Research On Exempting Post-weld Heat Treatment And Software Development Of Engineering Critical Fatigue Assessment Of Jacket T/K/Y Node

Jacket platform is a kind of common structures in the offshore oil and gas industry, which is widely used, and T/K/Y joints are quite familiar parts of jacket platform. The ocean environment is awful and with the influences of sea wave and sea water corrosion, the jacket platform may be destroyed. Fatigue failure is the main reason why jacket platform is destroyed and T/K/Y joints become the weak link of the jacket structure. People usually use post-weld heat treatment(PWHT) to decrease residual stress and increase the fracture toughness. However, recent research tells us that PWHT may not have those effects for high-thickness welded plates. Besides, with the development of the offshore oil and gas industry, the jacket structure becomes larger and larger, which increase the difficulty of PWHT. Therefore, evaluating the structural integrity by ECA technology based on the theory of fracture mechanics and discussing whether they are exempted from the post-weld heat treatment received widespread attention of the related industry.There are two parts in this article, including experiments and software development. In the part of experiments, we used FCAW to weld plates with the thickness of 80 mm. After conducting CTOD tests we found that the toughness values of vertical welding position specimens with high heat input were higher than those of horizontal welding position specimens with low heat input, which were the reverse results of general cognition, and the article focused on this point of view. Besides, we also measured the residual stresses and CTOD values of welded plates with the thickness of 80 mm before and after PWHT. As a result, after PWHT, the level of residual stress decreased distinctly, while the CTOD values of HAZ worsened, and HAZ area became the weak link of the structure. We studied on this phenomenon.In the part of software development, we developed a kind of ECA fatigue assessment software for T/K/Y joint of jacket platform according to BS7910 standard and FITNET standard with the use of VB.NET. In this development progress, we put the computational formulas of standards into different modules and connected them with the software interface. The user can cooperate with the software to finish the calculation. In this paper, we used the own development ECA software to evaluate one Y joint of jacket structure in Gulf of Mexico, and at the same time, we evaluated the same structure with the use of Crackwise developed by TWI. The results of these softwares indicated that the own developed software is really reliable and it could tell whether PWHT is necessary. Finally, we combined the experiments and the software and used the software to calculate the limit sizes of crack when the residual stress and CTOD value changed before and after PWTH. We could tell whether PWHT is necessary for the structure with the consideration of assessment results and the engineering situation.