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CONTENTS
Volume 9, Number 2, June 2019
 

Abstract
Flow through a scaled horizontal axis marine current turbine was numerically simulated after validation and the turbine design was optimized. The computational fluid dynamics (CFD) code Ansys-CFX 16.1 for numerical modeling, an in-house blade element momentum (BEM) code for analytical modeling and an in-house surrogate-based optimization (SBO) code were used to find an optimal turbine design. The blade-pitch angle (o) and the number of rotor blades (NR) were taken as design variables. A single objective optimization approach was utilized in the present work. The defined objective function was the turbine\'s power coefficient (Cp). A 3x3 full-factorial sampling technique was used to define the sample space. This sampling technique gave different turbine designs, which were further evaluated for the objective function by solving the Reynolds-Averaged Navier–Stokes equations (RANS). Finally, the SBO technique with search algorithm produced an optimal design. It is found that the optimal design has improved the objective function by 26.5%. This article presents the solution approach, analysis of the turbine flow field and the predictability of various surrogate based techniques.

Key Words
marine current turbine; CFD; BEM; single objective optimization; surrogate modeling

Address
Karthikeyan Thandayutham and Abdus Samad: Wave Energy and Fluids Engineering Laboratory (WEFEL),Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai, India
E.J. Avital : School of Engineering and Material Science, Queen Mary University of London, London, U.K.
Nithya Venkatesan: School of Electrical Engineering, VIT University, Chennai, India


Abstract
One of the safest and the most economical methods to transfer oil and gas is pipeline system. Prediction and prevention of pipeline failures during its assessed lifecycle has considerable importance. The dropped object is one of the accidental scenarios in the failure of the submarine pipelines. In this paper, using Monte Carlo Sampling, the probability of damage to a submarine pipeline due to a box-shaped dropped object has been calculated in terms of dropped object impact frequency and energy transfer according to the DNV-RP-F107. Finally, Reliability sensitivity analysis considering random variables is carried out to determine the effect intensity of each parameter on damage probability. It is concluded that impact area and drag coefficient have the highest sensitivity and mass and add mass coefficient have the lowest sensitivity on probability of failure.

Key Words
submarine pipeline; dropped object; probabilistic assessment; sensitivity analysis

Address
Sina Taghizadeh Edmollaii, Pedram Edalat: Department of Offshore Engineering, Petroleum University of Technology, Mahmoudabad, Iran
Mojtaba Dyanati: The University of Akron, USA

Abstract
Studies on motion response of a vessel is of great interest to researchers, since a long time. But intensive researches on stability of vessel during motion under dynamic conditions are few. A numerical model of vessel is developed and responses are analyzed in head, beam and quartering sea conditions. Variation of response amplitude operator (RAO) of vessel based on Strip Theory for different wave heights is plotted. Validation of results was done experimentally and numerical results was considered to obtain effect of damping on vessel stability. A scale model ratio of 1:125 was used which is suitable for dimensions of wave flume at National Institute of Technology Calicut. Stability chart are developed based on Mathieu\'s equation of stability. Ince-Strutt chart developed can help to capture variations of stability with damping.

Key Words
in-tact stability; Mathieu

Address
Poonam Mohan and A.P. Shashikala: Department of Civil Engineering, National Institute of Technology, Calicut, India

Abstract
This paper deals with the problem of the global stabilization for a class of ocean structure systems. It is well known that, in general, the global asymptotic stability of the ocean structure subsystems does not imply the global asymptotic stability of the composite closed-loop system. The classical fuzzy inference methods cannot work to their full potential in such circumstances because given knowledge does not cover the entire problem domain. However, requirements of fuzzy systems may change over time and therefore, the use of a static rule base may affect the effectiveness of fuzzy rule interpolation due to the absence of the most concurrent (dynamic) rules. Designing a dynamic rule base yet needs additional information. In this paper, we demonstrate this proposed methodology is a flexible and general approach, with no theoretical restriction over the employment of any particular interpolation in performing interpolation nor in the computational mechanisms to implement fitness evaluation and rule promotion.

Key Words
intelligent control function; fuzzy rule interpolation (FRI), interpolated rules

Address
Tim Chen: Faculty of Information Technology, Ton Duc Thang University, Ho Chi Minh City, Vietnam
C.Y.J. Cheng: Faculty of Engineering, King Abdulaziz University, Abdullah Sulayman, Jeddah 21589
Tahura Nisa: ECE Department, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15213, U.S.A.
Jonathan Olivera:School of Computer Science University of Nottingham Jubilee Campus, Room: C32 Wollaton Road Nottingham NG8 1BB, U.K.

Abstract
The tower-platform interface and mooring system of floating offshore wind turbines (FOWTs) are some of the most critical components with significant influences on overall project costs. In addition to satisfying strength requirements, it is typical and vital to meet fatigue criteria for a service life of 25 years or more. Wind spectra characteristics considered in analysis can penalize fatigue designs, leading to unnecessary costs. The International Electrotechnical Commission (IEC, 2009) recommends the use of site-specific wind data (spectrum, turbulence intensity, etc.) in design of FOWTs, but for offshore sites it is often the case that such data is unavailable and land-based data are used as surrogates in design. For such scenarios, it is worth investigating whether such alternative approach is suitable and accurate, and understanding the consequence of the selection of wind spectral characteristics on fatigue design. This paper addresses the impact of the subsequent selection on fatigue responses of towerbase and mooring system in a FOWT, as a sequel to the paper by Udoh and Zou (2018) which focused on impacts on strength design. The 5 MW semi-submersible FOWT platform with six mooring lines implemented in the preceding study is applied in analysis. Results indicate significant variations in resulting fatigue life with considered wind parameters. Thus, it is critical to apply proper wind spectra characteristics for analysis and design of FOWTs to avoid unnecessary conservatism and costs. Based on the findings of this study, more explicit guidance on the application of turbulence intensities for IEC-recommended models in offshore sites could lead to more accurate load estimates in design of FOWTs.

Key Words
towerbase; mooring line; fatigue; floating; wind turbine; spectra

Address
Ikpoto E. Udoh and Jun Zou: Houston Offshore Engineering / Atkins, a member of the SNC-Lavalin Group,
17220 Katy Freeway, Suite 200, Houston, TX 77094, USA



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