Min-Guk Seo

Systematic Experimental and Numerical Analyses on Added Resistance in Waves

국제적으로 친환경에 대한 관심이 증가하고, 국제해사기구 (International Maritime Organization, IMO)가 선박의 온실가스 방출을 규제하기 위해서 2013년부터 선박연비제조지수 (Energy Efficiency Design Index, EEDI)를 도입하면서 조선 및 해운업계 에서는 선박의 운항 효율이 주요 관심사가 되었다. 선박의 운항 효율을 향상시키기 위한 많은 방법 중에 하나로 파랑 중 저항을 감소시키는 방법이 고려되고 있다. 부가저항이란 선박이 실제 해 상에서 운항하는 경우 바람이나 파랑에 의해 정수 중 저항보다 증가하는 저항을 말하며, 이러한 부가저항은 선박에 따라서 정수 중에서 운항하는 경우의 저항보다 15~30%까지 커지는 경우도 있다. 따라서 부가저항을 정확히 예측하고 이를 효과적으로 감소 시키고자 하는 연구들이 수행되고 있다. 본 논문에서는 최근 수행되었던 부가저항의 예측을 위한 수조 실험 및 수치계산 기법에 대한 연구들에 대하여 체계적으로 살펴 보고, 그 결과들을 비교하여 여러 기법들에 대한 검증과 장단점, 그리고 실선 적용에 대한 활용성 등을 다루고자 한다. 파랑 중 부가저항의 추정을 위해 실험 및 수치적 연구는 1970~1980년대에 많이 수행이 되었으나 한동안 주목을 받지 못 하다가 최근 그린쉽에 대한 관심이 고조되며 근래들어 여러 연구 들이 수행되고 있다. 부가저항의 초기 연구는 실험에 기반하여 진행되었다. 부가저항 실험은 1970년대부터 Series 60 선형 (Gerritsma & Beukelman, 1972; Storm-Tejsen, et al., 1973), S175 컨테이너선 모델 (Fujii & Takahashi, 1975; Nakamura & Naito, 1977), Wigley 모델 (Journee, 1992) 등에 대한 연구가 진행되었다. 최근 Kuroda, et al. (2011)은 컨테이너선 기본선형 을 기준으로 선수부의 형상을 다양하게 변화시켜가면서 이에 따 른 부가저항 변화를 살펴보았다. 또한, Lee, et al. (2013)도 최 pISSN:1225-1143, Vol. 51, No. 6, pp. 459-479, December 2014

Comparative Study on Added Resistance for Different Hull Forms by using Weakly-Nonlinear Seakeeping Formulations

Recently, the design of commercial ships with less green-house gas is one of great interests in naval architecture fields. Ship designers are asked to find optimum hull forms with minimum resistance in ocean waves. The accurate computation of added resistance, therefore, is getting more important for the prediction of power increase in random ocean waves. This study focuses on the numerical computation of added resistance on ships with Ax-bow shapes which are designed to reduce added resistance. To this end, the time-domain Rankine panel methods based on weakly-nonlinear and weak-scatterer approaches are applied, which can reflect the influence of above-still-water bow shape. As computational models, KCS and KVLCC2 hull forms are considered. Each ship is combined with the three types of Ax-bow shape, and computational results are compared each other.

Analysis of Added Resistance in Short Waves

In this study, the added resistance of ships in short waves is systematically studied by using two different numerical methods - Rankine panel method and Cartesian grid method - and existing asymptotic and empirical formulae. Analysis of added resistance in short waves has been preconceived as a shortcoming of numerical computation. This study aims to observe such preconception by comparing the computational results, particularly based on two representative three-dimensional methods, and with the existing formulae and experimental data. In the Rankine panel method, a near-field method based on direct pressure integration is adopted. In the Cartesian grid method, the wave-body interaction problem is considered as a multiphase problem, and volume fraction functions are defined in order to identify each phase in a Cartesian grid. The computational results of added resistance in short waves using the two methods ar e systematically compared with experimental data for several ship models, including S175 containership, KVLCC2 and Series 60 hulls (CB = 0.7, 0.8). The present study includes the comparison with the established asymptotic and empirical formulae in short waves.

