Journal of Marine Science and Application 2012 No. 2
Guoxiong Wu
Journal of Marine Science and Application,2012(No. 2): 143-149
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A high order boundary element method was developed for the complex velocity potential problem. The method ensures not only the continuity of the potential at the nodes of each element but also the velocity. It can be applied to a variety of velocity potential problems. The present paper, however, focused on its application to the problem of water entry of a wedge with varying speed. The continuity of the velocity achieved herein is particularly important for this kind of nonlinear free surface flow problem, because when the time stepping method is used, the free surface is updated through the velocity obtained at each node and the accuracy of the velocity is therefore crucial. Calculation was made for a case when the distance S that the wedge has travelled and time t follow the relationship s=Dtα, where D and α are constants, which is found to lead to a self similar flow field when the effect due to gravity is ignored.

Cheng Chin and Michael Lau
Journal of Marine Science and Application,2012(No. 2): 150-163
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In this paper, numerical modeling and model testing of a complex-shaped remotely-operated vehicle (ROV) were shown. The paper emphasized the systematic modeling of hydrodynamic damping using the computational fluid dynamic software ANSYS-CFXTM on the complex-shaped ROV, a practice that is not commonly applied. For initial design and prototype testing during the developmental stage, small-scale testing using a free-decaying experiment was used to verify the theoretical models obtained from ANSYS-CFXTM. Simulation results are shown to coincide with the experimental tests. The proposed method could determine the hydrodynamic damping coefficients of the ROV.

Qiuxin Gao, Wei Jin and Dracos Vassalos
Journal of Marine Science and Application,2012(No. 2): 164-168
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In order to provide instructions for the calculation of the propeller induced velocity in the study of the hull-propeller interaction using the body force approach, three methods were used to calculate the propeller induced velocity: 1) Reynolds-Averaged Navier-Stokes (RANS) simulation of the self-propulsion test, 2) RANS simulation of the propeller open water test, and 3) momentum theory of the propeller. The results from the first two methods were validated against experimental data to assess the accuracy of the computed flow field. The thrust identity method was adopted to obtain the advance velocity, which was then used to derive the propeller induced velocity from the total velocity field. The results computed by the first two approaches were close, while those from the momentum theory were significantly overestimated. The presented results could prove to be useful for further calculations of self-propulsion using the body force approach.

Baoyu Ni, Shaoshi Dai, Rui Han, Longquan Sun and Hailong Chen
Journal of Marine Science and Application,2012(No. 2): 169-177
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In an atrocious ocean environment, the lateral propulsion hole could potentially be partly out of water and capture an air cavity. Bubbles would form when the captured air cavity escapes underwater and they may affect the performance of the sonar. The common commercial computational fluid dynamics software CFX was adopted to calculate the ambient flow field around the lateral propulsion hole generated by a moving vessel. The oscillation of the spherical bubble was based on the Rayleigh-Plesset equation and its migration was modeled using the momentum equation. The radiated noise of the oscillating bubble was also studied. The aim is that the results from this paper would provide some insight into corresponding fluid and acoustic study.

Yumin Su, Qingtong Chen, Hailong Shen and Wei Lu
Journal of Marine Science and Application,2012(No. 2): 178-183
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Planing vessels are applied widely in civil and military situations. Due to their high speed, the motion of planning vessels is complex. In order to predict the motion of planning vessels, it is important to analyze the hydrodynamic performance of planning vessels at high speeds. The computational fluid dynamic method (CFD) has been proposed to calculate hydrodynamic performance of planning vessels. However, in most traditional CFD approaches, model tests or empirical formulas are needed to obtain the running attitude of the planing vessels before calculation. This paper presents a new CFD method to calculate hydrodynamic forces of planing vessels. The numerical method was based on Reynolds-Averaged Navier-Stokes (RANS) equations. The volume of fluid (VOF) method and the six-degrees-of-freedom equation were applied. An effective process was introduced to solve the numerical divergence problem in numerical simulation. Compared with experimental results, numerical simulation results indicate that both the running attitude and hydrodynamic performance can be predicted well at high speeds.

