Numerical and Experimental Study of Shared Mooring Systems for Prototype Floating Wind Farms

Paper V is excluded in the paper due to copyright. Paper VI and VIII is excluded from the dissertation until they are published.Conventional mooring systems represent a substantial portion of the cost of a floating offshore wind farm (FOWF). The concept of shared mooring was initially proposed to reduce the overall usage of mooring lines and the number of anchors required by connecting adjacent floating offshore wind turbines (FOWTs) in an FOWF. Although such a concept offers cost-saving potential, extra complexity has been introduced to the floating system as the platform motions of FOWTs are coupled. To demonstrate the feasibility of shared mooring systems, it is of great interest and importance to understand the fundamental influence of shared mooring systems on the dynamic characteristics of FOWFs. Limited research has been conducted concerning detailed dynamic analyses of shared mooring systems and experimental investigations of FOWFs. This thesis addresses this knowledge gap by conducting numerical and experimental studies on prototype dual-spar FOWFs with shared mooring systems. Extensive investigations are carried out considering various shared mooring configurations, different turbine spacings and water depths, and varying mooring properties, configurations, and load scenarios. In the numerical study, a modeling method is developed for shared mooring systems utilizing Irvine’s elastic catenary theory for hanging cables, which serves as the basis for the mooring stiffness linearization and eigenvalue analysis of FOWFs with shared mooring systems. For a dual-spar FOWF, a shared line connecting two FOWTs is the basic shared mooring configuration. The influence of the shared line on the system’s natural periods and eigenmodes are investigated through a comparison to a single spar FOWT. The investigation reveals the significant influence of the shared line on the natural periods of surge and sway degrees of freedom (DOFs) of the spar platforms due to the dominance of mooring stiffness in the relevant terms of restoring stiffness. Moreover, the natural periods of these DOFs are found to be sensitive to variations in the mooring properties of both the single lines connecting the FOWTs to the seabed and the shared line connecting adjacent FOWTs. The basic shared mooring configuration is then modeled with a numerical simulation tool to perform fully coupled time-domain simulations under varying environmental conditions (ECs). A comparison with a single spar FOWT demonstrates larger motion ranges and dynamic motions of FOWTs in the FOWF, along with higher tension levels in the single lines. The shared line experiences high dynamic tension and snap load events. The studied mooring layout displays sensitivity to loading directions due to its inherent asymmetry. An alternative shared mooring system is considered by connecting neighboring FOWTs to a shared tethered buoy and by replacing the single lines of the FOWTs with hanging lines connected to individual tethered buoys. Through dynamic analyses under various load scenarios and comparison against the basic shared mooring configuration, it is concluded that the proposed tethered-buoy shared mooring system can achieve a significant reduction in tension levels across all mooring lines and alleviate potential threats associated with snap loads, with a trade-off for increased mean offset and dynamic motions of the FOWTs. Therefore, the proposed tethered-buoy shared mooring system is a preferred solution for future applications involving inter-array cables, where the excursion restrictions of FOWTs become less critical. A hydrodynamic model test campaign is conducted for dual-spar FOWFs with different shared mooring systems. Decay tests are carried out, and regular and irregular wave tests are performed under various ECs. Detailed physical modeling of the FOWTs and shared mooring systems are documented. Through a comparison to the results of a previously tested single spar FOWT, the findings from the numerical study are verified. A clump weight is introduced at the midpoint of the shared line in the basic shared mooring configuration, and its effects are investigated through a comparative analysis where the natural periods, motion response, and mooring tension before and after the inclusion of the clump weight are analyzed. The added clump weight leads to a decrease in the natural periods of horizontal DOFs and reduced dynamic motions of the FOWTs. Despite an increase in the pretension of all mooring lines, dynamic tension in these lines is reduced, and a reduced number of snap load events in the shared line is observed. In summary, this thesis investigates shared mooring systems for prototype dualspar FOWFs by numerical and experimental approaches. A quasi-static modeling method of shared mooring systems is developed and applied. The dynamic characteristics of FOWFs with five shared mooring configurations are revealed, and the station-keeping performance of the FOWF systems are highlighted in the comparison. These research outcomes augment the current understanding of and expand the existing knowledge base related to shared mooring systems. The analysis methods developed and employed in this study can be further utilized in the design and optimization of FOWFs.publishedVersio

