Showing papers in "Journal of Ocean Engineering and Technology in 2011"

Plasticity and Fracture Behaviors of Marine Structural Steel, Part III: Experimental Study on Failure Strain

Abstract: This is the third of several companion papers dealing with the derivation of material constants for ductile failure criteria under hydrostatic stress. It was observed that the ultimate engineering stresses and elongations at fracture from tensile tests for round specimens with various notch radii tended to increase and decrease, respectively, because of the stress triaxiality. The engineering stress curves from tests are compared with numerical simulation results, and it is proved that the curves from the two approaches very closely coincide. Failure strains are obtained from the equivalent plastic strain histories from numerical simulations at the time when the experimental engineering stress drops suddenly. After introducing the new concept of average stress triaxiality and accumulated average strain energy, the material constants of the Johnson-Cook failure criterion for critical energies of 100%, 50%, and 15% are presented. The experimental results obtained for EH-36 steel were in relatively good agreement with the 100% critical energy, whereas the literature states that aluminum fits with a 15% critical energy. Therefore, it is expected that a unified failure criterion for critical energy, which is available for most kinds of ductile materials, can be provided according to the used materials.

Motion Analysis of 5-MW Floating Offshore Wind Turbine

Abstract: The motion responses of a 5-MW floating offshore wind turbine were simulated in regular and irregular waves and its RAOs and significant motion responses were calculated, respectively. The floating offshore wind turbine employed in this simulation was the OC3-Hywind designed by the National Renewable Research Laboratory, USA. The numerical simulation was carried out using MOSES (Multi-Operational Structural Engineering Simulator), which is widely used to analyze and design floating offshore structures in the gas and oil industry.

Plasticity and Fracture Behaviors of Marine Structural Steel, Part I: Theoretical Backgrounds of Strain Hardening and Rate Hardening

Abstract: *Dep't of Naval Architecture and Ocean Engineering, Inha Univer sity, Incheon, Korea**Dep't of Naval Architecture and Marine Engineering, Mokpo Nat ional University, Mokpo, KoreaKEY WORDS: Plasticity 소성, Strain hardening 변형률 경화, Strain rate hardening 변형률 속도 경화, Fracture 파단, True stress 진응력ABSTRACT: In this paper, the global study trends for material behaviors a re investigated regarding the static and dynamic hardenings and final fractures of marine structural steels. In particular, after rev iewing all of the papers published at the 4th and 5th ICCGS (In ternational Conference on Collision and Grounding of Ship), the used hardening and fra cture properties are summarized, explicitly presenting the mate rial properties. Although some studies have attempted to employ new plasticity a nd fracture models, it is obvious that most still employed an i deal hardening rule such as perfect plastic or linear hardening and a simple shear fracture criterion with an assumed value of failure strain. HSE (2001) presented pioneering study results regarding the temperature dependency o f material strain hardening at various levels of temperature, b ut did not show strain rate hardening at intermediate or high strain rate range s. Nemat-Nasser and Guo (2003) carried out fully coupled tests for DH-36 steel: strain hardening, strain rate hardening, and temperature harden ing and softening at multiple steps of strain rates and tempera tures. The main goal of this paper is to provide the theoretical background for stra in and strain rate hardening. In addition, it presents the procedure and methodology needed to derive the material constants for the static hardenin g constitutive equations of Ludwik, Hollomon, Swift, and Ramber g-Osgood and for the dynamic hardening constitutive equations of power from Cowper-S ymonds and Johnson-Cook.교신저자 정준모: 인천광역시 남구 용현동 253, 032-860-7346, jmchoung@inha.ac.kr

Extraction of Wave Energy Using the Coupled Heaving Motion of a Circular Cylinder and Linear Electric Generator