Tuohan Chen, Jianli Zheng and Dong-Myung Bae
Journal of Marine Science and Application,2012(No. 2): 184-190
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Fatigue cracks and fatigue damage have been important issues for ships and offshore structures for a long time. However, in the last decade, with the introduction of higher tensile steel in hull structures and increasingly large ship dimensions, the greater attention should be paid to fatigue problems. Most research focuses on how to more easily access the fatigue strength of ships. Also, the major classification societies have already released their fatigue assessment notes. However, due to the complexity of factors influencing fatigue performances, such as wave load and pressure from cargo, the combination of different stress components, stress on concentration of local structure details, means stress, and the corrosive environments, there are different specifications with varying classification societies, leading to the different results from different fatigue assessment methods. This paper established the Det Norske Veritas(DNV) classification notes “fatigue assessment of ship structures” that explains the process of fatigue assessment and simplified methods. Finally, a fatigue analysis was performed by use data of a real ship and the reliability of the result was assessed.

Jawad Khan, Wenyang Duan and Salma Sherbaz
Journal of Marine Science and Application,2012(No. 2): 191-199
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Radar cross section (RCS) is the measurement of the reflective strength of a target. Reducing the RCS of a naval ship enables its late detection, which is useful for capitalizing on elements of surprise and initiative. Thus, the RCS of a naval ship has become a very important design factor for achieving surprise, initiative, and survivability. Consequently, accurate RCS determination and RCS reduction are of extreme importance for a naval ship. The purpose of this paper is to provide an understanding of the theoretical background and engineering approach to deal with RCS prediction and reduction for naval ships. The importance of RCS, radar fundamentals, RCS basics, RCS prediction methods, and RCS reduction methods for naval ships is also discussed.

H. K. Mohanty, M. M. Mahapatra, P. Kumar, P. Biswas and N. R. Mandal
Journal of Marine Science and Application,2012(No. 2): 200-207
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In marine application, marine grade steel is generally used for haul and superstructures. However, aluminum has also become a good choice due to its lightweight qualities, while rusting of aluminum is minimal compared to steel. In this paper a study on friction stir welding of aluminum alloys was presented. The present investigation deals with the effects of different friction stir welding tool geometries on mechanical strength and the microstructure properties of aluminum alloy welds. Three distinct tool geometries with different types of shoulder and tool probe profiles were used in the investigation according to the design matrix. The effects of each tool shoulder and probe geometry on the weld was evaluated. It was also observed that the friction stir weld tool geometry has a significant effect on the weldment reinforcement, microhardness, and weld strength.

Huoming Zhang, Wenjun Gao, Qiang Wang, Juan Jiang and Zhou Zhao
Journal of Marine Science and Application,2012(No. 2): 208-215
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At present, equivalent water depth truncated mooring system optimization design is regarded as the priority of hybrid model testing for deep sea platforms, and will replace the full depth system test in the future. Compared with the full depth system, the working depth and span are smaller in the truncated one, and the other characteristics maintain more consistency as well. In this paper, an inner turret moored floating production storage & offloading system (FPSO) which works at a water depth of 320m, was selected to be a research example while the truncated water depth was 80m. Furthermore, an improved non-dominated sorting genetic algorithm (INSGA-II) was selected to optimally calculate the equivalent water depth truncated system, considering the stress condition of the total mooring system in both the horizontal and vertical directions, as well as the static characteristic similarity of the representative single mooring line. The results of numerical calculations indicate that the mathematical model is feasible, and the optimization method is fast and effective.

Fengmei Jing, Liang Zhang and Zhong Yang
Journal of Marine Science and Application,2012(No. 2): 216-221
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As a kind of clean and renewable energy, tidal current energy is becoming increasingly popular all over the world with the shortage of energy and environmental problems becoming more and more severe. A floating tidal current power station is a typical type of tidal current power transformers which can sustain the loads of wind, waves, and current, and even the extreme situation of a typhoon. Therefore, the mooring system must be reliable enough to keep the station operating normally and to survive in extreme situations. The power station examined in this paper was installed at a depth of 40 m. A 44 mm-diameter R4-RQ4 chain was chosen, with a 2 147 kN minimum break strength and 50 kN pretension. Common studless link chain was used in this paper. Based on the Miner fatigue cumulative damage rule, S-N curves of chains, and MOSES software, a highly reliable mooring system was designed and analyzed. The calculation results show that the mooring system designed is reliable throughout a 10-year period. It can completely meet the design requirements of American Petroleum institution (API). Therefore, the presented research is significant for advancing the design of this kind of power station.

