Dynamic Analysis and Development of Floating Offshore Wind Turbines

팜탄담 Graduate School of Ulsan University 2020 국내박사

A floating offshore wind turbine offers tremendous potential benefits in near future with major challenges. Various research centers, institutes, and universities pay attention to and perform researches on FOWTs. Currently, the efficient performance and the economics of the floating wind systems are paid more attention. The South Korean Government releases an ambitious plan, known as “Renewable Energy 3020”, to increase new renewable energy source power generation by 48.7 GW by 2030. The target is to provide 16.5 GW from wind energy which includes 13 GW from offshore wind. Focusing on a water depth of 150 m in the East Sea gas field off the coast, Ulsan, Korean, floating offshore wind turbines are considered to be the best candidate to fulfill the target. The main types of floating foundations are the semi-submersible, spar, and tension leg platforms. Usually, a semi-submersible platform is composed of three or four slender columns and those are connected to each other through braces. The semisubmersible relies mainly on the waterplane area of columns and the distances between each column to achieve stability. The TLP type is stabilized by a high-tension mooring system; for this reason, the anchor system for TLP is complex. The spar type usually uses heavy ballast materials such as concrete at the bottom to provide a lower center of gravity for stabilization, however, the acceleration at the nacelle and the tower base bending moment are large. Several semi-submersibles and spar types were tested in scaled models in the Ocean Engineering Wide Tank of the University of Ulsan. Numerical models of these types were built and validated based on model test results. The NREL FAST code was used to conduct the fully coupled numerical simulation of floating offshore wind turbines. A set of in-house codes generate potential hydrodynamic coefficients to coupling with FAST code. The finding here is that the semi-submersible platform has a strong effect from second-order wave loads, but the spar platform has little effect from second-order wave loads. This thesis aims to develop a spar-type platform to support the NREL-5-MW reference wind turbine for 150 m water depth of the East Sea gas field off the coast, Ulsan, Korean. The Spar-type platform includes a moonpool at the center. Design optimization processes are three steps, the first step is using a spreadsheet to calculate the platform dimension, the second step is the frequency-domain analysis which calculates the responses in regular waves and the last is the fully coupled simulation time-domain analysis to obtain dynamic responses in combined wind, wave, and current conditions. By having a water column inside the open moonpool, the dynamic responses of the FOWT system are reduced significantly. The reduction of motions will lead to reducing the nacelle acceleration and tower base bending moment as well. The spar-type platform combined with a moonpool has good performances in both operational conditions and extreme conditions. Compared with another floating type, the proposed model promises a feasibility to apply for the offshore wind farm in the East Sea gas field off the coast, Ulsan, Korean.

Mechanical Properties of the Rubber Material Used in Elastomeric Bridge Bearing Considering the Effects of Aging.

