Nonparametric identification of nonlinear ship roll motion by using the motion response in irregular waves

In order to precisely predict the nonlinear roll motion of ships at sea, it is important to determine the nonlinear damping and restoring moment as accurately as possible. In this paper, a nonlinear mathematical model is used to describe the nonlinear roll motion of ships at sea. A novel nonparametric identification method based on a combination of random decrement technique (RDT) and support vector regression (SVR) is used to identify the nonlinear damping and restoring moments in the mathematical model simultaneously by using only the random rolling responses of ships in irregular waves. In the identification method, RDT is first used to derive the random decrement equation as well as the auto- and cross-correlation equations based on the established mathematical models, and the random decrement signatures are also obtained from the random roll responses. Then SVR is applied to identify the damping and restoring moments in the roll motion equation. For the purpose of verifying the applicability, accuracy and generalization ability of the identification method, it is applied to analyzing the simulated data with different wave excitations. The identification results show that the identification method can be applied to identify the damping and restoring moments of the nonlinear roll motion using the random responses of ships in irregular waves.