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CONTENTS
Volume 10, Number 1, March 2020
 

Abstract
Hydrodynamic Drag of Surface combatants pose significant challenges with regard to fuel efficiency and exhaust emissions. Stern flaps have been used widely as an energy saving device, particularly by the US Navy (Hemanth et al. 2018a, Hemanth Kumar and Vijayakumar 2018b). In the present investigation the effect of flap turning angle on drag reduction is numerically and experimentally studied for a high-speed displacement surface combatant fitted with a stern flap in the Froude number range of 0.17-0.48. Parametric investigations are undertaken for constant chord length & span and varying turning angles of 5 10 & 15. Experimental resistance values in towing tank tests were validated with CFD. Investigations revealed that pressure increased as the flow velocity decreased with an increase in flap turning angle which was due to the centrifugal action of the flow caused by the induced concave curvature under the flap. There was no significant change in stern wave height but there was a gradual increase in the stern wave steepness with flap angle. Effective length of the vessel increased by lengthening of transom hollow. In low Froude number regime, flow was not influenced by flap curvature effects and pressure recovery was marginal. In the intermediate and high Froude number regimes pressure recovery increased with the flap turning angle and flow velocity.

Key Words
ship hydrodynamics; stern flow; transom stern; stern flap

Address
Y. Hemanth Kumar and R. Vijayakumar: Department of Ocean Engineering, Indian Institute of Technology, Madras, India

Abstract
A study of the wave conditions for the Asabo offshore location at the Qua Iboe oil field in Eastern Nigeria has been carried out. Statistical analysis was applied to three (3) years of data comprising spectral periods, Tp and significant wave heights, Hs. The data was divided into two (2); data from October to April represents one set of data and data from May to September represents another set of data. The results were compared with similar studies at other locations offshore of West Africa. It was found that there is an absence of direct swellwaves from the Southern Ocean reaching the location under study (the Asabo site). This work suggests that the wave system is largely emanating from the North Atlantic storms. The presence of numerous islands near the Asabo location shields the site from effects of storms from south west and therefore swells from the Southern Ocean. It is noted that the local wind has little or no contribution. An Hs maximum of 2 m is noted at the Asabo offshore location. It is found that the Weibull distribution best describes the wave distribution at Asabo. Thus, the Weibull distribution is suggested to be adequate for long term prediction of extreme waves needed for offshore design and operations at this location.

Key Words
sea state; significant wave height; spectral density; time history; returns period; offshore Nigeria

Address
Agbakwuru A. Jasper and Akaawase T. Bernard: Center for Maritime and Offshore Studies, Federal University of Petr. Resources Effurun, Delta State Nigeria
Ove T. Gudmestad: University of Stavanger, Norway

Abstract
In this study, the effects of inclined shaft angle on the hydro-acoustic performance of cavitating marine propellers are investigated by a numerical method developed before and Brown\'s empirical formula. The cavitating blades are represented by source and vortex elements. The cavity characteristics of the blades such as cavitation form, cavity volume, cavity length etc., are computed at a given cavitation number and at a set advance coefficient. A lifting surface method is applied for these calculations. The numerical lifting surface method is validated with experimental results of DTMB 4119 model benchmark propeller. After calculation of hydrodynamic characteristics of the cavitating propeller, noise spectrum and overall sound pressure level (OASPL) are computed by Brown\'s equation. This empirical equation is also validated with another numerical results found in the literature. The effects of inclined shaft angle on thrust coefficient, torque coefficient, efficiency and OASPL values are examined by a parametric study. By modifying the inclination angles of propeller, the thrust, torque, efficiency and OASPL are computed and compared with each other. The influence of the inclined shaft angle on cavity patterns on the blades are also discussed.

Key Words
propeller; cavitation; noise; hydro-acoustic; shaft inclination angle

Address
Sakir Bal: Department of Naval Architecture and Marine Engineering, Istanbul Technical University, Istanbul, Turkey

Abstract
The main goal of this study is to investigate the free vibration analysis of a large sag catenary with application to the jumper in hybrid riser system. The equation of motion is derived by using the variational method based on the virtual work principle. The finite element method is applied to evaluate the numerical solutions. The large sag catenary is utilized as an initial configuration for vibration analysis. The nonlinearity due to the large sag curvature of static configuration is taken into account in the element stiffness matrix. The natural frequencies of large sag catenary and their corresponding mode shapes are determined by solving the eigenvalue problem. The numerical examples of a large sag catenary jumpers are presented. The influences of bending rigidity and large sag shape on the free vibration behaviors of the catenary jumper are provided. The results indicate that the increase in sag reduces the jumper natural frequencies. The corresponding mode shapes of the jumper with large sag catenary shape are comprised of normal and tangential displacements. The large sag curvature including in the element stiffness matrix increases the natural frequency especially for a case of very large sag shape. Mostly, the mode shapes of jumper are dominated by the normal displacement, however, the tangential displacement significantly occurs around the lowest point of sag. The increase in degree of inclination of the catenary tends to increase the natural frequencies.