Abstract: *Department of Ocean System Engineering, Jeju National Universi ty, Jeju, Korea**Department of Civil Engineering, Kyongju University, Kyongju, KoreaKEY WORDS: Heave motion 수직운동, Linear electric generator 선형발전기, Power absorbtion 에너지 흡수, Matched eigenfunction expansion method 고유함수전개법, Double resonance 이중 공진, Circular cylinder 원기둥ABSTRACT: The feasibility of wave energy extraction from a heaving trunca ted cylinder and the corresponding response of the linear elect ric generator (LEG) composed of spring, magnet, and coil has been i nvestigated in the frame of three-dimensional linear potential theory. The heaving motion of a circular cylinder is calculated by means of the matched eigenfunction expansion method. Further, the analytical results are validated by numerical results using the ANSYS AQWA commercial code. By the action of a heaving circular cylinder, the magnet suspended by a spring can slide vertically inside the heaving cylinder. The mechanical po wer is extracted from the magnet motion relative to the coil/st ator which is attached to the cylinder. The coupled ODE of a heaving cylinder and LEG sys tem in waves is derived to obtain the magnet motion relative to a cylinder. To maximize the relative motion of the magnet, both the buoy draft and the LEG system parameters (spring stiffness, damping) should be selected properly for generating the double resonance considering the pe ak frequency of the target spectrum.교신저자 조일형: 제주시 제주대학로 66, 064-754-3482, cho0904@jejunu.ac.kr

Numerical Analysis of Wave-induced Motion of Floating Pendulor Wave Energy Converter

Abstract: In this paper, the wave-induced motion characteristics of a floating pendulor are investigated numerically. A floating pendulor is a movable-body-type wave energy converter. This device consists of three main parts (floater, pendulum, and damping plates). In order to obtain the hydrodynamic coefficients and wave exciting forces acting on floating bodies, a higher-order boundary element method (HOBEM) using a wave Green function is applied to the present problems. The hinged motion of a pendulum is simulated by applying the penalty method. In order to obtain a more realistic motion response for a pendulor, numerical body damping is included. First, the wave force and motion characteristics of just a floater are observed with respect to different shape parameters. Then, a coupled analysis of a floater, pendulum, and damping plates is carried out. The relative pitch velocity and wave forces acting on the floating pendulor are compared with those of a fixed pendulor.

Plasticity and Fracture Behaviors of Marine Structural Steel, Part II: Theoretical Backgrounds of Fracture

Abstract: The main goal of this paper is to provide the theoretical background for the fracture phenomena in marine structural steels. In this paper, various fracture criteria are theoretically investigated: shear failure criteria with constant failure strain and stress triaxiality-dependent failure strain (piecewise failure and Johnson-Cook criteria), forming limit curve failure criterion, micromechanical porosity failure criterion, and continuum damage mechanics failure criterion. It is obvious that stress triaxiality is a very important index to determine the failure phenomenon for ductile materials. Assuming a piecewise failure strain curve as a function of stress triaxiality, the numerical results coincide well with the test results for smooth and notched specimens, where low and high stress triaxialities are observed. Therefore, it is proved that a failure criterion with reliable material constants presents a plastic deformation process, as well as fracture initiation and evolution.

Material Properties of Arctic Sea Ice during 2010 Arctic Voyage of Icebreaking Research Vessel ARAON: Part 1 - Sea Ice Thickness, Temperature, Salinity, and Density -

Abstract: *Department of Ocean Engineering, Korea Maritime University, Bu san, KoreaKEY WORDS: Icebreaking research vessel ARAON 쇄빙연구선 아라온, Arctic voyage 북극해 항해, Material properties of sea ice 해빙의 재료특성, Thickness of ice 빙판 두께, Temperature 온도, Density 밀도, Salinity 염도ABSTRACT: A field trial in an ice-covered sea is one of the most importan t tasks in the design of icebreaking ships and offshore structu res. To correctly estimate the ice load and ice resistance of a ship's hull, it is essential to understand the material properties of sea ice during ice field trials and to use the proper experimental procedure for gathering effe ctive ice data. The first Korean-made icebreaking research vess el, “ARAON,” had her second sea ice trial in the Arctic Ocean during the summer seas on of 2010. This paper describes the test procedures used to ob tain proper sea ice data, which provides the basic information for the ship's perfo rmance in an ice-covered sea and is used to estimate the correc t ice load and ice resistance of the IBRV ARAON. The data gathered from the sea ic e in the Chukchi Sea and Beaufort Sea during the Arctic voyage of the ARAON includes the temperature, density, and salinity of the sea ice, which was believed to be from two-year old ice floes. This paper analyses the gathered sea ice data in comparison with data from the first voyage of t he ARAON during her Antarctic Sea ice trial.교신저자 최경식: 부산광역시 영도구 동삼동 1번지, 051-410-4324, kchoi@hhu.ac.kr본 논문은 한국해양공학회 2010년 추계학술대회에서 발표되었음(박영진 등, 2010).

