• Ocean Systems Engineering
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• v.7 no.1
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• pp.53-74
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• 2017

Ship shaped FPSO (Floating Production, Storage and Offloading) units are the most commonly used floating production units to extract hydrocarbons from reservoirs under the seabed. These structures are usually much larger than general cargo ships and have their natural frequency outside the wave frequency range. This results in the response to first order wave forces acting on the hull to be negligible. However, second order difference frequency forces start to significantly impact the motions of the structure. When the difference frequency between wave components matches the roll natural frequency, the structure experiences a significant roll motion which is also termed as second order roll. This paper describes the theory and numerical implementation behind the calculation of second order forces and motions of any general floating structure subjected to waves. The numerical implementation is validated in zero speed case against the commercial code OrcaFlex. The paper also describes in detail the popular approximations used to simplify the computation of second order forces and provides a discussion on the limitations of each approximation.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.20 no.5
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• pp.439-451
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• 2008

Based on reflected and transmitted waves by a single step bottom, a new model of scatterer method is constructed which can be used to calculate wave transformation over a multi-step topography. The approximate results are tested by comparison with the more accurate results obtained from EFEM presented by Kirby and Dalrymple(1983). In the case of plane-wave approximation, solutions of the scatterer method and the EFEM are the same. Results obtained by the scatterer method with non-propagating modes are much better, in terms of phase for the calculated reflection and transmission coefficients, than those by plane-wave approximation. As the effect of non-propagating modes decreases, solutions of the scatterer method become closer to those of the EFEM.

• Journal of Navigation and Port Research
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• v.41 no.5
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• pp.345-352
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• 2017

Recently, a new port reserves deep water depth for safe navigation and mooring, following the trend of larger ship building. Larger port facilities include long and huge breakwaters, and mainly adopt vertical type considering low construction cost. A vertical breakwater creates stem waves combining inclined incident waves and reflected waves, and this causes maneuvering difficulty to the passing vessels, and erosion of shoreline with additional damages to berthing facilities. Thus, in this study, the researchers have investigated the response of stem waves at the vertical breakwater near the entrance channel and applied numerical models, which are commonly used for the analysis of wave response at the harbor design. The basic equation composing models here adopted both the linear parabolic approximation adding the nonlinear dispersion relationship and nonlinear parabolic approximation adding a linear dispersion relationship. To analyze the applicability of both models, the research compared the numerical results with the existing hydraulic model results. The gap of serial breakwaters and aligned angles caused more complicated stem wave generation and secondary stem wave was found through the breakwater gap. Those analyzed results should be applied to ship handling simulation studies at the approaching channels, along with the mooring test.

• Journal of Ocean Engineering and Technology
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• v.21 no.1 s.74
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• pp.31-39
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• 2007

Recently, the nonlinear dynamic responses among waves, submarine pipeline and seabed have become a target of analyses for marine geotechnical and coastal engineers. Specifically, the velocity field around the submarine pipeline and the wave-induced responses of soil, such as stress and strain inside seabed, have been recognized as dominant factors in discussing the stability of submarine pipeline. The aim of this paper is to investigate nonlinear dynamic responses of soil in seabed, around submarine pipeline, under wave loading. In order to examine wave-induced soil responses, first, the calculation is conducted in the whole domain, including wave field and the seabed, using the VOF-FDM method. Then, velocities and pressures, which are obtained on the boundary between the wave field and the seabed, are used as the boundary condition to compute the wave-induced stress and strain inside seabed, using the poro-elastic FEM model, which is based on the approximation of the Biot's equations. Based on the numerical results, the characteristics of wave-induced soil responses around submarine pipeline are investigated, in detail, inrelation to relative separate distance of the submarine pipeline from seabed. Also, the velocity field around the submarine pipeline is discussed.

• Journal of Ocean Engineering and Technology
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• v.33 no.1
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• pp.76-84
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• 2019

For decades, solitary waves have commonly been used to simulate tsunami conditions in numerical studies. However, the main component of a tsunami waveform acts at completely different spatial and temporal distributions than a solitary waveform. Thus, this study applied a 2-D numerical wave tank that included a non-reflected tsunami generation system based on Navier-Stokes equations (LES-WASS-2D) to directly simulate the run-up of a tsunami-like solitary wave on a slope. First, the waveform and velocity due to the virtual depth factor were applied to the numerical wave tank to generate a tsunami, which made it possible to generate the wide waveform of a tsunami, which was not reproduced with the existing solitary wave approximation theory. Then, to validate the applied numerical model, the validity and effectiveness of the numerical wave tank were verified by comparing the results with the results of a laboratory experiment on a tsunami run-up on a smooth impermeable 1:19.85 slope. Using the numerical results, the run-up characteristics due to a tsunami-like solitary wave on an impermeable slope were also discussed in relation to the volume ratio. The maximum run-up heights increased with the ratio of the tsunami waveform. Therefore, the tsunami run-up is highly likely to be underestimated compared to a real tsunami if the solitary wave of the approximation theory is applied in a tsunami simulation in a coastal region.