테일러 헨리 쿠오나 Graduate School of Ulsan University 2020 국내석사

Abstract Mechanical Properties of the Rubber Material Used in Elastomeric Bridge Bearing Considering the Effects of Aging University of Ulsan Department of Civil and Environmental Engineering Henry Quona Taylor Bridge systems are subjected to certain movements, induced by external or dead loading, which has necessitated the inclusion of support devices with the capabilities to efficiently dissipate energy and protect the bridge pier from unwanted stress concentration. These criteria are effectively met by elastomeric bearings, whose primary material, rubber, has elasticity properties suitable for base isolation. The performance of these bearings whose service life is about 100years is dependent on the behaviour of the rubber material over time. Pursuant with understanding the behaviour of elastomeric bearing overtime, this study sought to determine the effects of aging on the material properties of the rubber used in elastomeric bearings considering Natural Rubber and Neoprene; to conduct member level simulations of the elastomeric bearing blocks and determine the changes in stiffness of the aged rubber bearing and to evaluate the effects of the different assumptions surrounding the modes aging on the behaviour of aged rubber bearing. In its pursuit, the Study discovered that neoprene performs better than natural Rubber with respect to the physical changes, tensile and shear, in the vulcanizates and that the effects of the aging conditions applied were more prominent in their tensile behaviour than it was with their shear. Also evident from the study was that the models with neoprene had lesser changes in the shear stiffness as they aged than did Natural Rubber. Natural rubber produced a maximum shear stiffness increase of 18.73% while neoprene produced a maximum shear difference of a meagre 2.89%. it was noticed that the Bearing models under combined loading of compression and shear developed maximum shear at the interface between the rubber and steel plates 요약 노화의 영향을 고려한 교량 탄성받침의 고무 재료의 기계적 성질 울산대학교 대학원 건설환경공학과 핸리 쿠오나 테일러 교량의 교각은 지진하중에 의해 큰 외력(모멘트 및 전단력)을 받게 된다. 이러한 지진력에 의한 에너지를 효과적으로 소산시켜서 원치 않는 부재력(응력)이 집중되지 않도록 하는 것이 중요하다. 이를 위하여 고무재료를 기반으로 하는 탄성받침이나 지진보호받침을 적용하는 경우가 점차 증가하고 있다. 교량의 공용수명이 약 100년 정도임을 감안하면 교량의 고무받침의 성능은 시간이 경과함에 따라 역학적 거동 특성에 변화가 생긴다. 이 연구에서는 노화(열화)에 따른 탄성받침의 거동을 이해하기 위해 탄성받침에 사용되는 천연고무와 합성고무의 역학적 특성 변화를 실험을 통하여 분석하였으며, 이들 재료를 기반으로 하는 탄성받침의 부재 수준의 역학적 거동 특성변화를 해석을 수행하여 분석, 평가하였다. 이 연구를 통해, 합성고무가 천연고무에 비해 인장(tension) 및 전단(shear)의 물리적 변화가 훨씬 크다는 것을 알았으며, 또한 전단에 비해 인장의 역학적 특성이 노화에 크게 영향을 받는 것을 확인하였다. 노후화에 의한 강성의 변화는 천연고무에서는 최대 전단 강성이 18.73% 증가했고 합성고무는 2.89%로 증가하였다. 또한, 파단시의 최대 연신율은 감소하였다. 이러한 교량 받침의 역학적 특성 변화는 교량의 진동주기를 작게하여 교량에 발생하는 지진력을 증가시키기때문에 시간 경과에 따른 고무받침의 노후화가 교량의 내진성능을 악화시킬 수 있다는 것을 알 수 있다.

차량용 폐루프 유압 에너지 회생 시스템의 제안 모델링 및 제어에 관한 연구 : A Study on a Closed-loop Hydraulic Energy Saving Systems Proposition, Modeling and Control

Ho Triet Hung Graduate School, University of Ulsan 2010 국내박사

A survey of energy-saving activities in hydraulic systems for rotary loads indicates that hydraulic valve systems have disadvantages of system energy losses while hydraulic pump controlled systems are freely system energy losses but are not able to recover energy. Energy-recovery systems based on open loop are not only freely system energy losses but also are able to recover energy of the load during deceleration or lowering the load. However, some limits of hydraulic technology restrict applicability of the open recovery-energy hydraulic systems for mobile applications. A new closed-loop hydraulic energy-saving system is proposed in this research to overcome the drawbacks of the current open loop hydraulic energy-saving systems. The new system includes a variable displacement hydraulic pump, a directional control valve, two hydraulic accumulators and a hydraulic pump/motor, the secondary unit, which can work either as a hydraulic pump or a hydraulic motor. The new proposed system is discussed, modeled, analyzed and validated in this research. The proposed system based on closed-loop hydraulic circuit. Hydraulic accumulator is used as energy storage system in a novel configuration so that the system can recover the kinetic energy of the load without reversion of fluid flow. The secondary unit can work in only positive region for both functions, which is a novel characteristic of the system. Modeling of the system based on the components physical attributes is presented in this research. Reduced-order models of the system in different configurations are also built for the controller synthesis. Energy utilization and energy recovery potential of the system are analyzed via simulation. The simulation results indicate that influences on the energy-utilization and energy-recovery potential of the system are the load conditions and the parameters of the system. An adaptive robust control scheme using adaptive fuzzy sliding mode control is proposed for speed control of the system. The controller design is to consider nonlinearities of the control input, the dead-zone and the saturation. The controller is brought into simulation and the simulation results indicate that the controller is robust for uncertainties and external disturbance. Experiments are taken to evaluate the system in two aspects, the energy-recovery potential and the effectiveness of the designed controller. A proposed application of the system for hydraulic hybrid vehicle is investigated. The experimental results confirm the validity of qualitative and quantitative statements of the proposed system.