Data Acquisition of Time Series from Stationary Ergodic Random Process Spectrums

Abstract: The fatigue damages in structural details of offshore plants can be accumulated due to various environmental loadings such as swell, wave, wind and current. It is known that load histories acting on mooring and riser systems show stationary and ergodic bimodal wide-banded process. This paper provides refined approach to obtain time signals representing stress range histories from wide-banded bimodal spectrum which consists of ideally narrow-banded and fully separated two spectrums. Variations of the probabilistic characteristics for time signals according to frequency and sampling time increments are compared with the reference data to be the probabilistic characteristics such as zero-crossing period, peak period, and irregularity factor obtained from an assumed ideal spectrum. It is proved that the sampling time increment more affects on the probabilistic characteristics than frequency increment. The fatigue damages according to the frequency and sampling time increments are also compared with the ones with minimum increment condition which are thought to be exact fatigue damage. It is concluded that the maximum sampling time increment to obtain reliable time signals should be determined that ratio of applied maximum sampling time increment and minimum period is less than approximately 0.08.

Shear and CBR Characteristics of Dredge Soil-Bottom Ash-Waste Tire Powder-Mixed Lightweight Soil

Abstract: *Department of Ocean Engineering, Pukyong National University, Busan, Korea*Geotechnical Dept., Dodam E&C, Seoul, KoreaKEY WORDS:Recycling 재활용, Waste tire powder 폐타이어 분말, Lightweight soil 경량토, Triaxial compression test 삼축압축시험, CBR test 노상토 지지력비 시험ABSTRACT:This study investigated the shear and CBR characteristics of d redge soil-bottom ash-waste tire powder-mixed lightweight soil, which was developed to recycle dredged soil, bottom ash, and waste ti re powder. Test specimens were prepared with various contents o f waste tire powder ranging from 0 to 100% at 50% intervals by the weight of the dr y dredged soil. Several series of triaxial compression tests an d CBR tests were conducted. The shear strength characteristics of the lightweigh t soil were compared using two different shear tests (triaxial compression test and direct shear test). The experimental results indicated that the internal friction angle of the lightweight soil obtained by the direct shear tests was greater than that by the triaxial shear tests. However, the cohesion value obtained by the triaxial shear tests was greater th an that by the direct shear tests. The CBR value of the lightweight soil decreased from 35% to 15% as waste tire powder content increased교신저자 김윤태: 부산광역시 남구 대연3동 599-1, 051-629-6587, yuntkim@pknu.ac.kr

Motion Control of Omnidirectional Mobile Platform for Path Following Using Backstepping Technique

Abstract: This paper proposes a controller design for an omnidirectional mobile platform (OMP) with three wheels using backstepping control. A kinematic model and dynamic model of the system are presented. Based on the dynamic modeling, a backstepping controller is designed to stabilize the OMP when following a desired path. The controller is designed based on a backstepping control theory. It includes two steps: first, a virtual state and a stability function are introduced. Second, Lyapunov functions for the system are chosen and an equation for the virtual control that makes the system stabile is obtained. The system stability is guaranteed by the Lyapunov stability theory. The simulation and experimental results are presented to demonstrate the effectiveness of the proposed controller.

