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CONTENTS
Volume 1, Number 4, December 2011
 

Abstract
The performance of a rig tie-down system on a TLP (Tension Leg Platform) is investigated for 10-year, 100-year, and 1000-year hurricane environments. The inertia loading on the derrick is obtained from the three-hour time histories of the platform motions and accelerations, and the dynamic wind forces as well as the time-dependent heel-induced gravitational forces are also applied. Then, the connection loads between the derrick and its substructure as well as the substructure and deck are obtained to assess the safety of the tie-down system. Both linear and nonlinear inertia loads on the derrick are included. The resultant external forces are subsequently used to calculate the loads on the tie-down clamps at every time step with the assumption of rigid derrick. The exact dynamic equations including nonlinear terms are used with all the linear and second-order wave forces considering that some dynamic contributions, such as rotational inertia, centripetal forces, and the nonlinear excitations, have not been accounted for in the conventional engineering practices. From the numerical simulations, it is seen that the contributions of the second-order sum-frequency (or springing) accelerations can be appreciable in certain hurricane conditions. Finally, the maximum reaction loads on the clamps are obtained and used to check the possibility of slip, shear, and tensile failure of the tie-down system for any given environment.

Key Words
safety; tie-down system; derrick; wind-inertia-gravity loading; slip-shear-tensile failure; hurricane; second-order sum-frequency wave forces.

Address
Chan K. Yang : Technip, Houston, TX, USA
M.H. Kim : Department of Civil Engineering, Texas A&M University, College Station, TX, USA

Abstract
Spar platforms have several advantages for deploying wind turbines in offshore for depth beyond 120 m. The merit of spar platform is large range of topside payloads, favourable motions compared to other floating structures and minimum hull/deck interface. The main objective of this paper is to present the response analysis of the slack moored spar platform supporting 5MW wind turbine with bottom keel plates in regular and random waves, studied experimentally and numerically. A 1:100 scale model of the spar with sparD, sparCD and sparSD configuration was studied in the wave basin (30

Key Words
spar platform; wind turbine; keel plate; slack mooring; regular waves; random waves.

Address
T. Seebai and R. Sundaravadivelu : Department of Ocean Engineering, Indian Institute of Technology Madras Chennai-600036, Tamil Nadu, India

Abstract
New applications of streamlined Autonomous Underwater Vehicles require an AUV capable of completing missions with both high-speed straight-line runs and slow maneuvers or station keeping tasks. At low, or zero, forward speeds, the AUV

Key Words
autonomous underwater vehicles; AUV thrusters; through-body thrusters; tunnel thrusters; thruster modeling.

Address
Aaron Saunders : Boston Dynamics, Waltham MA 02451, USA
Meyer Nahon : Mechanical Engineering, McGill University, Montreal, Canada, H3A 2K6

Abstract
A time domain finite element based method is employed to analyze wave radiation by multiple cylinders. The nonlinear free surface and body surface boundary conditions are satisfied based on the perturbation method up to the second order. The first- and second-order velocity potential problems at each time step are solved through a finite element method (FEM). The matrix equation of the FEM is solved through an iteration and the initial solution is obtained from the result at the previous time step. The three-dimensional (3D) mesh required is generated based on a two-dimensional (2D) hybrid mesh on a horizontal plane and its extension in the vertical direction. The hybrid mesh is generated by combining an unstructured grid away from cylinders and two structured grids near the cylinder and the artificial boundary, respectively. The fluid velocity on the free surface and the cylinder surface are calculated by using a differential method. Results for various configurations including two-cylinder and four-cylinder cases are provided to show the mutual influence due to cylinders on the first and second waves and forces.

Key Words
second order wave radiation; time domain; multiple cylinders; finite element method; unstructured mesh.

Address
C.Z. Wang, S. Mitra and B.C. Khoo : Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore

Abstract
In the present study, an offshore platform having large partial porous cylindrical members, which are composed of permeable and impermeable cylinders, is suggested. In order to calculate the wave force on large partial porous cylindrical members, the fluid domain is divided into three regions: a single exterior region, N inner regions and N beneath regions, and the scattering wave in each fluid region is expressed by an Eigen-function expansion method. Applying Darcy

Key Words
large partial porous cylindrical members, eigen-function expansion method, wave excitation forces, substructure method, MCS method.

Address
Min-Su Park and Shuichi Nagata : Institute of Ocean Energy, Saga University, Saga 840-8502, Japan
Kenji Kawano : Department of Ocean and Civil Engineering, Kagoshima University, Kagoshima 890-8580, Japan

Abstract
Based on the fully nonlinear velocity potential theory, the liquid sloshing in a three dimensional tank under random excitation is studied. The governing Laplace equation with fully nonlinear boundary conditions on the moving free surface is solved using the indirect desingularized boundary integral equation method (DBIEM). The fourth-order predictor-corrector Adams-Bashforth-Moulton scheme (ABM4) and mixed Eulerian-Lagrangian (MEL) method are used for the time-stepping integration of the free surface boundary conditions. A smoothing scheme, B-spline curve, is applied to both the longitudinal and transverse directions of the tank to eliminate the possible saw-tooth instabilities. When the tank is undergoing one dimensional regular motion of small amplitude, the calculated results are found to be in very good agreement with linear analytical solution. In the simulation, the normal standing waves, travelling waves and bores are observed. The extensive calculation has been made for the tank undergoing specified random oscillation. The nonlinear effect of random sloshing wave is studied and the effect of peak frequency used for the generation of random oscillation is investigated. It is found that, even as the peak value of spectrum for oscillation becomes smaller, the maximum wave elevation on the side wall becomes bigger when the peak frequency is closer to the natural frequency.

Key Words
sloshing waves; random waves; desingularized BIEM; fully nonlinearity

Address
Gang Xu : Department of Mechanical and Industrial Engineering, Qatar University, Qatar, School of Naval Architecture and Ocean Engineering, Jiangsu University of Science and Technology, China
A.M.S. Hamouda : Department of Mechanical and Industrial Engineering, Qatar University, Qatar
B.C. Khoo : Department of Mechanical Engineering, National University of Singapore, Singapore


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