• Ocean Systems Engineering
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• v.13 no.2
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• pp.173-193
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• 2023

Reliable prediction of the motion of FOWT (floating offshore wind turbine) and associated mooring line tension is important in both design and operation/monitoring processes. In the present study, a 5MW OC4 semisubmersible wind turbine is numerically modeled, simulated, and analyzed by the open-source numerical tool, OpenFAST and in-house numerical tool, Charm3D-FAST. Another commercial-level program FASTv8-OrcaFlex is also introduced for comparison for selected cases. The three simulation programs solve the same turbine-floater-mooring coupled dynamics in time domain while there exist minor differences in the details of the program. Both the motions and mooring-line tensions are calculated and compared with the DeepCWind 1/50 scale model-testing results. The system identification between the numerical and physical models is checked through the static-offset test and free-decay test. Then the system motions and mooring tensions are systematically compared among the simulated results and measured values. Reasonably good agreements between the simulation and measurement are demonstrated for (i) white-noise random waves, (ii) typical random waves, and (iii) typical random waves with steady wind. Based on the comparison between numerical results and experimental data, the relative importance and role of the differences in the numerical methodologies of those three programs can be observed and interpreted. These comparative-study results may provide a certain confidence level and some insight of potential variability in motion and tension predictions for future FOWT designs and applications.

• Atmosphere
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• v.26 no.1
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• pp.85-95
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• 2016

In this study, we investigated the variabilities of wind speed of 850 hPa and precipitable water over the East Asia region using the NCEP Final Analysis data from December 2001 to November 2011. A large variance of wind speed was observed in northern and eastern China during the winter period. During summer, the regions of the East China Sea, the South Sea of Japan and the East Sea show large variances in the wind speed caused by an extended North Pacific High and typhoon activities. The large variances in the wind speed in the regions are shown to be correlated with the inter-annual variability of precipitable water over the inland region of windward side of the Korean Peninsula. Based on the investigation, sensitivity tests to the domain size were performed using the WRF model version 3.6 for heavy precipitation events over the Korean Peninsula for 26 and 27 July 2011. Numerical experiments of different domain sizes were set up with 5 km horizontal and 50 levels vertical resolutions for the control and the first experimental run, and 9 km horizontal for the second experimental run. We found that the major rainfalls correspond to shortwave troughs with baroclinic structure over Northeast China and extended North Pacific High. The correlation analysis between the observation and experiments for 1-h precipitation indicated that the second experiment with the largest domain had the best performance with the correlation coefficient of 0.79 due to the synoptic-scale systems such as short-wave troughs and North Pacific High.

• Ocean Systems Engineering
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• v.2 no.3
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• pp.217-228
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• 2012

As the size of container ships continues to increase, not many existing harbors can host the super-container ship due to its increased draft and the corresponding dredging requires huge budget. In addition, the minimization of waiting and loading/offloading time is the most important factor in harbor competitiveness. In this regard, mobile-harbor concept has been developed in Korea to achieve much improved harbor capacity and efficiency. In developing the concept, one of the most important elements is the operability of crane between two or more floating bodies in side-by-side arrangement. The container ship is to be stationed through a hawser connection to an outside-harbor fixed-pile station with the depth allowing its large draft. The mobile harbors with smart cranes are berthed to the sides of its hull for loading/offloading containers and transportation. For successful operation, the relative motions between the two or more floating bodies with hawser/fender connections have to be within allowable range. Therefore, the reliable prediction of the relative motions of the multiple floating bodies with realistic mooring system is essential to find the best hull particulars, hawser/mooring/fender arrangement, and crane/docking-station design. Time-domain multi-hull-mooring coupled dynamic analysis program is used to assess the hydrodynamic interactions among the multiple floating bodies and the global performance of the system. Both collinear and non-collinear wind-wave-current environments are applied to the system. It is found that the non-collinear case can equally be functional in dynamics view compared to the collinear case but undesirable phenomena associated with vessel responses and hawser tensions can also happen at certain conditions, so more care needs to be taken.

• Journal of Navigation and Port Research
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• v.32 no.4
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• pp.305-313
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• 2008

Recently it was known that the problems of nearshore processes and damage of berth and counter facilities frequently had appeared at the small fishery port, such as Daebang near Samcheonpo city, Korea. Here we try to analyze the impact of the rearrangement of counter facilities and berth layout adopted for tranquility of its inner harbor. Because this harbor is being connected to Daebang channel, the rearrangement of the structures might affect to the current speed and direction and wave height, so do to the sea bottom undulation. Therefore, we made model test for the several layouts of the berth and breakwater in this area. Numerical model result shows that the bottom was eroded by 1m by tidal currents and the speed of flow did not shrink, even after the construction work was completed. The direction of the sand movement was downdrift. Although the model study gave reasonable description of beach processes and approach channel sedimentation mechanism, it is necessary to compare with the field history, including the records of waves, tides and bottom materials, etc. for better prediction.

