• Ocean Systems Engineering
• /
• v.3 no.2
• /
• pp.137-147
• /
• 2013

Moonpools are vertical wells in a floating body used onboard many types of vessels like cable-laying vessels and offshore support vessels. Moonpool gives passage to underwater activities for different types of ships as per their mission requirements. It is observed that inside a moonpool considerable relative motions may occur, depending on shape, depth of the moonpool and on the frequency range of the waves to which the ship is exposed. The vessel responses are entirely different in zero and non-zero Froude number. Former situation is paid attention in this study as the mission requirement of the platform is to be in the particular location for long period of operation. It is well known that there are two modes of responses depending on the shape of the moonpool viz., piston mode for square shape and sloshing mode for rectangular shapes with different aspect ratios of opening like 1:1.5 and 1:2 ratios. Circular shaped moonpool is also tested for measuring the responses. The vessel moored using heavy lines are modeled and tested in the wave basin. The moored lines are provided with pre-tension and the dynamic tensions on the lines are measured. The different modes of oscillations of water column are measured using wave gauge and the vessel response at a particular situation is determined. RAOs determined for various situations provide better insight to the designer. The experiments done in the wave basin may also be compared with a software package meant for handling moored floating bodies.

• Ocean Systems Engineering
• /
• v.8 no.4
• /
• pp.441-459
• /
• 2018

A coupled dynamic analysis of a semisubmersible-type FOWT has been carried out in time domain under the combined action of irregular wave and turbulent wind represented respectively by JONSWAP spectrum and Kaimal spectrum. To account for the turbine-floater motion coupling in a more realistic way, the wind turbulence has been incorporated into the calculation of aerodynamic loads. The platform model was referred from the DeepCwind project and the turbine considered here was the NREL 5MW Baseline. To account for the operationality of the turbine, two different environmental conditions (operational and survival) have been considered and the aerodynamic effect of turbine-rotation on actual responses of the FOWT has been studied. Higher mean offsets in surge and pitch responses were obtained under the operational condition as compared to the survival condition. The mooring line tensions were also observed to be sensitive to the rotation of turbine due to the turbulence of wind and overestimated responses were found when the constant wind was considered in the analysis. Additionally, a special analysis case of sudden shutdown of the turbine has also been considered to study the swift modification of responses and tension in the mooring cables.

• Water for future
• /
• v.26 no.4
• /
• pp.35-46
• /
• 1993

The variations of water surface elevation due to bridge are studied using one_dimensional dynamic wave equation. The preissmann scheme is used to solve the dynamic wave equation and the bridges was treated as internal boundary conditions. Main causes of bridge backwater are the proportion of the contracted area due to bridge, roughness coefficient and discharge coefficient. The effect of discharge coefficient in weir flow condition is comparatively small. This model is verified by applying to the Suyoung River. which suffered a severe damage by typoon Gladys. The rise of water level through bridge is 1.53-1.08m in the reach of 4.25-6.20km from the downstream of river. The simulation results of the model have good agreements with the observed data.

• International Journal of Naval Architecture and Ocean Engineering
• /
• v.8 no.3
• /
• pp.252-261
• /
• 2016

A spar-type floating substructure that is being widely used for offshore wind power generation is vulnerable to resonance in the heave direction because of its small water plane area. For this reason, the stable dynamic response of this floating structure should be ensured by accurately identifying the resonance characteristics. The purpose of this study is to analyze the characteristics of the combination resonance between the excitation frequency of a regular wave and natural frequencies of the floating substructure. First, the nonlinear equations of motion with two degrees of freedom are derived by assuming that the floating substructure is a rigid body, where the heaving motion and pitching motions are coupled. Moreover, to identify the characteristics of the combination resonance, the nonlinear term in the nonlinear equations is approximated up to the second order using the Taylor series expansion. Furthermore, the validity of the approximate model is confirmed through a comparison with the results of a numerical analysis which is made by applying the commercial software ANSYS AQWA to the full model. The result indicates that the combination resonance occurs at the frequencies of ${\omega}{\pm}{\omega}_5$ and $2{\omega}_{n5}$ between the excitation frequency (${\omega}$) of a regular wave and the natural frequency of the pitching motion (${\omega}_{n5}$) of the floating substructure.

