• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2004.05a
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• pp.30-35
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• 2004

Several roll motion reduction devices are reviewed and suggested for the application in FPSO. The firstly suggested solution is the shape of the bilge. The next is a bilge keel. The last suggestion is the ART (anti-rolling tank). Typical U-tube type ART is designed for a FPSO and examined extensively by model experiment. The model section was made of transparent acryl. Free decay test, forced oscillation test and wave test were carried out at a two-dimensional wave flume. U-tube type ART is effective only when the natural periods of ART and ship are same. Therefore, the divided U-tube type ART with split plate is suggested for the reduction of the roll motion of a FPSO over the wide range of the roll period.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.2
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• pp.257-266
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• 1993

Computer Simulation based of divided element method was done to predict strip shape in20-high Sendzimir Mill and has been used to find a way for preventing quarter waves occurring in the wide and thin gaged strip rolling. The simulation showed that it was difficult to prevent quarter waves by the existing methods of controlling actuators such as the shifting of the first intermediate roll and the profile control of As-U-Roll in back up roll. It was, however, confirmed analytically and experimentally that quarter waves could be effectively reduce by changing taper mode at the barrel-end taper radius of the first intermediate roll.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.50 no.2
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• pp.147-153
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• 2014

The purpose of this paper is to examine the roll damping characteristics by bilge keels on the fishing vessel. Unlike other degree of freedom motions, roll motion is highly nonlinear. However the quantitative evaluation of roll damping combined with waves is very important for the fishing vessel. To reduce roll motion, roll motion stabilizers such as a bilge keel is used due to easy made and cheap cost rather than anti-rolling tank and fin-stabilizer. Authors paid attention to the shape of bilge keel and waves to grasp the roll damping for the fishing vessel and studied about the difference of tendencies of roll angle following the shapes of bilge keel. The model ship was the offshore large purse seiner and four types of bilge keel were used. The data from the experiments were provided and analyzed to investigate the rolling characteristics of the model ship being affected by the wave height, wave period and bilge keel shape. The results of the study showed that three types of the bilge keel have little effective, but only one has an effect on the roll damping. So bilge keel shape and its attachment method need to be a future study for the practical use in fishing vessel.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• 2006.10a
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• pp.134-143
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• 2006

Out of all types of motions the critical motions leading to capsize is roll. The dynamic amplification in case of roll motion may be large for ships as roll natural frequency generally falls within the frequency range of wave energy spectrum typical used for estimation of motion spectrum. Roll motion is highly non-linear in nature. Den are various representations of non-linear damping and restoring available in literature. In this paper an uncoupled non-linear roll equations with three representation of damping and cubic restoring term is solved using a perturbation technique. Damping moment representations are linear plus quadratic velocity damping, angle dependant damping and linear plus cubic velocity dependant damping. Numerical value of linear damping coefficient is almost same for all types but non-linear damping is different. Linear and non-linear damping coefficients are obtained form free roll decay tests. External rolling moment is assumed as deterministic with sinusoidal form. Maximum roll amplitude of non-linear roll equation with various representations of damping is calculated using analytical procedure and compared with experimental results, which are obtained form forced tests in regular waves by varying frequency with three wave heights. Experiments indicate influence of non-linearity at resonance frequency. Both experiment and analytical results indicates increase in maximum roll amplitude with wave slope at resonance. Analytical results are compared with experiment results which indicate maximum roll amplitude analytically obtained with angle dependent and cubic velocity damping are equal and difference from experiments with these damping are less compared to non-linear equation with quadratic velocity damping.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.08a
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• pp.27-36
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• 1999

To have high flatness quality of hot rolled strip in the hot strip finishing mill train, a new inter-stand tension profile measuring device of segmented looper roll type(coined as Flatness Sensing Inter-stand Looper, FlatSIL) and a new flatness control system have been developed in this study. The device measures the strip tension profile across the strip width and informs the strip wave pattern to new flatness control system where work roll bending mode to relieve the strip wave is determined. The existing automatic shape control system which uses laser type shape-meter installed at the outlet of the last finishing mill stand strip tension between down coiler and last finishig mill since the latent wave concealed by the strip tension between down coiler and last finishing mill stand cannot be measured by the laser distance-meter. Thus the existing shape control system is not able to control the flatness through the full strip length. The new flatness control system, however, works for full strip length during strip rolling as far as the tension profile measuring device and work roll bender are on. With the new flatness control system, work roll bender is automatically controller to minimize the latent wave of the running strip and the flatness quality as well as strip travelling stability has been noticeably improved from strip head through body to tail.

