A new method for radiation forces for floating platforms in waves

•New method developed for calculating the time domain radiation forces.•Extending the conventional Prony method.•The new method does not induce additional differential equations.•The developed recursive calculation is simple and easy.•Computational time saving, especially for the multiple motion modes.

The performance of wave energy converters and other offshore renewable energy platforms, non-linear effects is frequently assessed in time domain, especially when the nonlinear forces from the large amplitude motions, mooring forces, power take-off or the energy conversion control system are considered. One popular approach in the time-domain analysis is the so-called hybrid frequency–time domain analyses, in which the radiation forces must be calculated through a convolution integral of the motion history and the impulse response function. The direct integration of the convolution is not only time-consuming and inconvenient when a real time control is used, but also needs a storage and renewal of a certain history of motions. To this end, some approximation methods have been developed for calculating the memory effect, and these methods inevitably introduce additional equations into the dynamic system. In this paper, a new method is proposed for calculating the memory effect by extending the conventional Prony׳s function method so that the memory effect calculation can be carried out simply in a recursive manner. As a result of the recursive calculation, only one previous step of the values is needed, which can be renewed automatically in each time step calculation.