Time-domain simulation, fatigue and extreme responses for a fully flexible TLP floating wind turbine

Abstract

This study explores the dynamic responses of a tension leg platform (TLP) floating wind turbine (FWT) when all components of the floating platform are considered as flexible (elastic). Advanced aero-hydro-servo-elastic models modelling the platform with beam elements, which have been validated with model tests in previous work, are now extended to a TLP with column and pontoons. First, four coupled numerical models were established in the engineering tool SIMA. Two of them represent the entire platform (floater) as a fully flexible body, while the other two treat it as a rigid body for comparison. The tower, blades, and tendons are considered flexible for all models. The hydrodynamic loads are based on either Morison’s equation or potential flow theory, and are distributed along the body in the models with flexible platform. Fully coupled time domain simulations in still water, regular waves, and combined turbulent wind-irregular wave conditions are used to compare global motions and local sectional internal loads at different locations on the platform, tower, and tendons. Both fatigue damage and extreme axial stresses along the structure (obtained using a modified environmental contour approach) are examined. Platform flexibility influences the platform heave and pitch natural periods and motion amplitudes, particularly at the first bending mode natural frequency. Consequently, the sectional loads of all structure members at the first bending natural frequency are largely affected, and tendon axial stress at the wave-frequency also changes significantly. Overall, the adoption of a flexible platform model results in lower fatigue damage and extreme stress prediction along the tower and tendon. For this TLP FWT, Morison’s models predict larger responses for fatigue. For the extreme axial stresses in parked conditions, resonant responses at the first bending mode natural frequency are dominant.