Mooring control of semi-submersible structures for wind turbines

This paper deals with the mooring control of tri-floater support structures for wind turbines by means of a numerical parametric study; both catenary and vertical tensioned mooring lines are considered. Operational and extreme climate conditions are examined, in particular wave, wind and current parameters with return periods of 5 and 50 years are determined for three different locations in Southern Mediterranean Sea. Mooring system is dimensioned in such a way that the offset due to environmental actions is restrained within allowable limits. The response of the platform is assessed in terms of: static forces due to turbine thrusts, wind and current drag forces and steady drift forces; the wave frequency motion RAO curves; the low frequency second order wave motions. Finally, assuming the Dutch tri-floater structure as a test case, a parametric study is carried out to identify the most suitable mooring configurations, varying the water depth in the range between 50 and 200 m.