Study On Safety Assessment And Remaining Life Prediction For Existing Offshore Platforms

One third of the fixed offshore platforms which have been built in the world have entered into the evening of their design life. However, as the need for oil production, most of the platforms will continue to serve after they achieve the design life, which requires a comprehensive safety assessment of the platforms for existing situation and the extended service life. Based on structural determinitistic and reliability assessment theory, a systermatic safety assessment methodology of offshore platform structure is set up where effects of damage factors on the structure are considered. And the remaining life of the existing platform is predicted based on the safety assessment. The thesis focuses on the following:1. This thesis studies on the ultimate bearing capacity of the platform firstly. Combining the ANSYS software and structural nonlinearity theory, the ultimate bearing capacity of the platform is studied, and the accuracy of the calculation of ultimate bearing capacity is validated through the comparison of the numerical simulation and physical experiments, which is the basis of certainty assessment and reliability assessment for platforms.2. Determinitistic and reliability assessment theory are studied systematically. The impact of structural safety caused by damages such as cracks, dents and corrosion, as well as marine growth and soil foundation erosion is studied and analyzed. Reserve strength factor is introduced, and existing platform is assessed by the deterministic method. The ultimate state equations of different failure modes of offshore platform are analyzed. And the reliability assessment of the offshore platform is carried out considering the random and time-varying of load and resistance, and load criterion of evaluation of the structure.3. Based on the S-N curve and Miner linear cumulative damage theory, the fatigue life prediction of the platform is carried out considering fatigue caused by both wave load and ice load. The study result shows that the fatigue life of the platform is in the safe range.4. A safety assessment methodology of offshore platform structure is set up. The safety assessment system includes three parts:annual safety assessment and dynamic management, prediction and management of structural safety and structural maintenance decision. The structural multi-level safety assessment is divided into four levels:filter assessment, simple quantitative assessment, structural deterministic safety assessment and structural reliability safety assessment. And the specific implementation methods are described in detail. An index system for safety assessment is set up, including detailed assessment index at all levels. The safety threshold is listed under the industry experience or existing norms, which can be used as a basis for safety assessment.5. The remaining life prediction method is studied. The remaining life of the target platform is predicted, applying on remaining life prediction for aging platform method basing on both deterministic method and dynamic reliability theory.