Structural Strength And Life-cycle Reliability Assessment Of An Existing Deepwater Semi-submergible Offshore Platform

The south China sea is rich in oil and gas drawing from which is an inevitable trend. The deep ocean environment is extremely poor, so a very high demand for security and reliability of platform is necessary. Especially now, a lot of semi-submersible platform have been in service over decades, and those aged semi-submersible platform operate in the extreme sea conditions for a long time, the probability of failure of platform will greatly increase. Ultimate strength is the security indicators of semi-submersible platform, thus we need to accurately assess ultimate bearing capacity of semi-submersible platform. At the same time the strength of the pipe component reliability is important problem related to structural failure. Therefore, the correct estimation of the reliability for structural system and the allocation of platform subsystem and component reliability could be made after accurately specific reliability index of each component is obtained.In this paper, a study on semi-submersible platform serving a couple of years in south China sea is conducted using finite element method by the software ANSYS and AQWA. First of all,3D potential flow theory is adopted to calculate the first order hydrodynamic parameter in the frequency domain. Then the motion response is computed by the viscous damping method. Secondly, the results from combined action of wind, wave and flow are imported into AQWA-WAVE model to calculate the load under various working condition. Thirdly, the results from the AQWA-WAVE are transformed into surface pressure of boat loading in ANSYS model to estimate the corrosion and cracks. Based on these results the steel thickness is calculated and overall intensity is verified. The strength change of platform is calculated in the life-cycle according to38years of docking repair technical reports and the strength change law of platform during the service is summarized.The overall intensity is calculated by selecting seven kinds of sea condition described by JONWASP in the end. The equivalent stress is picked up in various sea state conditions to carry out the reliability calculation of each component life-cycle intensity. The law of change of reliability of semi-submersible platform is summarized in life-cycle. Life-extending measures such as optimization, reinforcement and anticorrosive treatment are proposed to protect the key components and points based on results. This paper suggests some advice for safety forecasting, monitoring and maintaining of semi-submersible platform by studying the life-cycle law of it.