Scour and scour protection around offshore gravity based foundations

The prediction of seabed scour around offshore gravity based foundations with complex geometries is currently a significant barrier to optimising and providing cost effective foundation designs. A significant aspect that has the potential to reduce the uncertainty and costs related to the design of these foundations is the understanding of the effect the structural geometry of the foundation has on scour. This thesis focuses on an experimental investigation of the scour and scour protection around complex structure geometries. The first part of this research considers scour under clear water conditions. During this study different foundation geometries were subjected to a range of different hydrodynamic forcings which enabled a better understanding of the scour process for these foundations. The second part of the research encompasses the design and execution of a series of experiments which investigated stability of the scour protection around such structures. The structures were tested against different combinations of wave and current conditions to determine the bed shear stress required to initiate sediment motion around each structure. This research has led to a number of novel results. The experimental investigation on scour around complex geometries showed that the scour depth around cylindrical structures (with both uniform and complex cross-sections) is linked to the depth averaged pressure gradient. Following a dimensional analysis, the controlling parameters were found to be the depth averaged Euler number, pile Reynolds number, Froude number, sediment mobility number and the non-dimensional flow depth. Based on this finding a new scour prediction equation was developed which shows good agreement with experimental and prototype scour measurements. The scour protection tests indicated that under wave dominated conditions the amplification of the bed shear stress around these structures does not exceed the value of 2. In the case of current dominated flow conditions the amplification of the bed shear stress is a function of the structure type and the Keulegan–Carpenter number. The results of these experiments were used to develop a “Shields type” diagram that can guide designers to select the appropriate rock armour size that will be stable for a certain set of flow conditions. The study also revealed that the long term persistence of flow conditions that just lead to incipient motion of the scour protection material can eventually lead to complete failure of the scour protection. The study provides a set of new design techniques that can allow designers to predict the scour depth around cylindrical and complex foundation geometries and also select the appropriate stone size for their scour protection system. Together, these techniques may allow for the reduction of costs associated with the scour protection of offshore and coastal structures

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

Foundation scour as a geohazard

Carrying out a hazard assessment for offshore structures can entail the consideration of a number of different factors. Scour hazard assessments are routinely undertaken, and scour development at offshore structures should be considered a time-varying process. However, scour may take place within a morphologically dynamic environment, the combination of which will impact on the soil–structure–fluid response. This paper presents the analysis of an unique data set that shows the partial collapse of a scour hole at a large monopile foundation within a morphologically active site. The collapse suggests a slope failure mechanism, resulting in the movement of around 450 m3 of material within a period of about 75 min. The paper analyzes the processes involved regarding formation and development of the collapse

3D wave transmission around permeable submerged breakwaters with the use of Artificial Neural Networks

Wave transmission is a very important design parameter for submerged porous breakwater design, as it defines the dimensions of the breakwater as well as the cost and construction process. The spatial distribution of the wave transmission parameter influences the current pattern on the lee of the breakwater and therefore the sediment transport process. For this reason it is important to create a design tool capable of predicting the variation in wave height around detached submerged breakwaters. This study has been conducted as an extension of the work completed by Amir Ahmadian for his PhD project the University College of London under the supervision of Professor Richard Simons. During his research a large number of experiments were conducted in order to create an extensive database on wave transmission around semi-infinite impermeable breakwaters. The results of these experiments where then used to create an ANN model capable of predicting the 3D wave transmission coefficients around submerged breakwaters. This thesis therefore aims to create an ANN model capable of predicting the 3D wave field around permeable submerged breakwaters, by using the algorithm architecture proposed by Ahmadian. To the author’s knowledge there are a limited number of experimental studies on the field of 3D wave transmission of permeable breakwaters and therefore creating an ANN model based on physical measurements is impossible. For this reason a large number of 3D experiments where performed using MIKE21 BW in order to create a database that will then could be used to train and test the ANN model. Important evidence of the significance of diffraction and breakwater permeability on the wave transmission phenomenon for submerged porous breakwaters where obtained. In addition the results of the simulations where then cross validated against the empirical formula provided by Vicinanza et al (2009). This analysis showed that the quality of the data was very good and could be used for training a Neural Network. During this process it was proposed that the empirical prediction formula of Vicinanza could be improved by introducing a correlation factor, as the numerical simulations showed strong evidence that the diffraction and wave transmission over and through the breakwater have a negative correlation. With regards to ANN modeling the algorithm showed that it has an excellent capability to predict the test dataset (obtained from MIKE21 BW simulation). The analysis of the ANN model revealed that the model predictions are in very good agreement with the prediction method of Vicinanza. Finally the sensitivity analysis of implemented showed that the permeability factor introduced to account for the effects of permeability has the most important contribution to the models performance. Concluding this thesis suggests that the proposed model has the potential to become a valuable design tool for engineering purposes in the field of submerged breakwater design.Coastal EngineeringHydraulic EngineeringCivil Engineering and Geoscience

Do Chinese Children With Math Difficulties Have a Deficit in Executive Functioning?

