• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2006년 창립20주년기념 정기학술대회 및 국제워크샵
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• pp.127-134
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• 2006

Based on a weakly non-Gaussian theory the occurrence probability of freak waves is formulated in terms of the number of waves in a time series and the surface elevation kurtosis. Finite kurtosis gives rise to a significant enhancement of freak wave generation in comparison with the linear narrow banded wave theory. For fixed number of waves, the estimated amplification ratio of freak wave occurrence due to the deviation from the Gaussian theory is 50% - 300%. The results of the theory are compared with laboratory and field data.

• 한국해안해양공학회지
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• 제19권4호
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• pp.313-319
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• 2007

장기간의 극치 및 평상시 파고는 연안 및 항만구조물의 계획 및 설계에서 매우 중요한 환경인자이다. 그러나, 한국 연안 심해파의 관측 자료가 한정되어 있기 때문에, 심해설계파의 정보는 기상정보로부터 사후추정 한 장기간의 파랑자료를 이용하고 있다. 본 연구에서는 한국해양연구원(2003)에서 제시한 1979년부터 1998년까지의 한국연안 67개 지점의 16방향별 최대 유의파 산출자료를 이용하여 극치분포 분석을 수행하였다. 특성분석에 사용된 극치분포함수는 FT-I과 Weibull 분포이며, 각 분포함수의 매개변수는 Goda(2004)의 방법을 이용하여 추정하였다. 또한 Goda and Gobune(1990)가 제안한 MIR 값을 산정하여 가장 적합한 확률분포형을 결정하였다. 분석결과 FT-I 분포가 886개 지점, Weibull(k=0.75) 분포가 81개 지점 및 Weibull(k=1.00) 분포가 105개 지점의 확률분포형으로 적합한 것으로 판단된다.

• Journal of Positioning, Navigation, and Timing
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• 제3권2호
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• pp.83-90
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• 2014

In this study, to improve an existing ground-wave based time broadcasting system, a study that predicts the field distribution and field strength of the transmitted signal of a new ground-wave based time broadcasting system was performed. The prediction area was assumed to be the Korean peninsula; and to reflect the mountainous terrain of the Korean peninsula in the prediction of the variations of field distribution and field strength, a new prediction method based on the Monteath model was proposed and utilized. As field distribution changes depending on the position of a transmitter station, potential sites for the transmitter station were selected considering the geographical characteristics. In this regard, the ground conductivity information of North Korea cannot be obtained, and thus, the ground conductivity of the North Korean region was reflected considering the geological characteristics of South Korea and North Korea. Based on this, the variations of field distribution and field strength were predicted by setting the Korean peninsula as the prediction area, and the prediction results depending on the position of the transmitter station were discussed.

• 한국해양공학회지
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• 제34권6호
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• pp.406-418
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• 2020

In this study, a nonlinear wave simulation code is developed using a higher-order spectral (HOS) method. The HOS method is very efficient because it can determine the solution of the boundary value problem using fast Fourier transform (FFT) without matrix operation. Based on the HOS order, the vertical velocity of the free surface boundary was estimated and applied to the nonlinear free surface boundary condition. Time integration was carried out using the fourth order Runge-Kutta method, which is known to be stable for nonlinear free-surface problems. Numerical stability against the aliasing effect was guaranteed by using the zero-padding method. In addition to simulating the initial wave field distribution, a nonlinear adjusted region for wave generation and a damping region for wave absorption were introduced for wave generation simulation. To validate the developed simulation code, the adjusted simulation was carried out and its results were compared to the eighth order Stokes theory. Long-time simulations were carried out on the irregular wave field distribution, and nonlinear wave propagation characteristics were observed from the results of the simulations. Nonlinear adjusted and damping regions were introduced to implement a numerical wave tank that successfully generated nonlinear regular waves. According to the variation in the mean wave steepness, irregular wave simulations were carried out in the numerical wave tank. The simulation results indicated an increase in the nonlinear interaction between the wave components, which was numerically verified as the mean wave steepness. The results of this study demonstrate that the HOS method is an accurate and efficient method for predicting the nonlinear interaction between waves, which increases with wave steepness.

