Effect of Mooring Line Modelling on Motions and Structural Fatigue Damage for a Semisubmersible Wind Turbine

摘要

Most current software applied for motion analysis of floating windturbines employ some kind of linearized mooring line model, ignoringmooring line dynamics. The impact of these assumptions are wellknown for offshore oil platforms, but for a floating wind turbine thewind-to-wave load ratio is higher, and the experiences from theoffshore industry cannot be applied directly. However, theseexperiences state that linearized models can provide good prediction offatigue damage applying frequency domain analysis, which cansignificantly shorten analysis time.In this paper a single semi-submersible wind turbine (SSWT) wasinvestigated with a linearized mooring line model and compared withan advanced finite element mooring line model for three differentmooring line configurations. The parameters investigated wereplatform motion characteristics and bending moment in the tower.These analyses were carried out by a numerical tool obtained byintegrating the hydrodynamic software SIMO/RIFLEX and AeroDynfor aerodynamic loads on the blades of the turbine.The results from these analyses showed significant differences in thestandard deviations for sway and yaw motions and sideways bendingmoment in the tower between the linearized model and the full mooringline model. The linearized model gave relatively good predictions ofsurge motions of the platform and fore-aft bending moment in thetower. Wave frequency response, which normally contributes most tofatigue, was not particularly influenced by mooring line modellig.Nevertheless, an improved model should include mooring line dampingand coupling effects.