Min-Su Park

Numerical analysis of an offshore platform with partial porous cylinders due to wave excitation forces and seismic forces

In order to reduce the wave excitation forces on structures, the partial porous cylinder, which is composed of a porous part located near free surface and a rigid part bounded top and bottom by impermeable end caps, is newly suggested. The fluid domain is thus divided into three regions: a single exterior region, N inner regions and N beneath regions to calculate the wave force on the partial porous cylinder, and the scattering wave in each fluid region is expressed by an Eigenfunction expansion method with using 3-dimension liner potential theory. In order to simplify the effect of porosity, Darcys law is also applied to the porous boundary condition. Using the wave excitation forces and seismic forces on the offshore platform with partial porous cylinders, the dynamic response evaluations of the platform are carried out through the modal analysis and the substructure method based on the effect of soil-structure interaction. The displacement and bending stress at critical structure members are computed using the various input parameters, such as the shear-wave velocity of soil, the porosity rate and the wave period.

Safety Evaluation of a Hybrid Substructure for Offshore Wind Turbine

Towers and rotor-nacelles are being enlarged to respond to the need for higher gross generation of the wind turbines. However, the accompanying enlargement of the substructure supporting these larger offshore wind turbines makes it strongly influenced by the effect of wave forces. In the present study, the hybrid substructure is suggested to reduce the wave forces by composing a multicylinder having different radii near free surface and a gravity substructure at the bottom of the multicylinder. In addition, the reaction forces acting on the substructure due to the very large dead load of the offshore wind turbine require very firm foundations. This implies that the dynamic pile-soil interaction has to be fully considered. Therefore, ENSOFT Group V7.0 is used to calculate the stiffness matrices on the pile-soil interaction conditions. These matrices are then used together with the loads at TP (Transition Piece) obtained from GH-Bladed for the structural analysis of the hybrid substructure by ANSYS ASAS. The structural strength and deformation are evaluated to derive an ultimate structural safety of the hybrid substructure for various soil conditions and show that the first few natural frequencies of the substructure are heavily influenced by the wind turbine. Therefore, modal analysis is carried out through GH-Bladed to examine the resonance between the wind turbine and the hybrid substructure.

Structural analysis of a gravity substructure for 5MW offshore wind turbines

The offshore wind power generation especially has gained attention from many countries because of its huge amount of energy and infinite growth potential of application of its energy. The substructure for offshore wind turbines is strongly influenced by the effect of wave forces since the size of a tower and a rotor-nacelle becomes larger to increase the gross generation of wind turbines. Therefore, it is very important to accurately calculate the wave force acting on substructures. In the present study Eigen-function expansion method with using three-dimensional linear potential theory is used to evaluate the wave forces. The comparison between wave forces obtained from Morison equation and wave forces obtained from this study is made to compare the diffraction theory with Morison equation in case of the small substructure compared to the wave length. The wave run-up acting on the substructure is also presented. Using the wave forces obtained from this study, the structural analysis of the gravity substructure is carried out through ANSYS mechanical. The structural behaviors of the strength and deformation are evaluated to investigate an ultimate structural safety and serviceability of gravity substructure. Moreover, the modal analysis is carried out to investigate the resonance between the wind turbine and the gravity substructure. It is found that the suggested gravity substructure can be an effective substructure for 5MW offshore wind turbines.

Hydrodynamic and oscillatory motions of hybrid floating structure with cylinders

In this study, in order to investigate hydrodynamic and oscillatory motions of hybrid floating structure with cylinders, analytical and experimental studies were carried out on hybrid floating structures. In order to investigate hydrodynamic motions, hydrodynamic analyses were performed on the hybrid and pontoon models. Hydrodynamic analyses were carried out to the different incident wave of 0°, 45°, and 90° and hydrodynamic motions of heave, pitch, and surge were analyzed. Also, in order to investigate oscillatory motions, three small-scale models of hybrid, tapered, and pontoon were fabricated and tested under the still-water condition. Finally, oscillatory motions of oscillation period, stabilizing time to steady-state were analyzed. As the results of this study, it was found that the hydrodynamic motions of hybrid model, especially surge motion, were significantly reduced over the wave period 6 sec comparing with the pontoon model. Also, oscillatory motions of hybrid model were presented about 1.7 times longer oscillation period and 0.37 times shorter stabilizing time to steady-state than the pontoon model. Therefore, it was expected that the hybrid model of this study contribute to improve serviceability and safety of offshore floating structures as decreasing hydrodynamic and oscillatory motions.