On the Application of Selected Wave Group Spectra for the Experimental Investigation of Low Frequency Motions of a Moored Structure

A moored structure shows both wave and low frequency motions in waves. Wave frequency motions are related to the wave elevation and wave power spectrum of the sea state while low frequency motions are driven by wave groups and the corresponding wave group spectrum. Wave power spectra can be calibrated for model tests. The corresponding wave group spectrum follows from the wave power spectrum together with the applied wave seed or phasing of the wave train. Thus, in common practice (both in simulations and model tests), the wave group spectrum follows from the arbitrary choice of a random seed. This can lead to an under- or overestimation of the resulting low frequency motions of the moored object as compared to the theoretical group spectrum. As an alternative approach, the seeds which give the highest and lowest wave group spectra can be applied in the tests. In this paper, first results of model tests with a moored tanker based on an intentional choice of wave group spectra are presented.