• Computational Structural Engineering
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• v.10 no.4
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• pp.239-249
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• 1997

The method to distinguish chaotic attractors in the perturbed response behaviors of a piecewise-linear system under combined regular and external randomness is provided and examined. In the noisy fields such as the ocean environment, excitation forces induced by wind, waves and currents contain a finite degree of randomness. Under external random perturbations, the system responses are disturbed, and consequently chaotic signatures in the response attractors are not distinguishable, but rather look just random-like. Mean Poincare map can be utilized to identify such chaotic responses veiled due to the random noise by averaging the noise effect out of the perturbed responses. In this study, the procedure to create mean Poincare map combined with the direct numerical simulations is provided and examined. It is found that mean Poincare maps can successfully distinguish chaotic attractors under stochastic excitations, and also can give the information of limit value of noise intensity with which the chaos signature in system responses vanishes.

• KEPCO Journal on Electric Power and Energy
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• v.2 no.3
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• pp.461-467
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• 2016

In order to install the floating transport type wind-turbine foundation, water pumping is used to sink the foundation. During this process, its mass and center of gravity, and buoyancy center become continuously changed so that the dynamic stability of the floating foundation become unstable. Dynamic stability analysis of the floating foundation is a complex problem since it should take into account not only the environmental wave, wind, and current loads but also its weight change effect simultaneously considering six-degree-of-freedom motion. In this study, advanced numerical method based on the coupled computational fluid dynamics (CFD) and multi-body dynamics (MBD) approach has been applied to the dynamic stability analysis of the floating foundation. The sloshing effect of foundation internal water is also considered and the floating dynamic characteristics are numerically investigated in detail.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.3 no.2
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• pp.92-99
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• 1991

A moored body in the sea is subjected to second-order wave forces as well as to linear oscillatory ones. The second-order farces contain slowly-varying components, of which the characteristic frequency can be as low as the natural frequency of horizontal motions of the moored body. As a consequence, the slowly-varying force can excite unexpectedly large horizontal excursion of the body, which may cause a serious damage on the mooring system. In design analysis of Turret-type mooring system which is one of the interesting mooring systems for a floating body. the slowly-varying drift forces and the transient motion of the system during weathervaning are very important. In this paper the slowly-varying drift forces were calculated by using the Quadratic Transfer Function with considering the second order free-wave contributions. Additionaly the transient surge motion of the moored body was simulated with including the roll of the time-memory effect. In this simulation the spring constant of the spread Turret mooring system is updated at every time step for considering the nonlinear effect.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.12 no.1
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• pp.75-82
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• 1999

일반 해양 구조물이나 해저면에 설치되는 해저 관로는 외력에 의한 변형이 발생된다. 구조물 형상이 복잡하거나, 구성 요소의 개수가 많을 경우 응력해석 시 많은 초기값이 필요하고 해석 시간 또는 장 시간 소요된다. 해양 구조물에 작용하는 대표적인 외력은 파도, 조류, 바람이고 이런 외력은 구조물의 사용 기간(operation life)동안 계속적으로 작용하기 때문에 구조물의 변형율은 항상 허용치 안에서 발생되도록 설계되어야 한다. 허용 변형은 탄성범위 내에 존재해야 하며, 비교적 큰 변형을 일으키는 구조물이나 해저파이프라인의 응력해석을 수치적으로 접근하는 방법을 고찰하였다. 평행상태의 하중 벡터값만 직각 좌표계에서 인트린직(intrinsic) 좌료로 변환시킬 때 변형이 발생함으로, 본 논문에서 소개하는 이차 요소(quadratic element)방법을 사용할 경우 수치해석 시 많은 장점이 있다는 것을 보여준다. 본 방법을 도입함으로써 비교적 큰 변형이 발생되는 구조물 해석 시 일반 수치해석 방법과 그 결과는 같으나 해석 시간을 단축시킬 수 있다는 장점이 있다. 응력 해석 시 국부 강도 행열(element stiffness matrix)은 방향과 무관하며 이차요소 방법을 사용하여 각 요소 벡터를 발생시켰다. 해저관로 일점 상승 시 관로에 작용하는 변형과 상승력에 따른 휨 모멘트를 산출하여 일반적으로 사용되는 선형이론과 비교하였다.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2003.05a
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• pp.94-98
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• 2003

Remotely Operated Vehicle. ROV is a new concept equipment being made to replace the manned systems for investigating the deep sea environment. This paper presents the dynamic cable response during ROV launching considering the coupling effects of ship motion. By the harmonic response analysis, the variations of cable tensions were obtained. Harmonic forces in head/beam sea states were calculated by the concept of relative acceleration which obtained by ship motion analysis.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.31 no.5A
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• pp.351-360
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• 2011

