Fatigue Analysis Of Offshore Gravity Platform

The gravity platform applied in shallow waters, a working platform mainly used for oil drilling in offshore waters, mainly depends on its own gravity to stand on the seabed. It comprises four parts, namely platform deck, vertical shafts, and oil storage caisson as well as drum-shaped skirting board. At present, most of the small and medium-sized oil fields are marginal fields based on the actual condition of the coastal waters in China. Considering the particularity and complexity of China’s marine environment as well as the various advantages of gravity platform, it is bound to play a significant role in the majority of oil fields in China.The analysis on the fatigue life of gravity platform is becoming more and more important due to the severe working environment. The conjunction part among the vertical shafts, platform deck and oil storage caisson usually bears alternating loading with high frequency, therefore it is inclined to cause cracks, leading to fatigue failure eventually. It is quite necessary to conduct a detailed calculation of its fatigue life.This thesis focuses on the basic theory and methods to analyze the fatigue life of the gravity platform applied in shallow waters. Based on the working environment for the platforms applied in the sea, a static and dynamic analysis on the target platform will be conducted to determine the stress concentration area; in addition, several panel points in the stress concentration area and several typical panel points will be chosen by using the method of spectrum-based fatigue analysis to estimate the fatigue life of the platform.The details of this thesis are as follows:First, this thesis will elaborate the basic theory of the four loadings as wind, wave, and ocean current as well as ice, what’s more, the basic theory and research methods of the fatigue analysis will also be included.Second, model the target platform based on the marine environment in Bohai Sea, and then a static and dynamic analysis on the gravity platform applied in shallow waters will be conducted, and then analyzing the modal of the platform, the inherent attributes of the platform will be obtained, such as the inherent frequency and mode of vibration. After that, a transient dynamic analysis on the platform will be made under the action of wave loading, during which the stress and displacement of the platform will also be analyzed. Then the dynamic amplification factor will be obtained by making a contrast of the static and dynamic analysis after checking the strength and stability of the platform. Finally, a static and dynamic analysis on the platform under the action of ice loading will be made to analyze how the displacement will change in accordance with the thickness and speed of the ice. After that this thesis will make a comparative analysis on the influence of these two factors.Third, taking the sea conditions in the Bohai Sea as the study subject, choosing several typical panel points located in the conjunction part between the vertical shafts and platform deck as well as the conjunction part between the upper and lower surface of the oil storage caisson, calculate the stress transfer function of the sea conditions to obtain the corresponding stress spectral density function, probability density of stress peak and statistical property of several alternating stress. Then based on S-N curve and the theory of linear cumulative damage, calculate the fatigue damage of the panel points so as to obtain the fatigue life, according which the security of the platform during its usage period can be decided in the end.Hopefully this thesis can provide certain help for the model building of the gravity platform applied in shallow waters and further calculation of its fatigue life as well.