On the Use of the White-Noise Approximation for Modelling the Slow-Drifts of a FOWT: An Example Using FAST

When designing the mooring system of a floating unit, performing extensive time-domain simulations in several sea conditions is common practice. For this, the second-order wave induced forces, expressed by QTF matrices, are most often precomputed in frequency domain diffraction codes. However, the computation of the full QTFs is quite demanding and it is also not uncommon for the designer to be in doubt as to the frequency limits and resolution required for their construction. Among the approximations that can be used to ease this burden, the most well-known is Newman’s approximation, which performs quite well as long as the natural periods of drift and the water depth are sufficiently large. The white noise approach, on the other hand, leads to an approximation of a different kind. Taking advantage of the fact that the slow-drift response is narrow-banded, it approximates the second-order force spectrum where it contributes the most, and in a way that is independent of the natural periods and depth. However, its original formulation, based on the force spectra, is certainly more convenient in frequency domain. This article presents an easy way to make use of the white noise approach in time domain simulations. For this, the well-known OC4 semi-submersible FOWT is taken as a case-study. Simulations in different wave conditions are performed with the software FAST using both, the original full QTFs and new ones, simplified according to the principle of the white noise approximation. It is shown that, with the latter, the simulations can be performed without significant loss of accuracy, indicating that the white-noise approach indeed is an interesting option for preliminary design stages.