Dong-Hui Ko

The estimation of probability distribution models for wave data enables an understanding of the probabilistic characteristics of wave conditions and the quantitative evaluation of engineering design variables. In this study, the optimal probability distributions were fitted using by significant wave height data observed at 20 points around the Korean coast, and the probabilistic characteristics and tail behaviors of the data were analyzed. A total of nine distribution models were considered, including the Normal, Log-normal, Weibull-3, Gamma-3, Generalized Gamma, Generalized Beta-2, Johnson SB, Generalized Hyperbolic, and Extended Generalized Inverse Gaussian distributions. The parameters for each distribution were estimated using the Maximum Likelihood Estimation (MLE) method. To evaluate the performance of each model, goodness-of-fit tests such as the Kolmogorov–Smirnov test and the Kullback–Leibler divergence metric were used, and Q-Q plots were employed to assess the fit in the tail regions of the distributions. As a result, the Generalized Hyperbolic, Johnson SB, and Generalized Gamma distributions showed high goodness-of-fit at many stations. Distributions with multiple shape parameters were found to effectively capture the asymmetry and tail characteristics of the significant wave height data. However, some distribution models tended to underestimate or overestimate the extreme quantile regions. Therefore, it is considered appropriate to select and apply the probability distribution model that best fits the wave data at each target site in order to more accurately describe the characteristics of significant wave heights.

Various mathematical models have been proposed to estimate the vertical tidal current profile. However, because the parameters of these models depend on specific site conditions such as seabed conditions and viscosity, it is recommended to estimate them using field-measured data. In particular, tidal current energy sites with fast tidal velocities exhibit different vertical tidal current profile characteristics from general sea areas, making it essential to estimate these profiles using field-measured data for practical purposes. In this study, layered tidal currents were measured over two months using an acoustic Doppler current profiler to analyze the vertical tidal current profiles at two sites in Jangjuk Strait, a candidate site for tidal current energy. Based on the field-measured data, the profiles were fitted using power law and logarithmic profile models, and the parameters were estimated for both flood and ebb tides with a velocity bin set at 1 m/s intervals. The analysis results indicated that at JSD-1, the power law parameter was estimated to be 7.50 during flood tide and 9.49 during ebb tide, with an average bed roughness of 0.42. Additionally, the roughness length, which represents seabed roughness in the logarithmic profile, was estimated at 0.022 m during flood tide and 0.011 m during ebb tide, while the friction velocity was estimated at 0.115 m/s during flood tide and 0.073 m/s during ebb tide. At JSD-2, the power law parameter was estimated at 7.40 during flood tide and 10.30 during ebb tide, with an average bed roughness of 0.41. Additionally, the roughness length was estimated at 0.020 m during flood tide and 0.005 m during ebb tide, while the friction velocity was estimated at 0.118 m/s during flood tide and 0.074 m/s during ebb tide. Finally, a correlation analysis between the parameters confirmed a positive correlation between the depth-averaged tidal current velocities and friction velocities. Tidal current energy sites display characteristics different from other sea areas; hence, it is necessary to consider field conditions when analyzing layered tidal characteristics or conducting numerical ocean modeling.

요 지 : 본 연구에서는 서해안 해상기상타워 2기에서 관측된 수면변동자료를 이용하여 해양특성을 구분하는 주요 인자인 첨두주기 T p 와 평균주기 T 02 와 T m1, 0 를 산정하고 이상자료의 비율, 상관관계 분석 및 최적 확률밀도함수를 추정하였다. 산정된 대표주기 중 첨두주기의 경우, 이상 자료의 비율은 각각의 지점에서 5.73 %, 0.67 %로 나타났 으며, T 02 는 4.35%, 0.01%, T m1, 0 는 2.82%, 0.03%로 나타났다. 한편, T 02 와 T p 사이의 관계를 분석한 결과 각 지점 별로 0.53, 0.63의 관계로 산정됐으며, T m1, 0 와 T p 의 관계는 각각 1.15, 1.32로 나타났다. T 02 와 T m1, 0 는 서로 1.18, 1.22의 관계를 보이고 있었다

In this study, a smoothing method was applied to improve the accuracy of peak wave period estimation using water surface elevation data collected from two oceanographic and meteorological observation towers situated on the western coast of the Korean Peninsula. The effectiveness of the smoothing method was validated by assessing the surface elevation variance and total wave energy. Subsequently, we examined the impact of the smoothing application. The results of our analysis revealed a reduction of approximately 27% in comparison to the confidence interval of the initially estimated spectrum upon applying the smoothing method. Furthermore, a slight variation of approximately 1–2 s was observed between the peak wave period calculated from the estimated spectrum and that obtained through the smoothing process. Utilizing statistical methods to determine the optimal smoothing parameter count, we observed an increasing trend as the range of significant wave heights decreased.