Analysis of Added Resistance: Comparative Study on Different Methodologies

This paper considers the comparative study on added resistance for different methodologies. An accurate prediction of added resistance and resultant power increase becomes an important issue in greenship design. There are several methodologies for the prediction of added resistance, and most of them are based on frequency domain approaches such as slender-body theory or wave Green-function approach. As the time-domain approaches becomes an alternative method for seakeeping analysis, the time-domain approaches are also applicable for added resistance prediction.In this paper, a few approaches have been applied for the prediction of added resistance on different hull forms. The methods to be considered in this study are (i) slender-body method, (ii) Rankine panel methods, (iii) Cartesian-grid-based Euler solver, and (iv) short-wave approximations. Both the far- and near-field formations are considered in the slender-body and Rankine panel methods, while the direct pressure integration is applied for the CFD method. The computational results are validated by comparing them with experimental data on Wigley hull, Series 60 hull, and S175 containership, showing reasonable agreements for all models. The study is extended to the analysis of added resistance in short wavelengths.© 2013 ASME

Effect of internal sloshing on added resistance of ship

The motion responses of ships carrying liquid cargo are affected not only by external wave excitation, but also by internal sloshing-induced forces and moments. Sloshing flow is coupled with the ship motion. This means the added resistance in waves may change when sloshing occurs inside the tank of the ship. In this study, the motion responses and added resistance of a ship, coupled with the sloshing-induced internal forces and moments are considered by using the linear potential theory. The three-dimensional Rankine panel method, in which the physical quantities are represented by using B-spline basis function, is applied. The sloshing flow of inner tanks is also simulated by using the Rankine panel method and linearized boundary value problem. To study the added resistance, a near-field method, which integrates the second-order pressure on a body surface, is applied. The model ship is a blunt modified Wigley model with two inner tanks. Numerical results obtained without inner tanks are compared with the experimental data, and then the effect of filling ratio of inner tanks on ship motion and added resistance are observed. The components that induce added resistance are examined, and the effects of surge motion on sloshing flow and added resistance are briefly considered. This study shows that the sloshing flow inside the inner tanks may significantly influence not only the motion responses, but also added resistance, especially, when the incident wave frequency approaches the resonance frequency of the sloshing flow.

Numerical Analysis of Turning Performance in Waves by Considering Wave Drift Forces

최근 국제적으로 친환경에 대한 관심이 증가하고, 국제해사기 구(International Maritime Organization, IMO) 산하의 해양환경보 전위원회(Marine Environmental Protection Committee, MEPC) 에서 선박의 온실가스 방출을 규제하기 위해서 EEDI(Energy Efficiency Design Index)를 도입하면서 선박의 효율에 대한 연구 가 활발히 진행되었다. 하지만 선박의 효율을 추구하다 보니 반 대급부로 해상환경조건이 좋지 않은 상황에서의 선박의 안정성 및 조종성이 문제가 되었으며, 이러한 문제를 예방하기 위해서 2013년에 MEPC에서는 황천시 선박의 조종성능을 유지하기 위 한 최소 마력에 대한 가이드라인을 제안하였다 (MEPC, 2013). 이러한 국제적인 이슈로 인해서 파랑 중 조종성능에 대한 연구가 활발하게 이루어지고 있으며, 유럽연합의 공동 프로젝트인 SHOPERA(Energy Efficient Safe SHip OPERAtion), 일본의 JASNAOE 주도의 연구가 대표적으로 진행되고 있다. SHOPERA 프로젝트에서는 benchmark study를 통해서 탱커와 컨테이너선 에 대한 파랑 표류력 및 파랑 중 조종 시험을 수행하였으며, 파 랑 중 조종성능을 계산할 수 있는 프로그램을 평가하기도 하였다 (SHOPERA, 2016). 파에 의한 영향을 평가하는 효과적인 방법은 파랑 중 자유항 주시험을 하는 것이며, 이에 대한 몇몇 결과가 발표된 바 있다. Uenoet et al. (2003)는 파랑 중에서 탱커 모형을 사용하여 선 회시험, 지그재그시험 및 정지시험을 수행하였다. Yasukawa (2006a, 2006b)는 컨테이너 선박에 대하여 규칙파 및 불규칙파 에서 선회시험을 수행하였으며, 파랑 중 조종 수치해석 또한 수 행하여 결과를 비교하였다. 또한 언급한 바와 같이 SHOPERA benchmark study에서 KVLCC2, DTC 선형의 규칙파 및 불규칙 파에서의 선회 시험을 수행한 바 있다. 파에 의항 영향을 평가하기 위한 또 다른 방법은 수치기법을 사용하는 것이며, 이를 사용하여 파랑 중 선박의 조종성능을 추 정하는 다양한 연구가 진행되어 오고 있다. Ottosson and Bystrom (1991)은 고정된 부가 질량(added mass)과 감쇠계수 (damping coefficient)를 사용하여 파랑 중 조종운동을 계산하였 으며, Fang et al. (2005)은 조우주파수에 따른 선형 유체력 미 파랑 표류력을 고려한 선박의 파랑 중 선회성능 해석 서민국 .남보우† .김연규

