studies of Hydrodynamic Properties and Screening of Riser Fairing Concepts for Deep Water Applications

2015 ◽  
Author(s):  
Rolf Baarholm ◽  
Kjetil Skaugset ◽  
Halvor Lie ◽  
Henning Braaten
Keyword(s):  
Experimental Studies ◽  
Real Life ◽  
Cross Flow ◽  
Free Vibrations ◽  
The Body ◽  
Scale Model ◽  
Small Scale ◽  
Structural Fatigue ◽  
And Performance ◽  
Set Up

The VIV oscillations of marine risers are known to increase drag, and lead to structural fatigue. One proven method of suppressing this vibration is the use of fairings and strakes. These coverings essentially modify the flow along the cylinder, tripping the production of Karman vortices so that they act less coherently or far enough downstream so they interact less with the body. The Norwegian Deepwater Programme (NDP) has conducted a project with the objective to develop and qualify effective low drag fairing concepts with respect to VIV mitigation and galloping. Furthermore, emphasis is put on easy handling and installation. This paper describes the work and findings in an early phase of the development. This includes small scale model test campaigns. In addition to the bare riser for reference, the behaviour and performance of a total of 10 different fairing concepts are evaluated. Free oscillation tests are performed in a towing tank, where 2D fairings were tested in a pendulum set-up. The set-up enables free vibrations in up to 3 DOF (in-line and cross-flow vibrations and yaw). Fix tests with the purpose of establishing hydrodynamic coefficients for the various fairings have been performed in a large cavitation tunnel. Clear differences in performance have been noticed; particular for drag and galloping responses. Based on the results from the 2D tests, a screening of the fairing designs has been performed and the findings have set the course for further development of the most promising candidates for real life applications.

scholarly journals Micromechanical study of potential scale effects in small-scale modelling of sinker tree roots

2021 ◽  
Author(s):  
Xingyu Zhang ◽  
◽  
Matteo Ciantia ◽  
Jonathan Knappett ◽  
Anthony Leung ◽  
...  
Keyword(s):  
Particle Size ◽  
Large Scale ◽  
Scale Effects ◽  
Distinct Element ◽  
Scaling Law ◽  
Scale Model ◽  
Small Scale ◽  
Tree Roots ◽  
Scale Modelling ◽  
Element Method

When testing an 1:N geotechnical structure in the centrifuge, it is desirable to choose a large scale factor (N) that can fit the small-scale model in a model container and avoid unwanted boundary effects, however, this in turn may cause scale effects when the structure is overscaled. This is more significant when it comes to small-scale modelling of sinker root-soil interaction, where root-particle size ratio is much lower. In this study the Distinct Element Method (DEM) is used to investigate this problem. The sinker root of a model root system under axial loading was analysed, with both upward and downward behaviour compared with the Finite Element Method (FEM), where the soil is modelled as a continuum in which case particle-size effects are not taken into consideration. Based on the scaling law, with the same prototype scale and particle size distribution, different scale factors/g-levels were applied to quantify effects of the ratio of root diameter (𝑑𝑟) to mean particle size (𝐷50) on the root rootsoil interaction.

scholarly journals CEOHYDRAULIC INVESTIGATIONS OF RUBBLE MOUND BREAKWATERS

1988 ◽  
Vol 1 (21) ◽  
pp. 166 ◽  
Author(s):  
W. Burger ◽  
H. Oumeraci ◽  
H.W. Partenscky
Keyword(s):  
Pore Pressure ◽  
Scale Effects ◽  
Numerical Models ◽  
Model Tests ◽  
Physical Models ◽  
Core Material ◽  
Scale Model ◽  
Small Scale ◽  
Rubble Mound ◽  
Rubble Mound Breakwaters