Key Words
catenary jumper; finite element method; free vibration analysis; large sag catenary; hybrid riser; natural frequency; variational method; virtual work

Address
Karun Klaycham: Department of Civil Engineering, Faculty of Engineering at Kamphaeng Saen, Kasetsart University,
Nakhon Pathom 73140, Thailand
Panisara Nguantud, Chainarong Athisakul and Somchai Chucheepsakul: Department of Civil Engineering, Faculty of Engineering, King Mongkut\'s University of Technology Thonburi, Bangkok 10140, Thailand

Abstract
Catamaran has recently been a choice to support a typical vertical axis turbine in floating tidal current energy conversion system. However, motion responses associated with the catamaran can reduce the turbines efficiency. The possibility to overcome this problem is to change the catamaran parameter by varying and simulating the demi-hull separations to have lower motion responses. This simulation was undertaken by Computational Fluid Dynamic (CFD) using potential flow analysis. Cases of demi-hull separation were considered, with ratios of demi-hull separation (S) to the breadth of demi-hull (B), S⁄B of 3.45, 4.95, 6.45, 7.2 and 7.95. In order to compare to the previous works in the literature, the regular wave was set with wave height of 0.8 m. Furthermore, the analysis was carried out by irregular waves with significant wave height, Hs, of about 0.09 to 1.5 m and the wave period, T, of about 1.5 to 6 s or corresponding to the wave frequency, w, of about 1.1 to 4.2 rad/s. The wave spectrum was derived from the equation of the International Towing Tank Conference (ITTC). For the case of turbines-loaded catamaran under consideration, the new finding is that the least significant amplitude response can be satisfied at the ratio S⁄B of 7.2. This study indicates that selecting a right choice of demi-hull separation ratio could contribute in reducing motion responses of the tidal current turbines-loaded catamaran.

Key Words
demi-hull separation; floating turbine; twin turbines; vertical axis; catamaran model; tidal current energy

Address
Sony Junianto, Mukhtasor and Rudi Walujo Prastianto: Department of Ocean Engineering, Institut Teknologi Sepuluh Nopember, Kampus ITS Sukolilo 60111, Surabaya, Republic of Indonesia
Chul Hee Jo: Department of Naval Architecture and Ocean Engineering, Inha University,
100 Inha-ro, Nam-gu, Incheon, Republic of Korea


Abstract
A simplified algorithm is proposed for fast estimation of the material quantities required for the construction of rubble-mound breakwaters. The proposed algorithm is able to employ only the data available at feasibility study phase such as the maximum draught of the design ship selected to transport the cargos to the harbor despite, because at the feasibility phase, information for the planned harbor is likely to be very limited. A linear-constant waterdepth model together with a proposed section configuration for the breakwaters, which is customary for harbors, is considered to calculate the quantity of materials. The numerical simulation of the wave characteristics has been verified using the recorded wave data collected by a buoy installed near the Neka harbor in Caspian Sea waters. A case study has been also applied to four harbors to validate the proposed algorithm. The estimated weights using the proposed linear-constant and multi-linear waterdepth models were compared using the bathymetry maps and layouts of these harbors. A computer program, written in QBasic language, has been developed to simulate the wave characteristics and to estimate the material quantities needed to construct a rubble-mound breakwater. The obtained results show that taking into account the acceptable accuracies normally applied to the feasibility study and conceptual design phases, the proposed algorithm is sufficiently accurate and highly effective for the conceptual estimation of materials

Key Words
rubble-mound; breakwater; analytical model; material quantity; conceptual estimate

Address
Kabir Sadeghi and Fatemeh Nouban: Civil and Environmental Engineering Faculty, Near East University, Near East Boulevard, ZIP: 99138, Nicosia, North Cyprus, via Mersin 10 – Turkey


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