Spectral Fatigue Analysis for the Topside Structure of Offshore Floating Vessel

摘要

In this study, spectral fatigue analysis was performed for the topsidestructure of offshore floating vessel. The topside structure was idealizedusing beam elements in SACS program. The fatigue analysis wascarried out considering wave and wind loads separately. For waveinduced fatigue damage calculation, motion RAOs calculated fromdirect wave load analysis and regular waves with different period andunit wave height were utilized. And then the transfer functions ofmember end force were generated covering all loading conditions.Stress response transfer functions at each joint were produced using thespecified SCFs and the transfer functions of member end force. Fatiguedamages were calculated using the obtained stress ranges, S-N curve,wave spectrum, heading probability of each loading condition and theircorresponding occurrences of the wave scatter diagrams. For windinduced fatigue damage calculation, the dynamic wind spectral fatigueanalysis was performed. Firstly, dynamic natural frequency analysiswas carried out to generate structural dynamic characteristics includingeigenvalues (natural frequencies), eigenvectors (mode shapes) and massmatrix. To adequately represent the dynamic characteristic of thestructure, the number of modes was determined appropriately in thelateral direction. Secondly, the wind spectral fatigue analysis wasperformed using the mode shapes and mass data obtained from theprevious results. In this analysis, the Weibull distribution of wind speedoccurrence, the probability of occurrence in each direction, dampingcoefficient, S-N curves and SCF of each joint are defined and used.Especially wind fatigue damages were calculated under the assumptionthat stress ranges follow Rayleigh distribution. Total fatigue damagesconsidering wave and wind were calculated from the combination withboth according to DNV rule.