Session: 01-01-01 Offshore Platforms - I

Submission Number: 156931

Sensitivity Analyses of Dominant Design Variables for Semi-Submersible Type Substructure of a 15MW Floating Offshore Wind Turbine 

This study aims to analyze the influence of key design parameters of a semi-submersible substructure for a floating offshore wind turbine (FOWT) on the motion performance of the FOWT.

The IEA 15 MW class reference turbine is used as a baseline model. According to the position where the semi-submersible substructure supports the turbine, the substructure was classified into center column type and side column type. Draft was selected as the main variable that determines the displacement. Roll and pitch stiffnesses were assumed to be strongly influenced by column spacing and column diameter. Pontoon breadth was selected as the dominant variable to dampen the heave motion. For the center and side column types, 220 cases were generated for each design variable variation. A Python script of the commercial finite element analysis code Abaqus was developed to automatically generate the frequency response analysis model for each case. In this study, the maximum inclination angle and maximum acceleration at the nacelle, and the natural period of the heave at the center of mass (CoM) were observed. Environmental loads corresponding to extreme sea state (ESS) and severe sea state (SSS) were applied as 50-year recurrence period uniform wind speed and significant wave height-peak period, respectively. In the case with loads corresponding to ESS, the maximum nucleus tilt angle was observed. Nacelle acceleration was observed after applying the SSS load.

The nacelle inclination angle was significantly higher for the center column type substructure than for the side one. The deep draft cases caused a smaller nacelle inclination angle compared to the low draft cases. The smaller the column spacing, the larger the pontoon width, and the smaller the column diameter, the smaller the nacelle inclination angle developed. The nacelle acceleration in the side column type developed more than that in center column type. Deep draft models caused smaller nacelle acceleration than low draft models. Larger column spacing, smaller pontoon width, and larger column diameter resulted in less developed nacelle acceleration. Column spacing was more sensitive than column diameter in terms of nacelle acceleration, whether center or side column type. The heave natural frequencies were larger for center than for side column type, and the heave natural frequencies for the low draft model were larger than those for the deep draft model. The smaller the column spacing, the larger the pontoon width, and the smaller the diameter, the higher the heave natural frequency. The column diameter had the largest effect on the heave motion, regardless of whether it was center or side column type.

Assuming that the weight of the Nacelle acceleration, which has the greatest influence on the power generation efficiency, is 50%, and the weights of the heave natural period and the Nacelle inclination angle are 25% each, one best case was selected from among 220 eccentric and 220 centric cases. Load analysis was performed on these cases using OpenFAST.

Presenting Author: Soobin Lee INHA University, Ship and Offshore Sturcture Engineering Lab

Presenting Author Biography: B.S 2020~2024
MS Course 2024~

I am conducting research on optimizing the shape of a floating body based on frequency response analysis of a floating offshore wind turbine.