Fracture Analysis Of Offshore Platform Structures

Due to the fact that most of oil production fields of our country are marginaloilfields, it is supposed to do more researches on the ocean platform applying tomarginal oilfields. On the other hand, with the development of technology, deep-seaoilfields are also being developed, and the oil exploitation has been graduallytransferred from shallow sea to deep sea. Ocean platform is the main device in marinepetroleum exploitation, the number of which is more and more owing to our urgentneeds to ocean oil resources. This dissertation selected two research objects: one isgravity platform in shallow sea applying to marginal and small oilfields, the other isSpar platform applying to deep sea. Gravity platform is a kind of reinforced concretestructure which keeps its stability by its weight and generally suitable for shallow sea.There are three main parts in gravity platform: oil storage tank at thebottom(caisson)， supporting column and deck floor. The oil storage tank is directlyin touch with seabed which is used for store crude oil and there are reinforcedconcrete columns on the top of the tank to support the upper deck. Since gravityplatform has the advantages including large deck, good adaptability to marineenvironment, fireproofing, corrosion-resistance and low maintenance costs, it can beused as multipurpose platform in marine petroleum exploitation. Spar platform iswidely used in deep sea and can be divided into six parts: upper body of platform,main part of platform, buoyancy system, central shaft, riser system and mooringsystem. Spar platform can do works including drilling, production, crude oilprocessing, oil storage and handling, etc, and is seen as main development direction ofnext generation of deep water platform by a lot of oil companies.The structures of ocean platforms are complex and of high costs. In the serviceprocess, ocean platforms can be in a very bad environment for a long time, hence thestructure’s function can be easily degraded and many complicated mechanical damagemay occur, such as corrosive wear, fatigue crack or local depression, etc. Firstly, thisdissertation simplified structures of oil tank in gravity platform, and tooktwo-dimensional crack into account, then discussed its fracture destruction in theloads of wind, wave, flow and ice. Secondly, with the application of finite elementsoftware ANSYS, a static and dynamic analysis to deep sea Spar platform was made, and then fracture analysis of its main structure on the basis of static and dynamicanalysis, then analysis of fatigue crack propagation and fatigue life. Finally, torsionalfracture analysis of Spar platform’s main body with edge cracks under different windloads was made with the boundary element method, laying foundation for actualengineering application of oil fields.Specific tasks and innovations of this dissertation are as follows:Firstly, fracture analysis of plane structure of gravity platform in shallow sea:surface crack’s maximum depth of gravity platform caisson was studied, and byconsidering of the inner surface crack in the caisson, this issue was simplified asgravity platform with no crack plus rectangular plate with crack. With the applicationof finite element method, a static analysis was made to gravity platform with no crack,and ring tensile stress corresponding to crack position was obtained. After calculationof the strength factor of rectangular plate with crack, the allowed maximum depth ofcaisson’s surface crack in the marine load environment was obtained.Secondly, static and dynamic analysis of deep sea Spar platform: according tothe specific situation of the south China sea, the platform model was built withapplication of large finite element software ANSYS. After that, the dissertationconducted the static and dynamic analysis to the whole platform based on itsenvironment, and a kind of wave load cases was considered for static analysis, whilemodal analysis was firstly made for dynamic analysis based on which the research onthe dynamic response of the Spar platform model under the wave and otherenvironmental load.Thirdly, fracture analysis of three-dimensional structure of the deep sea Sparplatform: on the basis of static and dynamic analysis, this dissertation used3D solidelement for modeling, and the model was meshed. In addition, a simplified method ofcalculating stress intensity factor of three-dimensional crack has been stated, and bycomparing computed results with existing theoretical results, the fatigue crackextension was finally discussed. This is one of innovations in the dissertation.Fourthly, torsional fracture analysis of Spar platform’s main body with boundarycracks under wind loads: In consideration of complex loads in marine environmentand fixed modality of the platform, it has been found that twisting action cannot beneglected. So the main structure of Spar platform was simplified to a cylinder whichconsists of one material, and with consideration of edge crack, this dissertation builtboundary integral equation applying to cylinder Saint-Venant torsion to study on the torsion crack of Spar platform’s main body, discussed the singularity of the crack tip.The issue was solved by integral equation on each boundary, and with analysismethod of boundary element and calculation of Fortran programming, the torsionfracture failure of cylinder with straight crack and curve crack has been analyzed. Theobtained results were in accordance with documents, showing accuracy andeffectiveness of methods this dissertation used. Meanwhile, torsion fracture of Spar’smain body with crack was calculated when the platform was under different windloads. This is the second sparking point of the dissertation.