• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.10
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• pp.7189-7195
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• 2015

The method of moments (MoM) is implemented to simulate scattering from a PEC (perfectly electric conductor) cylinder in the TE(transversw electric) EFIE (Electric Field Integral Equation) approach. The procedure expresses the singularity integral and the hypersingularity integral in terms of an analytic function and employs a singularity isolation process coupled with numerical technique along the discretized segment to evaluate the self terms. It is known that, in the MoM technique, the choice of base functions and test functions is very important for the accuracy and convergence of the numerical analysis. Thus, in this paper, three conditions, obtained from the combination of basis functions and test functions, are adopted to get the induced currents on the PEC surface. These currents are compared to the analytical one in the relative rms current error to get the condition that shows fast convergence rate. The fast order of convergence of the current error, 2.528, is obtained under the combination of pulse basis function/delta test function.

• Wind and Structures
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• v.11 no.2
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• pp.97-120
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• 2008

Wind and wave loadings have a predominant role in the design of offshore structures in general, and articulated tower in particular for a successful service and survival during normal and extreme environmental conditions. Such towers are very sensitive to the dynamic effects of wind and wind generated waves. The exposed superstructure is subjected to aerodynamic loads while the submerged substructure is subjected to hydrodynamic loads. Articulated towers are designed such that their fundamental frequency is well below the wave frequency to avoid dynamic amplification. Dynamic interaction of these towers with environmental loads (wind, waves and currents) acts to impart a lesser overall shear and overturning moment due to compliance to such forces. This compliancy introduces geometric nonlinearity due to large displacements, which becomes an important consideration in the analysis of articulated towers. Prediction of the nonlinear behaviour of these towers in the harsh ocean environment is difficult. However, simplified realistic mathematical models are employed to gain an important insight into the problem and to explore the dynamic behaviour. In this paper, various modeling approaches and solution methods for articulated towers adopted by past researchers are reviewed. Besides, reliability of articulation system, the paper also discussed the design, installation and performance of articulated towers around the world oceans.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.13 no.2
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• pp.109-121
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• 2001

This paper describes results of an experimental study to examine the effect of a submerged horizontal breakwater to sea water exchange. Flow measurements were taken by using a PIV(Particie Image Velocimetry) system, and mean currents and wave ellipses extracted through the harmonic analysis are presented. As results, the rates of circulating flow were closely connected with the volume flux of incident waves and the counter-rotating vortex pair was observed at the onshore side of a plate. The dye study showed that incoming sea water and polluted water body mixed up significantly due to turbulent motions induced by a jet-like flow.

• Smart Structures and Systems
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• v.16 no.4
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• pp.667-681
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• 2015

The scour induced by strong currents and wave action decreases the embedded length of monopiles and leads to a decrease of their structural stability. The objective of this study is the development and consideration of scour-monitoring techniques for offshore monopile foundations. Tests on physical models are carried out with a model monopile and geo-materials prepared in a cylindrical tank. A strain gauge, two coupled ultrasonic transducers, and ten electrodes are used for monitoring the scour. The natural frequency, ultrasonic reflection images, and electrical resistivity profiles are obtained at various scour depths. The experimental results show that the natural frequency of the model monopile decreases with an increase in the scour depth and that the ultrasonic reflection images clearly detect the scour shape and scour depth. In addition, the electrical resistivity decreases with an increase in scour depth. This study suggests that natural frequency measurement, ultrasonic reflection imaging, and electrical resistivity profiling may be used as effective tools to monitor the scour around an offshore monopile foundation.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.9 no.4
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• pp.201-207
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• 1997

Most of small harbours are suffering serious harbour siltation problems in the eastern coast of Korea. Also, many of them necessitate maintenance dredging every year. In order to solve these problems, we have to predict the amount of previous harbour siltation. In the present study, numerical prediction of the harbour siltation has been accomplished using numerical model of 3D beach deformation around a structure. And, also the validity of the model has been confirmed by the field investigation.

• Proceedings of the KSRS Conference
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• 2004.10a
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• pp.689-692
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• 2004

Hydrographic data obtained from several ship experiments have shown that internal waves are frequently observed in the East (Japan) Sea, mostly in the continental slopes located along the western side of the East Sea. It is well known that oceanic internal waves can be detected well in synthetic aperture radar (SAR) images. Interactions between surface capillary-gravity waves and horizontally varying surface currents induced by internal waves produce variations in sea surface roughness which can be detected by SAR. C-band SAR images from ERS, ENVISAT ASAR and RADARSAT have been used to study the characteristics of internal waves in the East Sea. The observed properties of internal waves from many SAR images were compared and verified from in-situ measurements and theories.

