Aging Offshore Platform Structural System Reliability Assessment Methods

One in third fixed offshore platforms which have been built in the world have entered into the evening in their design life. These aging offshore platforms have always been threatening by wind force, wave force, current force, ice force and earthquakes in the sea. With servicing in such a harsh environment, many disadvantages, such as corrosion and fatigue cracks, will lead to weaken the global ultimate strength of aging offshore platforms and affect the structures'safety and durability. Therefore, the reliability assessment research on aging offshore platforms has been pressing. Based on the structural reliability theory, a global reliability assessment approach for aging fixed offshore platforms with corrosion and fatigue cracks under extreme loads is presented in the paper. The main research including:Using Paris-Erdogen equation and nonlinear mathematical model simulate cracks propagating and material corrosion, respectively. And corrosion and fatigue crack growth rates in main members and key joints are modeled as random variables. Therefore, the mathematical prediction models for aging factors of platforms are established.With linear wave theory, Morison equation and wave force spectrum, the random wave force is calculated. Then according to the extreme wave loads acquired by every simulation, the statistical rule is obtained by Monte Carlo method. A extreme wave load prediction method is studied.An uncertainty analysis method on base shear capacity with corrosion and fatigue cracks is developed. Calculation flow of platform's base shear capacity with corrosion and cracks is also proposed, combining with the existing finite element software to accomplish automatic integration at the same time. Finally, using a typical jacket platform as a simulated example, the statistical character of platform's base shear capacity is obtained. Based on the limit state equation of global failure mode, the dynamic reliability assessment method is given in the paper, including instantaneous reliability and time dependent reliability assessment methods of offshore platform structure. Using a typical fixed platform as a simulated example, the index of reliability and failure probability of the jacket in its whole life is given at last.