Corrosion Resistance of Galvanized Steel by Treating Modified Si Organic/Inorganic Hybrid Coating Solution

Abstract: Galvanized steel has gone through a chemical process to keep it from corroding. The steel gets coated in layers of zinc because rust will not attack this protective metal. For countless outdoor, marine, or industrial applications, galvanized steel is an essential fabrication component. The reduction of the corrosion rate of zinc is an important topic. In the past, a very popular way to reduce the corrosion rate of zinc was to use chemical conversion layers based on . However, a significant problem that has arisen is that the use of chromium salts is now restricted because of environmental protection legislation. Therefore, it is very important to develop new zinc surface treatments that are environmentally friendly to improve the corrosion resistance of zinc and adhesion with a final organic protective layer. In this study, a Urethane solution (only Urethane 20 wt.%; S-700) and an organic/inorganic solution with Si (Si polysilicate 10 wt.% + Urethane 10 wt.%; LRO-317) are used. Based on the salt spray test of 72 h, S-700 and LRO-317 had a superior effect for the corrosion resistance on EGI and HDGI, respectively.

Buckling Analysis of Filament-wound Composite Towers for Large Scale Wind-Turbine

Abstract: *Pusan Educational Center for Computer Aided Machine Design, Pu san National University, Busan, KoreaKEY WORDS: Composite 복합재, Filament-wound 필라멘트 와인딩, Tower 타워, Wind-turbine 풍력, Buckling 좌굴 ABSTRACT: The purpose of this study was to investigate the buckling load of filament-wound composite towers for large scale wind-turbine s using the finite element method (FEM). To define the material p roperties, we used both the effective property method and stack ing properties method. The effective properties method assumes that a composit e consists of one ply. The stacking properties method assumes t hat a composite consists of several stacked plies. First, a linear buckling ana lysis of the tower, filament-wound with angles of [ ±30], was carried out using the two methods for composite material properties: the stacking method and effective method. An FE analysis was also performed for the composite towers using the filament winding angles of [±30], [±45], and [±60]. The FE analysis results using the stacking properties of t he composite were in good agreement with the results from the effective properties method . The difference between the FEM results and material propertie s method was approximately 0~2.3%. Above the angle of [±60], there was little change in the buckling load.교신저자 홍철현: 부산광역시 금정구 장전동 산30번지, 051-510-1665, chhong@pusan.ac.kr

CFD Analysis of Two-Dimensional Floating Body with Moon Pool under Forced Heave Motion

Abstract: A two-dimensional floating body with a moon pool under forced heave motion, including a piston mode, is numerically simulated. A dynamic CFD simulation is carried out to thoroughly investigate the flow field around a two-dimensional moon pool over various heaving frequencies. The numerical results are compared with experimental results and a linear potential program by Faltinsen et al. (2007). The effects of vortex shedding and viscosity are investigated by changing the corner shapes of the floating body and solving the Euler equation, respectively. The flow fields, including the velocity, vorticity, and pressure fields, are discussed to understand and determine the mechanisms of wave elevation, damping, and sway force.

Plasticity and Fracture Behaviors of Marine Structural Steel, Part V: Effects of Strain Rate and Temperature

Abstract: This is the fifth in a series of companion papers dealing with the dynamic hardening properties of various marine structural steels at intermediate strain rates. Five steps of strain rate levels (0.001, 1, 10, 100, 200/s) and three steps of temperature levels (LT (), RT, and HT ()) were taken into account for the dynamic tensile tests of three types of marine structural steels: API 2W50 and Classifications EH36 and DH36. The total number of specimens was 180 pieces. It was seen that the effects of dynamic hardening became clearer at LT than at RT. Dynamic strain aging accompanying serrated flow stress curves was also observed from high temperature tests for all kinds of steels. The dynamic hardening factors (DHFs) at the two temperature levels of LT and RT were derived at the three plastic strain levels of 0.05, 0.10, 0.15 from dynamic tensile tests. Meanwhile, no DHFs were found for the high temperature tests because a slight negative strain rate dependency due to dynamic strain aging had occurred. A new formulation to determine material constant D in a Cowper-Symonds constitutive equation is provided as a function of the plastic strain rate, as well as the plastic strain level. The proposed formula is verified by comparing with test flow stress curves, not only at intermediate strain rate ranges but also at high strain rate ranges.

