• 대한조선학회지
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• 제22권3호
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• pp.9-18
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• 1985

The numerical calculation for solving boundary-value problem related to potential flows with a free surface is carried out by application of the localized finite element method. Only forced motion of 2-D body in infinitely deep fluid is considered, although this schemes is equally applicable to any first order time-harmonic problems of similar nature. The infinite domain of the fluid is separated into the inner flow field and the outer flow field with common inter-surface boundary. The finite element method is applied to obtain the solution in the inner flow field and the Green functions are utilized to represent the solution in the outer flow field. At the inter-surface boundary, the continuity of the value of potential and the normal derivative of the potential(i.e. matching condition) is conserved. The present method has better computational efficiency than the previous LFEM and the integral equation method of Frank. This enhanced computational efficiency is presumably due to the fact that the present method gives a symmetric coefficient matrix and requires less computational time in calculating the influence coefficient matrix of Green function than the integral equation method. And the irregular frequency desen't exist because the uniqueness of the solution is assured by the such that the exact free surface condition is satisfied on the boundary of the localized finite element region(i.e. inner region). As an example of the above method, the hydrodynamic forces for the circular cylinder and the rectangular cylinders are calculated. In the computed results, the small number of singularity distribution segments($3{\sim}6$) give good result relative to Ursell's and Vugts'.

• 대한토목학회논문집
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• 제14권5호
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• pp.1145-1158
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• 1994

물의 유동(流動)에 관한 계산상 효율성(效率性)을 갖는 자유수면(自由水面)을 고려한 3차원(次元) 유한차분(有限差分) 수직모형(數値模型)을 개발하였다. 수직모형(數値模型)은 연직방향(鉛直方向)에 대해 정규화(正規化)한 좌표(座標)(${\sigma}$-coordinate)를 사용하며, 시간(時間) 적분방법(積分方法)으로는 반음해법(半陰解法)(semi-implicit)을 사용하여 계산시간(計算時間)의 효율성(效率性)을 도모하였다. 모드분리(mode-splitting)개념을 도입하여 내부모드(internal mode)에 대해서는 양해법(陽解法)을 사용하였으며, 외부모드(external mode)는 수평방향(水平方向) 운동량방정식(運動量方程式)들과 연속방정식(連續方程式)의 차분식(差分式)으로부터 구한 타원형(楕圓型) 차분방정식(差分方程式)을 SOR방법에 의하여 해석하였다. 이와 같은 방법은 계산(計算) 시간간격(詩間間隔)이 표면(表面) 중력파(重力波)에 대한 CFL(Courant-Fredrich-Lewy)조건에 의해 제약을 받지 않아 계산시간(計算時間)의 효율성을 도모할 수 있다. 개발된 모형은 1차원(次元) 수로(水路)에서 취송유(吹送流)의 연직분포(鉛直分布)에 대한 해석해(解析解)와 비교(比較) 및 연직(鉛直) 가변격자(可變格子)의 도입에 따른 오차분석(誤差分析) 정사각형(正四角形) 호수(湖水)에서 취송유(吹送流) 계산(計算) 및 차분화(差分化) 상수(常數)들의 민감도(敏感度) 분석(分析)을 수행하였다.

• 대한토목학회논문집
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• 제7권3호
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• pp.223-228
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• 1987

본 논문에서는 Tension Leg Platform(TLP)에 작용하는 파랑하중을 간단한 Morison 방정식을 이용하여 효율적으로 산정할 수 있는 방법에 대해서 연구하였다. 본 방법에서는 MacCamy-Fuchs 산란파이론에 기초를 둔 파동력 감소계수를 도입하여 파의 산란효과를 근사적으로 고려하였으며, Morison 방정식 상에서는 무시되는 연직기둥의 바닥에 작용하는 수직력을 이 면에서의 동압력과 수직방향의 부가질량에 관련된 관성력으로 산정하여 고려하였다. 수치해석은 1000 ft 수심에 위치한 가상적인 구조물에 $0^{\circ}$ 및 $45^{\circ}$로 입사하는 파에 대하여 전술한 방법 및 이론적으로 보다 정확한 산란파이론에 의한 방법을 사용하여 수행하였으며, TLP 운동 및 tether의 상단 인장력의 전달함수(RAO)를 구하여 비교 검토하였다.

