• 한국신재생에너지학회:학술대회논문집
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• 2007.06a
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• pp.423-426
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• 2007

Pohang Wind Energy Research Center (PoWER-C) is developing a 3 MW Radial Flux Permanent Magnet (RFPM) Synchronous Generator for offshore Wind Energy Converter (WEC). The rotor rpm is 15.7 and the gear ratio is set to be 92.93. The nominal generator rpm at the rated load is about 1459. To reduce the switching loss in the power electronics, the maximum frequency is limited to 100 Hz. This requirement limits the number of pole to six or eight. Permanent magnet excitation is assumed for higher energy yield and higher efficiency. In this report, the requirements and the first efforts for the physics design are described.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.31 no.6
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• pp.423-433
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• 2019

In order to predict the seabed topography change due to the construction of offshore wind power structures in the west-southern sea of Korea, field observations for tides, tidal currents, suspended sediment concentrations and seabed sediments were carried out at the same time. These data could be used for numerical simulation. In numerical experiments, the empirical constants for the suspended sediment flux were determined by the trial and error method. When a concentration distribution factor was 0.1 and a proportional constant was 0.05 in the suspended sediment equilibrium concentration formulae, the calculated suspended sediment concentrations were reasonably similar with the observed ones. Also, it was appropriate for the open boundary conditions of the suspended sediment when the south-east boundary corner was 11.0 times, the south-west was 0.5 times, the westnorth 1.0 times, the north-west was 1.0 times and the north-east was 1.0 times, respectively, using the time series of the observed suspended sediment concentrations. In this case, the depth change was smooth and not intermittent around the open boundaries. From these calibrations, the annual water depth change before and after construction of the offshore wind power structures was shown under 1 cm. The reason was that the used numerical model for the large scale grid could not reproduce a local scour phenomenon and they showed almost no significant velocity change over ± 2 cm/s because the jacket structures with small size diameter, about 1 m, were a water-permeable. Therefore, it was natural that there was a slight change on seabed topography in the study area.

• Journal of Advanced Marine Engineering and Technology
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• v.39 no.9
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• pp.947-952
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• 2015

To supplement energy needs and take advantage of renewable energy sources, many wind farms are currently being built in mountainous areas under the supervision of the Korean government. However, operation of these wind farms can cause serious threats to national security due to Doppler modulation from the wind turbines causing interference with military radar operating in the vicinity. Therefore it is necessary to develop methods to analyze the Doppler frequency during the operation of wind turbines and the effect on radar signals. Based on modeling of the mountainous region, blockage analysis, turbine motion and the radar signals, this paper proposes a signal processing method to extract and analyze the Doppler frequency. Simulation results showed the change of Doppler frequency over time caused by the geometry of the mountainous area and the wind turbine.

• Geomechanics and Engineering
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• v.37 no.4
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• pp.383-393
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• 2024

Monopile foundations of offshore wind turbines embedded in soft clay are subjected to the long-term cyclic lateral loads induced by winds, currents, and waves, the vibration of monopile leads to the accumulation of pore pressure and cyclic strains in the soil in its vicinity, which poses a threat to the safety operation of monopile. The researchers mainly focused on the hysteretic stress-strain relationship of soft clay and kinds of stiffness degradation models have been adopted, which may consume considerable computing resources and is not applicable for the long-term bearing performance analysis of monopile. In this study, a modified cyclic stiffness degradation model considering the effect of plastic strain and pore pressure change has been proposed and validated by comparing with the triaxial test results. Subsequently, the effects of cyclic load ratio, pile aspect ratio, number of load cycles, and length to embedded depth ratio on the accumulated rotation angle and pore pressure are presented. The results indicate the number of load cycles can significantly affect the accumulated rotation angle of monopile, whereas the accumulated pore pressure distribution along the pile merely changes with pile diameter, embedded length, and the number of load cycles, the stiffness of monopile can be significantly weakened by decreasing the embedded depth ratio L/H of monopile. The stiffness degradation of soil is more significant in the passive earth pressure zone, in which soil liquefaction is likely to occur. Furthermore, the suitability of the "accumulated rotation angle" and "accumulated pore pressure" design criteria for determining the required cyclic load ratio are discussed.

• Journal of the Society of Naval Architects of Korea
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• v.60 no.5
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• pp.375-387
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• 2023

The subsea power cables are increasingly important for harvesting renewable energies as we develop offshore wind farms located at a long distance from shore. Particularly, the continuous flexural motion of inter-array dynamic power cable of floating offshore wind turbine causes tremendous fatigue damages on the cable. As the subsea power cable consists of the helical structures with various components unlike a mooring line and a steel pipe riser, the fatigue analysis of the cables should be performed using special procedures that consider stick/slip phenomenon. This phenomenon occurs between inner helically wound components when they are tensioned or compressed by environmental loads and the floater motions. In particular, Vortex-induced vibration (VIV) can be generated by currents and have significant impacts on the fatigue life of the cable. In this study, the procedure for VIV fatigue analysis of the dynamic power cable has been established. Additionally, the respective roles of programs employed and required inputs and outputs are explained in detail. Demonstrations of case studies are provided under severely sheared currents to investigate the influences on amplitude variations of dynamic power cables caused by the excitation of high mode numbers. Finally, sensitivity studies have been performed to compare dynamic cable design parameters, specifically, structural damping ratio, higher order harmonics, and lift coefficients tables. In the future, one of the fundamental assumptions to assess the VIV response will be examined in detail, namely a narrow-banded Gaussian process derived from the VIV amplitudes. Although this approach is consistent with current industry standards, the level of consistency and the potential errors between the Gaussian process and the fatigue damage generated from deterministic time-domain results are to be confirmed to verify VIV fatigue analysis procedure for slender marine structures.

