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CONTENTS
Volume 5, Number 2, June 2015
 

Abstract
Of all the six degrees of freedom, the roll motion of a ship is the most poorly understood and displays complicated phenomena. Due to the low potential wave damping at the natural frequency, the effective analysis of ship roll dynamics comes down to the accurate estimation of the viscous roll damping. This paper provides overview of the importance of roll damping and an extensive literature review of the various viscous roll damping prediction methods applied by researchers over the years. The paper also discusses in detail the current state of the art estimation of viscous roll damping for ship shaped structures. A computer code is developed based on this method and its results are compared with experimental data to demonstrate the accuracy of the method. While some of the key references describing this method are not available in English, some others have been found to contain typographic errors. The objective of this paper is to provide a comprehensive summary of the state of the art method in one place for future reference.

Key Words
roll damping; viscous damping; quadratic damping; equivalent linearized damping

Address
Jeffrey Falzarano and Abhilash Somayajula: Marine Dynamics Laboratory, Texas A&M University, College Station, TX – 77843, USA
Robert Seah: Chevron Energy Technology Company, Houston, TX – 77002, USA

Abstract
As the exploitation of oil and gas resources advances into deeper waters and harsher environments, the design and analysis of the flexible risers has become the research focus in the offshore engineering filed. Due to the complexity of the components and the sliding between the adjacent layers, the bending response of the flexible risers is highly non-linear. This paper presents the finite element analysis of the flexible risers under bending loads. The detailed finite element model of the flexible riser is established in ABAQUS software. This finite element model incorporates all the fine details of the riser to accurately predict its nonlinear structural behavior. Based on the finite element model, the bending moment–curvature relationships of a flexible riser under various axisymmetric loads have been investigated. The results have been compared with the analytical ones obtained from the literature and good agreements have been found. Moreover, the stress of the tendon armors has been studied. The non-linear relationship between the armor tendons\' stress and the bending loads has been obtained.

Key Words
flexible risers; finite element model; non-linear; bending loads; axisymmetric loads; stress

Address
Chen Xiqia and Du Xiaying: Tianjin Branch, CNOOC Ltd, Tianjin, China
Fu Shixiao: State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, China
Gao Yun: 3State Key Laboratory of Oil and Gas Reservoir Geology and Exploration, Southwest Petroleum University, Chengdu, China


Abstract
Ship hull optimization is categorized as a bound, multi variable, multi objective problem with nonlinear constraints. In such analysis, where the objective function representing the performance of the ship generally requires computationally involved hydrodynamic interaction evaluation methods, the objective functions are not smooth. Hence, the evolutionary techniques to attain the optimum hull forms is considered as the most practical strategy. In this study, a parametric ship hull form represented by B-Spline curves is optimized for multiple performance criteria using Genetic Algorithm. The methodology applied to automate the hull form generation, selection of optimization solvers and hydrodynamic parameter calculation for objective function and constraint definition are discussed here.

Key Words
multi objective genetic algorithm; ship hull optimization; seakeeping; nonlinear programming; EEDI

Address
Amitava Guha and Jeffrey Falzarano: Marine Dynamics Laboratory, Department of Civil Engineering, Texas A&M University, College Station, TX, USA, 77843-3136


Abstract
In the present study, the coupled dynamic response of a Submerged Floating Tunnel (SFT) and mooring lines under regular waves is solved by using two independent numerical simulation methods, OrcaFlex and CHARM3D, in time domain. Variations of Buoyancy to Weight Ratio (BWR), wave steepness/period, and water/submergence depth are considered as design and environmental parameters in the study. Two different mooring-line configurations, vertical and inclined, are studied to find an optimum design in terms of limiting tunnel motions and minimizing mooring-line tension. The numerical results are successfully validated by direct comparison against published experimental data. The results show that tunnel motions and tether tensions grow with wave height and period and decrease with submergence depth. The inclined mooring system is more effective in restricting tunnel motions compared to the vertical mooring system. Overall, the present study demonstrates the feasibility of this type of structure as an alternative to traditional bridges or under-seabed tunnels.

Key Words
SFT (Submerged Floating Tunnel); tethers; coupled dynamics; FEM/BEM (Finite/Boundary Element Method); drag coefficient; BWR (Buoyancy to Weight Ratio); vertical/inclined mooring; line tensions

Address
Cristian Cifuentes, Seungjun Kim and M.H. Kim: Ocean Engineering Program, Department of Civil Engineering, Texas A&M University College Station, Texas, USA
W.S. Park: KIOST, Korea

Abstract
In this study, hydrodynamic performance of a 400 mm diameter horizontal axis marine current turbine model was tested in a cavitation tunnel with 1.21 m x 0.8 m cross-section for over a range of tip speed ratios. Torque and thrust data, as well as cavitation visualizations, for certain operating conditions were acquired. Experimental results indicated that the turbine can be exposed to significant amount of sheet and cloud cavitation over the blades along with vortex cavitation at the blade tips. Inception and distribution of cavitation along the blades of the model turbine were then modelled numerically for design operating conditions using a vortex lattice method. The method was also applied to a turbine tested previously and obtained results were compared with the data available. The comparison between simulation results and experimental data showed a slight difference in terms of span-wise extent of the cavitation region. The cloud and tip vortex cavity observed in experiments cannot be modelled due to the fact that the VLM lacks the ability to predict such types of cavitation. Notwithstanding, the use of such prediction methods can provide a reasonably accurate approach to estimate, therefore take the hydrodynamic effects of cavitation into account in design and analysis of marine current turbines.

Key Words
marine current turbine; cavitation; vortex lattice method

Address
Sakir Bal and Deniz Usar: Department of Naval Architecture and Marine Engineering, Istanbul Technical University,
Maslak 34469, Istanbul, Turkey
Mehmet Atlar: School of Marine Science and Technology, Newcastle University, Newcastle upon Tyne, NE2 1PT, United Kingdom



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