• International Journal of Naval Architecture and Ocean Engineering
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• 제8권5호
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• pp.487-495
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• 2016

In the offshore project such as semi-submersible FPU and FPSO, the free fall type life boat called TEMPSC (Totally Enclosed Motor Propelled Survival Craft) has been installed for the use of an emergency evacuation of POB (People on Board) from the topside platform. For the design of life boat arrangement for semi-submersible FPU in the initial design stage, the drop height and launch angle are required fulfill with the limitation of classification society rule and Company requirement, including type of approval as applicable when intact and damage condition of the platform. In this paper, we have been performed the numerical studies to find proper arrangement for the life boats consider drop height in various environmental conditions such as wave, wind and current. In the calculations, the contributions from static and low frequency (LF) motions are considered from the hydrodynamic and mooring analysis as well as damage angle from the intact and damage stability analysis. Also, Air-gap calculation at the life boat positions has been carried out to check the effect on the life boat arrangement. The air-gap assessment is based on the extreme air-gap method includes the effect of 1st order wave frequency (WF) motions, 2nd order low frequency roll/pitch motion, static trim/heel and set down.

• International Journal of Naval Architecture and Ocean Engineering
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• 제9권5호
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• pp.552-567
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• 2017

The floating crane vessel in waves gives rise to the motion of the lifted object which is connected to the hoisting wire. The dynamic tension induced by the lifted object also affects the motion responses of the floating crane vessel in return. In this study, coupled motion responses of a floating crane vessel and a lifted subsea manifold during deep-water installation operations were investigated by both experiments and numerical calculations. A series of model tests for the deep-water lifting operation were performed at Ocean Engineering Basin of KRISO. For the model test, the vessel with a crane control system and a typical subsea manifold were examined. To validate the experimental results, a frequency-domain motion analysis method is applied. The coupled motion equations of the crane vessel and the lifted object are solved in the frequency domain with an additional linear stiffness matrix due to the hoisting wire. The hydrodynamic coefficients of the lifted object, which is a significant factor to affect the coupled dynamics, are estimated based on the perforation value of the structure and the CFD results. The discussions were made on three main points. First, the motion characteristics of the lifted object as well as the crane vessel were studied by comparing the calculation results. Second, the dynamic tension of the hoisting wire were evaluated under the various wave conditions. Final discussion was made on the effect of passive heave compensator on the motion and tension responses.

• 동력기계공학회지
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• 제16권5호
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• pp.40-46
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• 2012

본 연구에서는 자유표면 유동문제를 효율적으로 계산하기 위한 방법으로서, 수치 파수조를 구현하여 잠수체에 의한 조파현상을 시간영역에서 다룰 수 있는 수치해법에 대하여 소개하였다. 그리고 이를 이용하여 수행된 연구내용들을 검토하고 양력물체의 경우를 포함하기 위하여 개선된 수학적 정식화 및 수치해법의 개요를 다루었다. 임의의 운동을 하는 양력물체에 의한 조파현상을 전산기로 구현하는 수치 파수조는 과중한 계산시간이 문제가 되는데, 이는 수치 Kutta 조건의 구현과 양력표면 후방의 wake 영역을 계산에서 고려해야 하기 때문이다. 따라서 한층 더 수치계산의 효율성이 중요하다고 판단되므로, 본 연구에서 소개된 3차원 고차 스펙트럴/경계요소법(High-Order Spectral/Boundary Element Method)은 자유표면 요소수를 N이라 할 때 그 계산량이 NlogN에 비례(N이 클 때는 거의 선형적으로 비례)하여 증가하므로 기존의 방법들 보다 매우 효율적인 수치해법이라 할 수 있다. 향후, 본 연구의 타당성 검증을 위한 수치코드의 개선과 여러 가지 수치계산결과 비교 등의 노력이 더 필요하다고 생각된다.

