Approximately 55 % of the FP(S)O fleet worldwide consist of converted tankers. These former tankers have already consumed a considerable part of their design life while sailing across the world. Depending on the sailing routes and the type and quantity of cargo, the spend fatigue design life can vary considerably. These factors must be taken into account when a new design service life of 20 years is targeted for a converted tanker and its future service life as FP(S)O. Furthermore if considering new build which is approximately 25% and redeployment which is approximately 20% of the FPSO fleet, the structural integrity of these assets could be also further enhanced based on the accumulated experience and new technology, as ultrasonic peening, increasing the possibility of drastically reduced costs on structural integrity maintenance.
Furthermore, if we consider any already deployed FP(S)O which probably have been on station for 20 years or more the topic of structural life extension becomes even more crucial. If any structural life extension project could be carried out on location and therefore avoiding any towing to shipyard and the pertaining time out of production, considerable maintenance cost saving could be achieved.
The two main damage mechanisms which needs to be taken into account for design life and eventually any life extension are corrosion and fatigue. Here we are concentrating in the fatigue damage and its influence on the structural integrity of the asset.
The service life of offshore installations is limited by its structural integrity. Furthermore the structural integrity is mainly governed by the fatigue resistance of critical welded details. In a FPSO installation these details are among others pallet stools weld joints to deck structure and bulkheads/web frames weld connections to longitudinal in ballast tanks, tank brackets, scallop connections, hopper knuckle, among others.
The ultrasonic peening treatment can improve the fatigue resistance of welded joints. Fatigue test results of welded specimens shows a life extension of four times for high stress range and up to ten times for high cycle fatigue or moderate to low stress ranges. For specimens which have already consumed half of their fatigue life the treatment resets the clock to zero, as a minimum value. This is an important information when taken into account that a life extension project is often proposed when the considered asset exceeds its design life.
Consequently ultrasonic peening treatment was applied on fatigue sensitive weld locations on several offshore installations with the aim to extend the service life and maintain their structural integrity. Finite Element Analysis carried out by classification societies for these offshore structures demonstrated critical fatigue lives for several weld connections. These weld connections were then treated by ultrasonic peening with the objective to extend their fatigue lives and by doing that achieve the targeted service life for the installation. The successful application of the ultrasonic peening treatment was a pioneering work which involved several partners.
A pilot project on a FPSO started already in 2005 and the treated critical weld connections are still intact and show not sign of crack initiation despite the fact the calculations then showed shorter fatigue lives than the life span already consumed. As a result the same ultrasonic peening procedure has been proposed to be applied for other fatigue sensitive locations on the installation.
Many offshore installations around the world are reaching their original design life. Most of the operators chose to extend the service life of their assets rather than scrape them and build new installation. The reasons for that are: improved oil recovering techniques, time required to get a new build installation on site, environment concerns, wiser management of energy and resources and an increased oil/gas price due to the current political and economical instability. Therefore the Life Extension of Offshore Installations is currently a subject of high interest for the upstream industry.