• Journal of Ocean Engineering and Technology
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• v.28 no.2
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• pp.119-125
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• 2014

A vertical planar motion mechanism(VPMM) test was used to increase the prediction accuracy for the maneuverability of an underwater glider model. To improve the accuracy of the linear hydrodynamic coefficients, the analysis techniques of a pure heave test and pure pitch test were developed and confirmed. In this study, the added mass and damping coefficient were measured using a VPMM test. The VPMM equipment provided pure heaving and pitching motions to the underwater glider model and acquired the forces and moments using load cells. As a result, the hydrodynamic coefficients of the underwater glider could be acquired after a Fourier analysis of the forces and moments. Finally, a motion control simulation was performed for the glider control system, and the results are presented.

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

• Bulletin of the Society of Naval Architects of Korea
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• v.13 no.1
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• pp.11-16
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• 1976

The effect of the bow shape on the ship motion response in longitudinal regular waves of water of finite depth is investigated by employing the strip theory. The two-dimensional hydrodynamic forces(added mass and damping) were calculated by close-fit method for water of finite depth. The models for investigation are U and V bow ship forms of block coefficient 0.8 with constant after body which were used by Yourkov [2] and recently by Kim [3] for their deep water investigations. The following results are obtained by the present numerical experiments. (1) It is confirmed that the damping coefficient of the V-bow ship is greater than that of U-bow ship and in consquence the amplitude of heave and pitch of V-bow ship is smaller than that of U-bow ship among longitudinal regular head waves in water of finite depth (2) The merit of the V-bow ship on the motion damping is more significant in heave than in pitch, and is decreasing with the shallowness of water depth. (3) The change of bow form gives little effect on the wave exciting force and moment compared with the motion responce.

• Journal of Korean Society for Geospatial Information Science
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• v.12 no.4 s.31
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• pp.35-43
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• 2004

Multibeam Echosounder system is the latest technology of a hydrographic survey utilized in producing an electronic nautical chart, obtaining a DEM with high precision, making a moving image by Swath surveying a wide area. As a fundamental study for improving the precision of MBES, we compared and analyzed measurements of DGPS and Motion sensor, and studied for the dynamic characteristics of vessel's movements. DGPS was installed in front and in the rear and on both side or the vessel and surveyed. The receiving precision of surveyed GPS results was obtained to the satisfactory extent that was possible to valuate the accuracy of Motion sensor as 0.0016$^{\circ}$ of the roll value and 0.0009$^{\circ}$ of the pitch value. The relationship between the values of heading, pitch, and roll in Motion sensor and the data of DGPS was proportional correlation. In addition, it is considered that deviations by elements like rapid turning and vibration of the vessel will be occurred, although the correlation of each deviation according to each amount or change is proportional. It is suitable that GPS installs in the central line of the vessel that is less affected than other places by waving because the amount of change in the tide level obtained from GPS survey and the value of heave are similar with the values taken by Motion sensor, and the velocity of GPS is different from installed places. The accuracy of the final result from MBES could be affected by the values of gyro and Motion sensor inputted to MBES processor because there were intervals of 15s and 13s of receiving time in gyro and Motion sensor respectively compared with the real-time measurements of DGPS.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2006.11a
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• pp.245-248
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• 2006

The development of offshore structures have been increased spectacularly, especially in oil rig structures. This study concerns with the effects of heave motion of spar platform that attached the helical fin. There are three models, namely, cylinder, cylinder-truss and cylinder-cell with different geometrical dimensions are examined. Finally, the interaction between structure and fluid is closely considered. As the results, it can be seen that the existence of helical fin does not influence on surging but it affects a little on heaving of spar platform.

• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.711-722
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• 2020

An optimization framework using genetic algorithms has been developed towards an automated parametric optimization of the Octabuoy semi-submersible design. Compared with deep draft production units, the design of the shallow draught Octabuoy semi-submersible provides a floating system with improved motion characteristics, being less susceptible to vortex induced motions in loop currents. The relatively large water plane area results in a decreased natural heave period, which locates the floater in the wave period range with more wave energy. Considering this, the hull design of Octabuoy semi-submersible has been optimized to improve the floater's motion performance. The optimization has been conducted with optimized parameters of the pontoon's rectangular cross section area, the cone shaped section's height and diameter. Through numerical evaluations of both the 1st-order and 2nd-order hydrodynamics, the optimization through genetic algorithms has been proven to provide improved hydrodynamic performance, in terms of heave and pitch motions. This work presents a meaningful framework as a reference in the process of floating system's design.

• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.297-313
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• 2020

An Arctic Spar is characterized by its conical shape near the waterline. In this case, the nonlinear effects from its irregular hull shape would be significant if there is either a large amplitude floater motion or steep wave conditions. Therefore, in this paper, the nonlinear effects of an Arctic Spar are numerically investigated by introducing a weakly nonlinear time-domain model that considers the time dependent hydrostatic restoring stiffness and Froude-Krylov forces. Through numerical simulations under multiple regular and irregular wave conditions, the nonlinear behavior of the Arctic Spar is clearly observed, but it is not shown in the linear analysis. In particular, it is found that the nonlinear Froude-Krylov force plays an important role when the wave frequency is close to the heave natural frequency. In addition, the nonlinear hydrostatic restoring stiffness causes the structure's unstable motion at a half of heave natural period.

• Bulletin of the Society of Naval Architects of Korea
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• v.12 no.2
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• pp.59-66
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• 1975

Recently, as the dimensions of energy carriers increase, especially in draft, a reliable prediction of the ship motions at finite depths of water becomes necessary. The purpose of this paper is to probe the effect of finite water depth on the hydrodynamic forces and ship motions, particularly heave and pitch, in longitudinal regular head waves, by comparing the experimental value of Freakes and Keay with the author's theoretical value obtained by applying the modified strip theory to the Mariner class ship. It is confirmed that generally the hydrodynamic coefficients in the equations of motion increase with decreasing water depth, and the wave exciting forces and moments decrease with decreasing water depth. Amplitudes of heave and pitch in longitudinal regular head waves decrease as the water depth in the range where the length of the incident wave is comparatively long. The effects of Froude Number on the hydrodynamic coefficients increase with decreasing water depth and is more noticeable in the case of heave than pitch. In heave, generally the discrepancy between the experimental value and the theoretical value is relatively small in the case of $F_n=O$, but it is very large in the case of $F_n=0.2$. It is considered that the trend stems from the ignorance of the three dimensional effect and the other effects due to shallowness of water on the hydrodynamic coefficients in the theoretical calculation. An extension of methods for calculating the two dimensional hydrodynamic forces to included the effect of forward speed should be recommended. It is required that more experimental works in finite water depths will be carried out for correlation studies between the theoretical calculation, according tp modified strip theory, and model experiments.

• Ocean Systems Engineering
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• v.3 no.3
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• pp.203-217
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• 2013

A PTO (power-take-off) mechanism by using relative heave motions between a floating buoy and its inner mass (magnet or amateur) is suggested. The inner power take-off system is characterized by a mass with linear stiffness and damping. A vertical truncated cylinder is selected as a buoy and a special station-keeping system is proposed to minimize pitch motions while not affecting heave motions. By numerical examples, it is seen that the maximum power can actually be obtained at the optimal spring and damper condition, as predicted by the developed WEC(wave energy converter) theory. Then, based on the developed theory, several design strategies are proposed to further enhance the maximum PTO, which includes the intentional mismatching among heave natural frequency of the buoy, natural frequency of the inner dynamic system, and peak frequency of input wave spectrum. By using the intentional mismatching strategy, the generated power is actually increased and the required damping value is significantly reduced, which is a big advantage in designing the proposed WEC with practical inner LEG (linear electric generator) system.

• Journal of the Society of Naval Architects of Korea
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• v.54 no.4
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• pp.329-334
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• 2017

As the motion response of heave for floating bodies on the water surface is relatively large near the natural frequency, it is necessary to predict its value accurately from the stage of initial design. Bodies accelerating in fluid experience force acted upon by the fluid, and this force is quantified by using the concept of added mass. For predicting the natural frequency of heave we need to know the added mass, which is given as a function of frequency, and hence the natural frequency can be obtained through only by iteration process, as was pointed out by Lee (2008). His method was applied to circular cylinders, and two dimensional cylinders of Lewis form by making use of the Ursell-Tasai method in the previous works, Lee and Lee (2013), Kim and Lee (2013), and Song and Lee (2015). In this work, a similar algorithm employing the concept of strip method is adopted for predicting the heave natural frequency of KCS(KRISO Container Ship), and the obtained computational result was compared with other existing experimental data, and the agreement seems reasonable. Furthermore, through the error analysis, it is shown that why the frequency corresponding to the local minimum of the added mass and the natural frequency are very close. And it seems probable that we can predict the heave natural frequency if we know only the local minimum of added mass and the corresponding frequency under a condition, which holds for ship-like bodies in general.