Numerical Study for Development of Submerged Seawater Lift Pump

Abstract: *Minreal Resources Research Division, KIGAM, Daejon, Korea**Naval Architecture, Ocean and IT Engineering, Kyungnam Univer sity, Changwon, KoreaKEY WORDS: Seawater lift pump 해수펌프, Mixed flow pump 사류펌프, Floating production storage and offloading 부유식원유생산설비, Total head 전양정ABSTRACT: Seawater lift pump systems are responsible for maintaining open canal levels to provide the suction flow of circulating water pumps at the set point. The objective of this paper is to design a 2- stage mixed flow pump (for seawater lifting), investigate the n ew impeller modeling method, and performance improvements of the impeller by using a commercial CFD code. The rotating speed of the impeller is 1,7 50 rpm with a flow rate of 2,700 m3/h. A finite volume method with a structur ed mesh and realized k-e turbulent model is used to guarantee a more accurate prediction of turbulent flow in the pump impeller. The performa nce variables such as the static head, brake horsepower, and ef ficiency of the mixed flow pump are compared based on changes in the impeller blade s hape.교신저자 우남섭: 대전광역시 유성구 과학로 124, 042-868-3134, nswoo@kigam.re.kr본 연구는 2011년 울산에서 개최된 해양플랜트설계연구회 춘계워크숍에 발표된 논문을 근간으로 하고 있음을 밝힙니다 .

UNSM Surface Technology for Manufacturing and Remanufacturing Torsion Bars for Crawler Vehicles

Abstract: The Ultrasonic Nanocrystal Surface Modification (UNSM) technology improves the fatigue life of a torsion bar by inducing compressive residual stress on the surface layer. The UNSM is applied to replace the presetting method and shot peening technology. The torsion bar must be changed periodically because of a lack of durability and a phenomenon related to the stress relaxation. The torsion fatigue test specimens were made of DIN17221 material, and the results showed that the fatigue life was 5 times more than under durability test conditions. A comparison test between the commercial vehicles' presetting method and shot peened torsion bar and the UNSM torsion bar showed that the UNSM could replace the presetting method and shot peening.

Friction Welding and AE Characteristics of Magnesium Alloy for Lightweight Ocean Vehicle

Abstract: In this paper, friction welded joints were constructed to investigate the mechanical properties of welded 15-mm diameter solid bars of Mg alloy (AZ31B). The main friction welding parameters were selected to endure reliable quality welds on the basis of visual examination, tensile tests, impact energy test, Vickers hardness surveys of the bonds in the area and heat affected zone (HAZ), and macrostructure investigations. The study reached the following conclusions. The tensile strength of the friction welded materials (271 MPa) was increased to about 100% of the AZ31B base metal (274 MPa) under the condition of a heating time of 1 s. The metal loss increased lineally with an increase in the heating time. The following optimal friction welding conditions were determined: rotating speed (n) = 2000 rpm, heating pressure (HP) = 35 MPa, upsetting pressure (UP) = 70 MPa, heating time (HT) = 1 s, and upsetting time (UT) = 5 s, for a metal loss (Mo) of 10.2 mm. The hardness distribution of the base metal (BM) showed HV55. All of the BM parts showed levels of hardness that were approximately similar to friction welded materials. The weld interface of the friction welded parts was strongly mixed, which showed a well-combined structure of macro-particles without particle growth or any defects. In addition, an acoustic emission (AE) technique was applied to derive the optimum condition for friction welding the Mg alloy nondestructively. The AE count and energy parameters were useful for evaluating the relationship between the tensile strength and AE parameters based on the friction welding conditions.

Design Optimization of Wake Equalizing Duct Using CFD

Abstract: In this paper, wake equalizing duct (WED) form optimization was carried out using computational fluid dynamics (CFD) techniques. A WED is a ring-shaped flow vane with a foil-type cross-section fitted to a hull in front of the upper propeller area. The main advantage of a WED is the power savings resulting from the uniformity of the velocity distribution on the propeller plane, a reduction in the flow separation at the aft-body, and lift generation with a forward force component on the foil section. This paper intends to evaluate these functions and find an optimized WED form for minimizing the viscous resistance and equalizing the wake distribution. In the optimization process, the study uses four WED parameters: the angle of the section, longitudinal location, and angles of the axes for the half rings against the longitudinal and transverse planes of the ship. KRISO 300K VLCC2 (KVLCC2) is chosen as an example ship to demonstrate the WED optimization. The optimization procedure uses genetic algorithms (GAs), a gradient-based optimizer for the refinement of the solution, and Non-dominated Sorting GA-II(NSGA-II) for Multiobjective Optimization. The results show that the optimized WED can reduce the viscous resistance at the expense of the uniformity of the wake distribution.