• Structural Engineering and Mechanics
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• v.59 no.3
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• pp.559-577
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• 2016

There are many theoretical analyses and experimental studies of the hydrodynamics for the tension leg platform (TLP) of a floating wind turbine. However, there has been little research on the arrangement of the TLP's internal structure. In this study, a TLP model and a 5-MW wind turbine model as proposed by the Minstitute of Technology and the National Renewable Energy Laboratory have been adopted, respectively, to comprehensively analyze wind effects and wave and current combinations. The external additional coupling loads on the TLP and the effects of the loads on variables of the internal structure have been calculated. The study investigates preliminary layout parameters-namely, the thickness of the tension leg body, the contact mode of the top tower on the tension leg, the internal stiffening arrangement, and the formation of the spoke structure-and conducts sensitivity analyses of the TLP internal structure. Stress is found to be at a maximum at the top of the tension leg structure and the maximum stress has low sensitivity to the load application point. Different methods of reducing maximum stress have been researched and analyzed, and the effectiveness of these methods is analyzed. Filling of the spoke structure with concrete is discussed. Since the TLP structure for offshore wind power is still under early exploration, arrangements and the configuration of the internal structure, exploration and improvements are ongoing. With regard to its research and analysis process, this paper aims to guide future applications of tension leg structures for floating wind turbine.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.20 no.5
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• pp.615-621
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• 2007

Wind power generation is one of the promising gateways that will solve the energy crisis in the future. The wind power generator studied so far is limited to static interpretation in the areas related to tower. This study broadly sets the form of tower as tubular and jacket, identifies the characteristics of each and aims to find and apply their trend to in actual design and manufacturing process. This paper identified the resonance frequency of tower at each mode and studied their features. Furthermore, this study identified the characteristics of the load that occurs in operation and the effect of additional mass incurring when installed in sea, and it compared the two types of tower and was able to predict their trend.

• 한국태양에너지학회:학술대회논문집
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• 2008.04a
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• pp.348-358
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• 2008

Unlike structures in the air, the vibration analysis of a submerged or floating structure such as offshore structures is possibly only when the fluid-structures is understood, as the whole or part of the structure is in contact with water. Through the comparision between the experimental result and the finite element analysis result for a simple cylindrical model, it was verified that an added mass effects on the cylindrical structure. Using the commercial FEA program ANSYS(v.11.0), underwater added mass was superposed on the mass matrix of the structure. A frequency response analysis of forced vibration in the frequency considered the dynamic load was also performed. It was proposed to find the several important modes of resonance peak for these fixed cylindrical type structures. Furthermore, it is expected that the analysis method and the data in this study can be applied to a dynamic structural design and dynamic performance evaluation for the ground and marine purpose of power generator by wind.

• Journal of Advanced Marine Engineering and Technology
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• v.31 no.8
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• pp.1020-1027
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• 2007

As a national enterprise has been expanded over and over, the worldwide energy consumption has been growing necessarily. Moreover, as recently energy spendings are on the increase in countries such as BRICs, it has resulted that a rise in the price of both oil and mineral resources and instability between supply and demand become serious issue in the world resources market. The recent high price of oil and mineral resources have a deep influence on economy and threaten energy security and even national prosperity of Korea. In addition to these, exhaustion of fossil fuels and the enhanced greenhouse effect which results from gases emitted as a result of fossil fuels has been in serious questions which occur a great deal of effort to secure clean energy resources all around the world. As it is considerably possible for Korea that the Kyoto protocol may come into effect on and after 2013, it is essential to require the technological development to promote energy efficiency as well as to develope safe and renewable energy resources. The wind energy technology which converts kinetic energy into electrical energy has been in the focus of the world's attention. In this study, two-dimensional numerical analyses were conducted to observe subsidence aspects of the sea bottom on differently applied loads and various ground conditions.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2023.05a
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• pp.191-192
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• 2023

In recent years, the use of high-strength grout has gained popularity in offshore wind power generation complexes for facility foundations and bridges. These marine wind farms require support for horizontal loads from wind and waves. To ensure the strength of the grout produced in environments similar to the actual placing site, this study investigated the curing of high-strength grout discharged through pump pressure in various environments, and examined the difference in strength according to different variables. Compressive strength measurements revealed that the core specimen collected from the bottom (3cm) and uppermost (50cm) of the specimen exhibited lower strength compared to other height specimens, while the core specimen obtained from the corner exhibited lower strength compared to the center. These findings suggest that the strength difference between the center and the corner is more pronounced when curing at low temperatures. This effect is greater than the strength reduction that typically occurs during low-temperature curing, and thus, necessitates careful attention in similar construction environments.