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CONTENTS
Volume 7, Number 2, June 2017
 

Abstract
A breakwater has always been an ideal option to prevent shoreline erosion due to wave action as well as to maintain the tranquility in the lagoon area. The effects of the impinging wave on the structure could be analyzed and evaluated by several physical and numerical methods. An alternate approach to the numerical methods in the prediction of performance of a breakwater is Artificial Intelligence (AI) tools. In the recent decade many researchers have implemented several Artificial Intelligence (AI) tools in the prediction of performance, stability number and scour of breakwaters. This paper is a comprehensive review which serves as a guide to the current state of the art knowledge in application of soft computing techniques in breakwaters. This study aims to provide a detailed review of different soft computing techniques used in the prediction of performance of different breakwaters considering various combinations of input and response variables.

Key Words
breakwaters; artificial neural networks; ANFIS; support vector machines; genetic algorithm; particle swarm optimization

Address
Suman Kundapura and Arkal Vittal Hegde: Department of Applied Mechanics and Hydraulics, NITK, Surathkal, India

Abstract
The study explores a novel design of wave energy converter (WEC) that utilizes the interaction between an inside heaving vertical cylinder with an outside fixed hollow cylinder. This design originates from the oscillating water column (OWC) type WEC but replaces the pneumatic power take off (PTO) through the Wells turbine with the hydrodynamic PTO through the inside heaving cylinder. To effectively evaluate the maximum power output, the system has been modeled in the hydrodynamic software AQWA (developed by ANSYS Inc) that has accumulated extensive offshore industry users. Ranges of the PTO parameters have been examined to make sure that proper linear damping can be implemented to simulate the PTO force. Comparing the efficiency of the pneumatic PTO with the hydrodynamic PTO, it appears that the hydrodynamic PTO is more promising than the traditional Wells turbine for an OWC system.

Key Words
power take off; two-body interaction; oscillating water column; damping; AQWA; point absorber

Address
Hao Wang and Jeffrey Falzarano: Department of Ocean Engineering, Texas A&M University, College Station, USA
Khairil Sitanggang: INTECSEA Inc, Worley Parsons Group, Houston, USA

Abstract
Spud-can is used for fixing jack-up rig on seabed. It needs to be inserted up to the required depth during the installation process to secure enough soil reaction and prevent overturning accidents. On the other hand, it should be extracted from seabed soils as fast as possible during the extraction process to minimize the corresponding operational cost. To achieve such goals, spud-can may be equipped with water-jetting system including monitoring and control. To develop such a smart spud-can, a reliable numerical simulation tool is essential and it has also to be validated against physical model tests. In this regard, authors developed a numerical simulation tool by using a commercial program ANSYS with extended Drucker-Prager (EDP) formula. Authors also conducted small-scale (1/100) physical model tests for verification and calibration purpose. By using the numerical model, a systematic parametric study is conducted both for sand and K(kaolin)-clay with varying important soil parameters and the best estimated soil properties of the physical test are deduced. Then, by using the selected soil properties, the numerical and experimental results for a sand/K-clay multi-layer case are cross-checked to show reasonably good agreement. The validated numerical model will be useful in the next-stage study which includes controllable water-jetting.

Key Words
jack-up platform; smart spud-can; 1/100 physical model test; numerical simulation; parametric study; SSI (Soil-Structure Interaction); soil properties/resistance

Address
Dong-Seop Han: R&D Center, Remitite Co. Ltd., Kyeongnam, South Korea
Moo-Hyun Kim: Ocean Engineering Department, Texas A&M University, College Station, Texas, USA

