Talei-Faz, Bijan; (2003) Fatigue and fracture of tubulars containing large cracks. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis presents an investigation into the performance of offshore tubular components containing large defects. The significance of the residual strength of cracked tubular members is considered with respect to inspection and maintenance of structural integrity. A series of nine destructive static strength tests were performed on full-scale precracked tubular welded T and Y-joints manufactured from a weldable high strength steel (Superelso 702), which is utilised in the construction of offshore Jack-Up platforms. All specimens had at least one through-thickness fatigue crack at the weld toe, from a previous fatigue-testing programme. Static strength tests on four large tubular sections manufactured from BS7191 355D were also carried out. The specimens contained either a through-thickness or a part- through-thickness defect. A novel digital photogrammetry technique was utilised to maximise the data collection from the destructive tests. The method is capable of the quantification of three-dimensional displacements, which subsequently allowed for a better understanding of the behaviour of the specimens during the tests. A fracture mechanics study of tubular components containing large cracks is presented. The limited number of stress intensity factor (SIF) solutions for cracks in tubular sections are considered and a new SIF solution for tubular T-joints, containing through-thickness cracks, under axial loading is provided. The method is based on the SIF at the crack tip and the non-uniform stress distribution present in an axially loaded tubular T-joint. The information has been integrated into the safety evaluation of all specimens using a failure assessment diagram (FAD) procedure. Finally, the local and the global responses of a structure to the presence of a large defect are reviewed. The importance of redundancy and multiple load paths are stressed and possible repair and maintenance options are considered.

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