Accelerated Hydrodynamic Analysis for Spar Buoys With Second-Order Wave Excitation

The simplified numerical models commonly employed for the pre-design of floaters for offshore wind only include linear wave loads, due to the higher computational effort required by second-order methods. Second-order hydrodynamics, on the other hand, need to be considered from an early stage, since they cause resonance of the moored structure.

In the present study, we introduce a new method to include second-order inviscid hydrodynamic loads at a computational cost similar to linear loads. We compare the accelerated method to standard second-order diffraction theory and to second-order Rainey forcing with Sharma & Dean wave kinematics. The comparison, based on the loads and response of a spar floating wind turbine in surge and pitch, is carried out for three different sea states.

We find that a good prediction of the second-order resonant response can be obtained with the accelerated method for medium and severe sea states, while the match is not as good for the mild sea state. The accelerated method is between 400 and 850 times faster than commonly used second-order approaches, for an 1-hour realization of a given sea state.

This speed up allows the application of the load model in the floater pre-design, where efficient numerical models are the key to achieve optimal designs and the consequent reduction in the cost of the floater.