Prediction Method Of The Ultimate Strength Of Of Semi-Submersible Platforms In Total Life Cycle

Semi-submersible offshore platform is one of the most widely used floatingoffshore platform currently. Under the influence of environmental loads,corrosion andfatigue crack in the service period, the global ultimate strength of offshore platformswill weaken and will affect the structures’ safety and durability. Therefore, it issignificant to study on residual ultimate strength of semi-submersible platform forevaluating the safety of semi-submersible platform in the lifecycle.A typical semi-submersible offshore platform structure is taken on thispaper.Firstly, the overall strength of the platform in dangerous conditions is analysedto determine the weaken components and joints of platform structure, and todetermine its main failure mode. On this base, it is proposed calculation method andanalysis process of residual ultimate strength to key components and joints with theinfluence of corrosion and cracks. The major works is as follows:(1) With design wave theory, design waves load value for the target platform aredetermined. Based on the finite element method, The typical semi-submersibleplatform was taken as calculation instance to calculate the global strength. The loadcases for the calculation included drilling and survival, which contains8typicaldangerous waves loads. Determined the weaken components and joints of platformstructure. On this base, extracting the main stress condition for key components andkey joints, then determine its main failure mode.(2) Paris-Erdogen equation and nonlinear mathematical model are used tosimulate cracks propagating and material corrosion respectively. Combining ANSYSsoftware to establish a parametric finite element model by taking corrosion thickness,crack size as the variational parameters for ultimate strength analysis.(3) Based on the finite element method, calculation method and analysis processof residual strength for key component and key joints of semi-submersible platform intotal life-cycle was developed, which consider corrosion, fatigue crack and thecooperation of corrosion and crack damage. At the same time, accomplishingautomatic integration with the existing finite element software. Finally, using a typicalsemi-submersible platform as a simulated example, the residual ultimate strength ofkey component and key joints of semi-submersible platform is influenced by corrosion and crack was studied. On this base, the residual ultimate strength of keycomponent and key joints changing with platform age in total life cycle was alsogiven.