Equilibrium Scour-Depth Prediction around Cylindrical Structures

Offshore gravity base foundations (GBFs) are often designed with complex geometries. Such structures interact with local hydrodynamics, creating an adverse pressure gradient that is responsible for flow and scour phenomena, including the bed shear stress amplification. In this study, a method is presented for predicting clear-water scour around cylindrical structures with nonuniform geometries under the force of a unidirectional current. The interaction of the flow field with the sediment around these complex structures is described in terms of nondimensional parameters that characterize the similitude of water-sediment movement. The paper presents insights into the influence the streamwise depth-averaged Euler number has on the equilibrium scour around uniform and nonuniform cylindrical structures. Here, the Euler number is based on the depth-averaged streamwise pressure gradient (calculated using potential flow theory), the mean flow velocity, and the fluid density. Following a dimensional analysis, the controlling parameters were found to be the Euler number, pile Reynolds number, Froude number, sediment mobility number, and nondimensional flow depth. Based on this finding, a new scour-prediction equation was developed. This new method shows good agreement with the database of scour depths acquired in this study (R2=0.91)(R2=0.91). Measurements of the equilibrium scour depth around nonuniform cylindrical structures were used to show the importance of the Euler number in the scour process. Finally, the importance of the remaining nondimensional quantities with respect to scour was also investigated in this study

Experimental observations of tsunami induced scour at onshore structures

Tsunami inundation of the coastal environment can induce scour at structure foundations leading to failure. A series of experiments are made using a unique Pneumatic Long Wave Generator to generate tsunami wave periods of 25 - 147 s equating to 3 - 17.3 mins at 1:50 Froude scale. The waves propagate over a sloping bathymetry and impinge upon a square structure founded onshore in a flat sediment bed. Flow velocity, height and scour are recorded as a function of time during tsunami inundation. The rate of scour is observed to be time dependent. Equilibrium, which is not attained, is argued to be an inappropriate measure for time-dependent transient flows such as tsunami in which the flow velocity, depth and direction are variable. The maximum scour depth is recorded and critically is observed not to be equal to the final depth due to significant sediment slumping when flow velocities reduce in the latter stages of inundation. Current and wave scour predictor equations over predict the scour, while the ASCE 7-16 method under predicts. Comparisons with available data in the literature show longer inundation durations increase the amount of scour

The influence of physical cohesion on scour around a monopile

© 2016 Taylor & Francis Group, London.We present experiments that systematically examine how the addition of physically cohesive clay to sand affects scour evolution around a monopile in a current. Repeated centreline transects are used to show the changes in scour depth and excavated material over time. Combined with 3D plots of the final equilibrium morphology, the results conclusively prove that clay content causes a progressive reduction in the equilibrium depth, excavated area and that timescales of scour increase with clay content. Winnowing of clay particles from the sand matrix is a pre-requisite for scour and differences in clay content influence the rate and extent of winnowing, ultimately controlling equilibrium scour morphology. The strong linear relationships between clay content and equilibrium scour parameters offers a simple index on which to modify existing scour prediction methods. It follows that improved predictions of scour development can reduce manufacturing costs and related logistical expenses of structure operations in fluvial, coastal or offshore environments

Hemispheric Asymmetries in Speech Perception: Sense, Nonsense and Modulations

Background: The well-established left hemisphere specialisation for language processing has long been claimed to be based on a low-level auditory specialization for specific acoustic features in speech, particularly regarding 'rapid temporal processing'.Methodology: A novel analysis/synthesis technique was used to construct a variety of sounds based on simple sentences which could be manipulated in spectro-temporal complexity, and whether they were intelligible or not. All sounds consisted of two noise-excited spectral prominences (based on the lower two formants in the original speech) which could be static or varying in frequency and/or amplitude independently. Dynamically varying both acoustic features based on the same sentence led to intelligible speech but when either or both acoustic features were static, the stimuli were not intelligible. Using the frequency dynamics from one sentence with the amplitude dynamics of another led to unintelligible sounds of comparable spectro-temporal complexity to the intelligible ones. Positron emission tomography (PET) was used to compare which brain regions were active when participants listened to the different sounds.Conclusions: Neural activity to spectral and amplitude modulations sufficient to support speech intelligibility (without actually being intelligible) was seen bilaterally, with a right temporal lobe dominance. A left dominant response was seen only to intelligible sounds. It thus appears that the left hemisphere specialisation for speech is based on the linguistic properties of utterances, not on particular acoustic features

