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CONTENTS
Volume 4, Number 1, March 2014
 

Abstract
Hydrodynamic coefficients strongly affect the dynamic performance of autonomous underwater vehicles (AUVs). A novel kind of underwater vehicle (Heavier-than-water AUV) with higher density than water is presented, which is different from conventional ones. RANS method and overlapping grids are used to simulate the flow field around the vehicle. Lifts, drags and moments of different attack and drift angles in steady state are calculated. The hydrodynamic performances and how the forces change with the attitude are analyzed according to the flow field structure. The steady-state results using overlapping grid method are compared with those of software FLUENT and wind tunnel tests. The calculation results show that the overlapping grid method can well simulate the viscous flow field around the underwater vehicle. Overlapping grid skills have also been used to figure out the planar-motion-mechanism (PMM) problem of Heavier-than-water AUV and forecast its hydrodynamic performance, verifying its effectiveness in dealing with the dynamic problems, which would be quite helpful for design and control of Heavier-than-water AUV and other underwater vehicles.

Key Words
hydrodynamic performance; heavier-than-water AUV; overlapping grid method;planar-motion-mechanism (PMM)

Address
Xiang Li, Min Zhao,Qingqing Yuan and Tong Ge : State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, China
Faming Zhao: China Ship Scientific Research Center, Wuxi, China

Abstract
This paper provides a practical stochastic method by which the burial and scour depths of truncated cones exposed to long-crested (2D) and short-crested (3D) nonlinear random waves can be derived. The approach is based on assuming the waves to be a stationary narrow-band random process, adopting the Forristall (2000) wave crest height distribution representing both 2D and 3D nonlinear random waves. Moreover, the formulas for the burial and the scour depths for regular waves presented by Catano-Lopera et al. (2011) for truncated cones are used. An example of calculation is also presented.

Key Words
burial; scour; truncated cones; long-crested waves; short-crested waves; nonlinear random waves; stochastic approach

Address
Dag Myrhaug :Department of Marine Technology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway
Muk Chen Ong: Norwegian Marine Technology Research Institute (MARINTEK), NO-7450 Trondheim, Norway

Abstract
Floatover technology has been widely used in offshore installation, which has many substantial advantages compared with the traditional derrick barge. During the topside offloading of a twin-barge floatover installation, the transport barge is side by side moored between two floatover barges. In this paper, the twin-barge model with the connecting hawsers and pneumatic fenders is established. Coupled dynamic analysis is carried out to investigate the motions of the barges under wind, wave and current environments. Particular attention is paid to the effects on system responses with different frictional performance of fender, axial stiffness of the hawsers and environmental conditions. The research results can be used for optimizing the parameters of the system and reducing the risk of topside offloading.

Key Words
twin-barge floatover; hawsers; pneumatic fenders; coupled dynamic analysis

Address
Shuqing Wang : Shandong Provincial Key Lab of Ocean Engineering, Ocean University of China, NO.238,
Songling Road, Laoshan District, Qingdao 266100,China
Xiliang Li: Shandong Tongzheng Design&Surveying Co., Ltd, No.603, Yihe Road, Dongying District,Dongying 257091,China

Abstract
The detection of ship wakes image can demonstrate substantial information regarding on a ship, such as its tonnage, type, direction, and speed of movement. Consequently, the wake target recognition is a favorable way for ship identification. This paper proposes a Local Binary Pattern (LBP) approach to extract image features (wakes) for training an Improved Back Propagation Artificial Neural Network (IBPANN) to identify ship speed. This method is applied to sort and recognize the ship wakes of five different speeds images, the result shows that the detection accuracy is satisfied as expected, the average correctness rates of wakes target recognition at the five speeds may be achieved over 80%. Specifically, the lower ship\'s speed, the better accurate rate, sometimes it\'s accuracy could be close to 100%. In addition, one significant feature of this method is that it can receive a higher recognition rate than the nearest neighbor classification method.

Key Words
ship wake; target classification; Local Binary Patterns; BP artificial neural network model

Address
Liu Bo and Lin Yan : State Key Laboratory of Structural Analysis for Industrial Equipment, School of Naval Architecture and Ocean Engineering, Dalian University of Technology, 116024 Dalian, P.R. China
Zhang Liang: Yantai HUF Automobile Lock Co. Ltd., 264006 Yantai, P.R. China

Abstract
Declutching control is applied to a hemispherical wave energy converter with direct linear electric Power-Take-Off systems oscillating in heave direction in both regular and irregular waves. The direct linear Power-Take-Off system can be simplified as a mechanical spring and damper system. Time domain model is applied to dynamics of the hemispherical wave energy converter in both regular and irregular waves. And state space model is used to replace the convolution term in time domain equation of the heave oscillation of the converter due to its inconvenience in analyzing the controlled motion of the converters. The declutching control strategy is conducted by optimal command theory based on Pontryagin\'s maximum principle to gain the controlled optimum sequence of Power-Take-Off forces. The results show that the wave energy converter with declutching control captures more energy than that without control and the former\'s amplitude and velocity is relatively larger. However, the amplification ratio of the absorbed power by declutching control is only slightly larger than 1. This may indicate that declutching control method may be inapplicable for oscillating wave energy converters with direct linear Power-Take-Off systems in real random sea state, considering the error of prediction of the wave excitation force.

Key Words
wave energy; direct linear electric PTO; declutching control; state space; optimal command

Address
Xian-tao Zhang, Jian-min Yang and Long-fei Xiao :State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China


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