1,721,074 research outputs found
Long-term performance assessment of offshore wind turbine structures based on monitoring data
Full text linkAs the world strives for cleaner and more sustainable energy utilisation, interests in the offshore wind turbine (OWT) industry have massively grown over the last decade. These have attracted sustained investment efforts from top global economies (even as emerging ones gradually embrace the technology), based mainly on the offshore wind farm’s (OWF) levelised cost of energy (LCOE). Foundation has been shown to have the most influence on both the LCOE and structural stability of OWTs, accounting for up to a third of the entire OWF project’s LCOE, with an equally high failure criticality rate. Therefore, to derive optimum benefits from OWTs, their foundations must be carefully selected based on the two factors of water depths and turbine ratings. While there is a wide range of OWT support foundations, the monopile and jacket are the most deployed. However, the foundation recommendations in the literature are often violated due to a lack of universal guidance for their selection, leading to cross-deployments of the two foundation types with implications on the integrity of OWT systems. By their nature, OWTs are almost always in states of continuous vibrations, especially under operational loads, leading to modal property and stability-condition changes. Where tolerable limits are breached, foundation-soil interaction-induced resonance and excessive foundation rotations (tilts) are two of the most likely consequences. These make the OWTs susceptible to unpredictable and sometimes difficult-to-locate structural or system damage(s), affecting their effective long-term functioning. In older OWTs (found today across many wind farms) that are at/close to their initial design lives, these damages are even more likely, increasing susceptibility to unexpected failures requiring unplanned and expensive maintenance. Two prominent solutions that can guarantee the safe and effective operations of OWTs, as identified in the literature, are their long-term comprehensive characterisation and the implementation of reliable structural health monitoring (SHM) regimes that monitor the conditions of the OWTs. The former solution contends with a gross shortage of sufficient track records due to the relatively young age of the OWT technology. In contrast, the latter solution is faced with measurement difficulties (especially bathymetric) based on access restrictions. Furthermore, with model updating-based SHMs, there is a limit on the maximum number of updating parameters an algorithm can effectively accommodate, thereby often forcing the use of oversimplified FEMs that may misrepresent the prototypes, especially real OWTs.This study investigates the dynamic implications of foundation cross-deployments in OWTs (focusing on monopile and jacket) to incorporate the findings into their future analyses and designs. Extensive FE modal analyses of the fixed and flexible-base cases of the monopile and jacket supported OWTs (MSOWT and JSOWT) are carried out with natural frequency chosen as the comparison parameter. Furthermore, this study develops a preliminary OWT foundation selection chart based on water depths and turbine ratings to avoid such cross-deployments. The comprehensive characterisation of OWTs is also achieved by conducting a range of model tests (medium and long-term) on two different-sized laboratory OWT (LOWT) models under varying amplitude high cycle application (HCA) operating loads. These loads are modelled after environmental field data. Acceleration and displacement sensor readings are respectively used to monitor the LOWTs’ structural responses leading to the natural frequency and foundation rotation (tilt) changes throughout the tests. To address the limitation on the allowable number of updating parameters for model updating, few-parameters soil-structure interaction (SSI) models are developed for the MSOWT and JSOWT with only two parameters (instead of seven). The relationship between the maximum lateral soil resistance and soil dept
An experimental investigation of transient dynamics of pile-supported structures in liquefiable soils.
Full text linkUnsatisfactory performance of pile supported structures in liquefiable areas (ranging from tilting/settlement to complete collapse) is still observed after most major earthquakes. As a result, further research is required in this subject. This thesis therefore aims to study the response of pile supported structures during seismic liquefaction. The ground liquefies progressively in a top down fashion when the soil transform from solid material to liquid-like material. This is referred to as transient behaviour (from no-liquefaction to full liquefaction state) and is particularly focused in this work. In practice, piles are usually analysed as laterally loaded beams using Beam on Nonlinear Winkler Foundation model where earthquake loading is applied in a pseudo-static way. Therefore, this study reviews methods of analysis of laterally loaded pile. Six different field case records were analysed using different approaches and the results were compared. Large scale shake table experiments were also conducted consisting of four pile models (two single piles and two pile groups of 2×2) placed in a rigid soil container with energy absorbing boundaries. Redhill-110 sand was used and earthquake motions were applied to liquefy the soil. It was observed that the bending moment along the piles changed with the progression of liquefaction and the maximum bending moment occurred in the transient phase. It was also observed that the time taken to reach liquefaction may affect the amplification of the bending moment. Design of piles requires soil parameters and as a result, a series of multi-stage soil element tests were carried out on four different types of sands; Redhill-110 sand, Japanese silica sand No. 8, Assam sand, and Ganga sand where the sands were first liquefied and then tests were carried out to obtain stress-strain of liquefied sand (post-liquefaction). The results showed that the post liquefaction behaviour of sand depends on the soil relative density. Furthermore, the results from the Redhill-110 sand were used to back analyse the shake table test results. Finally, a method has been proposed to incorporate transient behaviour of pile in liquefiable soils, based on an assessment of the estimated dynamics amplification factors in the shake table tests. Keywords: Dynamic soil-pile interaction, Liquefaction, Shake table test, multi-stage soil element test, transient dynamics, dynamic amplification factors
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Observed Dynamic Soil-Structure Interaction in scale testing of Offshore Wind Turbine Foundations
No full textMonopile foundations have been commonly used to support offshore wind turbine generators (WTGs), but this type of foundation encounters economic and technical limitations for larger WTGs in water depths exceeding 30m. Offshore wind farm projects are increasingly turning to alternative multipod foundations (for example tetrapod, jacket and tripods) supported on shallow foundations to reduce the environmental effects of piling noise. However the characteristics of these foundations under dynamic loading or long term cyclic wind turbine loading are not fully understood. This paper summarises the results from a series of small scaled tests (1:100, 1:150 and 1:200) of a NREL (National Renewable Energy Laboratory) on three types of foundations: monopiles, symmetric tetrapod and asymmetric tripod. The test bed used consists of either kaolin clay or sand and up to 1.4 million loading cycles were applied. The results showed that the multipod foundations (symmetric or asymmetric) exhibit two closely spaced natural frequencies corresponding to the rocking modes of vibration in two principle axes. Furthermore, the corresponding two spectral peaks change with repeated cycles of loading and they converge for symmetric tetrapods but not for asymmetric tripods. From the fatigue design point of view, the two peaks for multipod foundations broaden the range of frequencies that can be excited by the broadband nature of the environmental loading (wind and wave) thereby impacting the fatigue. The system life (number of cycles to failure) may effectively increase for symmetric foundations as the two peaks will tend to converge. However, for asymmetric foundations the system life may continue to be affected adversely as the two peaks will not converge. In this sense, designers should prefer symmetric foundations to asymmetric foundations
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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