Effect of hydrodynamic load modelling on the response of floating wind turbines and its mooring system in small water depths

A large number of offshore wind turbines have been installed recently, mostly in water depth up to 30 m based on monopile foundations. It is expected that floating wind turbine becomes more competitive than bottom fixed wind turbine when the water depth exceeds 50 m. In this paper, the focus is on the environmental loads and responses of mooring systems for a semi-submersible in water depths from 50 m to 200 m. Mooring design for moderate water depths is relatively easy to achieve, but it is challenging for shallow water. The effect of environmental load modelling should be studied based on designing a reasonable mooring system. With a mooring system design for 200 m water depth as a reference, two mooring system design concepts in 100 m and 50 m water depth have been proposed for a 5-MW-CSC semi-submersible floating wind turbine. Preliminary design has been carried out to determine mooring line properties, mooring system configurations and document the static performances. A fully coupled time domain dynamic analysis for extreme environmental conditions was performed using Simo-Riflex-AeroDyn. Four different load models were applied in order to check the influence of different load components including the effect of wind, current and second order wave forces by means of Newman's approximation and a full QTF method.