상표가 다른 국가와 한국의 사회망에 영향을 미치는 연구 : BUILDING BRAND TRUST, BRAND LOYALTY ON SOCIAL NETWORKING SITES BETWEEN KOREAN AND NON-KOREAN PEOPLE

팜레장 Graduate School, University of Ulsan 2011 국내석사

Consumers and users now define the brand and have increasing control. This level of consumers and user‘s control increase the need for a strategic brand trust, brand loyalty and satisfaction that are strong, well founded and sufficiently engaging to inspire your users to be brand advocates. And users add real value to companies and their social network sites. They are people with feelings and attitudes and they expect their brands to know what they are and reflect them. Direct communication between brands and their users has never been more active and our behavior has never had greater visibility or consequences. This is reason for deeply research on this topic. This study examines how brand trust and brand loyalty are affected by the following social network site factors: e-word of mouth, brand personality, perceived ease of use, experience, relationship commitment. The author also investigates the relationship between brand trust and brand loyalty on social network sites.

Synthesis and Characteristics of Immiscible Fe-Cu Nanoparticles using Electrical Explosion of Wire in Liquid

Chu dac phuc Graduate School of Ulsan University 2020 국내석사

Nowadays, with the fast development of science and technology, we require more advanced material. Nanomaterials, having unique, beneficial chemical, physical, and mechanical properties, nanomaterials have been studied long times ago from the 1980s but its applications were very limited still. In recent decades, the nanomaterials have been successfully commercialized using different manufacturing processes, and can be used in a variety of industrial products, bio-medical and healthcare, etc. Bimetallic nanoparticles (BNPs) are the nanometer-sized solid particle that formed by the combination of two different metals. The bimetallic nanoparticles have attracted huge attention as compared to monometallic nanoparticles in both technological and scientific view because BNPs show better properties. Because of the remarkable properties and performance of iron (Fe) - copper (Cu) alloys, there are a lot of research study on the synthesis and applications of these materials. Even though their atomic radiuses are similar, the Fe and Cu are practically immiscible in the equilibrium state. As non-equilibrium solid solutions, the metastable Fe-Cu alloys can be synthesized by special methods such as rapid quenching, vapor deposition, sputtering, ion-beam mixing, and mechanical alloying. The complexity of those methods (multiple steps, low productivity, high cost and non-eco-friendly) can be crucial problem for industrial applications. Electrical explosion of wire (EEW) is well known as an effective method for synthesis metallic and alloy nanoparticles. By explosive destruction of metal wires, materials of the wires turn into particles with nanosized range (10 – 100 nm). The extremely non-equilibrium conditions of EEW can cause some interesting properties of the nanopowders. In addition, fabrication by the EEW can be a simple and economical process, lead to more applications in industrial. In this study, Fe-Cu nanoparticles was fabricated by electrical explosion of wires. Powder shape and size distribution and alloying state will be analyzed and discussed according to the conditions of EEW. Fe nanoparticles, Cu nanoparticles, Fe and Cu mixed nanoparticles were also be prepared to be analyzed and compared with experimental conditions.

지능형 교통시스템을 위한 자동차번호판 인식 연구 : The Study of Vehicle License Plate Recognition for Intelligent Transportation Systems

Deb Kaushik Graduate School, University of Ulsan 2011 국내박사

In this thesis, a Hue-Saturation-Intensity (HSI) color model is adopted to select automatically statistical threshold value for detecting candidate regions. After the candidate regions are obtained by applying color segmentation then procedures for refining the candidate region are followed by using labeling and filtering. The dissertation focuses are on the implementation of a method to detect candidate regions when vehicle bodies and LP have similar color based on characteristics of color. A common drawback of color-based VLP detection is the failure to detect the boundaries or border of LPs. The proposed method lies on a modified recursive labeling algorithm for solving this problem and detecting candidate regions. The second part is for correcting LP tilt in both the horizontal and the vertical directions. In this thesis, according to the least square fitting with perpendicular offsets (LSFPO) the VLP region is fitted to a straight line. After the line slope is obtained, rotation angle of the VLP is estimated. Then the whole image is rotated for tilt correction in horizontal direction by this angle. Tilt correction in vertical direction by reorientation of the titled LP candidate through inverse affine transformation is proposed and implemented for removing shear from the LP candidates. The third part is for decomposing candidate region, which contains a predetermined alphanumeric character, by using position in the histogram to verify and detect vehicle license plate regions. The last part is mainly focused on conducting an experiment using template matching technique to perform recognition for different types of alphanumeric characters used in Korean VLP.

