Fatigue Analysis Of Semi-submersible Platform Based On Filed-measurement

The offshore platform is an important tool for human beings to explore the mysteries of ocean and utilize ocean resources,and fatigue failure is the main form of failure of deep-water semi-submersible platform.In order to research the fatigue damage at key nodes of semi-submersible platforms in real sea areas,and to calculate and forecast the fatigue damage based on the long-term measured results,this paper studies the distribution of pseudo wave surface elevation and the stress of measured points on the basis of a filed-measurement project of " HAI YANG SHI YOU 981" semi-submersible platform.The fatigue damage results in time domain and frequency domain are calculated separately and compared.In the analysis of wave input of the platform,the pseudo wave surface elevation is calculated according to the results of motion and air gap monitoring.Based on the related statistical analysis,the distributions of wave height and period are calculated,and the distribution characteristics of wave statistics under 1809 short-term sea states are studied.This pape also studies the combined action of wind wave and swell in typial sea states.The results show that the main component of the environmental waves play a decisive role in the form of the bimodal wave spectrum,and there is a stable linear relationship between the measured wave characteristics.The physical characteristics of the linear platform system are relatively clear.In the analysis of the stress response of the platform,the stress histories of the measured points on the four high-stress monitoring sections are obtained by the Fiber Bragg grating sensor.In this paper,the distribution of stress characteristics of each measuring point is studied and the relationship between stress and wave,stress and stress in frequency-domain are discussed.Finally,on the basis of measured stress,the fatigue damage is calculated in time-domain and frequency-domain respectively.The result of fatigue analysis shows that the damage with peak-value counting method is larger than that with rain flow counting method,and the ratio of the two outcomes are stable in the range of 2 ～ 3.While there is a stable linear relationship between the results of frequency-domain fatigue calculation models,due to the little consideration of the high-stress amplitude range,the fatigue damage is generally smaller than the time-domain results.When considering the platform response in real sea areas,the time-domain fatigue damage calculation method is more reliable.