Session: 09-01-02 Wind Energy: Aero-hydrodynamics 2

Paper Number: 131594

131594 - A Study on Efficient Evaluation Process of Stress Time Series of a Floater 

In general, a floater is designed for an operating stress that is less than the yield stress of the material, and has the characteristic that the dynamic amplification of the response to the applied load is not significant. In this study, we would like to propose a process that can efficiently calculate stress time series based on static analysis by utilizing these design and response characteristics of floaters.

The external loads acting on a floater can be classified into pressure loads such as radiation pressure, wave excitation pressure, and hydrostatic pressure, and concentrated loads such as turbine load transmitted through the lower part of the tower, mooring load, and inertial load due to acceleration. In this study, a stress RAO for each load type is obtained through linear static structural analysis, then combined with the time history of floating body movement, tower bottom load, and mooring load obtained through integrated load analysis.

Among the pressure loads acting on a floating body, the radiation pressure refers to the force induced by the movement of the floating body in plain water, and the radiation pressure has characteristics that are proportional to the movement of the floating body. On the other hand, wave excitation pressure is defined as the force acting on a floating body by waves incident on a fixed floating body, and the magnitude of the wave excitation pressure is proportional to the wave elevation or wave height.

In a meaningful frequency range, the wave excitation pressrue for a unit wave height and the radiation pressure for a unit body motion can be evaluated based on potential theory, which can be defined as the pressure response amplitude operator (RAO). Static structural analysis is performed by applying pressure RAO for each individual frequency to obtain stress RAO for that frequency, and then the stress RAO are combined with the integrated load analysis results converted to the frequency domain. Afterwards, the stress time series due to radiation and wave excitation pressure can be calculated through inverse Fourier transform.

The stress time series for the concentrated load acting on the floating body can be evaluated through a more direct procedure than the pressure load. After performing a static structural analysis using a concentrated load with unit magnitude for each concentrated load type, the stress time series can be calculated by multiplying the integrated load analysis result, that is, the concentrated load time history, with the stress value from the static structural analysis.

The stress time series calculation process proposed through this study is verified and compared with the transient dynamic analysis results for a semi-submersible floating structure.

Presenting Author: Kyu-Sik Park POSCO

Presenting Author Biography: Kyu-Sik PARK is a senior researcher in the steel solution R&D centre at POSCO in South Korea.
He has studied at Hanyang University (South Korea) for B.S. degree and KAIST (South Korea) for M.S. and PhD degrees.
He has beening conducting research related to the steel structures including substructures for wind power generation after joining POSCO in 2007.

Authors:

Eungsoo Kim POSCO
Kyu-Sik Park POSCO