Yongfeng Guo, Shaojun Ji, Changquan Tang, Jiansong Li, Huiquan Zhong and Ong Chin Yam Ian
Journal of Marine Science and Application,2012(No. 2): 222-227
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Mooring systems play an important role for semi-submersible rigs that drill in deepwater. A detailed analysis was carried out on the mooring of a semi-submersible rig that conducted a trial well drilling at a deepwater location in the South China Sea in 2009. The rig was 30 years old and had a shallow platform with a designed maximum operating water depth of 457 m. Following the mooring analysis, a mooring design was given that requires upgrading of the rig’s original mooring system. The upgrade included several innovations, such as installing eight larger anchors, i.e. replacing the original anchors and inserting an additional 600 m of steel wires with the existing chains. All this was done to enhance the mooring capability of the rig in order for the rig to be held in position to conduct drilling at a water depth of 476 m. The overall duration of the drilling was 50 days and the upgraded mooring system proved to be efficient in achieving the goal of keeping the rig stationary while it was drilling the trial well in the South China Sea. This successful campaign demonstrates that an older semi-submersible rig can take on drilling in deep water after careful design and proper upgrading and modification to the original mooring system.

Yannan Gao, Hongzhang Jin and Shengbin Zhou
Journal of Marine Science and Application,2012(No. 2): 228-235
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A zero-speed fin stabilizer system was developed for rolling control of a marine robot. As a robot steering device near the sea surface with low speed, it will have rolling motion due to disturbance from waves. Based on the working principle of a zero-speed fin stabilizer and a marine robot’s dynamic properties, a roll damping controller was designed with a master-slave structure. It was composed of a sliding mode controller and an output tracking controller that calculates the desired righting moment and drives the zero-speed fin stabilizer. The methods of input-output linearization and model reference were used to realize the tracking control. Simulations were presented to demonstrate the validity of the control law proposed.

Zheping Yan, Dongnan Chi, Jiajia Zhou and Yufei Zhao
Journal of Marine Science and Application,2012(No. 2): 236-243
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Wei Meng, Chen Guo1 and Yang Liu
Journal of Marine Science and Application,2012(No. 2): 244-250
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A robust adaptive control strategy was developed to force an underactuated surface vessel to follow a reference path, despite the presence of uncertain parameters and unstructured uncertainties including exogenous disturbances and measurement noise. The reference path can be a curve or a straight line. The proposed controller was designed by using Lyapunov’s direct method and sliding mode control and backstepping techniques. Because the sway axis of the vessel was not directly actuated, two sliding surfaces were introduced, the first one in terms of the surge motion tracking errors and the second one for the yaw motion tracking errors. The adaptive control law guaranteed the uniform ultimate boundedness of the tracking errors. Numerical simulation results were provided to validate the effectiveness of the proposed controller for path following of underactuated surface vessels.

Zhuang Li, Gang Qiao, Zongxin Sun, Haiyang Zhao and Ran Guo
Journal of Marine Science and Application,2012(No. 2): 251-257
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The existence of a multi-path channel under the water greatly decreases the accuracy of the short baseline positioning system. In this paper, the application of a time reversal mirror to the short baseline positioning system was investigated. The time reversal mirror technique allowed the acoustic signal to better focus in an unknown environment, which effectively reduced the expansion of multi-path acoustic signals as well as improved the signal focusing. The signal-to-noise ratio (SNR) of the time reversal operator greatly increased and could be obtained by ensonifying the water. The technique was less affected by the environment and therefore more applicable to a complex shallow water environment. Numerical simulations and pool experiments were used to demonstrate the efficiency of this technique.

Junjie Zhang, Chunhui Yuan, Xianming Zhu and Tianyun Li
Journal of Marine Science and Application,2012(No. 2): 258-264
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A general method was proposed to study the sound and vibration of a finite cylindrical shell with elastic theory. This method was developed through comprehensive analysis of the uncoupled Helmholtz equation obtained by the decomposition of elastic equations and the structure of the solution of a finite cylindrical shell analyzed by thin shell theory. The proposed method is theoretically suitable for arbitrary thickness of the shell and any frequency. Also, the results obtained through the method can be used to determine the range of application of the thin shell theory. Furthermore, the proposed method can deal with the problems limited by the thin shell theory. Additionally, the method can be suitable for several types of complex cylindrical shell such as the ring-stiffened cylindrical shell, damped cylindrical shell, and double cylindrical shell.