Jacket-type offshore structures are always exposed to severe environmental conditions such as salt, high speed of current, wave, and wind compared with other onshore structures. In spite of the importance of maintaining the structural integrity for offshore structure, there are few cases to apply structural health monitoring (SHM) system in practice. The impedance-based SHM is a kind of local SHM techniques and to date, numerous techniques and algorithms have been proposed for local SHM of real-scale structures. However, it still requires a significant challenge for practical applications to compensate unknown environmental effects and to extract only damage features from impedance signals. In this study, the impedance-based SHM was carried out on a 1/20-scaled model of an Uldolmok current power plant structure under changes in temperature and transverse loadings. Principal component analysis (PCA) was applied using conventional damage index to eliminate principal components sensitive to environmental change. It was found that the proposed PCA-base approach is an effective tool for long-term SHM under significant environmental changes.

• Journal of the Korean Society of Marine Environment & Safety
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• v.24 no.5
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• pp.628-636
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• 2018

Because large buoys are mainly made of steel, they are heavy and vulnerable to corrosion by sea water. This makes buoy installation and maintenance difficult. Moreover, vessel collision accidents with buoys and damage to vessels due to the material of buoys (e.g., steel) are reported every year. Recently, light buoys adopting eco-friendly and lightweight materials have come into the spotlight in order to solve the previously-mentioned problems. In Korea, a new lightweight buoy with a 7-Nautical Mile lantern adopting expanded polypropylene (EPP) and aluminum to create a buoyant body and tower structure, respectively, was developed in 2017. When these light buoys are operated in the ocean, the visibility and angle of light from the lantern installed on the light buoys changes, which may cause them to function improperly. Therefore, research on the performance of light buoys is needed since the weight distribution and motion characteristics of these new buoys differ from conventional models. In this study, stability estimation and motion analyses for newly-developed buoys under various environmental conditions considering a mooring line were carried out using ANSYS AQWA. Numerical simulations for the estimation of wind and current loads were performed using commercial CFD software, Siemens STAR-CCM+, to increase the accuracy of motion analysis. By comparing the estimated maximum significant motions of the light buoys, it was found that waves and currents were more influential in the motion of the buoys. And, the estimated motions of the buoys became larger as the sea state became worser, which might be the reason that the peak frequencies of the wave spectra got closer to those of the buoys.

• Journal of the Korean Geotechnical Society
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• v.30 no.1
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• pp.17-25
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• 2014

Offshore wind turbine foundations are subjected to wind, current and wave loadings. Hence, both static and cyclic behaviors of foundation's soil are important for the design of offshore wind turbine foundation. Undrained cyclic behaviors of soils depend upon the number of loading cycles, vertical effective stress, cyclic shear strain, relative density, and the combination of cyclic and average shear stresses. In order to evaluate the effect of average and cyclic shear stresses on the undrained cyclic behavior of marine silty sand, cyclic direct simple shear (CDSS) tests are performed with relative density of 85%, vertical effective stress of 200 and 300 kPa, and failure criteria of either 15% double amplitude cyclic shear strain (${\gamma}_{cyc}$) or permanent shear strain (${\gamma}_{p}$). The results are presented in the form of design graphs or contour diagrams. The undrained cyclic behavior of marine silty sand is found to be dependent on cyclic and average shear stresses and/or the combination of both shear stresses. It is found that when significant average shear stress exists the permanent or progressive shear strain is the govering failure criteria instead of cyclic shear strain.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• 2010.04a
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• pp.385-386
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• 2010

The ship's seakeeping performance in the pitching and roll motion was felt tired. These gives structural loads to loaded with cargo and hull facilities. Be to improve, small catamaran power yacht designed a data for ship research and ship's hull form to be decided. In this study, based on domestic release for coastal marine environment in the interpretation criteria were chosen based on the exercise performance. The seakeeping performance of marine leisure catamaran was based on voyage speed. The seakeeping performance estimates based on the encounter angle, the wave of the encounter frequency response amplitude ratio for exercise was assessed in the high performance area.

• Journal of Navigation and Port Research
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• v.34 no.10
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• pp.741-746
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• 2010

It is necessary to estimate more accurately the resistance of barge in still water and waves to compute the break load of towline and towing power for safety towing performance. The method proposed by government has calculated the total resistance of barge which is composed of frictional resistance, wave making resistance and air resistance considering the shape of hull and towing speed. However, the added resistance is equally applied with the significant wave height regardless of the type of vessels. In this study, we have carried out the numerical calculation to estimate the added resistance of wigley model in waves and compared with the experiment data to confirm the accuracy of the method. Then the computation was executed for the barge varying shape of the bow. As a result, added resistance of barge was differently occurred i.e. 0.3∼1.1 ton according to encounter angle, 0.4∼1.2 ton according to towing speed and 0.5∼1.1 ton according to shape of bow.