• Structural Engineering and Mechanics
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• 제53권3호
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• pp.393-410
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• 2015

Two dimensional numerical models and physical models have been developed to study the highly nonlinear interactions between waves and breakwaters, but several of these models consider the effects of the structural dynamic responses and the shape of the breakwater axis on the wave pressures. In this study, a multi-material Arbitrary Lagrangian Eulerian (ALE) method is developed to simulate the nonlinear interactions between nonlinear waves and elastic seawalls on a coastal rubble mound breakwater, and is validated experimentally. In the experiment, a solitary wave is generated and used with a physical breakwater model. The wave impact is validated computationally using a breakwater - flume coupling model that replicates the physical model. The computational results, including those for the wave pressure and the water-on-deck, are in good agreement with the experimental results. A local breakwater model is used to discuss the effects of the structural dynamic response and different design parameters of the breakwater on wave loads, together with pressure distribution up the seawall. A large-scale breakwater model is used to numerically study the large-scale wave impact problem and the horizontal distribution of the wave pressures on the seawalls.

• Ocean Systems Engineering
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• 제10권1호
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• pp.25-47
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• 2020

A study of the wave conditions for the Asabo offshore location at the Qua Iboe oil field in Eastern Nigeria has been carried out. Statistical analysis was applied to three (3) years of data comprising spectral periods, Tp and significant wave heights, H_s. The data was divided into two (2); data from October to April represents one set of data and data from May to September represents another set of data. The results were compared with similar studies at other locations offshore of West Africa. It was found that there is an absence of direct swellwaves from the Southern Ocean reaching the location under study (the Asabo site). This work suggests that the wave system is largely emanating from the North Atlantic storms. The presence of numerous islands near the Asabo location shields the site from effects of storms from south west and therefore swells from the Southern Ocean. It is noted that the local wind has little or no contribution. An H_s maximum of 2 m is noted at the Asabo offshore location. It is found that the Weibull distribution best describes the wave distribution at Asabo. Thus, the Weibull distribution is suggested to be adequate for long term prediction of extreme waves needed for offshore design and operations at this location.

• 한국전기전자재료학회논문지
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• 제33권1호
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• pp.65-69
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• 2020

The aim of this study is to improve the fault decision ability of FRTU (Feeder remote terminal unit) in DAS (Distribution automation system). FRTU uses the FI (Fault indicator) algorithm based on fault current pickup and operation of the protection device. Even if the inrush current flows or the protection device is sensitive to the transient current, FRTU may indicate incorrect fault information. To address these problems, we propose an improved fault recognition algorithm that can be applied to FRTU. We will detect a specific wave that is indicative of a fault, and use this information to identify a fault wave. The specific wave-detection algorithm is based on the duration and periodicity of the voltage, current, and harmonic variations. In addition, we propose fault recognition algorithms using voltage factor variation analysis and DWT (Discrete wavelet transform). All the wave data used in this study were actual data stored in FRTU.

• 한국정보전자통신기술학회논문지
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• 제1권1호
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• pp.41-45
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• 2008

교류 전원이 인가된 절연체가 열화되면 방전에 의한 방사 전자파는 주기성을 갖는 특징적 발생시간 분포를 보인다. 이러한 분포의 특징을 최적으로 분별하기 위해 방사전자파의 수신 주파수 및 수신 대역폭을 결정하기 위한 실험을 하였다. 방사전자파의 스펙트럼은 발생대역은 넓지만 시변 특징을 가지므로 수신 주파수보다는 수신 대역폭이 수신기의 성능에 영향을 미치고 적어도 900kHz 이상의 수신 대역폭이 요구된다.

• 한국항해학회지
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• 제25권4호
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• pp.423-433
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• 2001

As a part of the development program of Ulsan Port, construction of detached breakwater is planned. In Ulsan Port, several oil-buoys exist. If the detached breakwaters have been constructed, these oil-buoys will be located within 1 km from the planned breakwaters. Construction of the breakwaters gives rise to changes of wave conditions on the sea areas, especially in front of the breakwater and it affects mooring of tankers, which supply oil to the oil-buoy In this study, in order to calculate standing wave distribution after construction of a breakwater, numerical model is proposed based on unsteady mild slope equation. Calculation is performed by testing different wave heights, directions and reflection coefficients of breakwater. In addition, the influence to working condition of tanker moored at the oil-buoy is evaluated by using measured wave conditions and calculated results.

• 한국해양공학회지
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• 제11권4호
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• pp.76-89
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• 1997

The hydrodynamic interaction characteristics between multiple floating bodies of semisubmersible type are examined to present the basic data for the design of huge offshore structures supported by a large number of the floating bodies in multi-directional irregular waves. The numerical approach is based on a combination of a three-dimensional source distribution method, the wave interaction theory and the spectral analysis method. The effects of wave directionality on the wave exciting forces acting on multiple floating bodies in multi-directional irregular waves also have been pointed out.