• Bulletin of the Korean Chemical Society
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• v.19 no.5
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• pp.539-544
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• 1998

A new theoretical method, named the SCF-DWB-IOS approximation, is suggested to investigate the vibrational predissociation of triatomic van der Waals complexes. The meta stable vibrational excited states are described with SCF (self-consistent-field) approximation and the fragmented diatomic continuum states are determined by using IOS (infinite order sudden) approximation. The dissociation process itself is studied by using DWB (distorted wave Born) approximation. As a test case, the predissociation rates, rotational state distributions of products, and the lifetimes of vibrationally excited states of $Ne-I_2$ are all computed which are in reasonable agreements with other theoretical and/or experimental results. The suggested SCF-DWB-IOS approximation scheme is found to be a very simple but efficient theoretical tool to investigate the vibrational predissociation dynamics of small van der Waals complexes.

• Journal of Ocean Engineering and Technology
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• v.26 no.2
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• pp.8-13
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• 2012

This paper investigates the statistical properties of waves in the sea area of Uljin, which is located in the East Sea area of Korea. The wave data were measured using AWAC (Acoustic Wave and Current Meter), which was installed at a 16-m water depth from November 2010 to March 2011. The wave data acquisition rate, Hmax, monthly mean Hs, Tz, Tp, and wave direction are summarized. The distributions of Hs and Tz were analyzed using the Hs-Tz scatter diagrams. The measurement wave data were analyzed to investigate freak wave characteristics. By comparing the wave spectrum using the measurement wave data with the wave spectrum obtained by varying the JONSWAP wave spectrum, it was possible to approximate the wave spectrum shape at the Uljin Sea area.

• Proceedings of the Korea Water Resources Association Conference
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• 1983.07a
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• pp.78-79
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• 1983

It is well known that small amplitude wave theory, a first approximation to the complete theoretical description of wave behavior, yields a maximum investment in mathematical endeavor. But, if the wave amplitude is large, the small amplitude considerations are not valid, and finite amplitude wave theory which retains higher-order terms to obtain an accurate representation of the wave motion is numercal theory. The Stream function wave theory, one of the numerical methods, was developed by Dean for use with asymmetric measured wave profiles and with symmetric theoretical wave profiles. Dalrymple later improved the comjputational procedure by adding two Lagrangian constraints so that more efficient convergence of the iterative numerical method to a specified wave heigh and to a zero mean free surface displacement resulted. This paper introduces in details the Dean and Darlymple Stream Function Method in case of the symmetric theoretical wave, because in design purposes, wave height and wave period are given.

• Journal of the Korean Society for Nondestructive Testing
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• v.24 no.6
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• pp.559-565
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• 2004

Two different methods were used for the scattering analysis of side-drilled holes(SDH). The scattering models include an explicit model based on the Kirchhoff approximation and the solution of the exact separation of variables. The far-field scattering amplitude was calculated and their time-domain results were compared for the case of shear vertical wave. The exact solution predicts the existence of the creeping wave. The Kirchhoff approximation agreed to the exact solution, except the case of the creeping wave. Two measurement models were introduced to predict the response from the SDHs for the case of immersion, pulse-echo testing. The received voltage was calculated for the case of the shear vertical waves with the incident angle of $45^{\circ}$ to the SDH with the diameter of 1mm, and compared with the experimental results.

• Ocean Systems Engineering
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• v.3 no.4
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• pp.327-348
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• 2013

An Oscillating Water Column (OWC) is a relatively practical and convenient device that converts wave energy to a usable form, which is electricity. The OWC is kept inside a fixed truncated vertical cylinder, which is a hollow structure with one open end submerged in the water and with an air turbine at the top. This research adopts potential theory and Galerkin methods to solve the fluid motion inside the OWC. Using an air-water interaction model, OWC design for energy extraction from regular wave is also explored. The hydrodynamic coefficients of the scattering and radiation potentials are solved for using the Galerkin approximation. The numerical results for the free surface elevation have been verified by a series of experiments conducted in the University of New Orleans towing tank. The effect of varying geometric parameters on the response amplitude operator (RAO) of the OWC is studied and modification of the equation for evaluating the natural frequency of the OWC is made. Using the model of air-water interaction under certain wave parameters and OWC geometric parameters, a computer program is developed to calculate the energy output from the system.