• Journal of Korea Water Resources Association
• /
• v.34 no.5
• /
• pp.543-549
• /
• 2001

This research estimated stream discharges indirectly by hydraulic channel routing. Only stage data from three stage stations and river cross section data were used to estimate Manning roughness coefficients and to compute stream discharges. When the discharges were estimated a stage-stage set of conditions was used for upstream-downstream boundary conditions. The research used the data from the upper Mississippi River. The hydraulic channel routings were performed by DWOPER (operational dynamic wave model). The global optimization program of SCE-UA was used to improve the roughness coefficient estimation module of the modified Newton-Raphson method in DWOPER. The results from SCE-US were better. For the case study of a flood, most estimated discharges except a few show errors within 10%.

• Geomechanics and Engineering
• /
• v.19 no.3
• /
• pp.241-254
• /
• 2019

A finite element approach is presented to examine ground vibration characteristics under various moving loads in a homogeneous half-space. Four loading modes including single load, double load, four-load, and twenty-load were simulated in a finite element analysis to observe their influence on ground vibrations. Four load moving speeds of 60, 80, 100, and 120 m/s were adopted to investigate the influence of train speed to the ground vibrations. The results demonstrated that the loading mode in a finite element analysis is reliable for train-induced vibration simulations. Additionally, a three-dimensional finite element model (3D FEM) was developed to investigate the dynamic responses of a track-ballast-embankment-ground system subjected to moving loads induced by high-speed trains. Results showed that vibration attenuations and breaks exist in the simulated wave fronts transiting through different medium materials. These tendencies are a result of the difference in the Rayleigh wave speeds of the medium materials relative to the speed of the moving train. The vibration waves induced by train loading were greatly influenced by the weakening effect of sloping surfaces on the ballast and embankment. Moreover, these tendencies were significant when the vibration waves are at medium and high frequency levels. The vibration waves reflected by the sloping surface were trapped and dissipated within the track-ballast-embankment-ground system. Thus, the vibration amplitude outside the embankment was significantly reduced.

• International Journal of Naval Architecture and Ocean Engineering
• /
• v.11 no.2
• /
• pp.980-992
• /
• 2019

This paper describes a model test and numerical simulation of a 750-kW-semi-submersible platform wind turbine under several wind and wave conditions for validation of the numerical simulation model. The semi-submersible platform was designed to support the 750-kW-wind turbine class and operate at a water depth of 50 m. The model tests were performed to estimate the performance characteristics of the wind turbine system in the wide tank of the University of Ulsan. Motions and loads of the wind turbine system under the wind and wave conditions were measured and analyzed. The NREL-FAST code was used to simulate the wind turbine system, and the results were compared with those of the test model. The results demonstrate that the numerical simulation captures noticeably the fully coupled floating wind turbine dynamic responses. Also, the model shows a good stability and small responses during waves, wind, and operation of the 750-kW-floating offshore wind turbine.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.12 no.1
• /
• pp.19-26
• /
• 2000

A numerical model is applied for predicting two-dimensional beach and dune erosion during severe storms. The model uses equation of sediment continuity and dynamic equation, governing the on-offshore sediment transport due to a disequilibrium of wave energy dissipation. And the model also uses sediment transport rate parameter K from dimensional analysis instead of that recommended by Kriebel. During a storm, a beach profile evolves to a form where the depth at the surf zone is related to the distance seaward of the waterline. In general, the erosion in the beach profile is found to be sensitive to equilibrium profile parameter, sediment transport rate parameter, storm surge level and breaking wave height.

• Journal of the Korean Geotechnical Society
• /
• v.37 no.6
• /
• pp.39-50
• /
• 2021

Recently, as the occurrence of earthquakes with a magnitude of 5.0 or higher in Korea increases, many studies and interests in seismic design are increasing. A lot of damage was caused by the Pohang earthquake in 2017, and port facilities such as a breakwater were also damaged. This study analyzed the dynamic behavior of the upright breakwater, an external facility, based on a centrifugal model experiment. A series of centrifugal model test was conducted by three different seismic waves such as Pohang Earthquake Wave, Artificial Wave I, and II. As a result, the dynamic behavior of upright breakwater was analyzed. The review showed that acceleration amplification tends to be suppressed as breakwater foundation ground increases support and stiffness through DCM reinforcement and riprap replacement.

• Proceedings of the KSR Conference
• /
• 2008.06a
• /
• pp.831-834
• /
• 2008

The spectral element model is known to provide very accurate structural dynamic characteristics, while reducing the number of degree-of-freedom to resolve the computational and cost problems. Thus, the spectral element model with variational method for an axially moving Bernoulli-Euler beam subjected to axial tension is developed in the present paper. The high accuracy of the spectral element model is the verified by comparing its solutions with the conventional finite element solutions and exact analytical solutions. The effects of the moving speed and axial tension the vibration characteristics, wave characteristics, and the static and dynamic stabilities of a moving beam are investigated.