A multivariate approach to estimate design loads for offshore wind turbines

The design of offshore wind farms is a complex process which requires a detailed study of the oceanographic, meteorological, geological and electrical conditions at the site of location. The main environmental conditions which may contribute to structural damages, operation disturbances or other failures are: wind, waves, currents and sea ice. Thus, the combination of the different parameters can produce many different critical situations for the stability of the structure, being required the calculation of long time series corresponding to long term historical data situations. The most accurate techniques available at the moment to estimate loads over a structure are the numerical and physical models, however they are very time consuming and the calculation of long time series of data is unfeasible. Therefore, a hybrid methodology to transfer sea-wind-current conditions to forces over a structure is proposed. The methodology consists of a selection of a subset of representative cases of wave-wind-current climate at the structure location by a maximum dissimilarity algorithm, the estimation of loads over the structure for the selected cases and the reconstruction of loads corresponding to historical data using an interpolation technique based on radial basis function. In order to validate the proposed methodology, a current manual (IEC 61400-3 (2009)) has been applied to estimate the loads for the complete reanalysis time series of waves, winds and currents. The validation of the results confirms the ability of the developed methodology to reconstruct time series of forces over the structure. This methodology enables to apply the numerical and physical models in the offshore wind farms design with a considerable reduction in the computational effort.