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CONTENTS
Volume 2, Number 4, December 2012
 

Abstract
Analysis of ship parametric roll has generally been restricted to simple analytical models and sophisticated time domain simulations. Simple analytical models do not capture all the critical dynamics while time-domain simulations are often time consuming to implement. The model presented in this paper captures the essential dynamics of the system without over simplification. This work incorporates various important aspects of the system and assesses the significance of including or ignoring these aspects. Special consideration is given to the fact that a hull form asymmetric about the design waterline would not lead to a perfectly harmonic variation in metacentric height. Many of the previous works on parametric roll make the assumption of linearized and harmonic behaviour of the time-varying restoring arm or metacentric height. This assumption enables modelling the roll motion as a Mathieu equation. This paper provides a critical assessment of this assumption and suggests modelling the roll motion as a Hills equation. Also the effects of non-linear damping are included to evaluate its effect on the bounded parametric roll amplitude in a simplified manner.

Key Words
parametric roll; container ships; head seas rolling; regular waves

Address
Hisham Moideen, Jeffrey M. Falzarano and S.Abhilash Sharma : Department of Ocean Engineering, Texas A&M University, College Station, Texas, USA, 77843

Abstract
This paper provides a practical stochastic method by which the maximum equilibrium scour depth around spherical bodies exposed to long-crested (2D) and short-crested (3D) nonlinear random waves can be derived. The approach is based on assuming the waves to be a stationary narrow-band random process, adopting the Forristall (2000) wave crest height distribution representing both 2D and 3D nonlinear random waves, and using the regular wave formulas for scour and self-burial depths by Truelsen et al. (2005). An example calculation is provided.

Key Words
scour depth; self-burial depth; shear stress; long-crested waves; short-crested waves; nonlinear random waves; stochastic method

Address
Dag Myrhaug and Muk Chen Ong : Department of Marine Technology, Norwegian University of Science and Technology NO-7491 Trondheim, Norway

Abstract
Due to its large size, a ship is first divided into scores of blocks and then each block is constructed through various shops, such as the assembly shop, the painting shop, and the outfitting shop. However, each block may not be directly moved to the next shop and may be temporarily laid at a block stockyard because the working time in each shop is different from each other. If blocks are laid at the block stockyard without any planning, the rearrangement of the blocks by a transporter is required because the blocks have the different in and out time. In this study, a block layout method based on the genetic algorithm was proposed in order to minimize the rearrangement of the blocks in the block stockyard. To evaluate the applicability of the proposed method, it was applied to simple layout problems of the block stockyard. The result shows that the proposed method can yield a block layout that minimizes the total relocation cost of moving obstacle blocks in the block stockyard.

Key Words
block layout; block stock yard; transporter; genetic algorithm; optimization; shipbuilding

Address
Myung-Il Roh : School of Naval Architecture and Ocean Engineering, University of Ulsan, Mugeo-Dong, Ulsan, 680-749, Korea

Abstract
Assume fluid eddy viscosity in the vertical direction is parabolic. Sediment particles diffuse with the given fluid eddy viscosity. However, when the vertical diffusion coefficient profile is computed from the suspended sediment concentration profile, the coefficient shows lager values than the fluid mixing coefficient values. This trend was explained by using two sizes of sediment particles. When fine sediment particles like wash load are added in water column the sediment mixing coefficient looks much larger than the fluid mixing coefficient.

Key Words
vertical diffusion coefficient; suspended sediment concentration profile; sediment mixing coefficient; fluid mixing coefficient

Address
Hyoseob Kim : Civil and Environmental Engineering, Kookmin University, Sungbuk-gu, Seoul 136-702, Korea
Changhwan Jang : Construction Technology Examination Division, Korea Intellectual Property Office, Seo-gu, Daejeon 302- 701, Korea
Namjae lhm : Hyosung Ebara Engineering, Seocho-gu, Seoul 137-060, Korea

Abstract
Power-take-off through inner dynamic system inside a floating buoy is suggested. The power take-off system is characterized by mass, stiffness, and damping and generates power through the relative heave motion between the buoy and inner mass (magnet or amateur). A systematic hydrodynamic theory is developed for the suggested WEC and the developed theory is illustrated by a case study. A vertical truncated cylinder is selected as a buoy and the optimal condition of the inner dynamic system for maximum PTO (power take off) through double resonance for the given wave condition is systematically investigated. Through the case study, it is seen that the maximum power can actually be obtained at the optimal spring and damper condition, as predicted by the developed WEC theory. However, the bandwidth of high performance region is not necessarily the greatest at the optimal (maximum-power-take-off) condition, so it has to be taken into consideration in the actual design of the WEC.

Key Words
heave motion; linear electric generator; power absorption; matched eigenfunction expansion method; double resonance; power take off; high performance band-width

Address
I.H. Cho : Department of Ocean System Engineering, Jeju National University, Jeju 690-756, Korea
M.H. Kim : Department of Civil Engineering, Texas A&M University, College Station, Texas, 77843, USA
H.M. Kweon : Department of Civil Engineering, Kyongju University, Kyongju, Korea


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