Subsea Responses to the BP Oil Spill in the Gulf of Mexico

Abstract: On April 20, 2010, a well control event allowed hydrocarbon (oil and gas) to escape from the Macondo well onto Deepwater Horizon (DWH), resulting in an exploration and fire on the rig. While 17 people were injured, 11 others lost their lives. The fire continued for 36 hours until the rig sank. Hydrocarbons continued to flow out from the reservoir through the well bore and blowout preventer (BOP) for 87 days, causing an unprecedented oil spill. Beyond Petroleum (BP) and the US federal government tried various methods to prevent the oil spill and to capture the spilled oil. The corresponding responses were very challenging due to the scale, intensity, and duration of the incident that occurred under extreme conditions in terms of pressure, temperature, and amount of flow. On July 15, a capping stack, which is another BOP on top of the existing BOP, was successfully installed, and the oil spill was stopped. After several tests and subsea responses, the well was permanently sealed by a relief well and a bottom kill on September 19. This paper analyzes the subsea responses and engineering efforts to capture the oil, stop the leaking, and kill the subsea well. During the investigation and analysis of subsea responses, information was collected and data bases were established for future accident prevention and the development of subsea engineering.

A Laboratory Study for Erosional Properties of Cohesive Sediments from Mokpo Coast

Abstract: The purpose of this study is to quantify the erosional parameters, such as the critical shear stress for erosion and the erosion rate coefficient for cohesive sediments from the Mokpo coast. Using Chonbuk annular flume, five erosion tests were conducted under the uniform bed condition but a different bed density respectively. Erosion test results for Mokpo sediments have shown increases in the range of 0.16~0.43 N/m2 but decreases exponentially in the range of 272~4.64 mg/cm 2 ·hr for the given bed shear stress of 1.14~1.34 g/cm 3 . The erosional parameters of Mokpo sediments are found to vary remarkably in quantity compared with those for cohesive sediments from other sites. On the whole, the value of Mokpo coast sediments appears to be similar to Kunsan sediments but smaller than Saemankeum and Okeechobee sediments. On the other hand, Mokpo sediments have been shown to be larger than Saemankeum and Okeechobee sediments but smaller than Kunsan sediments.

Basic Experiment Using Taguchi method for Vertical Wind Turbine with Wind-shield

Abstract: This study aimed to develop a wind turbine system for the domestic wind environments. The Taguchi method was applied to obtain the optimal design for a wind turbine with a wind-shield. The design parameters were defined to look for the shape of the wind turbine. Optimal parameters were determined on the basis of the analyzed level averages of the characteristics. According to the test results to which the optimal parameters were applied, the rpm improved. It was also found that a windshield 3/4 the size contributes to improving the efficiency of existing turbines.

The Estimation of the Propulsion Performance of a UUV Using Commercial Thruster

Abstract: *Ocean Engineering Research Department, KORDI, Daejon, Korea**Marine Safety & Pollution Response Research Department, KORDI , Daejon, KoreaKEY WORDS: Autonomous underwater vehicles(AUV) 자율무인잠수정, Thrust 추력, Propulsion performance 추진성능, Properller open water(POW) test 추진기 단독성능 시험, Maximun speed 최고속도ABSTRACT: The previously developed method of estimating the propulsion pe rformance of a UUV was applied to the high speed UUV, which is propelled by commercial thrusters. The thrusters were selected with an overdesign mentality; in other words, their capacities were excessive. At that point, the designer's concern was focused on a question regardi ng at what rpm the UUV could reach the design speed. Because the developed method required thrust coefficient curve data, the researchers asked for the POW data of the thrusters from the manufacturer. From the data, the researchers extracted the thrust coefficient and estimated the rpm value of design speed for the UUV. Finally, the researchers compared the estimated value and the result from a self-propulsion test usin g a VPMM (Vertical planar motion mechanism) test at a towing ta nk in MOERI.교신저자 이종무 : 대전광역시 유성구 장동 171, 042-866-3811 jmlee@moeri.re.kr본 연구는 방사청 사업에서 수행된 연구결과 중 일부임을 밝히며 , 대상이 되는 무인잠수정의 상세한 제원을 밝히지 않는 점에 대한 양해를 바라는 바이다.

