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Fatigue Life Assessment of the Joints in Offshore Structures:Considering Residual Stress Caused By Welding Process and Elliptical Crack

Yari Boroujeni, Ayoub | 2010

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 40578 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholam Hossein
  7. Abstract:
  8. Offshore structures are valuable structures used in the offshore petroleum and gas fields for the purpose of hydrocarbon production operation. These structures are influenced by wave loads. Sea waves are inherently alternative and for this reason fatigue failure is considered as one of the most important modes of failure in these structures. Joints in these structures are made by tubes and they are called tubular joints. The joining process in manufacturing of these joints is welding. Welding process induces residual stresses, mostly tensional, in the substructure of materials. This tensional residual stress has a destructive effect on fatigue life of the joint and increases probability of crack nucleation and propagation in it. One of the crack shapes that happen in the material structure is elliptical or emi-elliptical crack. The crack front in this type is in the shape of a half of an ellipse. In the research presented, one of the offshore structures used in Persian Gulf is studied and its life is estimated and one of its joints with least life duration is indicated as critical joint. After indicating the critical joint, this joint is deeply analyzed and the effects of residual stress and semi-elliptical crack on fatigue life are studied. The results show that firstly, in the fatigue analysis of the whole structure, to achieve more accurate life estimation, besides considering corrosion effects, dynamic effects of the jacket and the topside both should be considered. Secondly, residual stress and semi-elliptical crack existence, dependent on quality and magnitude of exerting load, decrease the fatigue life of the joint as much as almost two orders of power of ten.
  9. Keywords:
  10. Residual Stress ; Jacket Platform ; Top Side ; X-Joint ; Elliptical Crack

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