As problems such as difficulties in securing stable energy resources and global warming due to the emission of greenhouse gases due to the use of fossil fuels have emerged, interest in the development of renewable energy is increasing. Since the tidal phenomenon has a regularity that occurs regularly with a certain period, it is possible to predict accurately in advance, which has a advantage in terms of energy recovery. Therefore, various methods have been devised to utilize the tide as an energy source. Tidal power using barrages is a representative method that is widely operated, but the promotion of tidal power generation projects is being delayed or stopped due to the decrease in the level of water in the tidal basin, changes in water quality and in the ecosystem. In this study, a field experiment was conducted to develop and verify the performance of a tidal power device applicable to sea areas where dykes are already installed. As a result of carrying out four cases of experiments using two water tanks, pipe lines, open channels, weirs, and water turbine and generator, the possibility of developing a power generation system capable of 10 kW output or more and 60% efficiency or more was confirmed. These research results can be used for small-scale tidal power by utilizing the existing dykes.

Lee, U.-J.; Cho, H.-Y.; Lee, B.W., and Ko, D.-H., 2023. Joint probability distribution of significant wave height and peak wave period using Gaussian copula method. In: Lee, J.L.; Lee, H.; Min, B.I.; Chang, J.-I.; Cho, G.T.; Yoon, J.S., and Lee, J. (eds.), Multidisciplinary Approaches to Coastal and Marine Management. Journal of Coastal Research, Special Issue No. 116, pp. 96-100. Charlotte (North Carolina), ISSN 0749-0208. In this study, the joint probability distribution was estimated using the significant wave height-peak wave period data observed for 3 years for 3 stations along the coast of Korea, and an environmental contour line drawing was performed. For accurate estimation, an optimal probability distribution model was calculated for each wave parameter, and the Kolmogorov-Smirnov test and Kullback-Leibler divergence were used for the distribution fit test. Using each estimated optimal probability distribution, a Gaussian copula function was applied considering the correlation between the two wave parameters. As a result of the analysis, it was found that the significant wave height was suitable for the ‘Log-normal’ and ‘3-P Weibull’ distributions, and the peak wave period was suitable for the ‘Log-normal’ distribution. Then, as a result of correlation analysis between significant wave height and peak wave period at all points, it was confirmed that there was a significant correlation. As a result of performing environmental contour line drawing through this, observation data exceeding 3-year recall were found to be less than 1.0% of the total data. In addition, the environmental contour line calculated through the Gaussian Copula showed a limit to reproduce the area where no wave exists as a concave tendency appeared.

In this study, the two horizontal shroud tidal current energy converter, which can generate power even under low flow speed conditions, was developed. In order to determine the shape of the shroud system, a three-dimensional numerical simulation test was conducted, and a 1/6 scale down model was made to perform a hydraulic model experiment. The hydraulic model experiment was performed under four flow conditions, and the flow speed, torque, and RPM were measured for each experimental case. As a result of the numerical simulation test, it was found that the flow speeds passing through the nozzle were increased by about 2~3 times in the cylinder, and when the extension ratio was 2:1, the highest flow speed was shown. In addition, it was found that the flow speeds increased 2.8 times when the diameter ratio between the nozzle and the cylinder was 1.5:1. Meanwhile, as a result of the hydraulic model experiment, it was found that when the tip speed ratio was between 1.75 and 2, the power coefficient was 0.32 to 0.34.

본 연구에서는 조류발전시스템의 설계근거(design basis) 작성을 위해 2003년부터 2019년까지 장죽수도 인근해역의 장기 파랑 후측 자료를 이용하여 극치파랑조건을 분석하였다. 장기파랑자료의 극치모델로는 Weibull-3 분포형을 사용하였으며, 매개변수 추정법으로는 확률가중모멘트법과 최소자승법을 사용하였다. 또한 극치분포함수의 적합도 검정을 위해 95%의 신뢰수준으로 K-S 검정법(Kolmogorov-Smirnov test)을 사용하였으며, POT(Peaks-Over-Threshold) 기법을 이용하여 재현기간별 설계파고를 산정하였다. 그 결과, 파랑 후측 자료는 Weibull-3 분포형에 잘 접합하는 것으로 나타났으며, 확률가중모멘트법을 이용하여 추정한 극치파고가 최소자승법으로 추정한 극치파고보다 높게 산정되었다. 또한 재현주기별 극치파고에 상응하는 파주기를 산정하기 위해 파고와 파주기에 대한 관계식을 추정하였다.

As global energy consumption continues to increase, the negative impact of global warming also grows. Therefore, eco-friendly energy policy is being established all over the world. Korea’s energy consumption problems are further complicated by the country’s high dependence on energy sourced overseas. Korean energy policy is evolving rapidly to address these problems. Korea has begun to phase out nuclear power and is focusing on developing new sources of renewable energy. So there has been substantial interest in the development of ocean energy. Of all ocean energy technologies, tidal current energy is the closest to the commercialization. Especially, the southwestern coast of Korea is the most promising candidate site for the development of tidal current energy owing to the abundant tidal current energy resource. Meanwhile, ocean energy policy is an important factor in determining the development of ocean energy. Thus, this paper presents the overview of the current status of policy and technology for the tidal current energy system in Korea. These policies explained in this paper can provide significant interest and motivation for the use of tidal current energy resources.