Influence of soil stiffness on the dynamic response of an offshore wind turbine

La industria de la energía eólica marina ha crecido de forma significativa durante los últimos 15 años, y se espera que siga creciendo durante los siguientes. La construcción de torres en aguas cada vez más profundas y el aumento en potencia y tamaño de las turbinas han creado la necesidad de diseñar estructuras de soporte cada vez más fiables y optimizadas, lo que requiere un profundo conocimiento de su comportamiento. Este trabajo se centra en la respuesta dinámica de una turbina marina con cimentación tipo monopilote y sobre la que actúa la fuerza del viento. Se han realizado cálculos con distintas propiedades del suelo para cubrir un rango de rigideces que va desde una arena muy suelta a una muy densa. De este modo se ha analizado la influencia que tiene la rigidez del suelo en el comportamiento de la estructura. Se han llevado a cabo análisis estáticos y dinámicos en un modelo de elementos finitos implementado en Abaqus. El desplazamiento en la cabeza de la torre y la tensión en su base se han obtenido en función de la rigidez del suelo, y con ellos se ha calculado la amplificación dinámica producida cuando la frecuencia natural del sistema suelo‐cimentación torre se aproxima a la frecuencia de la carga. Dos diferentes enfoques a la hora de modelizar el suelo se han comparado: uno utilizando elementos continuos y otro utilizando muelles elásticos no lineales. Por último, un análisis de fiabilidad se ha llevado a cabo con un modelo analítico para calcular la probabilidad de resonancia del sistema, en el que se han considerado las propiedades de rigidez del suelo como variables aleatorias. Offshore wind energy industry has experienced a significant growth over the past 15 years, and it is expected to continue its growth in the coming years. The expansion to increasingly deep waters and the rise in power and size of the turbines have led to a need for more reliable and optimized support designs, which requires an extensive knowledge of the behaviour of these structures. This work focuses on the dynamic response of an offshore wind turbine founded on a monopile and subjected to wind loading. Different soil properties have been considered in order to cover the range of stiffness from a very loose to a very dense sand. In this way, the influence of stiffness on the structure behaviour has been assessed. Static and dynamic analyses have been carried out by means of a finite element model implemented in Abaqus. Head displacement and stress at the tower base have been obtained as functions of soil stiffness, and they have been used to calculate the dynamic amplification that is produced when the natural frequency of the system soil‐foundation‐tower approaches the load frequency. Two different approaches of soil modelling have been compared: soil modelled as a continuum and soil simulated with non linear elastic springs. Finally, a reliability analysis to assess the probability of resonance has been performed with an analytical model, in which soil stiffness properties are considered as stochastic variables.

Aggregate community properties and the strength of species’ interactions

Although humanity depends on the continued, aggregate functioning of natural ecosystems, few studies have explored the impact of community structure on the stability of aggregate community properties. Here we derive the stability of the aggregate property of community biomass as a function of species’ competition coefficients for a two-species model. The model predicts that the stability of community biomass is relatively independent of the magnitude of the interaction strengths. Instead, the degree of asymmetry of the interactions appears to be key to community stability.

Aggregate auto travel forecasting: state of the art and suggestions for future research. Final report.

Transportation Department, Office of the Assistant Secretary for Systems Development and Technology, Washington, D.C.

Performance of electric and microwave batch plant moisture meters in determining the moisture contents of fine-aggregate materials for concrete production /

Mode of access: Internet.

The shape of road aggregate and its measurement.

At head of title: Dept. of Scientific and Industrial Research and Ministry of Transport.

The investigation of present aggregate gradation control practices and the development of short-cut or alternative test methods /

Mode of access: Internet.

Time series analysis in the assessment of ICT impact at the aggregate level – lessons and implications for the new economy

The major role of information and communication technology (ICT) in the new economy is well documented: countries worldwide are pouring resources into their ICT infrastructure despite the widely acknowledged “productivity paradox”. Evaluating the contribution of ICT investments has become an elusive but important goal of IS researchers and economists. But this area of research is fraught with complexity and we have used Solow's Residual together with time-series analysis tools to overcome some methodological inadequacies of previous studies. Using this approach, we conduct a study of 20 countries to determine if there was empirical evidence to support claims that ICT investments are worthwhile. The results show that ICT contributes to economic growth in many developed countries and newly industrialized economies (NIEs), but not in developing countries. We finally suggest ICT-complementary factors, in an attempt to rectify possible flaws in ICT policies as a contribution towards improvement in global productivity.

