• Korean Journal of Remote Sensing
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• v.21 no.1
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• pp.91-98
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• 2005

On board satellite magnetometer measures all possible magnetic components, such as the core and crustal components from the inner Earth, and magnetospheric, ionospheric and' its coupled components from the outer Earth. Due to its dipole and non-dipole features, separation of the respective component from the measurements is most difficult unless the comprehensive knowledge of each field characteristics and the consequent modeling methods are solidly constructed. Especially, regional long wavelength magnetic signals of the crust are strongly masked by the main field and dynamic external field and hence difficult to isolate in the satellite measurements. In particular, the un-modeled effects of the strong auroral external fields and the complicated behavior of the core field near the geomagnetic poles conspire to greatly reduce the crustal magnetic signal-to-noise ratio in the polar region relative to the rest of the Earth. We can, however, use spectral correlation theory to filter the static lithospheric and core field components from the dynamic external field effects that are closely related to the geomagnetic storms affecting ionospheric current disturbances. To help isolate regional lithospheric anomalies from core field components, the correlations between CHAMP magnetic anomalies and the pseudo-magnetic effects inferred from satellite gravity-derived crustal thickness variations can also be exploited, Isolation of long wavelengths resulted from the respective source is the key to understand and improve the models of the external magnetic components as well as of the lower crustal structures. We expect to model the external field variations that might also be affected by a sudden upheaval like tsunami by using our algorithm after isolating any internal field components.

• Structural Engineering and Mechanics
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• v.57 no.5
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• pp.951-968
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• 2016

Long-span cross-strait bridges extending into deep-sea waters are exposed to complex marine environments. During the construction stage, the flexible freestanding bridge towers are more vulnerable to environmental loads imposed by wind and wave loads. This paper presents an experimental investigation on the dynamic responses of a 389-m-high freestanding bridge tower model in a test facility with a wind tunnel and a wave flume. An elastic bridge model with a geometric scale of 1:150 was designed based on Froude similarity and was tested under wind-only, wave-only and wind-wave combined conditions. The dynamic responses obtained from the tests indicate that large deformation under resonant sea states could be a structural challenge. The dominant role of the wind loads and the wave loads change according to the sea states. The joint wind and wave loads have complex effects on the dynamic responses of the structure, depending on the approaching direction angle and the fluid-induced vibration mechanisms of the waves and wind.

• Journal of the Society of Naval Architects of Korea
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• v.53 no.2
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• pp.85-91
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• 2016

The most important factor in the structural design of ships and offshore structures operating in arctic region is ice load, which results from ice-structure interaction during the ice collision process. The mechanical properties of ice related to strength and failure, however, show very complicated aspect varying with temperature, volume fraction of brine, grain size, strain rate and etc. So it is nearly impossible to establish a perfect material model of ice satisfying all the mechanical characteristics completely. Therefore, in general, ice collision analysis was carried out by relatively simple material models considering only specific aspects of mechanical characteristics of ice and it would be the most significant cause of inevitable errors in the analysis. Especially, it is well-known that the most distinctive mechanical property of ice is high dependency on strain rate. Ice shows brittle attribute in higher strain rate while it becomes ductile in lower strain rate range. In this study, the simulation method of ice collision to ship hull using the nonlinear dynamic FE analysis was dealt with. To consider the strain rate effects of ice during ice-structural interaction, strain rate dependent constitutive model in which yield stress and hardening behaviors vary with strain rate was adopted. To reduce the huge amount of computing time, the modeling range of ice and ship structure were restricted to the confined region of interest. Under the various scenario of ice-ship hull collision, the structural behavior of hull panels and failure modes of ice were examined by nonlinear FE analysis technique.

• Journal of Ship and Ocean Technology
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• v.12 no.3
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• pp.36-54
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• 2008

A jet stream applied tangential to a curved surface in fluid increases lift force by strengthening circulation around the surface and this phenomenon is known as the Coanda effect. Many experimental and numerical studies have been performed on the Coanda effect and the results found to be useful in various fields of aerodynamics. Recently, preliminary studies on Coanda control surface are in progress to look for practical application in marine hydrodynamics since various control surfaces are used to control behaviors of ships and offshore structures. In the present study, the performance of a Coanda control surface with different geometries of the jet injection nozzle was surveyed to assess applicability to ship rudders. A numerical simulation was carried out to study flow characteristics around a section of a horn type rudder subjected to a tangential jet stream. The RANS equations, discretized by a cell-centered finite volume method were used for this computation after verification by comparing to the experimental data available. Special attentions have been given to the sensitivity of the lift performance of a Coanda rudder to the location of the slit (outlet) and intake of the gap between the horn and rudder surface at the various angles of attack. It is found that the location of the water intake is important in enhancing the lift because the gap functions as a conduit of nozzle generating a jet sheet on the rudder surface.