Teacher incentive pay programs : characteristics and association with instructional practices

"July 2011"Title from PDF of title page (University of Missouri--Columbia, viewed on May 17, 2012).The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file.Dissertation advisor: Dr. Motoko AkibaVita.Ph. D. University of Missouri--Columbia 2011.This dissertation research examined the characteristics of teacher incentive pay programs in the United States. Using the 1999, 2003, and 2007 SASS datasets, this study found that large and ethnically diverse districts in urban areas with less union influence were more likely to offer PRP. However, highly qualified teachers in high demand were no more likely to receive a larger amount of PRP. The 2010 Teacher Compensation Programs survey data also showed that poor rural districts were less likely to offer teacher incentive pay programs. Using statewide longitudinal Teachers' Opportunity to Learn survey data, this study found that most of the PRP recipients were evaluated by school principals, who conducted classroom observations and face-to-face meetings to assess the teachers' teaching practice and professional development activities. After controlling for teacher and school characteristics, this study found a modest yet positive association between PRP and improvement in teacher practice of constructivist instruction.Includes bibliographical reference

Newly recognized mosquito-associated viruses in mainland China, in the last two decades

There are four principal arboviruses in mainland China. Two kinds of them are mosquito-borne viruses, namely Japanese encephalitis virus and dengue virus, which lead to Japanese encephalitis, and dengue fever/dengue hemorrhagic fever respectively; the other two are tick-borne viruses, namely tick-borne encephalitis virus and Crimean-Congo hemorrhagic fever virus (also known as Xinjiang hemorrhagic fever virus), which contribute to tick-borne encephalitis and Xinjiang hemorrhagic fever respectively. With exception of these four main arboviruses, many other mosquito-associated viruses have been isolated and identified in recent years. These newly isolated and identified mosquito-associated viruses are probably responsible for human and animal infections and diseases. The purpose of this review is to describe the newly isolated mosquito-associated viruses in mainland China which belong to five viral families, including their virological properties, phylogenetic relationships, serological evidence, as well as to appeal the public health concentration worldwide

The Neglected Arboviral Infections in Mainland China

The major arboviral diseases in mainland China include Japanese encephalitis, dengue fever, Crimean-Congo hemorrhagic fever (also known as Xinjiang hemorrhagic fever), and tick-borne encephalitis. These and other newly found arbovirus infections due to Banna virus and Tahyna virus contribute to a large and relatively neglected disease burden in China. Here we briefly review the literature regarding these arboviral infections in mainland China with emphasis on their epidemiology, primary vectors, phylogenetic associations, and the prevention programs associated with these agents in China

Factors responsible for the emergence of arboviruses; strategies, challenges and limitations for their control

International audienceSlave trading of Africans to the Americas, during the 16th to the 19th century was responsible for the first recorded emergence in the New World of two arthropod-borne viruses (arboviruses), yellow fever virus and dengue virus. Many other arboviruses have since emerged from their sylvatic reservoirs and dispersed globally due to evolving factors that include anthropological behaviour, commercial transportation and land-remediation. Here, we outline some characteristics of these highly divergent arboviruses, including the variety of life cycles they have developed and the mechanisms by which they have adapted to evolving changes in habitat and host availability. We cite recent examples of virus emergence that exemplify how arboviruses have exploited the consequences of the modern human lifestyle. Using our current understanding of these viruses, we also attempt to demonstrate some of the limitations encountered in developing control strategies to reduce the impact of future emerging arbovirus diseases. Finally, we present recommendations for development by an international panel of experts reporting directly to World Health Organization, with the intention of providing internationally acceptable guidelines for improving emerging arbovirus disease control strategies. Success in these aims should alleviate the suffering and costs encountered during recent decades when arboviruses have emerged from their sylvatic environment

Influence of Aerodynamic Loads on a Dual-Spar Floating Offshore Wind Farm With a Shared Line in Parked Conditions