Due to the increase of ship sizes in recent decades a number of harbours and terminals have been built in deeper waters. Accordingly, the structures which have to provide protection against wave action become higher, too. In most cases, these protective structures are of the rubble mound type. Under such conditions the flow induced by waves within the breakwater and the related geotechnical behaviour of the rubble mound fill become more significant fcr the overall stability and should be considered in the design. In addition, it is known that the scales usually adopted in hydraulic models (1:30 to 1:60) for investigating the stability of large rubble mound breakwaters generally lead to scale effects with respect to the flow field inside the breakwater. This means that small-scale model tests are not appropriate for investigating the internal flow patterns or for evaluating the pore pressure field induced by the incident waves in,the core material. because of the uncontrolled conditions in the prototype, and since the actual permeability of the prototype rubble mound fill cannot be predicted (segregation, settlement, variation in grading, etc.), the use of large-scale physical models seems to be the most promising method for basic investigations of this kind. Moreover, the results of such largescale model tests may be used to validate the usual smaller scale models and to calibrate numerical models. Therefore, it is one of the objectives of our research programme on rubble mound breakwaters, which started in 1987, to concentrate on the evaluation of the wave-induced flow and pore pressure distribution within the breakwater.

scholarly journals EULER EQUATIONS WITH SEVERAL INDEPENDENT PRESSURE LAWS AND ENTROPY SATISFYING EXPLICIT PROJECTION SCHEMES

2006 ◽  
Vol 16 (09) ◽  
pp. 1469-1504 ◽  
Author(s):  
CHRISTOPHE CHALONS ◽  
FRÉDÉRIC COQUEL
Keyword(s):  
Stokes Equations ◽  
Scale Effects ◽  
Numerical Procedure ◽  
Approximate Solutions ◽  
Navier Stokes ◽  
Small Scale ◽  
Stability Estimates ◽  
Jump Conditions ◽  
Riemann Solutions ◽  
Navier Stokes Equations

This work aims at numerically approximating the entropy weak solutions of Euler-like systems asymptotically recovered from the multi-pressure Navier–Stokes equations in the regime of an infinite Reynolds number. The nonconservation form of the limit PDE model makes the shock solutions to be sensitive with respect to the underlying small scales. Here we propose to model these small scale effects via a set of generalized jump conditions expressed in terms of the independent internal energies. The interest in considering internal energies stems from the presence of solely first-order nonconservative products by contrast to other variables. These nonconservative products are defined in the now classical sense proposed by Dal Maso, LeFloch and Murat. We show how to enforce the generalized jump conditions at the discrete level with a fairly simple numerical procedure. This method is proved to satisfy a full set of stability estimates and to produce approximate solutions in good agreement with exact Riemann solutions.

scholarly journals Non-Repeatability, Scale- and Model Effects in Laboratory Measurement of Impact Loads Induced by an Overtopped Bore on a Dike Mounted Wall

2019 ◽  
Author(s):  
Maximilian Streicher ◽  
Andreas Kortenhaus ◽  
Corrado Altomare ◽  
Steven Hughes ◽  
Krasimir Marinov ◽  
...  
Keyword(s):  
Large Scale ◽  
Scale Effects ◽  
Vertical Wall ◽  
Scale Factor ◽  
Scale Model ◽  
Small Scale ◽  
Length Scale ◽  
Force Measurements ◽  
Impact Forces ◽  
Small Scale Model

Abstract Overtopping bore impact forces on a dike mounted vertical wall were measured in similar large-scale (Froude length scale factor 1-to-4.3) and small-scale (Froude length scale factor 1-to-25) models. The differences due to scale effects were studied, by comparing the up-scaled force measurements from both models in prototype. It was noted that if a minimum layer thickness, velocity of the overtopping flow and water depth at the dike toe were maintained in the small-scale model, the resulting differences in impact force due to scale effects are within the range of differences due to non-repeatability and model effects.

scholarly journals Prediction of Seabed Scour Induced by Full-Scale Darrieus-Type Tidal Current Turbine

2019 ◽  
Vol 7 (10) ◽  
pp. 342 ◽  
Author(s):  
Sun ◽  
Lam ◽  
Dai ◽  
Hamill
Keyword(s):  
Experimental Data ◽  
Numerical Model ◽  
Tidal Current ◽  
Full Scale ◽  
Scale Model ◽  
Small Scale ◽  
Scour Depth ◽  
Equilibrium Scour Depth ◽  
Tidal Current Turbine ◽  
Current Turbine