• 대한조선학회논문집
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• 제52권3호
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• pp.222-235
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• 2015

The purpose of this study is to suggest guidelines for riser fatigue analysis in terms of selection of reasonable analysis method. Three analysis methods (spectral, regular wave, rain-flow counting) are introduced and compared. As the riser systems give non-linear response, the time-domain analysis method is more preferred than frequency-domain analysis method. The spectral fatigue analysis method, however, is still useful for identifying fatigue prone areas. Once stress RAO is established, fatigue damage can be calculated very quickly. The regular wave method and the rain-flow counting method are more time consuming but give more exact results compare to spectral method. In case of regular wave method, a set of regular waves which represent random sea states is considered for dynamic analysis. The rain-flow counting method is the most intuitive and exact method because it refers time history stresses containing most of non-linear effects of the riser system. However, it is not common for early design stage to use rain-flow counting method because of its high cost. In this study, it was confirmed that the regular wave method is the most cost effective way in specific cases. However, if the system is highly non-linear, it seems that the regular wave method gives less accurate results than rain-flow counting method. Therefore, it is imperative that the engineers select appropriate analysis method based on design stage and given engineering period. This paper also discusses the theoretical background of each calculation method and hydrodynamic aspects of marine riser systems. A steel catenary riser (SCR) line on FPSO was considered and marine dynamic program (OrcaFlex) was used for static and dynamic analysis.

• 대한조선학회논문집
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• 제47권2호
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• pp.150-162
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• 2010

Assessment of hydrodynamic performance of a ship hull has been focused on a model ship rather than a full-scale ship. In order to design the propeller of a ship, model-scale wake is often extended to full-scale based upon an empirical method or designer's experience, since wake measurement data for a full-scale ship is very rare. Recently modern CFD tools made some success in reproducing wake field of a model ship, which implicates that there are some possibilities of the accurate prediction of full-scale wakes. In this paper firstly the evaluation of model-scale wake obtained by Fluent package was performed. It was found that CFD calculation with the Reynolds-stress model (RSM) provided much better agreement with wake measurement in the towing tank than with the realizable k-$\varepsilon$ model (RKE). In the next full-scale wake was calculated using the same package to find out the difference between model and full-scale wakes. Three hull forms of KLNG, KCS, KVLCC2 having measurement data open for the public, were chosen for the comparison of resistance, form factor, and propeller plane wake between model ships and full-scale ships.

• 대한조선학회지
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• 제27권4호
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• pp.32-42
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• 1990

선체표면 및 자유표면에 Rankine Source를 분포하는 방법에 의하여 선체 주위의 유동의 수치계산을 수행하였다. 선체표면 및 자유표면은 사각형 Panel들로 표시되며 자유표면 조건은 이중모형 흐름에 의해 선형화 되어 C.W. Dawson의 유한차분법에 따라 교란없는 자유표면에 적용되었다. Wigley 선형 및 Series 60, $C_B=0.6$ 선형에 대한 Fixed Condition에서의 조파저항, 선측파고, 압력분포 및 Trim & Sinkage 등을 계산하였으며 계산된 결과는 국내외 수조에서의 계측치와 비교하였다. 또한, 선체표면과 자유표면의 Panel 분할조건 및 자유표면의 설정영역의 변화에 따른 계산치의 영향도 아울러 조사하였다.

• 한국해양공학회지
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• 제29권2호
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• pp.154-162
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• 2015

In this study, experimental and numerical methods were applied to observe sloshing impact phenomena. A two-dimensional rectangular tank filled with water and air was considered with a specific excitation condition that induced a hydrodynamic impact without an air pocket at the top corner of the tank. High-speed cameras and a pressure measurement system were synchronized, and a particle image velocimetry (PIV) technique was applied to measure the velocity field and corresponding pressure. The experimental condition was implemented in a numerical computation to solve incompressible two-phase flows using a Cartesian-grid method. The discretized solution was obtained using the finite difference and constraint-interpolation-profile (CIP) methods, which adopt a fractional step scheme for coupling the pressure and velocity. The tangent of the hyperbola for interface capturing (THINC) scheme was used with the weighed line interface calculation (WLIC) method to capture the interface between the air and water. The calculated impact pressures and velocity fields were compared with experimental data, and the relationship between the local velocity and pressure was investigated based on the computational results.