Static And Dynamic Analysis For The Structure Of Gravity Platform In Shallow Sea

Gravity platforms rest directly on the ocean floor by virtue of their own weight, which are usually suitable for shallow sea. The gravity platform in shallow sea consists of three parts: the bottom caissons, the columns and the upper deck. The bottom caissons contact the seafloor which can store oil. And the bottom part is composed of one or more caissons, on which in most cases, there are columns made of reinforced concrete to support the upper deck. Although gravity platforms appeared comparatively late in oil and gas development, they have many advantages due to they have the advantages of low maintenance cost and saving steel materials, larger decker areas than other platforms, adapted to various marine environment. Their major component can be prefabricated in the land and the construction technology is simple with low maintenance cost. Also they are in outstanding performance and merit in fireproof and anticorrosive. Gravity platforms can not only be used to drill, product, transport and store oil and gas but also can be used as a multifunctional platform in the oil and gas development.According to the actual situation of oil and gas development in our country, most of the middle and small oilfields is marginal oilfield. Because there are so many marginal oilfields are waiting for developed in CHINA, the research of gravity platform is very important. There are many studies about gravity platform all over the world. Borrowing successful international experiences and consider their various advantages, gravity platforms will play an important part in our oil and gas development in the future.This dissertation is mainly to study and research the gravity platform which is applicable to China offshore. And do some static and dynamic analysis for the structure of gravity platform in shallow sea. First of all, do some static and dynamic analysis of the whole structure of gravity platform. Secondly, the torsion problems of a gravity platform column with cracks were investigated. Finally, do some physical tests and mathematical model calculations to analyze the foundation stability of gravity platform under alternating load. The major contents of this dissertation are shown as follow:Firstly, static and dynamic analysis of the whole structure of gravity platform was employed. Consider the effects of various environment loads, modeling a gravity platform by the finite element analysis software ANSYS. Get the static and dynamic mechanical behavior data of the platform by meshing, solving and post-processing method. The static analysis is mainly to calculate the stresses and displacement of gravity platform under various environmental loads.And then do some research on the dynamic analysis. When the model of gravity platform is established, analyze the essential modal. Based on the dynamic characteristics, analyze the first three steps model to get the natural frequencies and mode shapes of the gravity platform. Then do some transient analysis of the platform under ice loading. And analyze the random wave load to get the dynamic responses of the platform. The results show that gravity platform satisfies the request of strength and stability.Secondly, fracture analysis for torsion problems of a gravity platform column with cracks was considered. Consider the effects of various environmental loads and natural mode of gravity platform, the torsion failure of the gravity platform column can not be ignored. The column was assumed to be a composite cylinder with cracks. Therefore the torsion fracture problem of the composite cylinder should be considered. And new boundary integral equations for the Saint-Venant torsion problem of a composite cylinder with curvilinear cracks were derived. The problem was reduced to solving the boundary integral equations on every boundary. By using the new boundary element method and programming with fortran, some typical torsion problems of a composite cylinder involving a straight or kinked crack were calculated. The obtained results were compared with the literature data to prove the validity and applicability of the present method. And also by using the method, under various wind loads, the torsion problem of the gravity platform column with a straight crack can be calculated.Thirdly, under alternating load, stability analysis of foundation soil of gravity platform was employed. Model a physical model of gravity platform as 1/25 of the real model. Simulate the environment of marine, and research the stability of the foundation under alternating load which is different by the oil volume. Model the foundation model of gravity platform by ANSYS. The obtained results were compared with the results which obtained from the physical model to prove the applicability and validity of the research method. The results from the laboratory experiment and the numerical simulation show that the foundation of our gravity platform is stable.