Interlinking Converter Control Techniques to Improve Power Quality in Hybrid AC-DC Microgrids

판장민 Graduate School of Ulsan University 2020 국내박사

Hybrid AC-DC microgrids (HMGs) have been studied recently in order to simultaneously exploit AC and DC microgrids (MGs). This dissertation develops control techniques for interlinking converter (IC) which is utilized to link MGs in HMGs. With the proposed control techniques, IC simultaneously manages various AC and DC voltage quality issues along with power flow among MGs. In islanded HMGs, where utility grid is not considered, power quality is seriously degraded with highly distorted bus voltage when nonlinear loads are applied. In order to obtain sinusoidal bus voltage, we propose an IC control scheme which maintains bus voltage by providing harmonic currents caused by the nonlinear load. Moreover, the active power between DC and AC distributed generations (DGs) is also accurately balanced by IC. In the proposed algorithm, the desired IC current is determined by only considering the IC terminal voltage instead of the load current. Consequently, power converters in HMGs are operated autonomously with low system cost because communication links among them are not needed. When AC MG connects to the utility grid, HMGs become grid-connected HMGs. In grid-connected HMGs, we have considered different conditions such as: the abnormal grid voltage, the utilized nonlinear load and the wide variation of DC load power. A versatile IC control strategy is proposed in order to enhance HMGs power quality under such severe conditions. Without implementing any additional hardware compensator, IC with the proposed control scheme can mitigate various issues such as distortion, sag/swell and imbalance in AC voltage and DC bus voltage restoration despite of wide load power variation. Both the desired power and the current among MGs are provided by IC since IC current references are generated by only detecting buses voltages. Therefore, AC bus voltage is balanced and sinusoidal with proper magnitude while DC bus voltage is constantly kept at the nominal voltage. In grid-connected HMGs, DC bus voltage is highly ripple when single-phase inverter with variable frequencies (SPI-VF) is operated in DC MG. In order to eliminate the ripple voltage, we present an IC control strategy which can adaptively deal with ripple voltage at different frequencies by controlling IC ripple current. In the proposed approach, instead of SPI current/voltage, only DC bus voltage is required to generate the IC ripple current reference. Besides that, power among MGs is also balanced by controlling IC fundamental current. As a consequence, AC MG can support DC MG in case DC load power changes widely. And DC bus voltage is well maintained at the nominal voltage without ripple. Simulation and experimental results are carried out to validate performance of the proposed IC control techniques.

린 생산방식의 적용에 대한 연구 : 몽골 캐시미어 산업을 중심으로 = A Study on the Adoption of Lean Manufacturing Method in Mongolian Cashmere Industry

아마라 Graduate School, University of Ulsan 2011 국내석사

In the current era of globalization, industries are adopting new tools and techniques to produce goods to compete and survive in the market. The most daunting issue faced by manufacturers today is how to deliver their products or materials quickly at low cost and good quality. One promising method for addressing this issue is the application of lean manufacturing principles and techniques. “Lean manufacturing” is a leading manufacturing paradigm being applied in many sectors of many countries economy, where improving product quality, reducing production costs, and being “first to market” and quick to respond to customer needs are critical to competitiveness and success. Now, Lean manufacturing has become a widely acceptable and adoptable best manufacturing practice across countries and industries [11]. Implementing Lean in textile companies may dramatically enhance performance, leading to a breakthrough that saves time, improves quality, lowers costs, reduces inventory and lead times, and gives increased revenue [9]. But Lean manufacturing is still a new concept in Mongolia where there are not any studies that have been done in Mongolia based on Lean manufacturing implementation. Therefore, this research is to further discuss the Lean manufacturing implementation in Mongolian cashmere industries. The Mongolian cashmere industry operates within the context of the world cashmere industry which in turn operates within the context of the world garment industry. Mongolia supplies the world market with raw, semi-processed, and finished cashmere. Mongolia is the second biggest raw cashmere producer in the world after China. Most of the world's cashmere processing factories are situated in Mongolia and China. This research was intended to identify the general knowledge of Lean manufacturing within the cashmere industry. The results are based on data collected through a questionnaire from 27 cashmere producers. In the research, it was found that many cashmere producers are somewhat familiar with the Lean Manufacturing concepts, but there seems to be a lack of knowledge of how these concepts can be beneficial for this industry. There also appears to be a lack of knowledge of areas with opportunities for improvements in process performances. Though there is a fairly high level of knowledge, only 22.2% of the participants have implemented some degree of Lean Manufacturing in their operations. The main barriers of these firms are the lack of real understanding of lean manufacturing concept. The best approach for implementing Lean Manufacturing in cashmere production operations varies from company to company, depending on the amount of resources available within the company, and the management’s ability to implement changes. Regardless of the path of implementation, if tools are implemented without corresponding changes in management style, the benefits gained will not be continuous. The management needs to spend as much time in enforcing cultural changes as on implementing the actual tools.

