Identification Method Of Hydrodynamic Coefficients Of Ship Structure Nonlinear Rolling Motion Based On Hilbert Transform

Under the interference of external factors such as wind,wave and current,the floating structure on the sea will have six degrees of freedom of violent motion.Among them,the rolling motion is the largest swing amplitude,which is also a common motion mode.The rolling motion will seriously threaten the safety of offshore floating structures.Therefore,it is of great significance to study how to avoid the dangerous rolling motion of floating structure on the sea,and to accurately obtain the hydrodynamic coefficient of floating structure in rolling motion.Therefore,how to accurately obtain the rolling damping moment coefficient and restoring moment coefficient in the hydrodynamic coefficient is the main research content of this paper.At present,the empirical formula method,ship model test method and numerical simulation method are the most widely used methods to determine the rolling damping moment coefficient.There are many kinds of floating structures on the sea.This paper mainly studies the rolling motion of ships.Two identification methods of nonlinear rolling hydrodynamic coefficients based on Hilbert theory,i.e.identification based on first-order solution and identification considering high-order super harmonics(IPS and PISH),are studied to estimate the restoring moment coefficient and damping moment coefficient of nonlinear rolling.The two methods are compared with the nonlinear rolling coefficient identification method of transmission,and the accuracy of the nonlinear rolling hydrodynamic coefficient identification method based on Hilbert transform is verified.In this paper,a novel two-stage identification method of hydrodynamic coefficients is proposed to identify the damping moment coefficient and restoring moment coefficient of ship rolling in irregular waves.Firstly,the random decrement technique(RDT)is used to transform the response of ships in irregular waves into random decrement features.Then,the nonlinear system identification method based on Hilbert transform(HT)is used to identify damping moment coefficient and restoring moment coefficient from the obtained random decrement features.The rolling motion data of two physical models of barges are extracted,and the nonlinear rolling coefficient identification method based on Hilbert transform is used to identify the hydrodynamic coefficients.The damping moment coefficient and restoring moment coefficient of two barge models are obtained.The results show that the two identification methods based on HT are effective for nonlinear roll damping moment and restoring moment identification using free roll response.More accurate nonlinear coefficients can be obtained when super harmonics are considered.In addition,only the free rolling response is used to identify the damping moment and restoring moment,so some special tests and calculations are not needed.Under the condition of resonance,the nonlinear dynamic characteristics of the ship can be identified successfully.The significant wave height has some influence on the identification accuracy of damping moment.In addition,the peak period has a significant effect on the random attenuation signal of random rolling response,which directly affects the identification results,especially the damping torque.