Multiaxial fatigue assessment of floating offshore wind turbine blades operating on compliant floating platforms

Fatigue damage is assessed for blades of floating offshore wind turbines operating on three different hull configurations using two different strain-based methods. Lifetime fatigue damage is predicted by summing damage over a range of wind-wave conditions conforming to a long-term Weibull distribution. The three hull configurations are selected to cover a wide range of rotational stiffness. Nonlinear beam theory is applied to compute the 3-D beam displacements along the turbine blades. Time histories of the resulting beam displacements are converted to maximum principal strains and to normal strains, and each resulting strain is used to compute the fatigue damage to the blade shells. Results show that the two strain-based methods yield different predictions of fatigue lives and locations of fatigue failures. The normal strain method significantly underpredicts fatigue damage compared to the maximum principal strain method. Fatigue of blades on floaters with very low rotational stiffness is found to be worse than on those with moderate rotational stiffness.