FEA calculations demonstrated critical fatigue lives for some welded connections in WBT (water ballast tank) both in P and S (Portside and Starboard). A total of six WBT will be treated by ultrasonic peening with the aim to extend the fatigue life of these critical weld connections.
The locations of the weld connections are: side shell longitudinal in the way off typical web frame and side shell longitudinal in the way off transverse bulkhead. These connections showed fatigue lives (15 and 7 years respective) well below the targeted structural service life of 20 years for FPSO Triton. Therefore it was decided to extend the fatigue lives of these connections by ultrasonic peening treatment.
The life extension achieved for UP treated welds is 4 times, conservative estimate, for high stress ranges and up to ten times for low stress ranges. The comparison is based on own fatigue testing, SN Design Curves and IIW Recommendations for improved welds.
The estimated benefit of UP treatment is calculated under the assumption that the concerned welded details do not contain fatigue cracks at the time of blast cleaning and/or posterior ultrasonic peening. The ultrasonic peening treatment of these details will improve and/or restore their original fatigue life.