• Journal of the Korean Institute of Navigation
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• v.13 no.2
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• pp.37-55
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• 1989

When a ship is sailing on the sea, she has the six-degrees of freedom of motion. It means that she meets a lot of dangerous situations. Especially, when the VLCC is travelling in irregular sea, the slamming, the deck-wetness and the propeller racing are occured with the sea state she is on. These are the representative steps that a heave-to and a scudding are used for a ship building , but for a predominance in both. The author intends to clarify this problem theoretically. The methods of statistical calculation are based with the ITTC spectral formulation and with the assumption that the wave height histogram follows the Rayleigh distribution. In this study, the author gives an attention on the relative bow motion to a wave in the irregular sea. It is verified that the relative diplacement at the bow to sea level in the following sea is less than that in the head sea. It is confirmed that, therefore, one have to sail with scudding when he is threatened to heave-to at a rough sea. But he must bear the propeller racing in mind in the cases.

• Journal of Ocean Engineering and Technology
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• v.21 no.1 s.74
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• pp.7-17
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• 2007

Based on the Boussinesq wave model, the wave distribution in the Chagui-Do sea area in Jeju was simulated by applying the directional irregular waves at an incident boundary. The time and spatial variations of monthly mean wave height and period were investigated, which aims to provide basic information on optimal sites for wave power generation. The grid size and time interval of the Boussinesq wave model were validated by examining wave distributions around a surface piercing wall, fixed at sea bottom with a constant slope. Except for the summer season, the significant wave height is dominated by wind waves and appears to be relatively high at the north sea of Chagui-Do, which is open to the ocean, while it is remarkably reduced at the rear sea of Chagui-Do because of its blocking effect on incident waves. In the summer, the significant wave height is higher at the south sea, and it is dominated by the swell waves, which is contributed by the strong south-west wind. The magnitude of significant wave height is the largest in the winter and the lowest in the spring. Annual average of the significant wave height is distinctively high at the west sea close to the Chagui-Do coast, due to a steep variation of water depth and corresponding wave focusing effect. The seasonal and spatial distribution of the wave period around Chagui-Do sea reveals very similar characteristics to the significant wave height. It is suggested that the west sea close to the Chagui-Do coast is the mast promising site for wave power generation.

• International Journal of Naval Architecture and Ocean Engineering
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• v.11 no.1
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• pp.409-421
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• 2019

Investigation of hydroelastic responses of high-speed vessels in irregular sea state is of major interest in naval applications. A three dimensional nonlinear time-domain hydroelastic method in oblique irregular waves is developed, in which the nonlinear hydrostatic restoring force caused by instantaneous wetted surface and slamming force are considered. In order to solve the two technical problems caused by irregular sea state, the time-domain retardation function and Proportional, Integral and Derivative (PID) autopilot model are applied respectively. Besides, segmented model tests of a high-speed trimaran in oblique waves are performed. An oblique wave testing system for trimarans is designed and assembled. The measured results of main hull and cross-decks are analyzed, and the differences in distribution of load responses between trimarans and monohull ships are discussed. Finally, from the comparisons, it is confirmed that the present concept for dealing with nonlinear hydroelastic responses of ships in oblique irregular waves is reliable and accurate.

• Proceedings of KOSOMES biannual meeting
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• 2003.05a
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• pp.31-40
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• 2003

There are winds and waves in the sea, and they are changed frequently in accordance with the weather. By analyzing them which have the closest relation to the ship's safe voyage. evaluating the seakeeping performance and then taking a proper action, navigators should carry out safe navigation on the sea. A ship in seaways suffers continuous disturbances by irregular waves, and ship motions with irregular waves cannot be easily described as a system model which is adequate to a control system. But, in general, for seakeeping analysis, ship motions in irregular seas can be estimated by the superposition of the motion responses in regular wave components of the sea spectrum. After comparing and analyzing the winds and waves in major sea areas, this paper evaluates the navigational safety of ships on the Korean coast with potential dangerous seakeeping performance using the weather information provided by land. The conclusion is as follows : (1) It is possible that the safety of ships could be secured more accurately by evaluating the seakeeping performance of ships. (2) When the weather is bad, the departure of ships could be controlled by evaluating the navigational safety of ships. (3) When a ship is placed in commission in any area, this evaluation could be used to decide the type and size of ship in use.