• International Journal of Naval Architecture and Ocean Engineering
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• v.13 no.1
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• pp.208-222
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• 2021

Managing with the presence of sea ice is the primary challenge in the operation of floating platforms in the Arctic region. It is widely accepted that offshore structures operating in Arctic conditions need station-keeping methods as well as ice management by icebreakers. Dynamic Positioning (DP) is one of the station-keeping methods that can provide mobility and flexibility in marine operations. The presence of sea ice generates complex external forces and moments acting on the vessel, which need to be counteracted by the DP system. In this paper, an icevaning control algorithm is proposed that enables Arctic offshore vessels to perform DP operations. The proposed icevaning control enables each vessel to be oriented toward the direction of the mean environmental force induced by ice drifting so as to improve the operational safety and reduce the overall thruster power consumption by having minimum external disturbances naturally. A mathematical model of an Arctic offshore vessel is summarized for the development of the new icevaning control algorithm. To determine the icevaning action of the Arctic offshore vessel without any measurements and estimation of ice conditions including ice drift, task and null space are defined in the vessel model, and the control law is formulated in the task space. A backstepping technique is utilized to handle the nonlinearity of the Arctic offshore vessel's dynamic model, and the Lyapunov stability theory is applied to guarantee the stability of the proposed icevaning control algorithm. Experiments are conducted in the ice tank of the Korea Research Institute of Ships and Ocean Engineering to demonstrate the feasibility of the proposed approach.

• Journal of IKEEE
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• v.26 no.3
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• pp.396-400
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• 2022

In this study, an ICT-based underwater communication monitoring device for underwater structures is implemented based on lifting fixture that transport human bodies found on the seabed to sea level. The lifting fixture is packaged with a retback, sideback, and cartridge that injects air. Monitoring systems are developed in a mobile manner in a portable structure. The underwater ultrasonic sensor signal is supplied using a USB port, and the O/S consists of Linux. For the underwater communication dong test, a measurement test was conducted in real time from 6m to 40m in depth on the east coast. The ultrasonic sound sensor is converted to 2,400 bps to verify the transmission error according to the duality. The communication speed of sensor to monitoring is 115,200 bps, and the speed of communication from controller to receiver is 2,400 bps. In the commercialization stage of the lifting device, it is easy to develop a low-end type and the compatibility is wide.

• Advances in nano research
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• v.13 no.1
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• pp.97-111
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• 2022

In the present article, silica nanoparticles (SNPs) were exploited to improve the tribological and mechanical properties of vinyl ester/glass fiber composites. To the best of our knowledge, there hasn't been any prior study on the wear properties of glass fiber reinforced vinyl ester SiO₂ nanocomposites. The wear resistance is a critical concern in many industries which needs to be managed effectively to reduce high costs. To examine the influence of SNPs on the mechanical properties, seven different weight percentages of vinyl ester/nano-silica composites were initially fabricated. Afterward, based on the tensile testing results of the silica nanocomposites, four wt% of SNPs were selected to fabricate a ternary composite composed of vinyl ester/glass fiber/nano-silica using vacuum-assisted resin transfer molding. At the next stage, the tensile, three-point flexural, Charpy impact, and pin-on-disk wear tests were performed on the ternary composites. The fractured surfaces were analyzed by scanning electron microscopy (SEM) images after conducting previous tests. The most important and interesting result of this study was the development of a nanocomposite that exhibited a 52.2% decrease in the mean coefficient of friction (COF) by augmenting the SNPs, which is beneficial for the fabrication/repair of composite/steel energy pipelines as well as hydraulic and pneumatic pipe systems conveying abrasive materials. Moreover, the weight loss due to wearing the ternary composite containing one wt% of SNPs was significantly reduced by 70%. Such enhanced property of the fabricated nanocomposite may also be an important design factor for marine structures, bridges, and transportation of wind turbine blades.