Nd도핑Bi계의 전계인가 고변위 압전세라믹 연구 : Large Electric-Field-Induced Strain in Nb-doped lead-free Bi_(0.5)Na_(0.5)TiO_(3) piezoelectric ceramics

Nam, Pham-Ky Graduate School, University of Ulsan 2010 국내석사

Electric-filed-induced strain is one of the most important parameter among material properties for its applications in electromechanical devices. Lead oxide based piezoelectric ceramics such as PZT, possess much higher electric-field-induced strain as compared to that of lead-free piezoelectric ceramics and therefore dominate the piezoelectric market. But, they are restricted by environmental issues that call for the use of environmentally benign materials, i.e. lead-free ceramics, with comparable properties to that of the lead oxide based. Therefore, research is now focused on lead-free systems with perovskite structure, especially compositions based on bismuth sodium titanate, Bi_(0.5)Na_(0.5)TiO_(3) (BNT) and sodium potassium niobate Na_(0.5)K_(0.5)NbO_(3) (NKN). In this study, solid solutions based on bismuth sodium potassium titanate with a morphotropic phase boundary (MPB) composition, Bi_(0.5)Na_(0.5)TiO_(3) (BNKT), Niobium (Nb^(5+)) was substituted on B-site, for titanium (Ti^(4+)) in BNKT system, with an aim to enhance the electric-field-induced strain response for its applications in electromechanical devices. The phase formation, crystalline crystal, density, microstructure, dielectric, ferroelectric and electric-field-induced strain behavior of these materials were analyzed and discussed. All of the studied materials were found to have a perovskite structure. The Nb^(5+)-substituted BNKT ceramics the field-induced strain was enhanced from 0.18% to 0.46% at an applied electric fields of 70 kV/㎝. The normalized strain d_(33)* (S_(max)/E_(max)) obtained for Nb modified ceramics were 641 pm/V. This value of normalized strains were much higher than pure BNKT as well as other lead-free BNT-based materials. These results suggest that the systems BNKTN are environmentally friendly candidate material classes for application in electromechanical devices and provide an alternative way in the quest for potential lead-free actuator materials.

NANOSCALE THERMAL AND FLUID TRANSPORT PHENOMENON IN POROUS MEDIA : A MOLECULAR DYNAMICS STUDY

하산 모하마드 라세둘 Graduate School of Ulsan University 2020 국내석사

The small length scales and large specific surface areas associated with the nanostructures play a key role in the molecular level thermal and fluid transport. In such nanometer length scales, the local intermolecular interaction creates temperature discontinuities between the solid-like interfaces and their neighboring fluid molecules. This phenomenon often referred to as interfacial thermal resistance (i.e., Kapitza resistance) during nanoscale thermal transport. There is also evidence that the fluid molecules are absorbed by the wall molecules promoting structural ordering of fluid at the solid/fluid interface. The local dynamic properties of this fluid layered structure are substantially different from the fluid properties at macroscale. Therefore, the continuum transport theories break down near the material interfaces at nanoscale. In this thesis, we investigate the unique transport behaviors of fluid molecules in confining nanoenvironments using Molecular Dynamics (MD) simulations. Firstly, heat transfer across an interface between a monolayer coated solid substrate and fluid has been analyzed by varying the atomic mass (mM) and interaction energy between monolayer molecules (εMM). In that case, the mutual combination of atomic mass (mM) and interaction energy (MM) of monolayer lead to a significant influence in heat transport at the interfacial region. It was found that Kapitza resistance monotonically increases with the increase of mM irrespective of εMM without any further change in the fluid-structure near the solid surface. This indicates the vibrational coupling between the molecules at the solid/fluid interface largely depend on the mass of monolayer molecules. We also investigate the pressure-driven transport mechanism of liquid argon through nanoporous graphene membrane (NPGM) using MD simulations. In this study we check the validity and limitations of the assumptions of continuum flow equation. We present a thorough characterization of the density and pressure distribution of liquid argon based on the respective flow region to elucidate the unique fluid transport behaviors. The argon velocity adjacent to the pore edge was found lower than pore center suggesting the influence of the interaction between argon and carbon molecules at the pore boundary. In that case, we consider the argon velocity closest to the pore edge as slip velocity, which provides an update in the continuum flow equation. The local viscosity was also calculated from the thin argon film flows sheared by graphene walls. Our study shows that the entrance interfacial pressure and higher local viscosity in the vicinity of graphene membrane associated with the optimized definition of wall/fluid boundary near the pore edge play a critical role for the permeation of argon through NPGM.