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CONTENTS
Volume 2, Number 2, June 2012
 

Abstract
In the present work, design procedure and computer simulation of an AUV are documented briefly. The design procedure containing the design of propulsion system and CFD simulation of hydrodynamics behavior of the hull leads to achieve an optimum mechanical performance of AUV system. After designing, a comprehensive one dimensional model including motor, propeller, and AUV hull behavior simulates the whole dynamics of AUV system. In this design, to select the optimum AUV hull, several noses and tails are examined by CFD tools and the brushless motor is selected based on the first order model of DC electrical motor. By calculating thrust and velocity in functional point, OpenProp as a tool to select the optimum propeller is applied and the characteristics of appropriate propeller are determined. Finally, a computer program is developed to simulate the interaction between different components of AUV. The simulation leads to determine the initial acceleration, final velocity, and angular velocity of electrical motor and propeller. Results show the final AUV performance point is in the maximum efficiency regions of DC electrical motor and propeller.

Key Words
autonomous underwater vehicle; optimum design; contra-rotating propeller; computer simulation

Address
Azadeh Kebriaee : Mechanical Engineering school, Sharif University of Technology, Tehran, Iran
Hamidreza Nasiri : Electrical Engineering school, Sharif University of Technology, Tehran, Iran

Abstract
Steel Catenary Risers (SCR) are the simplest and often the most economic solution compared to other riser types such as flexible pipe, riser towers, top tensioned risers, etc. The top of a SCR is connected to the host platform riser porch. The other end of the SCR connects to flowlines from subsea wells. The riser touchdown point (TDP), which is the location along the riser where contact with the sea floor first occurs, exhibits complex behaviors and often results in compression and fatigue related issues. Heave dynamic responses of semisubmersibles in extreme and operating sea states are crucial for feasibility of SCR application. Recent full field measurement results of a deep draft semisubmersible in Hurricane Gustav displayed the considerable discrepancies in heave responses characteristics between the measured and the simulated results. The adequacy and accuracy of the simulated results from recognized commercial software should be examined. This finding raised the awareness of shortcomings of current commercial software and potential risk in mega investment loss and environmental pollutions due to SCR failures. One main objective of this paper is to attempt to assess the importance and necessity of accounting for viscous effects during design and analysis by employing indicator of viscous parameter. Since viscous effects increase with nearly third power of significant wave height, thus newly increased metocean criteria per API in central Gulf of Mexico (GoM) and even more severe environmental conditions in Western Australia (WA) call for fundamental enhancements of the existing analysis tools to ensure reliable and robust design. Furthermore, another aim of this paper is to address the impacts of metocean criteria and design philosophy on semisubmersible hull sizing in WA and GoM.

Key Words
SCRs TDP; terminal velocity; viscous effects; viscous parameter; diffraction parameter; heave motion RAOs; cancelation zone

Address
Houston Offshore Engineering, LLC, Houston, Texas, USA

Abstract
The very low natural frequencies of tension leg platforms (TLP\'s) have raised the concern about the significance of the action of hydrodynamic wave forces on the response of such platforms. In this paper, a numerical study using modified Morison equation was carried out in the time domain to investigate the influence of nonlinearities due to hydrodynamic forces and the coupling effect between surge, sway, heave, roll, pitch and yaw degrees of freedom on the dynamic behavior of TLP\'s. The stiffness of the TLP was derived from a combination of hydrostatic restoring forces and restoring forces due to cables and the nonlinear equations of motion were solved utilizing Newmark\'s beta integration scheme. The effect of wave characteristics such as wave period and wave height on the response of TLP\'s was evaluated. Only uni-directional waves in the surge direction was considered in the analysis. It was found that coupling between various degrees of freedom has insignificant effect on the displacement responses. Moreover, for short wave periods (i.e., less than 10 sec.), the surge response consisted of small amplitude oscillations about a displaced position that is significantly dependent on the wave height; whereas for longer wave periods, the surge response showed high amplitude oscillations about its original position. Also, for short wave periods, a higher mode contribution to the pitch response accompanied by period doubling appeared to take place. For long wave periods, (12.5 and 15 sec.), this higher mode contribution vanished after very few cycles.