Estimation of Wave Energy Extraction Efficiency for a Compact Array System of Small Buoys

Abstract: *School of Naval Architecture and Ocean Engineering, University of Ulsan, Ulsan, KoreaKEY WORDS: Wave energy extraction 파랑에너지 추출, Compact array system of buoys 밀집 배열 부이시스템, Higher order boundary element method 고차경계요소법, Hydrodynamic interaction of multi-body system 다물체시스템의 유체역학적 상호작용ABSTRACT: A compact array system of small buoys is used for wave energy e xtraction. To evaluate the performance of this system, hydrodynamic analysis is carried out in regular waves using the higher order boundary element method. The motion response of e ach buoy is calculated considering hydrodynamic interactions caused by othe r buoys. The effect of energy extraction device is modeled as a linear damping load. The efficiencies of energy conversion are compared using the va rious sizes and arrangements of the array system and the dampin g coefficients for energy extraction. The increase in size or the packing ratio of the system gives better efficiency. However, the wave conditio n and the cost for the system should be considered to optimize performance from the pe rspective of engineering and economics. The proposed nondimensi onalized damping coefficient for energy extraction is 0.1~0.5.교신저자 최윤락: 울산광역시 남구 대학로 93, 052-259-2158, yrchoi@ulsan.ac.kr본 연구는 2010년도 한국해양과학기술협의회 공동학술대회에서 발표된 논문을 근간으로 하고 있음을 밝힙니다 .

Variation of Rotating Bending Fatigue Characteristics by UNSM on Ti-6Al-4V

Abstract: In order to analyze feasibility of replacing a conventional 6-mm Ti bar with a 5-mm bar, a series of rotating bending fatigue tests were carried out on Ti-6Al-4V bars by strengthening the fatigue performance using a special technique called UNSM (Ultrasonic Nanocrystal Surface Modification). The results of S-N curves clearly showed that the performance of the 5-mm titanium specimen was similar to that of the 6-mm specimen when the UNSM treatment was applied. The 5-mm treated specimen converged with small scattering band into the linear line of the non-treated 6-mm one. Below the fatigue life of cycles, the UNSM treatment did not show any significant superiority in the bending stress and fatigue life. However, over the fatigue life of cycles, the effect of UNSM was superior for each fatigue life, and the bending stress became longer and higher than that of the untreated one. In the case of 6-mm Ti-bar with UNSM, the fatigue limit was about 592 MPa, and there was fatigue strength increase of about 30.7% at the fatigue life of cycles compared to the untreated 6-mm bar. Therefore, the compressive residual stress made by the UNSM in Ti-6Al-4V increased the fatigue strength by more than 30%.

The Characteristics of Painted Coating on Aluminum Sheet by Sending Conditions

Abstract: Researchers suggested painted coating characteristics of aluminum sheet by various sending condition. Painted coating is used to protect the surface of the blocks from the interior and exterior. It is also used to enhance the value of automobiles and ship materials. This process makes the materials surface for protection and elegance. Although analysis of the roughness and critical scratch load of painted coating is important, the effects of this material have not progressed much. In this study, the characteristics of painted coating in the other conditions of the polished aluminum sheet were investigated. The results of both various conditions and initial roughness values were almost similar to final values after painted coating.

Future Vision through NL Tensioner Technology Development and Domestic Equipment Fabrication

Abstract: The following Technology Report was written with respect to recent offshore technology development. This report was prepared with the purpose of suggesting recent offshore technology and upcoming visions by describing the principles of an offshore drilling operation, the NL Tensioner Cylinder Package R & D, and the general technology related to three offshore patent areas, as major contents.