• Journal of the Society of Naval Architects of Korea
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• v.43 no.1 s.145
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• pp.1-14
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• 2006

To improve maritime safety, it is very important not only to make safer design and operation but also to do proper response in case of maritime casualty. The large-scaled casualties will be caused by loss of structural strength and stability due to the progressive flooding and enlargement of damage by the effect of waves and wind. To prevent foundering and structural failure, the prediction of ship motion behavior of damaged ship in wave is necessary. This paper describes the motion behavior of damaged ship in waves through theoretical and experimental studies. A time domain theoretical model of damaged ship motions and accidental flooding, which can be applied to any type of ship or arrangement and considers the effects of flooding of compartments, has been developed. The model tests have been carried out in regular and irregular waves with different wave heights and directions in ship motion basin. Those were performed for three different damaged conditions such as engine room bottom damage, side shell damage and bow visor damage of a Ro-Ro ship. Comparison of theoretical and experimental results was performed.

• Journal of Ocean Engineering and Technology
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• v.26 no.1
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• pp.47-53
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• 2012

A coastal flood area was predicted using the empirical superposition of the typhoon surge level and typhoon wave height along the Busan coastal area. The historical typhoon damages were reviewed, and the coastal topography was measured using VRS-GPS. A FEMA formula was applied to estimate the coastal flood area in a typhoon case when the measured and predicted data of typhoon waves are not available. The results in the area of Haeundae beach and Gwangalli beach were verified using the flood area data from the case of Typhoon Maemi (2003). If a Hurricane Katrina class typhoon were to pass through the Maemi trajectory, the areathat would be flooded along theBusan coastal area was predicted and compared with the results of the Maemi case. Because of the lack of ocean environment data such as data for the sea level, waves, bathymetry, wind, pressure, etc., it is hard to improve the prediction accuracy for the coastal flood area in the typhoon case, which could be reflected in the policy to mitigate a typhoon's impact. This paper discusses the kinds of ocean environment information that is needed to predict a typhoon's impact with better accuracy.

• Journal of Ocean Engineering and Technology
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• v.27 no.5
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• pp.63-69
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• 2013

The mooring lines of a floating type offshore plant are known to show wide banded and bimodal responses. These phenomena come from a combination of low and high frequency random load components, which are derived from the drift-restoring motion characteristic and wind- sea, respectively. In this study, fatigue models were applied to predict the fatigue damage of mooring lines under those loads, and the result were compared. For this purpose, seven different fatigue damage prediction models were reviewed, including mathematical formula. A FPSO (floating, production, storage, and offloading) with a $4{\times}4$ spread catenary mooring system was selected as a numerical model, which was already installed at an offshore area of West Africa. Four load cases with different combinations of wave and wind spectra were considered, and the fatigue damage to each mooring line was estimated. The rain flow fatigue damage for the time process of the mooring tension response was compared with the results estimated by all the fatigue damage prediction models. The results showed that both Benasciutti-Tovo and JB models could most accurately predict wide banded bimodal fatigue damage to a mooring system.

• The Journal of the Acoustical Society of Korea
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• v.21 no.4
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• pp.421-429
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• 2002

Sea surface backscattering signal measurements were conducted in the shallow waters off the east coast of Korea to study the acoustic wave scattering from the sea surface. The grazing angles of wave range from 20° to 40° with a frequency of 60 kHz. The wind speed and surface roughness of the experiment area were 3 m/os and below 1 m, respectively. The measured acoustic backscattering strengths greatly exceed the composite roughness predictions at low grazing angles. To account for this discrepancy, the scattering strengths due to a near-surface bubble layer were considered. The prediction with bubble contribution was found to be in good agreement with the experimental measurement.

• Journal of Ocean Engineering and Technology
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• v.37 no.5
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• pp.181-189
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• 2023

The fuel consumption of marine diesel engines holds paramount importance in contemporary maritime transportation and shapes energy efficiency strategies of ocean-going vessels. Nonetheless, a noticeable gap in knowledge prevails concerning the influence of ship hull conditions and propeller roughness on fuel consumption. This study bridges this gap by utilizing artificial intelligence techniques in Matlab, particularly convolutional neural networks (CNNs) to comprehensively investigate these factors. We propose a time-series prediction model that was built on numerical simulations and aimed at forecasting ship hull and propeller conditions. The model's accuracy was validated through a meticulous comparison of predictions with actual ship-hull and propeller conditions. Furthermore, we executed a comparative analysis juxtaposing predictive outcomes with navigational environmental factors encompassing wind speed, wave height, and ship loading conditions by the fuzzy clustering method. This research's significance lies in its pivotal role as a foundation for fostering a more intricate understanding of energy consumption within the realm of maritime transport.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.8 no.2
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• pp.146-150
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• 1996

The waves are most important dynamical factors for the analyses of structural stability and topographical changes on coastal engineering field. However, wind-generated waves are very irregular in shape and transformed through refraction, diffraction and shoaling when they propagate into shallow water where bottom topography and water depth vary significantly. Recently, Vincent and Briggs (1989) reported hydraulic model experiments for the transformation of monochromatic and directionally-spread irregular waves passing over a submerged elliptical mound. They concluded that for the case of combined refraction-diffraction of waves by a shoal, the propagation characteristics of the irregular and equivalent regular wave conditions can be vastly different. On the irregular wave transformation have been made theoretical and numerical studies for several years. Although theoretical and laboratory studies on wave transformation have progressed considerably, field measurement and comparison of numerical results with related theories are still necessary for the prediction of the phenomena in reality. In this study, field measurement of both incident and transformed waves in Youngil Bay were made using various kinds of equipments, and numerical computations were made on the transformed frequency spectra of large waves propagating over the shoal using Chae and Jeong's (1992) elliptic model. It is shown that this model results agree very well with field data, and thus the applicability of the model is now validated.