Fatigue failure of an offshore condensate recycle line in a natural gas production field

After 2 years of operation, a condensate recycle line at a natural gas production field failed due to fatigue cracking. Fatigue was caused by a combination of both cyclic stresses related to an intermittant operational mode and stress concentration at the base of a fillet weld. Stress concentration was increased by weld surface defects, including undercut and overlap, that produced a sharp transition at the fillet weld base. Internal welding defects, such as porosity and lack of fusion, did not play a role in crack formation. On the other hand, the failure could be assisted by corrosion after exposing the crack surface to the atmospheric condition. oblem was solved by improving the geometry of the fillet weld so that acceptable contouring with a smooth transition at fillet weld base was obtained, reducing the stress concentration. Periodic inspection was scheduled to detect the existence of outer surface cracks and determine the necessary action to be done in order to avoid sudden failure and unexpected shut down.