• Ocean Systems Engineering
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• v.10 no.2
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• pp.131-161
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• 2020

The objective of this study was to evaluate the downtime cost of side-by-side offloading operations in Malaysian waters. With the help of a numerical time domain tool, the structure and cable response of moored FPSO vessel was simulated for heading and beam sea-states under irregular waves. The weather downtime was assessed by comparing the response under operational wave condition with the pre defined industrial safe offloading criteria. Additionally, two cases of cable failure were simulated for each sea-state. The novel study on downtime cost was presented for three different location of Malaysia subcontinent for which the location specific wave scatter diagram facilitated to estimate the probability of occurrence of operational wave condition. It was concluded that an unpredictable increment in wave height by 0.5 m can significantly impact the production cost.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2003.05a
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• pp.105-108
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• 2003

The local motion at the top of an A-frame fixed on a research vessel for deep sea ROV floating in irregular waves is studied in the time-domain. The motion is analyzed in the time-domain using the convolution integral of the radiation forces. The memory effect functions and infinite frequency added masses are obtained from the solution of the three dimensional improved Green integral equation in the frequency domain by making use of the Fourier transformation.

• Journal of the Society of Naval Architects of Korea
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• v.42 no.3
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• pp.212-219
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• 2005

Real sea waves in a towing wave basin have been generated using random periodic motion of the segmented wave makers and the wave reflections of sidewalls. Theoretically, the real sea waves can be described by the superposition of many random oblique waves. This paper introduces numerical real sea wave generation in a rectangular wave basin using spectral method that uses a superposition of orthogonal functions which have to satisfy the Laplace equation. Oblique regular waves, long crested irregular waves and real sea waves were simulated and met the requirement of sidewall wave reflection and wave absorption. MLM (Maximum Likelihood Method) and Spatial Fourier Transform were used in order to obtain propagated wave direction characteristics. The estimated results proved the usefulness of the method and the performances showed reasonable directional patterns comparing with generating patterns.

• Ocean Systems Engineering
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• v.12 no.1
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• pp.23-38
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• 2022

In this paper, a pitch-type wave energy converter (WEC-rotor) is investigated in irregular wave conditions for the real sea testing at the west coast of Jeju Island, South Korea. The present research builds on and extends our previous work on regular waves to irregular waves. The hydrodynamic characteristics of the WEC-rotor are assessed by establishing a quasi-two-dimensional numerical wave tank using computational fluid dynamics by solving the Reynolds-averaged Navier-Stokes equation. The numerical solution is validated with physical experiments, and the comparison shows good agreement. Furthermore, the hydrodynamic performance of the WEC-rotor is explored by investigating the effect of the power take-off (PTO) loading torque by one-way and two-way systems, the wave height, the wave period, operational and high sea wave conditions. Irrespective of the sea wave conditions, the absorbed power is quadratic in nature with the one-way and two-way PTO loading systems. The power absorption increases with the wave height, and the increment is rapid and mild in the two-way and one-way PTO loading torques, respectively. The pitch response amplitude operator increases as the wave period increases until the maximum value and then decreases. For a fixed PTO loading, the power and efficiency are higher in the two-way PTO loading system than in the one-way PTO loading system at different wave periods.

• International Journal of Naval Architecture and Ocean Engineering
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• v.8 no.1
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• pp.13-21
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• 2016

Parametric pitch instability of a Deep Draft Semi-submersible platform (DDS) is investigated in irregular waves. Parametric pitch is a form of parametric instability, which occurs when parameters of a system vary with time and the variation satisfies a certain condition. In previous studies, analyzing of parametric instability is mainly limited to regular waves, whereas the realistic sea conditions are irregular waves. Besides, parametric instability also occurs in irregular waves in some experiments. This study predicts parametric pitch of a Deep Draft Semi-submersible platform in irregular waves. Heave motion of DDS is simulated by wave spectrum and response amplitude operator (RAO). Then Hill equation for DDS pitch motion in irregular waves is derived based on linear-wave theory. By using Bubnov-Galerkin approach to solve Hill equation, the corresponding stability chart is obtained. The differences between regular-waves stability chart and irregular-waves stability chart are compared. Then the sensitivity of wave parameters on DDS parametric pitch in irregular waves is discussed. Based on the discussion, some suggestions for the DDS design are proposed to avoid parametric pitch by choosing appropriate parameters. The results indicate that it's important and necessary to predict DDS parametric pitch in irregular waves during design process.

• International Journal of Naval Architecture and Ocean Engineering
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• v.8 no.4
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• pp.409-421
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• 2016

Container ships are prone to move at a greater speed compared to other merchant ships. The slenderness of the hull of container vessel is for better speed, but it leads to unfavorable motions. The pitch and roll are related and sometimes the vessel might be forced to parametric roll condition which is very dangerous. A fin attached to the ship hull proves to be more efficient in controlling the pitch. The fin is fitted at a lowest possible location of the hull surface and it is at the bow part of the ship. Simulations are done using proven software package ANSYS AQWA and the results are compared. Simulations are done for both regular and irregular seas and the effect of fin on ship motion is studied. P-M spectrum is considered for various sea states.