Key Words
compliant structures; tension leg platforms; hydrodynamic wave forces; coupling effect; wave period; wave height

Address
A.M. Abou-Rayan and Amr R. El-gamal :Civil Engineering Tec. Dept., Faculty of Engineering, Benha Univ., Egypt
Ayman A. Seleemah :Structural Engineering Dept., Faculty of Engineering, Tanta Univ., Egypt

Abstract
Recent developments in underwater image recognition methods have received large attention by the ocean engineering researchers. In this paper, an improved bi-dimensional empirical mode decomposition (BEMD) approach is employed to decompose the given underwater image into intrinsic mode functions (IMFs) and residual. We developed a joint algorithm based on BEMD and Canny operator to extract multi-pixel edge features at multiple scales in IMFs sub-images. So the multiple pixel edge extraction is an advantage of our approach; the other contribution of this method is the realization of the bi-dimensional sifting process, which is realized utilizing regional-based operators to detect local extreme points and constructing radial basis function for curve surface interpolation. The performance of the multipixel edge extraction algorithm for processing underwater image is demonstrated in the contrast experiment with both the proposed method and the phase congruency edge detection.

Key Words
underwater image; bi-dimensional empirical mode decomposition; edge feature detector; phase congruency; multiple pixel edge extraction

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

Abstract
A time-domain simulation of a land-based Oscillating Water Column (OWC) with various irregular waves as a form of PM spectrum is performed by using a two-dimensional fully nonlinear numerical wave tank (NWT) based on the potential theory, mixed Eulerian-Lagrangian (MEL) approach, and boundary element method. The nonlinear free-surface condition inside the OWC chamber was specially devised to describe both the pneumatic effect of the time-varying pressure and the viscous energy loss due to water column motions. The quadratic models for pneumatic pressure and viscous loss are applied to the air and free surface inside the chamber, and their numerical results are compared with those with equivalent linear ones. Various wave spectra are applied to the OWC system to predict the efficiency of wave-energy take-off for various wave conditions. The cases of regular and irregular waves are also compared.

Key Words
oscillating water column; numerical wave tank; irregular waves; pneumatic chamber, equivalent linear damping; viscous energy loss; available power

Address
Weoncheol Koo : School of Naval Architecture and Ocean Eng., Univ. of Ulsan, 680-749, Ulsan, Korea
Moo-Hyun Kim : Department of Civil Engineering, Texas A&M Univ., College Station, TX,77843, USA

Abstract
Reinforced concrete structures in cold coastal regions are subjected to coupled effects of service load, freeze-thaw cycles and seawater corrosion. This would significantly degrade the performance and therefore shorten the service life of these structures. In the current paper, the mechanical properties of concrete material and the structural behaviour of eccentrically loaded reinforced concrete columns under multiple actions of seawater corrosion, freeze-thaw cycles and persistent load have been studied experimentally. Results show that when exposed to alternating actions of seawater corrosion and freezethaw cycles, the compressive strength of concrete decreases with the increased number of freeze-thaw cycles. For reinforced concrete column, if it is only subjected to seawater corrosion and freeze-thaw cycles, the load resistance capacity is found to be reduced by 11.5%. If a more practical service condition of reinforced concrete structures in cold coastal regions is simulated, i.e., the environmental factors are coupled with persistent loading, a rapid drop of 15% - 26.9% in the ultimate capacity of the eccentrically loaded reinforced concrete column is identified. Moreover, it is observed that the increase of eccentric load serves to accelerate the deterioration of column structural behavior.

Key Words
freeze-thaw; sea water; reinforced concrete; deterioration; eccentric load; concrete columns

Address
Bo Diao, Yang Sun and Yinghua Ye: Department of Civil Engineering, Beihang Univ., Beijing, P.R.China
Shaohong Cheng : Department of Civil and Environmental Engineering, Univ. of Windsor, Windsor, Ontario, Canada


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