A new vessel motion based method for parametric estimation of the waves encountered by the ship in a seaway

Inference of sea wave spectrum based on vessel motions records is a relatively cheap monitoring strategy that has potential to improve the vessel's day-to-day operational efficiency and the safety of its crew. For this reason, several different approaches have been considered in recent years for turning this technique into practical tools. The methods available in the literature for implementing this strategy may be broadly categorized into parametric and non-parametric approaches. Apart from other aspects, they share the characteristics of generally performing the wave inference in the earth fixed reference frame and not in the ship's own referential. This procedure has the obvious advantage of providing the absolute wave spectrum as the output, but it comes at the expense of a greater complexity of the estimation algorithm when dealing with the motions of a vessel with forward speed. In a large part, this complexity arises from the uncertainties in the sea state estimation associated with the well-known triple-value problem when dealing with waves reaching the ship from the stern. The present article proposes a new parametric approach for vessel motion-based wave estimation. In this approach, a parametric description of the encounter wave spectrum is presented, thus enabling the algorithm to execute the wave inference in the ship's reference frame. For dealing with the large shape variability that is inherent to the encounter wave spectra, the proposed parametrization is based on the Weibull's distribution mathematical function. A first verification of performance of the method is also provided for the case-study of an oceanographic vessel. At this point, such verification is restricted to model-scale tests comprising conditions with and without forward speed. The results obtained are promising and indicate that the methodology is indeed capable of providing good estimates of the waves encountered by the ship under different operating conditions.