Safety Assessment Of Jacket Offshore Platforms In Active Service In Chengdao Oilfield

As the demand for energy of the human society is soaring and land oil and gas reserves are decreasing, the ocean with abundant resources become the priority of oil and gas exploitation. A platform is indispensable in the offshore oil and gas exploitation. However, due to the extremely harsh environment in which an offshore platform is put into service, several disastrous offshore platform accidents have happened in ocean development projects in both China and other foreign countries. The accident not only caused the pollution of the marine environment but also brought about enormous economic losses as well as adverse social influence. Accurate assessment and prediction on the safety and reliability of a platform is an effective measure to ensure the safe operation of the platform structure and to maximally avoid accidents.In this study, the large-scale finite element software ANSYS was used in the static analysis, transient dynamic response analysis, and ultimate bearing capacity and fatigue reliability analysis on platform structure of the jacket offshore platform in Chengdao Oilfield. The safety, applicability and durability of the structure were comprehensively and objectively reflected, which provided an important decision-making basis for reasonable arrangement of the production of the platform in Chengdao Oilfield. The study included the following aspects:(1). The nonlinearity of the soil was considered by adopting P-Y, Q-Z and T-Z Curves, and the determination methods of P-Y, Q-Z and T-Z Curves were summarized. And the soil body around the pile was simulated with nonlinear soil spring element. A platform finite element model was established which took the interaction of the structure-pile-soil into account. With the interaction between the pile and the soil considered, the flexibility of the platform structure increased while the rigidity decreased.(2). Static analysis was conducted on the platform to ensure that the strength and rigidity of the platform structure satisfy the requirements under extreme wave conditions. Study results showed that both the values of maximal displacement and the equivalent stress of the platform were in the tolerable value range under the extreme wave conditions that occurred once in 50 years. The static safety reserve was relatively great. (3). Modal analysis and transient dynamic response analysis were conducted on the platform. Results of modal analysis showed that the natural frequency of the platform was effectively kept away from that of the wave, which prevented the resonance between the platform and the wave. The displacement, speed, acceleration and stress-time curve were obtained through the transient response analysis, providing more valid and reliable data(4). The ultimate bearing capacity of both the defect-free platform and the platform with the damages of sea-water corrosion, damaged or missing components, foundation soil erosion, etc. was calculated respectively, and finally the strength reserve coefficient was calculated. Results showed that among all the damages it was the corrosion that damaged the platform most severely, and special attentions should be paid to strengthen the approaches in anti-corrosion of the platform in service.(5). The basic methods of fatigue life assessment on marine construction structures were studied. Structural dynamics analysis was conducted on the platform from the perspective of energy. Spectral method was applied to the structural fatigue analysis on the platform to predict its overall service life. Analytical results showed that the fatigue life of the CB273 Platform satisfied the designed requirement.