Abstract
The purpose of this paper is to understand and model the slow current (~2 m/s) effects on the global response of a floating offshore platform in waves. A time-domain numerical simulation of full wave-current-body interaction by a quadratic boundary element method (QBEM) is applied to compute the hydrodynamic loads and motions of a floating body under the combined influence of waves and current. The study is performed in the context of linearized potential flow theory that is sufficient in understanding the leading-order current effect on the body motion. The numerical simulations are validated by quantitative comparisons of the hydrodynamic coefficients with the WAMIT prediction for a truncated vertical circular cylinder in the absence of current. It is found from the simulation results that the presence of current leads to a loss of symmetry in flow dynamics for a tension-leg platform (TLP) with symmetric geometry, resulting in the coupling of the heave motion with the surge and pitch motions. Moreover, the presence of current largely affects the wave excitation force and moment as well as the motion of the platform while it has a negligible influence on the added mass and damping coefficients. It is also found that the current effect is strongly correlated with the wavelength but not frequency of the wave field. The global motion of a floating body in the presence of a slow current at relatively small encounter wave frequencies can be satisfactorily approximated by the response of the body in the absence of current at the intrinsic frequency corresponding to the same wavelength as in the presence of current. This finding has a significant implication in the model test of global motions of offshore structures in ocean waves and currents.

Key Words
wave-current-body interaction; platform motions; quadratic boundary element method

Address
Meng Shen and Yuming Liu: Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Abstract
For complex flexible structures such as nets, the determination of drag forces and its deformation is a challenging task. The accurate prediction of loads on cages is one of the key steps in designing fish farm facilities. The basic physics with a simple cage, can be addressed by the use of experimental studies. However, to design more complex cage system for various environmental conditions, a reliable numerical simulation tool is essential. In this work, the current load on a cage is calculated using a Morison-force model applied at instantaneous positions of equivalent-net modeling. Variations of solidity ratio (Sn) of the net and current speed are considered. An equivalent array of cylinders is built to represent the physical netting. Based on the systematic comparisons between the published experimental data for Raschel nets and the current numerical simulations, carried out using the commercial software OrcaFlex, a new formulation for Cd values, used in the equivalent-net model, is presented. The similar approach can also be applied to other netting materials following the same procedure. In case of high solidity ratio and current speed, the hybrid model defines Cd as a function of Re (Reynolds number) and Sn to better represent the corresponding weak diffraction effects. Otherwise, the conventional Cd values depending only on Re can be used with including shielding effects for downstream elements. This new methodology significantly improves the agreement between numerical and experimental data.

Key Words
fish cage; equivalent net model; Morison drag formula; strong current; large deformation; solidity ratio; drag coefficient; weak diffraction effect; shielding effect

Address
Cristian Cifuentes: Institute of Naval Architecture and Ocean Engineering Universidad Austral de Chile, Valdivia, Chile
M.H. Kim: Department of Ocean Engineering Texas A&M University, College Station, TX, USA


Abstract
This research is based on the concept of safety airbag to design a self-rescue system for the autonomous underwater vehicle (AUV) using micro inertial sensing module. To reduce the possibility of losing the underwater vehicle and the difficulty of searching and rescuing, when the AUV self-rescue system (ASRS) detects that the AUV is crashing or encountering a serious collision, it can pump carbon dioxide into the airbag immediately to make the vehicle surface. ASRS consists of 10-DOF sensing module, sensing attitude algorithm and air-pumping mechanism. The attitude sensing modules are a nine-axis micro-inertial sensor and a barometer. The sensing attitude algorithm is designed to estimate failure attitude of AUV properly using sensor calibration and extended Kalman filter (SCEKF), feature extraction and backpropagation network (BPN) classify. SCEKF is proposed to be used subsequently to calibrate and fuse the data from the micro-inertial sensors. Feature extraction and BPN training algorithms for classification are used to determine the activity malfunction of AUV. When the accident of AUV occurred, the ASRS will immediately be initiated; the airbag is soon filled, and the AUV will surface due to the buoyancy. In the future, ASRS will be developed successfully to solve the problems such as the high losing rate and the high difficulty of the rescuing mission of AUV.

Key Words
autonomous underwater vehicle; extended Kalman filtering; BPN classifier; AUV airbag

Address
Yi-Ting Yang and Sheng-Chih Shen: Department of Systems and Naval Mechatronic Engineering, National Cheng Kung University, No.1, Daxue Rd., East Dist., Tainan City 701, Taiwan (R.O.C.)


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