Ultrasonic peening has been applied to fatigue sensitive weld connections on Neptune TLP. The fatigue sensitive weld connections consisted of six brackets which transfer loads from pontoons to main structure. All the brackets were re-shaped by plasma cutting to improve their shape and by that reduce stress concentration previous ultrasonic peening.
Some weld connections containing fatigue cracks were weld repaired, disc grinded and pencil grinded previous to the application of the ultrasonic peening. Connections with no fatigue cracks were disc grinded and pencil grinded previous to ultrasonic peening The fatigue life extension obtained by the ultrasonic peening is a factor 1.22 in strength which gives a factor 2.7 in life. This factor is on top of the improvement in fatigue life of
the weld connection due to grinding.
It is recommended that periodic NDT inspections, besides acoustic emission monitoring, are carried out at P1R, P1L and P3L.