A Life Extension Program include a range of engineering solutions which combined will help to achieve the targeted service life and maintain the structural integrity of the installation.
A FPSO installation is subjected to more frequent loading and off-loading cycles than a normal oil tanker. These frequent loading and off-loading induces changes in pressure differential between ballast and cargo tanks in the case of double hull tankers. By optimizing the volumes of ballast and cargo it is possible to reduce the pressure differential for common bulkheads and hence reduce the high stresses which would otherwise lead to premature fatigue cracking.
Ultrasonic peening can be one of the engineering solutions used to avoid premature fatigue cracking in high stressed areas. High stressed welds showed too short fatigue lives in as-welded condition and consequently ultrasonic peening treatment was used to increase the fatigue resistance for the sensitive areas.
The aim with the ultrasonic peening treatment was to avoid any further weld repair and by that contribute to the structural integrity of the installation during its remaining service life. Fatigue life extension has been achieved by the application of ultrasonic peening to high stressed areas on the pallet stool- and on longitudinal-weld details on the ballast tanks on a FPSO installation.
The fatigue lives for the treated welds were extended to twenty years which is the targeted service life for the installation. Quality Assurance and Quality Control were covered by Ultrasonic Peening Procedure Specification, applied for every treated weld. It ensures that the treatment is exactly reproduced to achieve the expected life extension.
Blasting and re-coating with high-strain capacity coating
Ballast tank originally coated with Coal Tar Epoxy, which is relatively rigid, could show early signs of degradation or coating damage. These damages are located in areas with high stresses and therefore are exposed to high strain which could lead to coating damage.
The coating damage will lead to potential corrosion damage which would combine, whit the high stresses, to potential fatigue crack initiation and growth.
Therefore during re-coating operations or even after ultrasonic peening of high stressed weld it is highly recommended to apply a high capacity strain coating which will allow for high deformations without cracking.
Upgrading of pallet stool configuration for increased production equipment
One important outcome from a FE model developed in connection to a Life Extension Program is the possibility to carry out analysis of the structural implications of increased weight on pallet stool structure due to upgrading of production equipment. The ability to predict and prevent unforeseen structural integrity consequences due to production equipment upgrading is crucial to a life extension program success.