Non-linear Slack-Mooring Modelling of a Floating Two-Body Wave Energy Converter

Most floating oscillating-body wave energy converters that have been proposed and developed so far are two-body systems where the power is extracted from the relative translational motion between the two bodies. As any floating device, floating point absorbers are subject to drift forces due to waves, currents and wind, and therefore need to be kept in place by a proper mooring system. The mooring cables can be approximately modelled as catenary lines in a quasi-static analysis. The use of a time-domain analysis allows for nonlinear mooring forces of slack chain cables to be considered. Numerical results for motion, mooring tensions and absorbed power are presented for a two body system consisting of a hemispherical floater and a submerged body and slack bottom moorings, for regular and irregular waves. Comparisons are given with the unmoored two-body heaving system, the moored heaving twobody system and with the simplified one body linear PTO model. Results show the possibility of occurrence of low-frequency horizontal oscillations of large amplitude, and non linear motions, even for regular waves. Some differences are seen in comparison with the simplified one body model and with the heave two-body system. The moorings were found not to affect very significantly the power absorbed

Optimization of mooring configuration parameters of floating wave energy converts

Floating point absorbers devices are a large class of wave energy converters for deployment offshore, typically in water depths between 40 and 100m. As floating oil and gas platforms,the devices are subject to drift forces due to waves, currents and wind, and therefore have to be kept in place by a proper mooring system. Although similarities can be found between the energy converting systems and floating platforms, the mooring design requirements will have some important differences between them, one of them associated to the fact that, in the case of a wave energy converter, the mooring connections may significantly modify its energy absorption properties by interacting with its oscillations. It is therefore important to examine what might be the more suitable mooring design for wave energy devices, according to the converters specifications. When defining a mooring system for a device, several initial parameters have to be established, such as cable material and thickness, distance to the mooring point on the bottom, and which can influence the device performance in terms of motion,power output and survivability. Different parameters, for which acceptable intervals can be established, will represent different power absorptions,displacements from equilibrium position, load demands on the moorings and of course also different costs. The work presented here analyzes what might be, for wave energy converter floating point absorber, the optimal mooring configuration parameters, respecting certain pre-established acceptable intervals and using a time-domain model that takes into account the non-linearities introduced by the mooring system. Numerical results for the mooring forces demands and also motions and absorbed power, are presented for two different mooring configurations for a system consisting of a hemispherical buoy in regular waves and assuming a liner PTO

A time domain analysis of arrays of floating point-absorber wave energy converters including the effect of nonlinear mooring forces

The extensive exploitation of the offshore wave energy resource may require the deployment of dense arrays of point absorbers, the distance between elements being possibly tens of meters. In such cases, it may be more convenient and economical that only elements in the periphery of the array are directly slack-moored to the sea bottom,while the other elements are prevented from drifting and colliding by connections to adjacent elements. Previous work was done in a base configuration of three floating point absorbers located at the grid points of an equilateral triangular, with a solid weight located at the centre of the triangle, which was extended to more complex equilateral triangular grid arrays. The study was based on frequency domain analysis which requires, not only the power take-off system (PTO) to be linear, but also linear mooring forces, which is quite unrealistic as a model of slack moorings. In the present paper those restrictions are removed by using a time-domain, rather than a frequency domain, analysis, which allows nonlinear mooring forces to be considered. The mooring cables are approximately modelled as catenary lines in a quasi-static analysis. The results show very different behaviour for the horizontal and vertical motions of the floating converters, namely the possibility of occurrence of low-frequency horizontal oscillations of large amplitude. Even in the case of incident regular waves, such horizontal motions were found to be non-periodic, a behaviour that is typical of nonlinear systems

Portuguese average cost of capital

The oldest Portuguese share index still being calculated is the BVL/PSI-General, one which started the daily series on 5/Jan/1988 with a base value of 1000 points. Everyday a single value is computed based on the closing prices of all the shares included in the sample. Also, all corporate events affecting the price of any share beyond market sentiment are taken into account through proper adjustments, either in the numerator or the denominator of the formula. However, for dates before January 1988, there is nothing comparable to this index since the two different series known either never disclosed the methodology adopted to calculate the index or followed solutions not compatible with the above index. The present paper explains the solutions adopted to replicate as closely as possible the methodology of the BVL-General index to the main market of the Lisbon Exchange for the period 1978 – 1987. This is the first estimate of the historical Equity Risk Premium in Portugal above short-term risk-free rate from the re-opening of the market following the Carnation Revolution (and the accompanying nationalizations), to the present. In showing a value of the same order of magnitude found in other countries, the paper invites further studies on the effects of political decisions such as privatizations and joining the European Union

Sensitivity of the sorghum yield to individual changes in climate parameters: modelling based approach.