The Influence of Treatment Condition During Low Temperature Plasma Carburizing of AISI304L Stainless Steel

Abstract: A low temperature plasma carburizing process was performed to AISI 304L austenitic stainless steel to achieve the enhancement of surface hardness without a compromise in their corrosion resistance. Attempts were made to investigate the influence of the processing temperatures on the surface-hardened layer during low temperature plasma carburizng in order to obtain the optimum processing conditions. The expanded austenite ( γC) was formed on all the treated surfaces. Precipitates of chromium carbides were detected in the hardened layer (C-enriched layer) only for the specimen treated at 500 o C. The hardened layer thickness of γC increased up to about 35 µm, with increasing treatment temperature. The surface hardness reached about 1000 HK0.05, which is about 4 times higher than that of the untreated sample (250 HK0.05). Minor loss in corrosion resistance was observed for the specimens treated at temperatures of 310 o C-450 o C compared with untreated austenitic stainless steel. Particularly, the precipitation of chromium carbides at 500 o C led to a significant decrease in the corrosion resistance.

Analytical Research of Topside Installation in Mating phase with Crane Vessel

Abstract: The installation of a topside structure can be categorized into the following stages: start, pre-lifting, lifting, lifted, rotating, positioning, lowering, mating, and end of installation. The transfer of the module onto the floating spar hull occurs in the last three stages, from lowering to the end. The coupled multi-body motions are calculated in both calm water and in irregular waves with a significant wave height (1.52 m). The effects of the hydrodynamic interactions between the heavy lifting vessel and the spar hull during the lowering and mating stages are considered. The internal forces caused by the load transfer and ballasting are derived for the mating phases. The results of the internal forces for the calm water condition are compared with those in the irregular sea condition. Although the effect of the pitch motion on the relative vertical motion bet ween the deck of the floating structure and the topside module is significant in the mating phases, the internal force induced pitch motion is too small to have this influence. However, the effect of the internal force on the wave-induced heave responses in the mating phases is noticeable in the irregular sea condition because transfer mass-induced draught changes for the floating structure are observed to have higher amplitudes than the external force induced responses. The impacts of the module on the spar hull in the mating phase are investigated.

Long-term Consolidation Characteristics of Busan Clay considering OC or NC States

Abstract: Samsung Heavy Industries, KoreaKEY WORDS: Busan clay 부산점토, Long-term consolidation 장기압밀, Secondary compression index 이차압축지수, Creep rate 크리프 속도ABSTRACT: Numerouslong-term consolidation and secondary compression settl ements may occur in Busan clay, which is astructured soft clay and consists of a thick clay deposit. As a surcharge load is ap plied to soils, soils experience different stress paths with de pth. Therefore, it is necessary to study the long-term consolidation behavior of Busa n clay considering stress conditions such as OC or NC states. I n this study, a series of long-term consolidation tests were performed to inves tigate the consolidation characteristics of Busan clay for 20 d ays. The undisturbed clay samples were taken from 3 sites located in the Nakdong Riv er estuary. The results showed that the creep rate of the Busan clay gradually decreased with time, which indicated that the secondary compres sion settlement decreased with time. In addition, the experimen tal results for 3 samples showed that the ratios were about 0.0363 and 0.051, res pectively.교신저자: 김윤태 부산광역시 남구 대연동, 051-629-6587, yuntkim@pknu.ac.kr

Calibration of Water Velocity Profile in Circular Water Channel Using Particle Image Velocimetry

Abstract: This experimental study was performed to find rpms of the impeller and the surface flow accelerator to make a uniform velocity vertical distribution in the circular water channel. PIV technique was employed to measure the water velocity profiles into the water depth from the free surface. The number of instantaneous velocity profiles was decomposed into mean and turbulence velocity components, and the distribution of velocity fluctuation and turbulence intensity were computed for each experimental condition. From these results, the velocity uniformity was quantitatively determined to present the flow quality in the measuring section of the circular water channel. It has been shown that the proper operation of the surface flow accelerator would make the uniform velocity profiles and reduce the velocity fluctuation near the free surface.