The strength and stiffness of slabs to column joints

DUE TO COPYRIGHT RESTRICTIONS ONLY AVAILABLE FOR CONSULTATION AT ASTON UNIVERSITY LIBRARY AND INFORMATION SERVICES WITH PRIOR ARRANGEMENT

State advances and private retreats? Evidence from the decomposition of the Chinese manufacturing aggregate productivity decomposition in China

This paper is motivated by the recent debate on the existence and scale of China's 'Guo Jin Min Tui' phenomenon, which is often translated as 'the state sector advances and the private sector retreats'. We argue that the profound implication of an advancing state sector is not the size expansion of the state ownership in the economy per se, but the likely retardation of the development of the already financially constrained private sector and the issues around the sustainability of the already weakening Chinese economy growth. Drawing on recent methodological advances, we provide a critical analysis of the contributions of the state and non-state sectors in the aggregate Total Factor Productivity and its growth over the period of 1998-2007 to verify the existence of GJMT and its possible impacts on Chinese economic growth. Overall, we find strong and consistent evidence of a systematic and worsening resource misallocation within the state sector and/or between the state sectors and private sectors over time. This suggests that non-market forces allow resources to be driven away from their competitive market allocation and towards the inefficient state sector. Crown Copyright © 2014.

A beruházási projektek aggregált tőkeigénye = The Aggregate Capital Needs of Investment Projects

A tanulmány abból indul ki, hogy a beruházási projektek értékelése során egyidejűleg szükséges figyelembe venni a projektben lekötött tőkét és a lekötési időt mint jövedelemtermelési lehetőséget. Definiálja a projekt aggregált tőkeigényének fogalmát és megszerkeszti a vonatkozó mérőszámot. Az aggregált tőkeigény új vállalatgazdasági kategória, mely a beruházási projektek értékelésének egy új megközelítését teszi lehetővé. A projekt aggregált tőkeigénye azt a tőkeösszeget jelenti, mely a projekt működtetéséhez annak teljes élettartama alatt szükséges. A három meghatározó tényező: a kezdőtőke, a megtérülési idő (illetőleg az élettartam) és a megtérülés gyorsasága. A számszerűsítéshez minden évre vonatkozóan meg kell határozni az adott évben lekötött tőkét, ami az adott évig még meg nem térült tőkerészt jelenti, majd ezek összegzése révén adódik az aggregált tőkeigény. A mértékegység egységnyi tőke egyévi lekötése. A tanulmány az összefüggések modellszerű levezetése mellett gazdag példaanyagot is tartalmaz. Az elemzés bővíti a nettó jelenérték tartalmára vonatkozó ismereteket, rávilágít az aggregált tőkeigény ismeretének fontosságára mind a nettó jelenérték, mind a belső kamatláb esetében. _____ The starting point of this paper is that in the evaluation process of investment projects necessary to take into account simultaneously the tied-up capital and tiedup time as the income-generating potential. For this, it defines a special content of aggregate capital needs of investment projects, and elaborates an index. The aggregate capital needs is a new business economics category, which provides a new aspect to evaluate investment projects. This means the amount of capital needed for the operation of the project during its full duration. Three factors determine the aggregate capital needs for investments projects. These are the amount of initial investment, the payback period (or the duration) and the rapidity of capital payback. The solution is to sum up the yearly tied-up capital, that is, the notreturned parts of the capital for each year. The measurement unit is one unit tied-up capital for one year. The paper formulates the main relationships as models and by way of explanation presents some examples. The analysis highlights the importance of considering the aggregate capital needs furthermore widens knowledge regarding the net present value and internal rate of return.

(Table 1, page 171) Thorium and cobalt content of the total aggregate of manganese nodules from various locations

A criterion is suggested for discrimination between ferromanganese oxide minerals, deposited after the introduction of manganese and associated elements in sea water solution at submarine vulcanism, and minerals which are slowly formed from dilute solution, largely of continental origin. The simlultaneous injection of thorium into the ocean by submarine vulcanism is indicated, and its differentiation from continental thorium introduced into the ocean by runoff is discussed.

Inverse identification of the bending stiffness of a braided polyethylene twine subject to large deformation: application to the identification of the mesh opening rigidity of fishing nets.

The evaluation of the mesh opening stiffness of fishing nets is an important issue in assessing the selectivity of trawls. It appeared that a larger bending rigidity of twines decreases the mesh opening and could reduce the escapement of fish. Nevertheless, netting structure is complex. A netting is made up of braided twines made of polyethylene or polyamide. These twines are tied with non-symmetrical knots. Thus, these assemblies develop contact-friction interactions. Moreover, the netting can be subject to large deformation. In this study, we investigate the responses of netting samples to different types of solicitations. Samples are loaded and unloaded with creep and relaxation stages, with different boundary conditions. Then, two models have been developed: an analytical model and a finite element model. The last one was used to assess, with an inverse identification algorithm, the bending stiffness of twines. In this paper, experimental results and a model for netting structures made up of braided twines are presented. During dry forming of a composite, for example, the matrix is not present or not active, and relative sliding can occur between constitutive fibres. So an accurate modelling of the mechanical behaviour of fibrous material is necessary. This study offers experimental data which could permit to improve current models of contact-friction interactions [4], to validate models for large deformation analysis of fibrous materials [1] on a new experimental case, then to improve the evaluation of the mesh opening stiffness of a fishing net