Bed shear stress distribution around offshore gravity foundations

Offshore gravity foundations are often designed with complex geometries. Such structures interact with the local hydrodynamics and generate enhanced bed shear stresses and flow turbulence capable of scouring the seabed or destabilizing bed armour where deployed. In the present study a novel bed shear stress measurement method has been developed from the camera and laser components of a Particle Image Velocimetry (PIV) system. The bed shear stress amplification was mapped out around six models of gravity foundations with different geometries. Tests were repeated for two bed roughness conditions. The structures tested included uniform cylinders, cylindrical base structures and conical base structures. The flow field around the models was also measured using PIV. The results of this study reveal that the conical base structures generate a different hydrodynamic response compared to the other structures. For uniform cylinders the maximum bed shear stress amplification occurs upstream, at an angle of 45° relative to the flow direction, and measurements were found to agree well with numerical results obtained by Roulund et al. (2005). In the case of the cylindrical base structure the maximum amplification occurs upstream at a similar location to the uniform cylinder case. For the conical base structures the maximum amplification of the bed shear stress occurs on the lee side of the structure, with the magnitude dependent on the side slope of the cone. The bed shear stress results were validated against stresses derived from analysis of the flow fields obtained by the PIV measurements performed under the same test conditions. Conclusions from the study are that the structure with the cylindrical base foundation produces the lowest bed shear stress amplification and that an increase in the bed roughness results in an increase in the amplification of the bed shear stress. These findings have direct implications for design of scour protection. In addition the flow reattachment point behind the foundation is dependent on pile Reynolds number (ReD). This suggests that the results of this study may be extrapolated for higher pile Reynolds using the method described in Roulund et al. (2006)

CFD simulation of clearwater scour at complex foundations

Offshore Gravity Base Foundations (GBFs) are often designed with complex geometries. Such structures interact with local hydrodynamics, creating an adverse pressure gradient resulting in bed shear stress amplification and scour in erodible soils. At present, physical modelling and simple prediction equations have been the only practical engineering tool for evaluating scour around these support structures. However, with the increasing computational power of computers and the development of new Computational Fluid Dynamics (CFD) solvers, scour prediction around foundations using CFD is becoming more practical and accurate. In the present work, three-dimensional (3D) numerical modelling has been applied to reproduce local scour around a complex cylindrical base structure under the forcing of a unidirectional current in clearwater scour conditions. The simulations are carried out using a state-of-the-art multi-phase 3D Euler-Lagrange model based on the open source CFD software OpenFOAM. The fluid phase is resolved by solving modified Navier-Stokes equations, which take into consideration the influence of the solid phase, i.e., the soil particles. The solid phase is solved using the multi-phase particle-in-cell (MP-PIC) approach, a method which takes into account the sediment-sediment interaction, while the particles follow Newton’s Law of Motion. The present paper also presents physical modelling results for scour around the same type of structure which were conducted for the same hydrodynamic forcing conditions as in the CFD model. The results of the experimental campaign are used to evaluate the ability of the CFD model to predict the: time evolution of scour; the equilibrium scour depth; and, the 3D characteristics of the scour hole. The results show that the present CFD solver has the capacity to predict with good accuracy the evolution of the scour hole and equilibrium scour depth while capturing key morphological features which are not captured by similar software packages

Modelling of foundation response to scour and scour protection for offshore wind turbine structures

Local and global scour around offshore monopile wind turbine structures can cause a reduction in foundation strength and stiffness, and a consequential reduction in the structure’s natural frequency. To devise optimal scour remediation schemes, it is necessary to develop an understanding of the interactions between scour development, soil-foundation stiffness, and the dynamic response of the structure. This paper describes a research project that develops a framework to assess the influence of scour and scour remediation systems on the dynamic characteristics of offshore wind turbine structures. The research project includes one-dimensional (1D) finite element modelling, calibrated using three-dimensional (3D) finite element analysis, experimental measurements, and monitored field data. The paper outlines the development of the 1D and 3D finite element models, as well as a novel experimental programme carried out on a 1:20 scale offshore wind structure in the Fast Flow Facility at HR Wallingford, UK

Modelling of foundation response to scour and scour protection for offshore wind turbine structures

Local and global scour around offshore monopile wind turbine structures can cause a reduction in foundation strength and stiffness, and a consequential reduction in the structure’s natural frequency. To devise optimal scour remediation schemes, it is necessary to develop an understanding of the interactions between scour development, soil-foundation stiffness, and the dynamic response of the structure. This paper describes a research project that develops a framework to assess the influence of scour and scour remediation systems on the dynamic characteristics of offshore wind turbine structures. The research project includes one-dimensional (1D) finite element modelling, calibrated using three-dimensional (3D) finite element analysis, experimental measurements, and monitored field data. The paper outlines the development of the 1D and 3D finite element models, as well as a novel experimental programme carried out on a 1:20 scale offshore wind structure in the Fast Flow Facility at HR Wallingford, UK

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The design for fatigue for offshore wind turbine structures is characterized by uncertainty, resulting from both loading specifications and numerical modelling. At the same time, fatigue is a main design driver for this type of structures. This study presents a strategy to monitor the accumulated fatigue damage in real-time, employing a joint input-state estimation algorithm. Measuring the operational vibrations at well-chosen locations enables the estimation of strain responses at unmeasured locations. The estimation algorithm is applied to a wind turbine on a lattice support structure, for which the response estimates of the lattice members are based on measurements on the turbine tower only. This restriction follows from the difficulty to reliably and robustly measure at locations on the lattice structure. Artificial measurement data is generated with a full-order finite element model, while the strains are estimated with an erroneous reduced-order design model, after inclusion of measurement noise. The strain estimates show that the main frequency content can be captured relatively accurately, except for a small bias and some high frequency disturbance, corresponding to a weakly observable higher mode. This second aspect shows the importance of a trade-off between the accuracy of the reduced-order finite element model and the ill-conditioning of the observability matrix.Hydraulic EngineeringCivil Engineering and Geoscience