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.

The effect of Service Quality, Food Quality and Perceived Price on Customer Satisfaction

ERKUL ASLI Ulsan University 2009 국내석사

ABSTRACT This research examines the effects of service quality, food quality and perceived price on customer satisfaction and repurchase intention for the case of service and goods mix products. These effects are to be tested for two different service categories of the same company. One of the chain-operated pizza stores was chosen as a service. The satisfaction for restaurant service and delivery service was tested with the same sample, so as to be comparable. The relation between service quality and satisfaction, food quality and satisfaction, perceived price and satisfaction were found highly important for restaurant service of pizza store. Also, the path from satisfaction to repurchase intention was highly significant. The comparison hypotheses between restaurant service type and delivery service type, were much more informative because of novelty in the literature. The results showed that service quality has a greater effect on satisfaction for restaurant service than delivery service.

Hardware in the loop simulation for a quadrotor unmanned aerial vehicle

Nguyen, Dang Khoa Ulsan University 2018 국내박사

Assessment of Adhesion Characteristics of Thermal Barrier Coatings Using Scratch Test

웬 꽝 프엉 Ulsan University 2018 국내석사

Thermal barrier coatings (TBCs) are advanced materials usually applied to a variety of applications involving high temperature and oxidation such as gas turbines. It is commonly used as protective coatings to improve performance, protect metallic components at elevated temperature and allow higher operating temperatures beyond the limiting temperature of structural components. TBCs structure consists of four layers such as ceramic topcoat, usually 8wt% of YSZ; thin layer of thermally grown oxide (TGO) formed during the service; the metallic bond coat, usually MCrAlY and the metal substrate, usually Ni-based superalloy. The adhesion between the top coat/bond coat interface is an important property in TBCs system. The TGO formed during the service between the top coat/bond coat interface is a necessary layer of a TBC system, providing an oxidation barrier for the underlying component. Unfortunately, TGO is also the main driver of failure in TBC systems, which affects the adhesion of top coat/bond coat. However, the adhesion characteristic of TBCs are not well understood. In this work, new approaches to better understand the adhesion of TBCs by using the scratch test based on the Burnett and Rickerby model was used. The relationship between the adhesion of the top coat/bond coat and TGO formed during service was investigated. The cycle thermal fatigue test was used to reproduce the working condition of the TBCs system in the hot section. TBCs were also investigated the lifetime and failure characteristics after the thermal cycle fatigue test. The outcomes of this work may provide a useful guideline for work of adhesion characteristic of TBCs using the scratch test. In addition, the results of this work were expected to be useful to gain an in-depth understanding of the most probable combination and identifying a relationship of the work of adhesion with the thermal cycle fatigue test to use in a life prediction model.

Nearly open sets

서동진 Ulsan University 국내박사

Toeplitz operators and toeplitz algebras : Toeplitz 작용소와 Toeplitz 대수

이진이 (李鎭伊) Ulsan University 1996 국내박사

On the class of hyponormal operators : Hyponormal 작용소의 집합의 대한 연구

김국현 (金國顯) Ulsan University 1999 국내박사

Isometries of semi-Euclidean spaces and self-adjoint operators. : 준 유클리드 공간의 등장변환과 자기수반 작용소

김태환 Ulsan University 1996 국내박사

A study on star operations and related topics : Star 연산과 관련된 주제에 관한 연구

박철환 (朴哲煥) Ulsan University 1996 국내박사