Sea Spectra Estimation from Hull Motions: Experimental Assessment of Regular and Irregular Sea Response Amplitude Operators

The present work deals with data analysis of the results of the experimental campaign within the project MADELEINE (MeAsurement of sea state conDitions to monitor the ship fatiguE Life and Enhance IN real-time conditions the safEty of navigation). The main project goal is the development of a tool for the real-time assessment of sea spectra, based on the measurement and analysis of ship motions, integrated with sea state monitoring by on-board photogrammetric devices and tropospheric monitoring by GNSS. We present the preliminary experiments at the Naples towing tank, involving both regular and irregular sea tests on a scaled model. The main objective of this experimental campaign is to assess the hull model heave response amplitude operator (RAO). In this work we present the data pre and post processing pipeline, from the timeseries joining algorithm to the comparison between regular and irregular sea and different approaches for computing irregular sea RAO. Novel approaches to data analysis for RAO estimation and comparisons among different techniques are presented, based both on time domain and frequency domain analysis. The magnitude-squared coherence spectra estimate of wave and motion response has been employed to provide the frequency range where the obtained RAO can be considered valid and linear. A hybrid time-frequency domain method based on empirical mode decomposition EMD approach for irregular RAO assessment is compared with the classical frequency domain approach.