Nonparametric Identification Model of Coupled Heave-Pitch Motion for Ships by Using the Measured Responses at Sea

In order to simulate or control the coupled heave-pitch motion of ships in waves as realistically as possible, an appropriate mathematical model must be established in advance. In this paper, a nonparametric identification method, based on a combination of a random decrement technique (RDT) and support vector regression (SVR), was proposed to model the coupled heave-pitch motion of ships by only using the measured random responses at sea. First, a mathematical model was established to describe the coupled heave-pitch motion of ships in irregular waves. Second, the random decrement equation and the random decrement signatures were obtained by using RDT. Third, the damped frequency of the coupled heave-pitch motions were obtained by analyzing the random decrement signatures. Fourth, SVR was applied to identify the unknown hydrodynamic functions in the established mathematical model. The applicability and validity of the proposed nonparametric identification method were verified by case studies which were designed based on the simulated data and the model test data, respectively. Results of the study showed that the nonparametric identification method can be applied to identify the coupled heave-pitch motion of ships by only using the measured random responses in irregular waves.