Study On The Nolinear Dynamic Behaviors For SYS Of FPSO

Many experiments and research work indicate that the restore stiffness of FPSO mooring system is piece-wise linearity. In actual use process, oil tankers can encounter waves of different frequencies which induce sudden changes of motion condition and instantaneous increase of mooring load, thus the mooring lines may be destroyed. This article sets up the controlling equation considering the piece-wise linearity of the FPSO single point system, studies the inherent dynamic characteristics and reveals dynamic behaviors of mooring system motion in different waves, which lays the theory foundation for the determining mooring loads of FPSO.The development state at present, system constitution and study progress for FPSO system are summarized in this thesis. The amplitude of the wave is calculated by experience formula, using SYS system of ALP Company as a model, the restore force and displacement relation curve of SPM is gained by experiment, and the experiment curve is piece-wise linearity, the two pieces of restore stiffness are respectively k1 = 18t /m k 2 =33.8t /m . The double linear math analysis model and kinetic equation of SPM is set up by considering the surge of tanker. The period of the FPSO system is 529.96s through analysis of natural motion for SPM-250000 t tanker system. The dynamic responses of tanker's surge and steady analysis are carried out through considering different waves parameters, and the no damp amplitude-frequency response curves and damp amplitude-frequency response curves at the bilinear restore stiffness ratios are obtained.The results indicates that, the motion behaviors of piece-wise linearity is same as linear system when the resonance stiffness k1 = k2. The amplitude of surge will appear jump phenomenon when the restore stiffness ratio declines, the motion of the FPSO system will suddenly change from small-amplitude to large-amplitude motion in random waves so the mooring system will suffer from impact loads. The main resonant response will occur, when the frequency ratio isΩ=1. The 1/2 sub-harmonic resonance will occur, when the frequency ratio isΩ= 3.The 1/3 sub-harmonic resonance will occur; when the frequency ratio isΩ= 4. The 1/4 sub-harmonic resonance will occur, when the frequency ratio isΩ= 5, which indicate the sub-harmonic resonance will occur for the mooring system in the high seas.