The concept of a shared mooring system, in which adjacent wind turbines are coupled by sharing mooring lines, has been proposed to reduce the mooring costs of floating offshore wind farms. This work investigates the influence of aerodynamic loads on a floating offshore wind farm of two spar wind turbines connected with a shared line in extreme environmental conditions. A case study is performed for the floating offshore wind farm under parked conditions using a numerical simulation tool. The environmental conditions are determined from environmental contours with a return period of 50 years. Turbulent wind and irregular waves are simulated in dynamic analyses. Wind and waves are aligned and two loading directions are considered. Floater motions and structural response are analyzed. The influence of aerodynamic loads is studied by comparing the simulation results under both wind and waves with those under wave-only. It is concluded that the aerodynamic loads affect the horizontal motions of floating offshore wind turbines, the mooring response and the tower-base bending moment significantly in extreme environmental conditions, especially when the loading direction is 90 deg. The findings from this study improve understanding of the design loads of shared mooring systems.acceptedVersio

Dynamic analysis of a dual-spar floating offshore wind farm with shared moorings in extreme environmental conditions

The concept of a shared mooring system was proposed to reduce mooring and anchoring costs. Shared moorings also add complexity to the floating offshore wind farm system and pose design challenges. To understand the system dynamics, this paper presents a dynamic analysis for a dual-spar floating offshore wind farm with a shared mooring system in extreme environmental conditions. First, a numerical model of the floating offshore wind farm was established in a commercial simulation tool. Then, time-domain simulations were performed for the parked wind farm under extreme wind and wave conditions. A sensitivity study was carried out to investigate the influence of loading directions and shared line mooring properties. To highlight the influence of the shared line, the results were compared to those of a single spar floating wind turbine, and larger platform motions and higher tension loads in single lines are observed for the wind farm with shared moorings. The loading direction affects the platform motions and mooring response of the floating offshore wind farm. Comparing the investigated loading directions to the 0-deg loading direction, the variation of mean mooring tension at the fairlead is up to 84% for single lines and 16% for the shared line. The influence of the shared line properties in the platform motions and the structural responses is limited. These findings improve understanding of the dynamic characteristics of floating offshore wind farms with a shared mooring system.Dynamic analysis of a dual-spar floating offshore wind farm with shared moorings in extreme environmental conditionspublishedVersio

Modeling of a Shared Mooring System for a Dual-Spar Configuration

Author's accepted manuscript.acceptedVersio

Primary Channel Gain Estimation for Spectrum Sharing in Cognitive Radio Networks

In cognitive radio networks, the channel gain between primary transceivers, namely, primary channel gain, is crucial for a cognitive transmitter (CT) to control the transmit power and achieve spectrum sharing. Conventionally, the primary channel gain is estimated in the primary system and thus unavailable at the CT. To deal with this issue, two estimators are proposed by enabling the CT to sense primary signals. In particular, by adopting the maximum likelihood (ML) criterion to analyze the received primary signals, a ML estimator is first developed. After demonstrating the high computational complexity of the ML estimator, a median based (MB) estimator with proved low complexity is then proposed. Furthermore, the estimation accuracy of the MB estimation is theoretically characterized. By comparing the ML estimator and the MB estimator from the aspects of the computational complexity as well as the estimation accuracy, both advantages and disadvantages of two estimators are revealed. Numerical results show that the estimation errors of the ML estimator and the MB estimator can be as small as $0.6$ dB and $0.7$ dB, respectively.Comment: Submitted to IEEE Transactions on Communication

Teacher Incentive Pay Programs in the United States: Union Influence and District Characteristics

This study examined the characteristics of teacher incentive pay programs in the United States. Using the 2007–08 SASS data set, it found an inverse relationship between union influence and districts’ incentive pay offerings. Large and ethnically diverse districts in urban areas that did not meet the requirements for Adequate Yearly Progress as defined under the No Child Left Behind Act are more likely to offer a larger number of economic incentives. Although rural districts are likely to reward teachers in hard-to-staff schools, they are not more likely to reward teachers who are certified by the National Board or who teach in the subject areas of shortage, nor are they more likely to offer multiple financial incentives