• Ocean and Polar Research
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• v.43 no.4
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• pp.307-320
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• 2021

Udo Rhodolith Beach is a small-scale, mixed sand-and-gravel beach embayed on the N-S trending rocky coast of Udo, Jeju Island, South Korea. This study analyzes the short-term topographic changes of the beach during the extreme storm conditions of four typhoons from 2016 to 2020: Chaba (2016), Soulik (2018), Lingling (2019), and Maysak (2020). The analysis uses the topographic data of terrestrial LiDAR scanning and drone photogrammetry, aided by weather and oceanographic datasets of wind, wave, current and tide. The analysis suggests two contrasting features of alongshore topographic change depending on the typhoon pathway, although the intensity and duration of the storm conditions differed in each case. During the Soulik and Lingling events, which moved northward following the western sea of the Jeju Island, the northern part of the beach accreted while the southern part eroded. In contrast, the Chaba and Maysak events passed over the eastern sea of Jeju Island. The central part of the beach was then significantly eroded while sediments accumulated mainly at the northern and southern ends of the beach. Based on the wave and current measurements in the nearshore zone and computer simulations of the wave field, it was inferred that the observed topographic change of the beach after the storm events is related to the directions of the wind-driven current and wave propagation in the nearshore zone. The dominant direction of water movement was southeastward and northeastward when the typhoon pathway lay to the east or west of Jeju Island, respectively. As these enhanced waves and currents approached obliquely to the N-S trending coastline, the beach sediments were reworked and transported southward or northward mainly by longshore currents, which likely acts as a major control mechanism regarding alongshore topographic change with respect to Udo Rhodolith Beach. In contrast to the topographic change, the subaerial volume of the beach overall increased after all storms except for Maysak. The volume increase was attributed to the enhanced transport of onshore sediment under the combined effect of storm-induced long periodic waves and a strong residual component of the near-bottom current. In the Maysak event, the raised sea level during the spring tide probably enhanced the backshore erosion by storm waves, eventually causing sediment loss to the inland area.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.3
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• pp.595-605
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• 1994

The field observations. data analyses and numerical experiments are performed to investigate the short and long term beach deformation mechanisms in Haeundae beach. The schematic diagrams of deposition and erosion mechanism due to the attack of typhoons are described from the analysis on the beach widths and profiles. The short term beach deformation depends strongly on the characteristics of incident waves and wave-induced currents. The main incident wave and the calibration parameters of the shoreline change model are determined using the beach width data. Beacause the main incident wave approaches obliquly from the SE direction, the net westward longshore sediment transport occurs. Therefore the unbalance of longshore sediment budget in the east of the beach where the sediment source dose not exist causes a beach erosion. On the other hand, the deposited sand in the west is lost offshore by the storm wave action.

• Journal of Fisheries and Marine Sciences Education
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• v.21 no.2
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• pp.291-305
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• 2009

As you known well, Ulsan port is very famous for handling chemical products which occupies about 80% of quantities of all Korean ports. Many ship's operators prefer to handle liquid cargo es at proper anchorages instead of the berth for saving port expenses. Ulsan M-10 anchorage was assigned for handling liquid cargoes, however this anchorage's space is restricted by the oil pipeline which lays under seabed about 400m off from the center of M-10 anchorage, for which we have to consider of the external force and counter force for keeping the safety of anchoring. Where, external force is induced by wind, tidal currents and wave while counter force is induced by holding power of anchor/chain. In this study, author evaluated a method to analyze theoretically the limit of external force condition up to which an anchoring ship can keep her position without dragging, and for which applied to many kinds of combined Ships as mother ship of 50,000 DWT Tanker and 4 sizes of Tanker as alongsided ship.

• Ocean Systems Engineering
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• v.6 no.1
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• pp.23-60
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• 2016

The major objective of this study was to develop further understanding of 3D nearshore hydrodynamics under a variety of wave and tidal forcing conditions. The main tool used was a comprehensive 3D numerical model - combining the flow module of Delft3D with the WAVE solver of XBeach - of nearshore hydro- and morphodynamics that can simulate flow, sediment transport, and morphological evolution. Surf-swash zone hydrodynamics were modeled using the 3D Navier-Stokes equations, combined with various turbulence models (${\kappa}-{\varepsilon}$, ${\kappa}-L$, ATM and H-LES). Sediment transport and resulting foreshore profile changes were approximated using different sediment transport relations that consider both bed- and suspended-load transport of non-cohesive sediments. The numerical set-up was tested against field data, with good agreement found. Different numerical experiments under a range of bed characteristics and incident wave and tidal conditions were run to test the model's capability to reproduce 3D flow, wave propagation, sediment transport and morphodynamics in the nearshore at the field scale. The results were interpreted according to existing understanding of surf and swash zone processes. Our numerical experiments confirm that the angle between the crest line of the approaching wave and the shoreline defines the direction and strength of the longshore current, while the longshore current velocity varies across the nearshore zone. The model simulates the undertow, hydraulic cell and rip-current patterns generated by radiation stresses and longshore variability in wave heights. Numerical results show that a non-uniform seabed is crucial for generation of rip currents in the nearshore (when bed slope is uniform, rips are not generated). Increasing the wave height increases the peaks of eddy viscosity and TKE (turbulent kinetic energy), while increasing the tidal amplitude reduces these peaks. Wave and tide interaction has most striking effects on the foreshore profile with the formation of the intertidal bar. High values of eddy viscosity, TKE and wave set-up are spread offshore for coarser grain sizes. Beach profile steepness modifies the nearshore circulation pattern, significantly enhancing the vertical component of the flow. The local recirculation within the longshore current in the inshore region causes a transient offshore shift and strengthening of the longshore current. Overall, the analysis shows that, with reasonable hypotheses, it is possible to simulate the nearshore hydrodynamics subjected to oceanic forcing, consistent with existing understanding of this area. Part II of this work presents 3D nearshore morphodynamics induced by the tides and waves.