Based on sensitivity analyses the effect of changing in climate on sorghum has been investigated. This has been achieved by conducting crop modeling experiments carried out with weather observations and output of global climate models. As can be anticipated results demonstrated that the sorghum yield is more sensitive to rainfall, solar radiation and CO2 concentration. However, the increase in CO2 concentration has a positive effect on yield in years that the crop is submitted to water stress. This effect is extremely reduced under well watered conditions and therefore is subject to the climate inter-annual variability. Accumulated solar radiation along the sorghum cycle below 1900 MJ m?2 has also been shown to reduce the yield. Compared to other weather variables the sorghum yield is less sensitive to changes in relative humidity. In this sense, changes in stomatal closure and therefore photosyntesis is not highly dependent on water vapor pressure. These finds can be applied for both climate observations and global models output

Relacionamento entre filhos adolescentes e seus pais e mães em um contexto de violência doméstica e familiar contra a mulher

Este trabalho buscou investigar, com base na perspectiva dos filhos, como se estabelece o relacionamento entre filhos adolescentes e seus pais e mães em famílias que vivenciam/vivenciaram a situação de violência doméstica e familiar contra a mulher. Para tanto, procedeu-se à entrevistas com 11 adolescentes de 12 a 16 anos, de ambos os sexos, membros de famílias em que tenha ocorrido a identificação de violência doméstica e familiar contra a mulher, perpetrada pelo pai contra a mãe do adolescente. Houve a mediação institucional da Coordenação de Atendimento a Vítimas de Violência Doméstica e Discriminação (CAVVID), localizada em Vitória-ES, e da Secretaria de Políticas Públicas para a Mulher (SEPPOM), localizada na Serra-ES para identificação dos participantes. As entrevistas ocorreram com base em um roteiro semiestruturado que teve por inspiração a Escala de Qualidade da Interação Familiar (EQIF). As questões abordaram a avaliação dos filhos acerca do ambiente familiar, especificamente do relacionamento do adolescente com a mãe e com o pai, sobre o modo como os adolescentes avaliavam a relação entre a mãe e o pai e como essa relação afetava a parentalidade. Recorreu-se à gravação e transcrição integral das entrevistas e depois à categorização e análise dos dados com base no método de análise de conteúdo. A discussão dos resultados ocorreu à luz da Teoria Bioecológica do Desenvolvimento Humano e da literatura sobre o tema. Os dados indicaram que os adolescentes percebiam as situações de conflito entre o pai e a mãe dentro do microssistema familiar, as quais os afetavam. Especificamente sobre o relacionamento com o pai, os adolescentes ressaltaram sentimentos negativos, queixaram-se da falta de disponibilidade do pai para estar com os filhos, dos seus comportamentos agressivos, não só com relação à mãe, mas com relação aos outros membros da família e do consumo de bebidas alcoólicas feito por ele. No que se refere aos aspectos positivos, os adolescentes relataram o suporte material, momentos de cuidado e de expressão de afeto. Por outro lado, quase todos os adolescentes descreveram a relação com a mãe como positiva. Ocorreu menção dos momentos de cuidado, monitoramento, expressão de afeto, realização de atividades conjuntas e sentimento de apoio na relação. Os aspectos negativos descritos referiram-se à ocorrência de desentendimentos por motivos relacionados a limites impostos pela mãe, opiniões divergentes, além de situações que envolviam a relação com o pai. Considerando que, em sua maioria, os adolescentes avaliaram o relacionamento com a mãe positivamente, a análise realizada apontou que os processos proximais com a mãe podem contribuir para atenuar os efeitos dos aspectos negativos da convivência na família, descritos pelos adolescentes. Os dados obtidos reforçam a importância da ampliação de estudos sobre o impacto da violência conjugal na parentalidade do ponto de vista de todos os membros da família, bem como a ampliação da rede de apoio também aos filhos que vivenciam os conflitos envolvendo as figuras parentais, auxiliando as famílias na interrupção do ciclo da violência

Hydrodynamics of triangular-grid arrays of floating point-absorber wave energy converters with inter-body and bottom slack-mooring connections

It may be convenient that dense arrays of floating point absorbers are spread-moored to the sea bottom through only some of their elements (possibly located in the periphery), while the other array elements are prevented from drifting and colliding with each other by connections to adjacent elements. An array of identical floating point absorbers located at the grid points of an equilateral triangular grid is considered in the paper. A spread set of slack-mooring lines connect the peripheric floaters to the bottom. A weight is located at the centre of each triangle whose function is to pull the three floaters towards each other and keep the inter-body moorings lines under tension. The whole system – buoys, moorings and power take-off systems – is assumed linear, so that a frequency domain analysis may be employed. Equations are presented for a set of three identical point absorbers. This is then extended to more complex equilateral triangular grid arrays. Results from numerical simulations, with regular and irregular waves, are presented for the motions and power absorption of hemispherical converters in arrays of three and seven elements and different mooring and PTO parameters, and wave incidence angles. Comparisons are given with the unmoored and independently-moored buoy situations