Critical Assessment of Reverse-MISO Techniques for System Identification of Coupled Roll Motion of Ships
Abhilash Somayajula and Jeffrey Falzarano
Of the six degrees of freedom of a vessel at sea, roll motion is the most poorly understood. While the other modes such as heave and pitch motions behave fairly linearly, the roll motion exhibits a nonlinear behavior due to the effect of softening stiffness and viscous damping. This makes predicting the roll motion parameters from experimental time series fairly difficult. The identification of system parameters from the input and output time histories is termed as system identification and is the usual approach to predict parameters from experimental data. This paper investigates the application of a reverse Multiple Input Single Output (R-MISO) method to analyze the nonlinear roll motion of a ship. The paper provides a detailed theoretical description of the method followed by an example demonstrating its application to coupled roll and sway motions of a ship at sea. While the method's validity is demonstrated by application to simulated data, a discussion on the limitations of the method is also provided which we believe has not received adequate emphasis in the existing literature on the subject.
Keywords: System Identification; Roll Damping; Multiple Input/Single Output Problem; Coupled Equations; Nonlinear Roll; Partial Coherence; Ordinary Coherence; Conditional Spectra; Hydrodynamic Added Mass; Hydrodynamic Radiation Damping; Nonlinear Roll Damping Prediction; Nonlinear Stiffness.