Numerical simulation of Sewol ferry capsize

In this study, numerical simulations are conducted in order to analyze the capsizing of roll-on/roll-off passenger ship, Sewol, which occurred on 16 April 2014. Since little is known about the reason of ship capsize, numerical simulation aims to the finding of possible scenarios which can cause this tragic accident. To this end, 4-degree-of-freedom (surge, sway, yaw, and roll motions) maneuvering equations are solved. The hydrodynamic coefficients obtained from a similar ship are applied and the WISH-Maneuver program of Seoul National University is used to perform the numerical simulation. The present analysis utilizes the probabilistic approach, which considers various physical variables and involves the simulation of a large number of cases using different combinations of the variables. The total number of simulation cases is over 70 millions, and the simulation results that are close to the conditions of the actual accident situation are selected. Through this process, the effects of the simulation variables are investigated and the potential causes of the accident are identified. The results show the hydrostatic instability of the ship in the operational condition owing to cargo overload and insufficient ballast water, which raised the vertical center of gravity. The reduced stability combined with large starboard steering angle and cargo movement during large heel may have been the decisive reason of capsizing.

Panel generation framework for seakeeping analysis of multiple bodies and offshore structures

This paper presents a panel generation framework for seakeeping analysis of multiple bodies and offshore structures. The configurations of multiple bodies and offshore structures are different from those of a single ship. In particular, the topological structure of the free surfaces becomes complicated due to the multiple floating bodies, resulting in multiple classifications for the free surfaces based on their genus. The multi-body configuration consists of two floating bodies placed in two configurations, i.e., side by side and tandem, which would generate two holes in the free surface. For the offshore structure case, multiple holes are generated in the free-surface domain due to the legs of the offshore structure. In this work, strategies for generating body and free-surface panels are provided, and the results are analyzed. A software prototype that implements the proposed methods is developed to provide efficient panel generation for multiple bodies and offshore structures. Examples demonstrate that the proposed framework can be successfully used for seakeeping analysis of multiple bodies and offshore structures.

Effects on Nonlinear Ship Motions on Ship Maneuvering in Large Amplitude Waves

This paper considers a numerical analysis of ship maneuvering performance in the high amplitude incident waves by adopting linear and nonlinear ship motion analysis. A time-domain ship motion program is developed to solve the wave-body interaction problem with the ship slip speed and rotation, and it is coupled with a modular type 4-DOF maneuvering problem. Nonlinear Froude-Krylov and restoring forces are included to consider weakly nonlinear ship motion. The developed method is applied to observe the nonlinear ship motion and planar trajectories in maneuvering test in the presence of incident waves. The comparisons are made for S-175 containership with existing experimental data. The nonlinear computation results show a fair agreement of overall tendency in maneuvering performance. In addition, maneuvering performances with respect to wave slope is predicted and reasonable results are observed.