Session: 09-01-07 Wind Energy: Aero-hydrodynamics 7

Paper Number: 132890

132890 - Fully-Coupled Time Domain Shell Stresses With Integrated Load Analyses of a Floating Offshore Wind Turbine 

In an integrated load analysis (ILA) for the floating offshore wind turbine (FOWT), the superstructure can be composed of beam elements or point elements to implement aeroelasticity. On the other hand, the substructure of the FOWT is composed of plates, which makes it difficult to implement structural elasticity. In addition, the equations of motion are solved based on rigid body motion because the equations of motion due to hydrodynamic forces are defined at the center of the floating body mass. Therefore, it is impossible to evaluate the structural strength of the substructure of the FOWT using software such as OpenFAST, OrcaFlex, and BLADED, which are widely used to perform ILA. The transient finite element analyses based on direct integration or linear superposition have been understood as the most explicit and conventional way to obtain stress history. However, it has been proven that those approaches required extremely long computing time to complete design load cases specified in some references. Therefore, it is necessary to develop a technique to obtain stress processes for the purposes of ultimate limit state and fatigue limit state. This study performed and validated a new approach that performs static structural analysis in the frequency domain and converts it to a time series to obtain the stress history. For a 10 MW floating wind turbine, frequency response analysis using Wamit was performed to obtain the wave excitation- and motion-induced pressure response amplitude operators (RAOs). The wave elevation, motion components, mooring tension, tower-floater interface load components, and the motion acceleration components were obtained through OpenFAST analyses. The Matlab code to obtain stress processes was developed. The stress RAOs under unit amplitude loads were obtained through structural analysis using Abaqus/standard. The coarse mesh finite element model with an element size of a representative longitudinal stiffener spacing. The boundary conditions were formed by constraining the bottom of the columns in the structural model. For mooring tensions, tower-float interface loads, and inertial forces, unit loads were applied to the structural analysis model to obtain the stress response, which was then multiplied by the load history to derive the stress history. For hydrodynamic forces, the pressure mapping was performed on the structural model because the element size of the frequency response analysis model and the element size of the structural analysis model are typically different. Static structural analysis was performed on the mapped pressure RAOs to obtain the stress RAOs. Fourier transforms were performed on the wave history and the motion history to obtain the wave and motion spectra, respectively, which were then multiplied by the stress RAO to obtain the wave excitation-induced and motion-induced stress spectra. The hydrodynamic force-induced stress history was then derived using the inverse Fourier transform. Those results were in a good agreement with the results of linear static and linear dynamic structural analyses. It is expected that introduction of this approach can reduce the design period of an FOWT.

 

Presenting Author: Sungjun Park Department of Ocean Engineering and Naval Architecture, Inha University

Presenting Author Biography: Sungjun Park is master student in the Department of Naval Architecture and Ocean Engineering at INHA University, South Korea.

His research focuses on spectral fatigue analysis for hot spot stresses and direct strength analysis (DSA) to assess the structural safety of ships and offshore structures.

Today, he will present a procedure to obtain fully coupled time domain stresses with integrated load analyzes.

Authors:

Sungjun Park Department of Ocean Engineering and Naval Architecture, Inha University
Joonmo Choung Department of Ocean Engineering and Naval Architecture, Inha University
Eungsoo Kim Steel Structure Research Group, POSCO
Kyu-Sik Park Steel Structure Research Group, POSCO