Several studies have shown that Executive Functioning (EF) is a unique predictor of mathematics performance. However, whether or not children with mathematics difficulties (MD) experience deficits in EF remains unclear. Thus, the purpose of this study was to examine if Chinese children with MD experience deficits in EF. We assessed 23 children with MD (9 girls, mean age = 10.40 years), 30 children with reading difficulties and MD (RDMD; 12 girls, mean age = 10.82 years), and 31 typically-developing (TD) peers (16 girls, mean age = 10.41 years) on measures of inhibition (Color-Word Stroop, Inhibition), shifting of attention (Planned Connections, Rapid Alternating Stimuli), working memory (Digit Span Backwards, Listening Span), processing speed (Visual Matching, Planned Search), reading (Character Recognition, Sentence Verification), and mathematics (Addition and Subtraction Fluency, Math Standard Achievement Test). The results of MANOVA analyses showed first that the performance of the MD children in all EF tasks was worse than their TD peers. Second, with the exception of the shifting tasks in which the MD children performed better than the RDMD children, the performance of the two groups was similar in all measures of working memory and inhibition. Finally, covarying for the effects of processing speed eliminated almost all differences between the TD and MD groups (the only exception was Listening Span) as well as the differences between the MD and RDMD groups in shifting of attention. Taken together, our findings suggest that although Chinese children with MD (with or without comorbid reading difficulties) experience significant deficits in all EF skills, most of their deficits can be accounted by lower-level deficits in processing speed

Time evolution of scour: the importance of event duration

The assessment of scour risk at offshore foundations is dependent on a number of different factors including metocean conditions, water depth, soil conditions and structure dimensions and layout. At foundations where the soils are dominated by non-cohesive soils the scour risk is potentially greater than for clay dominated sites. However, even in those locations where sand overlies less erodible soils, it is important to understand the possible time-scale for the loss of this material. Further, as the location of foundation structures moves further offshore into deeper water, not only does the scour process change, the scour development is likely to become event duration limited. However, this latter effect may not only be confined to deeper water sites, but to sites where the metocean conditions are constrained (e.g. weak tidal currents, limited fetch lengths etc). This paper explores the importance of this effect through the use of a time-evolution model of scour

CFD simulation of clear water 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 that is responsible for flow and scour phenomena, including the bed shear stress amplification and scour. 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 computation power of modern computers and the development of new Computational Fluid Dynamics (CFD) solvers, scour prediction around foundations has become possible. 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 clear water scour conditions. The simulations are carried out using a state of the art three-dimensional Euler-Lagrange scour 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 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; Equilibrium scour depth; 3D characteristics of the scour hole

Heilige Texte. Formen und normative Grenzen der Übertragung in Judentum, Christentum und Islam, Hermeneutische Untersuchungen zur Theologie

Many offshore foundations are composed of non-uniform cylindrical geometries such as cones and different diameter composite cylinders. However, little is known about how these types of structure respond with regards to scour under the forcing of a unidirectional current. The present paper describes a series of laboratory experiments that have been performed to examine the effect that the geometry of a marine structure has on the evolution and equilibrium depth of scour under different hydrodynamic conditions. It was found that the hydrodynamic scour response of the non-uniform cylindrical structures is fundamentally different to that of a uniform cylinder

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)

Different Subcomponents of Executive Functioning Predict Different Growth Parameters in Mathematics: Evidence From a 4-Year Longitudinal Study With Chinese Children

Executive functioning (EF), an umbrella term used to represent cognitive skills engaged in goal-directed behaviors, has been found to be a unique predictor of mathematics performance. However, very few studies have examined how the three core EF subcomponents (inhibition, shifting, and working memory) predict the growth parameters (intercept and slope) in mathematics skills and even fewer studies have been conducted in a non-Western country. Thus, the purpose of this study was to examine how inhibition, shifting, and working memory predict the growth parameters in arithmetic accuracy and fluency in a group of Chinese children (n = 179) followed from Grade 2 (mean age = 97.89 months) to Grade 5 (mean age = 133.43 months). In Grade 2, children were assessed on measures of nonverbal IQ, number sense, speed of processing, inhibition, shifting, and working memory. In addition, in Grades 2–5, they were assessed on arithmetic accuracy and fluency. Results of structural equation modeling showed that nonverbal IQ, speed of processing, and number sense predicted the intercept in arithmetic accuracy, while working memory was the only EF subcomponent to predict the slope (rate of growth) in arithmetic accuracy. In turn, number sense, speed of processing, inhibition, and shifting were significant predictors of the intercept in arithmetic fluency. None of the EF subcomponents predicted the slope in arithmetic fluency. Our findings reinforce those of previous studies in North America and Europe showing that EF contributes to mathematics performance over and above other key predictors of mathematics, and suggest that different EF subcomponents may predict different growth parameters in mathematics