Scour induced by a Darrieus-type tidal current turbine was investigated by using a joint numerical and experimental method with emphasis on the scour process of a full-scale turbine. This work proposes a new numerical method to estimate turbine scour developments, followed by model validation through experimental data in the initial stage. The small-scale numerical model was further extended to a full-scale model for the prediction of turbine scour. The numerical model consists of (1) k-ω turbulence closure, (2) a sediment transport model, and (3) a sediment slide model. The transient-state model was coupled with a morphologic model to calculate scour development. A dynamic mesh updating technique was implemented, enabling the autoupdate of data for the grid nodes of the seabed at each time step. Comparisons between the numerical results and the experimental measurements showed that the proposed model was able to capture the main features of the scour process. However, the numerical model underestimated about 15%–20% of the equilibrium scour depth than experimental data. An investigation of the temporal and spatial development of seabed scour around a full-scale Darrieus-type tidal current turbine is demonstrated. This work concludes that the proposed numerical model can effectively predict the scour process of tidal current turbines, and the rotating rotor has a significant impact on the equilibrium scour depth for full-scale turbines.

Pipeline plough performance in sand waves. Part 2: kinematic calculation method

2010 ◽  
Vol 47 (1) ◽  
pp. 65-77 ◽  
Author(s):  
Mark Fraser Bransby ◽  
Michael John Brown ◽  
Keith Lauder ◽  
Andrew Hatherley
Keyword(s):  
Calculation Method ◽  
Wave Amplitude ◽  
Scale Model ◽  
Sand Wave ◽  
Small Scale ◽  
Offshore Pipelines ◽  
Sand Waves ◽  
Calculation Methodology ◽  
Small Scale Model ◽  
Good Agreement

Offshore pipelines can be buried in the seabed by ploughing a trench, placing the pipe at its base, and then backfilling. The presence of sand waves or megaripples on the seabed surface can affect the progress of the plough and prevent the plough from generating a level trench with a uniform trench depth. A calculation method has been presented that makes assumptions about the motion of the plough to predict the kinematics of ploughs through regions of nonuniform seabeds. Results from the calculation methodology are compared with those from small-scale model tests with good agreement, and the detailed kinematics of ploughs are then examined. The calculation method suggests that as a plough moves through a sand-wave field, the oscillation of the plough about the skids results in the trench base being formed alternately by the share tip and heel. The new method allows prediction of likely offshore plough performance given known plough geometry, sand wavelength, and wave amplitude and may be used as a tool for assessing the feasibility of pipeline ploughing in zones of sand waves or megaripples.

A Flexible Multi-Scale Two-Dimensional Model for Calculating the Radial Heat Transfer in Grooved Heat Pipes

2008 ◽  
Author(s):  
Se´verine Rossomme ◽  
Ce´cile Goffaux ◽  
Koen Hillewaert ◽  
Pierre Colinet
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Scale Effects ◽  
Saturation Temperature ◽  
Apparent Contact Angle ◽  
Scale Model ◽  
Small Scale ◽  
Interface Temperature ◽  
Multi Scale ◽  
Radial Heat

This paper describes a multi-scale model for evaluating the radial heat transfer within a grooved heat pipe evaporator. The model is composed of two parts, macroscopic and microscopic, which cannot be decoupled from each other. In the macroscopic part, we solve the heat conduction problem in the solid and in the liquid phases, thanks to a finite-element method allowing high flexibility in the definition of the groove geometry. In order to avoid the classical singularity problem at the contact line (where the liquid-vapor interface meets the groove wall), in addition to taking the solid thermal conductivity into account, we do not impose the saturation temperature but a mixed condition along the interface. We show in particular that the interface temperature equals the saturation temperature (at given vapor pressure), except in the microscopic region where it increases and reaches the solid temperature. In this microscopic zone, a classical lubrication-type theory allows to determine the apparent contact angle, taking into account the influence of small-scale effects, such as the variation of the saturation temperature with the disjoining pressure and with the meniscus curvature. In particular, analytical relationships and correlations are presented for the apparent contact angle, which allow an efficient coupling between macroscopic and microscopic scales. In this paper, attention is devoted to the numerical treatment of both regions, their coupling, and the influence of the macroscopic heat flux and local small-scale effects on the distribution of temperature in the groove.

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