Comparison of stiffener effect on fatigue crack in KT-type pipe joint by FEA

Abstract

Jacket structure in offshore structures is generally manufactured using steel pipe members. The jacket structure must maintain its performance under a harsh marine environment. The jacket structure is subjected to repeated wave pressure as well as wind loads. These repetitive waves and wind loads generate fatigue cracks in the structure and shorten the fatigue life of the structure. These fatigue cracks usually occur at the welded intersections. Due to the complicated shape of jacket structures, welding is mostly used for connecting tubular members. It is very difficult to predict where fatigue cracks will initiate first because of the complex shape and presence of many welded joints in the jacket structure. Besides, the welding part generates welding deformations as well as welding residual stresses. The initial imperfections at the weld zone, along with the stress concentrations, make the structure more vulnerable to fatigue failure, so in these circumstances, it is very significant to find the position of the initial fatigue crack to improve the fatigue life. By identifying the location of the initial fatigue crack and the mechanism of fatigue cracking, ways to prevent fatigue cracking can be found. In this study, the effect of stiffeners to prevent fatigue crack initiations was verified by finding the change of initial location of fatigue cracks in the jacket structure. As a verification method, fatigue life and fatigue cracks were analyzed by using a fatigue FEM analysis method based on an iterative elastic-plastic model and continuous damage mechanics. Besides, 3D thermo elastic-plastic analysis was performed to reproduce the welded part including the stiffener and 3D non-steady heat conduction analysis was performed to calculate the heat history. The performance of the ring stiffener inside the steel pipe member and the stiffener welded outside the steel pipe member was compared. As a result, it was quantitatively revealed that the two stiffeners are effective in preventing fatigue cracking, improving fatigue life, and decreasing stress concentration.