FPSO Conversion – Ultrasonic peening
The majority of FPSO installations currently in service are converted oil tankers. As a result these hull structures have been reassessed taken into consideration the updated requirements in terms of hull- and fatigue strength, previous or during every conversion project. There is a number of very well know typical critical areas in these hulls which always needs to be carefully assessed. These areas are presented in the picture below.
The original hull structure would normally have been in service for a number of years and hence many of the critical areas considered above have already consumed a considerable part of their original design life when the conversion work occurs. This would specifically apply to welded connections which are subjected to fatigue loads. The fatigue life of welded connections consist mainly of the crack propagation of small welding imperfections always present in any shipyard produced weld. Furthermore the fatigue life of any welded connection is mainly governed by the stress range to which it is subjected. It should be apparent the difficulties posed by the inherent uncertainties arising from unknown local stress ranges occurred during its previous service life as a tanker or future fatigue loads which the FPSO installation will be subjected to. Even if these two were to be well known, which is highly improbable, a problem would further arise when making the necessary fatigue life estimations under variable stress range service regimes possibly arising from many different sailing routes and/or on-station(s). Om top of these uncertainties the accuracy of fatigue propagation analysis hinges on the load sequence’s reproducibility of the actual sea-state history. Ship owners’ concern about fatigue damage is growing since any ship sailing operation/route is changed constantly according to the weather conditions, Pasetyo F. A. et Al, 2013. furthermore, the load sequence obtained from the wave scatter diagram in classification society rules may be different from that of the actual load , Mao W. OMAE 2013. This may affect the accuracy of the crack propagation analysis.
Even if current numerical fatigue assessment techniques including the spectral fatigue analysis enhances the accuracy of a fatigue design significantly, it remains the current reality that many FPSO hull structures suffer from unexpected fatigue cracking. In the figure above the most fatigue critical details and ways to enhance their resistance against fatigue cracking are presented. However, many FPSO operators and we in LETS Global have extensive experience in fatigue cracked welded connections in just exactly the same locations presented above.
FPSO Conversion – Prevention of fatigue cracking by ultrasonic peening
Therefore and besides the great effort and immense knowledge currently applied during a FPSO conversion project to achieve a fatigue crack free service life, reality shows that it is still a long way to go. It is our understanding that despite the enormous accuracy of current fatigue assessment tools an increased reliability will be difficult to achieve due to the uncertainties in the assessment of the external loads as explained above. The only way to achieve further reliability and safety margins against premature fatigue cracking would necessarily be to increase the local fatigue strength of any welded connection which are prone to premature cracking. Since, and due to the extensive experience of the all involved parties these locations are very well known for almost everybody. Now, having the ultrasonic peening treatment proven to be an efficient method to prevent these unexpected fatigue failures it would be just commonsense to use it in the FPSO industry to decrease maintenance costs. The application of ultrasonic peening to increase the fatigue cracking safety margins in order to avoid premature failures, mainly in the above presented fatigue sensitive details, would further complement and improve the reliability and structural integrity of any FPSO and of any offshore installation subjected to fluctuating loads.
FPSO Conversion and arctic environment – Ultrasonic Peening
Ultrasonic Peening can effectivley replace conventional PWHT. The HSE Guidance Notes require PWHT for highly stressed nodal welds greater than 40 mm thick and other welds greater than 50 mm thick unless fracture mechanics calculations show that the joint posses adequate toughness in the as-welded condition. Consequentely, there is interest for an alternative way to relieve residual stresses. The replacement of conventional PWHT can be done by ultrasonic peening.
CTOD increased values by ultrasonic peening treatment
The treatment has demonstrated its contribution to the reduction of tensile residual stresses and the introduction of compressive stresses. Clear improvement on CTOD values, which is a measure of brittle fracture resistance, has been achieved as a result of ultrasonic peening treatment of the weld and HAZ.
The increased CTOD values due to the application of ultrasonic peening after every weld layer in thick welds have a wide field of applications in the artic offshore industry, where increased toughness and/or resistance to brittle fracture in welds including HAZ is of great importance.
Advantages of Ultrasonic Peening Treatment when replacing conventional PWHT;
- Ultrasonic peening systems are small and versatile even for places of difficult access.
- Less energy used
- Much less time required for treatment
- Treatment can be applied locally
- Environment friendly due to saving of energy
- Residual stresses are relieved and fatigue resistance is improved during the same working operation.
- Fatigue life increased fourfold.