Study On Reliability Assessment Of Critical Structural And Overturned Risk For Deepwater Semi-submersible Platform Under Extreme Sea Conditions

With the exploration of marine resources,the safety of platform is paid more and more attention.The structure reliability of deep-water semi-submersible platform is an important indicator to ensure its normal production operations and personnel survival.At the same time,the event of platform overturning occurs frequently in the extreme sea conditions.Therefore,the mechanism of platform capsizing risk caused by harsh environment need to be analyzed.This article is aimed at forecasting the wave load of deepwater semi-submersible platform?analyzing the Mov of the platform,assess the ultimate strength of key components,analyzing the reliability of key structures under extreme sea conditions,researching the reliability of mooring system and the probability of platform overturning during mooring failure,the main contents include:A method of forecasting the random distribution of the wave load extreme value of the semi-submerged platform in deepwater is given at first.According to the multivariate extreme value joint probability analysis method,the extreme sea condition of the once-in-a-century in the South China Sea is calculated and compared with the normative values.And the Morison equation is used to forecast the main wave loads of the semi-submersible platform through SESAM / WADAM software.The hydrostatic load of the semi-submersible platform is calculated according to the formula of hydrostatic load of the platform.Based on this,the influences of wave spectrum and wave height uncertainty on short-term results are studied.The effect of long-term load uncertainty on long-term prediction results and long-term extreme value distribution is studied.Finally,the statistical parameters of environmental load variables are obtained.Srcond,the calculation method of the critical strength of the critical structrues of deepwater semi-submersible offshore platform is established.The statistical parameters and distribution forms of the ultimate strength are obtained.The calculation methods for the critical failure probability of the semi-submersible offshore platform are also given.Choosing the design wave and the limit regular wave conditions to analyze the strength of the platform structure,and use FMEA to research the failure modes of the platform structure to get the critical structrues and failure modes;Determining the basic Random variables of structural capacity,including structure size and material properties,and analyzing the nonlinear finite element of the critical structrues;Geting the statistical parameters and distribution of the ultimate strength of the structure based on the ansys APDL language and Rosenbluth method;Using the linear response surface method and the improved first-order second-order matrix method,the failure probability of the key components in the once-in-a-century situation in the South China Sea is obtained.Finally,the reliability calculation method of the mooring system is studied,and the analysis method of the platform overturning risk when the mooring system fails under extreme sea conditions is given.Taking the semi-submersible platform in deep water as the research object,a coupled analysis model of the mooring system was established.The mooring system was analyzed in the nonlinear time domain,and the relationship between the tension of each section of the mooring chain and its breaking strength was established,and the mooring chain limit state equation was also established.The probabilistic mechanics theory is used to establish the load probability model and the resistance probability model,and the reliability index of the mooring chain under a hundred-year sea condition is studied.Further study the platform overturning risk when the partial mooring system fails,and obtain the failure path and failure probability of the platform overturning risk when the local mooring system fails under extreme sea conditions.