Sungkyun Lee

Experimental Study on the Six Degree-of-Freedom Motions of a Damaged Ship Floating in Regular Waves

One of the most critical issues for ship owners, shipbuilders, and insurance companies is the operational safety. In particular, keeping damaged ships stable in waves is of great interest, because more nonconventional hull forms are being introduced for military and passenger vessels while international rules and regulations are becoming stricter. However, ship stability for damaged ships is quite different from that for intact ships as the assessment is very complicated and difficult due to the highly nonlinear behavior. Computational fluid dynamics (CFD) methods that solve the Navier-Stokes equations are acknowledged as the only viable approach to simulate and analyze these complex physical phenomena. Although there have been a number of research activities reported on damaged ship stability recently, most of them are not designed to validate CFD studies. For a data set to be valuable for CFD validation and development, model tests should eliminate unclear factors as much as possible. The main objective of this study is to establish an experimental database for CFD validation by collecting data from towing tank tests of a ship hulls six degree-of-freedom (6DOF) motion responses in regular waves for both intact and damaged conditions. A mooring system was designed to prevent drift motions of the ship model. Parametric roll was not observed when the ship was damaged, although it was observed for the intact ship in the same wave conditions. The mooring force acting on the ship model due to spring tension was also calculated.

Depth Control of a Submerged Body Near the Free Surface by LQR Control Method

AbstractThe submerged body near the free surface is disturbed by the 1s t and 2nd order wave forces, which results in unstable movements when no control is applied. In this paper, the vertical motions of the submerged body are analyzed, and the time-variant nonlinear system for the vertical motions of the submerged body is transformed to the time-invariant linear system in state space. Next, depth controller of the submerged body is designed by using LQR control, one of the modern optimal control technique. Numerical simulation shows that effective depth controls can be achieved by LQR control. ※Keywords: Submerged body(수중운동체), Vertical motion(연직면 운동), 1st and 2nd order Wave forces and moments(1,2차 파 강제력 및 파 강제 모멘트), Free surface effects(자유표면 효과), LQR control(LQR 제어) 1. 서론 수중운동체가 수면 근처에서 운항할 경우 파도에 의한 외력을 받게 되는데, 파 강제력을 파 진폭에 비례하는 1차항 힘과 2차항 힘인 시간 평균 표류력으로 나타내었다. 이 중에서 2차항 힘과 모멘트는 운동체를 수면으로 끌어올리는 역할을 한 접수일: 2009년 3월 24일, 승인일: 2009년 7월 20일✝교신저자: kdj8277@snu.ac.kr, 010-3994-4007 다. 따라서 수중운동체가 잠망경 심도로 운항할 때 원하는 심도를 유지 및 제어하기 위해서는 이러한 2차항 힘과 모멘트를 고려하여 선수와 선미의 수평타를 적절히 제어할 필요가 있다.수면 근처에서 파 강제력 및 파 강제 모멘트를 고려하여 수중운동체의 심도를 제어하는 연구는 이전부터 진행되어 왔다. Richards and Stoten (1982)은 수중운동체의 형상을 고려한 파강제력 및 파 강제 모멘트 수학 모형을 정립하였다. Dumlu and Istefanopulos(1995)는 수중운동체 심

Experimental Study on Free Roll Decay Motions of a Damaged Ship for CFD Validation Database

Among many factors to be considered for higher safety level requirements, the hull stability in intact and damaged conditions in seaways is of utmost importance. Since the assessment of a damaged ship is complicated due to the highly non-linear behavior, it is widely acknowledged that computational fluid dynamics (CFD) methods are one of the most feasible approaches. Although many research activities are being reported on the damaged ship stability recently, most of them are not designed for validation of CFD studies. In this study, well-designed model tests were performed to build a CFD validation database, which is essential in developing better CFD methods for the damage stability assessment. The geometry of the damaged compartment and test conditions were determined based on preliminary CFD simulations. Free roll decay tests in calm water with both intact and damaged ships were performed and the roll motion characteristics were compared. The damaged ship showed a larger roll damping coefficient and more rapid decrease of roll amplitude than the intact ship. The primary reason of these efforts can be explained by the movement of the flooded water.

The Effect of the Turning Rate of the Pod Propeller on the Roll Control System of the Cruise Ship

Recently, the application and installation of the pod propeller to the cruise ship is dramatically increased. It is because pod propulsion system allows a lot of flexibility in design of the internal arrangement of a ship. To reflect this trend, many researches have conducted to use the pod propeller for the roll stabilization of a ship. In the paper, a roll stabilization controller is designed by using fins and pod propellers as the control actuators for cruise ships. Two kinds of control algorithms are adopted for the roll control system; LQR (Linear Quadratic Regulator) algorithm and frequency-weighted LQR algorithm. Through the numerical simulation, the effect of the turning rate of the pod propeller on the roll control system is analyzed. Analysis of the simulation results indicated that the turning rate of the pod propellers is one of the important parameters which give the significant effects on the roll stabilization.

Study on Coning Motion Test for Submerged Body

A submerged body is sensitive to changes in the roll moment because of the small restoring moment and moment of inertia. Thus, a method for predicting the roll-related hydrodynamic coefficients is important. This paper describes a deduction method for the hydrodynamic coefficients based on the results of a coning motion test. A resistance test, static drift test, and coning motion test were performed to obtain the coefficients in the towing tank of Seoul National University. The sum of the hydrodynamic force, inertial force, gravity, and buoyancy was measured in the coning motion test. The hydrodynamic force was deduced by subtracting the inertial force, gravity, and buoyancy from the measured force. The hydrodynamic coefficients were deduced using the regression method.

The Effect of Appendages of a Water-Jet Propelled High Speed Vessel on the Course Keeping Ability

It has been often reported that a water-jet propelled high speed vessel lost the course keeping ability in seaway. In this study, model tests of a high speed vessel were performed to measure the running attitude and to check the course keeping ability. The model ship may lose the course keeping ability due to bad running attitudes such as bow drop. So model tests were carried out to improve the running attitude by changing the position of longitudinal center of gravity and using appendages at the bow and the stern of a model. The position of lateral center of pressure moved toward stern and the course keeping ability was improved by modifying the transom wedge angle.

Free Roll Decay Study of a Damaged Ship for CFD Validation

Among many factors to be considered for higher safety level requirements, the hull stability in intact and damaged conditions in seaway is of utmost importance. Since the assessment of the damaged stability is complicated due to the highly non-linear behavior, it is widely acknowledged that computational fluid dynamics (CFD) methods are the only viable approach. Although many research activities are being reported on the damaged stability recently, most of them are not designed for validation of CFD studies. In this study, well-designed model tests are performed to build a CFD validation database, which is essential in developing better CFD methods for the damaged stability assessment. The geometry of the damaged compartment and test conditions are determined based on preliminary CFD simulations. The damping coefficient of the model becomes large when the ship is damaged. The amplitude of roll motion is decreased more rapidly in damaged condition because the flooding water motion. Detailed description of the tests and corresponding conditions are described to constitute a CFD validation database.Copyright