Fatigue crack growth in stiffened steel tubular joints in seawater environment

Welded steel tubular structures commonly used for offshore platforms are susceptible to fatigue failure due to stress concentration at welded joints and environmental loading which is cyclic in nature. Offshore structures contain flaws ranging from microscopic material defects to macroscopic geometric imperfections, due to fabrication and construction process. Such flaws can develop into cracks due to influence of cyclic wave loading in an aggressive marine environment. Due to this and many other causes, maintaining structural integrity of these structures has been a major concern for the oil industry. Hence a model for crack initiation life and crack growth life estimation has been developed by conducting constant amplitude corrosion fatigue tests on internally ring-stiffened steel tubular T and Y joints under freely corroding conditions. Biologically inert synthetic seawater was used as the corrosive environment. Tests in seawater were carried out with a loading frequency of 0.2 Hz. The studies were made with respect to tropical environment similar to Indian offshore. The corrosion fatigue crack growth data obtained during the tests and the Fracture Mechanics (FM) model developed are discussed in this paper.