Long-Term Reliability Analysis of a Spar Buoy-Supported Floating Offshore Wind Turbine

Most offshore wind turbines constructed to date have support structures for the turbine towers that extend to the seabed. Such bottom-supported turbines are confined to shallow waters closer to the shore. Sites farther offshore provide a better wind resource (i.e., stronger wind and less turbulence) while also reducing concerns related to visual impact and noise. However, in deeper waters, bottom-supported turbines are less economical. Wind turbines mounted atop floating platforms are, thus, being considered for deepwater sites. Several floating platform concepts are being considered; they differ mainly in how they provide stability to counter the large mass of the rotor-nacelle assembly located high above the water. One of these alternative concepts is a spar buoy floating platform with a deep draft structure and a low center of gravity, below the center of buoyancy. The reliability analysis of a spar-supported 5MW wind turbine based on stochastic simulation is the subject of this study. Environmental data from a selected deepwater reference site are employed in the numerical studies. Using time-domain simulations, the dynamic behavior of the coupled platform-turbine system is studied; statistics of tower and rotor loads as well as platform motions are estimated and critical combinations of wind speed and wave height identified. Long-term loads associated with a 50-year return period are estimated using statistical extrapolation based on loads derived from the simulations. Inverse reliability procedures that seek appropriate load fractiles for the underlying random variables consistent with the target return period are employed; these include use of: (i) the 2D Inverse First-Order Reliability Method (FORM) where an extreme load is selected at its median level (conditional on a derived critical wind speed and wave height combination); and (ii) the 3D Inverse FORM where variability in the environmental and load random variables is fully represented to derive the 50-year load.