Structural – Hull Integrity for FPSO’s
Floating Producing Storage Offshore (FPSO) structures are affected to a large extend by fluctuating loads or fatigue loads. The most important sources for these fatigue loads are operational loads (frequent loading and off-loading operations) and the local set of weather conditions or metocean hind cast. Combinations thereof leads to unavoidable uncertainties on the load spectrum for any specific installation. Due to improved oil recovery technologies or for redeployment reasons, FPSO installations are with increased frequency the object of life extension projects. As a result the study and understanding of relevant load spectra as well as hull’s structural response for these specific installations have become one important area within life extension projects.
Ultrasonic peening to avoid premature fatigue cracking on pallet stool welds
The above mention uncertainties in load spectra and hull’s response are the main reason to early signs of degradation as premature fatigue cracking. Further signs of early hull degradation could be coating damage in high stressed areas in ballast tanks among others.
A Life Extension Program could include geometrical shape improvement of pallet stool brackets and subsequent ultrasonic peening treatment of the welds. However, the replacement of brackets will include hot-work and substantial disruptions in the operations of the installation. If the fatigue cracking can be prevented to occur for example by ultrasonic peening treatment of high stressed welds, the operation’s disruption, hot work planning, bed space shortage, etc. could be avoided.
Ultrasonic peening supported by FE Analysis to find high stress locations and optimal bracket radius
A bracket replacement or any other steel reinforcement should need to be carefully studied before the structural changes are implemented. Here we se the result of a FE Analysis performed with the intention to remove the potential crack initiation as far away from main deck, cargo tank, as possible. As the targeted life was 20 years a double radius finely ground finish could be acceptable.
Ultrasonic peening of welds to prevent fatigue cracking
Furthermore, Ultrasonic Peening was used on the side welds of the shape modified bracket, to ensure crack would not occur there; the ultrasonic peening treatment increases fatigue life of a weld by up to five times when compared to the fatigue life in the as-welded condition. The weld quality uncertainties if any, could be then improved by the ultrasonic peening treatment.