• International Journal of Naval Architecture and Ocean Engineering
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• v.3 no.1
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• pp.53-64
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• 2011

The present study considers multi-level approach for the analysis of parametric roll phenomena. Three kinds of computation method, GM variation, impulse response function (IRF), and Rankine panel method, are applied for the multi-level approach. IRF and Rankine panel method are based on the weakly nonlinear formulation which includes nonlinear Froude-Krylov and restoring forces. In the computation result of parametric roll occurrence test in regular waves, IRF and Rankine panel method show similar tendency. Although the GM variation approach predicts the occurrence of parametric roll at twice roll natural frequency, its frequency criteria shows a little difference. Nonlinear roll motion in bichromatic wave is also considered in this study. To prove the unstable roll motion in bichromatic waves, theoretical and numerical approaches are applied. The occurrence of parametric roll is theoretically examined by introducing the quasi-periodic Mathieu equation. Instability criteria are well predicted from stability analysis in theoretical approach. From the Fourier analysis, it has been verified that difference-frequency effects create the unstable roll motion. The occurrence of unstable roll motion in bichromatic wave is also observed in the experiment.

• Journal of Navigation and Port Research
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• v.28 no.5
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• pp.353-358
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• 2004

A transient irregular wave was designed based on ISSC spectrum The designed wave was generated in the towing tank and ,the roll motion of a model was measured A method to predict the maximum roll motion, expected in the short-term sea state, was investigated with comparison of the theoretical and experimental results.

• International Journal of Naval Architecture and Ocean Engineering
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• v.5 no.3
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• pp.333-347
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• 2013

The present study numerically models the interaction between a regular wave and the roll motion of a rectangular floating structure. In order to simulate two-dimensional incompressible viscous two-phase flow in a numerical wave tank with the rectangular floating structure, the present study used the volume of fluid method based on the finite volume method. The sliding mesh technique is adopted to handle the motion of the rectangular floating structure induced by fluid-structure interaction. The effect of the wave period on the flow, roll motion and forces acting on the structure is examined by considering three different wave periods. The time variations of the wave height and the roll motion of the rectangular structure are in good agreement with experimental results for all wave periods. The present response amplitude operator is in good agreement with experimental results with the linear potential theory. The present numerical results effectively represent the entire process of vortex generation and evolution described by the experimental results. The longer wave period showed a different mechanism of the vortex evolution near each bottom corner of the structure compared to cases of shorter wave periods. In addition, the x-directional and z-directional forces acting on the structure are analyzed.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.197-200
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• 2002

In this paper, roll damping coefficients for a non-conventional cross section, which is herein named as 'step' model, are investigated numerically and experimentally. Experiments are extensively carried out to estimate the roll damping coefficients. Numerical estimations are also made with the help of numerical codes. For convenience, the roll damping is divided into wave-making component and viscous component. The wave-making component is determined using a potential code and the viscous component using a viscous flow code, in which the fluid domain is taken as unbounded. In order to validate the present approach, a typical cross section with bilge is considered and our results are compared with published data. The comparison shows a good agreement qualitatively. For the step model, numerical results are compared well with experimental data besides some quantitative discrepancies at a certain range of frequency. It is thought that the discrepancy might be caused by the ignorance of the free surface in viscous computations. It is found in the case of the step model that not only the viscous component but also the wave component increases considerably compared to the section with bilge.

• Journal of Advanced Research in Ocean Engineering
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• v.2 no.2
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• pp.81-85
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• 2016

Nonlinear wave loads can lead to resonant responses of offshore structures in sum or difference frequencies. In this study, the roll motion of an FPSO with a low natural frequency is simulated in the time domain. To generate the time signals of wave loads, the quadratic transfer functions of the second-order excitations are calculated in the frequency domain. The equations of motions based on the time memory functions are used to evaluate the roll responses in irregular waves. The roll damping in empirical form is accounted for in the simulation.