  A novel approach is proposed to reduce, compared to the conventional binning
approach, the large number of aeroelastic code evaluations that are necessary
to obtain equivalent loads acting on wind turbines. These loads describe the
effect of long-term environmental variability on the fatigue loads of a
horizontal-axis wind turbine. In particular Design Load Case 1.2, as
standardized by IEC, is considered. The approach is based on numerical
integration techniques and, more specifically, quadrature rules. The quadrature
rule used in this work is a recently proposed "implicit" quadrature rule, which
has the main advantage that it can be constructed directly using measurements
of the environment. It is demonstrated that the proposed approach yields
accurate estimations of the equivalent loads using a significantly reduced
number of aeroelastic model evaluations (compared to binning). Moreover the
error introduced by the seeds (introduced by averaging over random wind fields
and sea states) is incorporated in the quadrature framework, yielding an even
further reduction in the number of aeroelastic code evaluations. The reduction
in computational time is demonstrated by assessing the fatigue loads on the
NREL 5MW reference offshore wind turbine in conjunction with measurement data
obtained at the North Sea, both for a simplified and a full load case.
