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CONTENTS
Volume 4, Number 3, September 2014
 

Abstract
For the reliable design of substructure supporting offshore wind turbines it is very important to reduce the effects of wave forces. Since the substructure is strongly influenced by the effects of wave forces as the size of substructure increases. In the present study, the hybrid substructure with multi-cylinder is newly suggested to reduce the effects of wave forces. Using diffraction theory the scattering waves in a fluid region are expressed by an Eigenfunction expansion method with three dimensional potential theory to calculate the wave force acting on the hybrid substructure. The wave force and wave run-up acting on the hybrid substructure is presented to examine the water wave interaction according to the variation of cylindrical size and the distance among cylinders. It is found that the suggested hybrid substructure with multi-cylinder is very useful to reduce the effects of wave forces acting on the substructure for offshore wind turbines.

Key Words
hybrid substructure; offshore wind turbine; Eigenfunction expansion method; wave force;wave run-up

Address
Min-Su Park, Youn-Ju Jeong, Young-Jun You, Du-Ho Lee and
Byeong-Cheol Kim:Structural Engineering Research Division, Korea Institute of Civil Engineering and Building Technology, 283Goyangdae-ro, Ilsanseo-gu, Goyang-si411-712, Republic of Korea

Abstract
The objective of the present simulations is to evaluate the applicability of the standard k-e turbulence model in engineering practice in the subcritical to supercritical flow regimes. Two-dimensional numerical simulations of flow around a circular cylinder at Re=1x105, 5x105 and 1x106 , had been performed using Unsteady Reynolds-Averaged Navier Stokes (URANS) equations with the standard k-e turbulence model. Solution verification had been studied by evaluating grid and time step size convergence. For each Reynolds number, several meshes with different grid and time step size resolutions were chosen to calculate the hydrodynamic quantities such as the time-averaged drag coefficient, root-mean square value of lift coefficient, Strouhal number, the coefficient of pressure on the downstream point of the cylinder, the separation angle. By comparing the values of these quantities of adjacent grid or time step size resolutions, convergence study has been performed. Solution validation is obtained by comparing the converged results with published numerical and experimental data. The deviations of the values of present simulated quantities from those corresponding experimental data become smaller as Reynolds numbers increases from 1x105 to 1x106. This may show that the standard k-e model with enhanced wall treatment appears to be applicable for higher Reynolds number turbulence flow.

Key Words
solution verification; validation; cylinder; k-e; higher Reynolds number

Address
Xian-tao Zhang, Zhi-yu Li, Shi-xiao and Ying Chen: State Key Laboratory of Ocean Engineering, Min Hang District, Dong Chuan Road No.800, Shanghai, 200240, China
Muk chen Ong : Norwegian Marine Technology Research Institute (MARINTEK), NO-7450 Trondheim, Norway

Abstract
Wave run-up is an important issue in offshore engineering, which is tightly related to the loads on the marine structures. In this study, a series of physical experiments have been performed to investigate the wave run-up around a vertical cylinder in transitional water depth. The wave run-ups of regular waves, irregular waves and focused waves have been presented and the characteristics in frequency domain have been investigated with the FFT and wavelet transform methods. This study focuses on the nonlinear features of the wave run-up and the interaction between the wave run-up and the cylinder. The results show that the nonlinear interaction between the waves and the structures might result wave run-up components of higher frequencies. The wave run-ups of the moderate irregular waves exhibit 2nd order nonlinear characteristics. For the focused waves, the incident waves are of strong nonlinearity and the wavelet coherence analysis reveals that the wave run-up at focal moment contains combined contributions from almost all the frequency components of the focused wave sequence and the contributions of frequency components up to 4th order harmonic levels are recommended to be included.

Key Words
wave run-up; transitional water depth; vertical cylinder; model tests; wavelet transform

Address
Yanfei Deng, Jianmin Yang, Longfei Xiao and Yugao Shen: State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang, Shanghai, China


Abstract
Steel catenary riser (SCR) is a popular/economical solution for the oil/gas production in deep and ultra-deep water. The behavioral characteristics of SCR have a high correlation with the motion of floating production facility at its survival and operational environments. When large motions of surface floaters occur, such as FPSO in 100-yr storm case, they can cause unacceptable negative tension on SCR near TDZ (touch down zone) and the corresponding elastic deflection can be large due to local dynamic buckling. The generation, propagation, and decay of the elastic wave are also affected by SCR and seabed soil interaction effects. The temporary local dynamic buckling vanishes with the recovery of tension on SCR with the upheaval motion of surface floater. Unlike larger-scale, an-order-of-magnitude longer period global buckling driven by heat and pressure variations in subsea pipelines, the sub-critical local dynamic buckling of SCR is motion-driven and short cycled, which, however, can lead to permanent structural damage when the resulting stress is greatly amplified beyond the elastic limit. The phenomenon is extensively investigated in this paper by using the vessel-mooring-riser coupled dynamic analysis program. It is found that the moment of large downward heave motion at the farthest-horizontal-offset position is the most dangerous for the local dynamic buckling.

Key Words
vessel-mooring-riser coupled simulation; steel catenary riser (SCR); touch down zone (TDZ); sub-critical local dynamic buckling; 100-yr & 10-yr storm; riser-seabed interaction; elastic wave propagation/decay; turret-moored FPSO

Address
T.S. Eom, M.H. Kim, Y.H. Bae and C. Cifuentes: Texas A&M University, College Station, Texas, USA

Abstract
Aqua is an underwater biomimetic vehicle designed and built at McGill University that uses six paddles to produce control and propulsion forces. It has the particularity of having time-periodic thrust due to its oscillating paddles. Using an existing model of the vehicle, two types of controller were developed: a PD controller and a Floquet controller. The Floquet controller has the advantage of explicitly addressing the time-periodicity of the system. The performance of the controllers was assessed through simulation and experimentally in the Caribbean Sea. We find that the vehicle was able to follow the prescribed trajectories with relative accuracy using both controllers, though, the Floquet controller slightly outperforms the PD controller. Furthermore, a key advantage of the Floquet controller is that it requires no tuning while the PD controller had to be tuned by trial and error.

Key Words
biomimetic; underwater vehicle; Floquet theory; control

Address
Nicolas Plamondon and Meyer Nahon: Center for Intelligent Machines, McGill University, Montreal, Quebec, Canada


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