Development of a Comprehensive Design Methodology and Fatigue Life Prediction of Composite Turbine Blades under Random Ocean Current Loading

A comprehensive study was performed to overcome the design issues related to
Ocean Current Turbine (OCT) blades. Statistical ocean current models were developed in
terms of the probability density function, the vertical profile of mean velocity, and the
power spectral density. The models accounted for randomness in ocean currents, tidal
effect, and ocean depth. The proposed models gave a good prediction of the velocity
variations at the Florida Straits of the Gulf Stream.
A novel procedure was developed to couple Fluid-Structure Interaction (FSI) with
blade element momentum theory. The FSI effect was included by considering changes in
inflow velocity, lift and drag coefficients of blade elements. Geometric non-linearity was
also considered to account for large deflection. The proposed FSI analysis predicted a
power loss of 3.1 % due to large deflection of the OCT blade. The method contributed to
saving extensive computational cost and time compared to a CFD-based FSI analysis. The random ocean current loadings were calculated by considering the ocean
current turbulence, the wake flow behind the support structure, and the velocity shear. The
random ocean current loadings had large probability of high stress ratio. Fatigue tests of
GFRP coupons and composite sandwich panels under such random loading were
performed. Fatigue life increased by a power function for GFRP coupons and by a linearlog
function for composite sandwich panels as the mean velocity decreased. To accurately
predict the fatigue life, a new fatigue model based on the stiffness degradation was
proposed. Fatigue life of GFRP coupons was predicted using the proposed model, and a
comparison was made with experimental results.
As a summary, a set of new design procedures for OCT blades has been introduced
and verified with various case studies of experimental turbines.