• Korean Journal of Materials Research
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• v.32 no.3
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• pp.161-167
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• 2022

A cold roll-bonding process is applied to fabricate an AA6061/AA5052/AA6061/AA5052 layered sheet. Two AA6061 and one AA5052 sheets of 2mm thickness, 40mm width and 300mm length are alternately stacked, then reduced to a thickness of 2.0 mm by multi-pass cold rolling after surface treatment such as degreasing and wire brushing. The rolling is performed at ambient temperature without lubricant using a 2-high mill with a roll diameter of 400 mm at a rolling speed of 6.0 m/sec. The roll-bonded AA6061/AA5052/AA6061/AA5052 layered sheet is then hardened by natural aging (T4) and artificial aging (T6) treatments. The microstructure of the as-roll bonded and the age-hardened Al sheets was revealed by SEM observation; the mechanical properties were investigated by tensile testing and hardness testing. After T4 and T6 aging treatment, the specimens had a recrystallization structure consisting of coarse equiaxed grains in both AA5052 and AA6061 regions. The as-roll-bonded specimen showed a clad structure in which the hardness of AA5052 regions was higher than that of AA6061 regions. However, after T4 and T6 aging treatment, specimens exhibited different structures, with hardness of AA6061 regions higher than that of AA5052 regions. Strengths of T6 and T4 age-treated specimens were found to increase by 1.55 and 1.36 times, respectively, compared to the value of the starting material.

• Smart Structures and Systems
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• v.31 no.3
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• pp.283-299
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• 2023

Structural stability of floating platforms has long since been a crucial issue in the field of marine engineering. Excessive motions would not only deteriorate the operating conditions but also seriously impact the safety, service life, and production efficiency. In recent decades, several control devices have been proposed to reduce unwanted motions, and an attractive one is the tuned heave plate (THP). However, the THP system may reduce or even lose its effectiveness when it is mistuned due to the shift of dominant wave frequency. In the present study, a novel adaptive tuned heave plate (ATHP) is proposed based on inerter by adjusting its inertance, which allows to overcome the limitation of the conventional THP and realize adaptations to the dominant wave frequencies in real time. Specifically, the analytical model of a representative semisubmersible platform (SSP) equipped with an ATHP is created, and the equations of motion are formulated accordingly. Two optimization strategies (i.e., J₁ and J₂ optimizations) are developed to determine the optimum design parameters of ATHP. The control effectiveness of the optimized ATHP is then examined in the frequency domain by comparing to those without control and controlled by the conventional THP. Moreover, parametric analyses are systematically performed to evaluate the influences of the pre-specified frequency ratio, damping ratio, heave plate sizes, peak periods and wave heights on the performance of ATHP. Furthermore, a Simulink model is also developed to examine the control performance of ATHP in the time domain. It is demonstrated that the proposed ATHP could adaptively adjust the optimum inertance-to-mass ratio by tracking the dominant wave frequencies in real time, and the proposed system shows better control performance than the conventional THP.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.57 no.1
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• pp.79-87
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• 2024

We seasonally examined marine macroalgal community structures in the subtidal zones at Gapado Island, Jeju, Korea, from February 2013 to November 2019. A total of 164 macroalgal species were identified, including 15 green, 40 brown, and 109 red algae. Species richness ranged from 54-106 species, with the maximum observed in 2013 and the minimum observed in 2015. Cladophora wrightiana var. minor, Ecklonia cava, Amphiroa anceps, Corallina aberrans, Corallina crassisima, Synarthrophyton chejuense, Sonderophycus capensis and Plocamium telfairiae occurred in all seasons. The average annual seaweed biomass was 1,258.22 g wet wt./m², ranging from 917.51 g/m² in 2017 to 1,551.95 g/m² in 2014. E. cava was the most dominant species, accounting for 46.57% of the total seaweed biomass. The subdominant species were P. telfairiae and A. anceps, comprising 8.64% (108.75 g/m²) and 6.65% (83.61 g/m²) of the total biomass, respectively. The vertical distribution of subtidal seaweeds were represented by E. cava and P. telfairiae at 5-20 m, A. anceps at 10-20 m, C. aberrans and C. crassisima at 5 m and C. wrightiana var. minor at 10 m. Cluster analysis revealed three distinct groups: group A (2014-2018), group B (2019) and group C (2013), indicating significant differences in the annual seaweed community.