The 31st International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2012)
Rio de Janeiro - Brazil July 1-6, 2012.
The use of fatigue life improvement techniques and specifically ultrasonic peening treatment to extend the service life of offshore structures has become an accepted practice during the last five years. The understanding of the process as well as equipment’s upgrading for treatment in-situ including quality control and assurance have been developed up to a level that it has become a current practice in many parts of the world.
The 30th International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2011)
June 19-24, 2011, Rotterdam, The Netherlands
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. Ultrasonic peening can improve the fatigue resistance of welded joints. Fatigue test results shows an increase of four times for high stress ranges and up to ten times for high cycle fatigue. For specimens which have already consumed half of their fatigue life the treatment resets the clock to zero, as a minimum value. Consequently ultrasonic peening treatment was applied to several offshore installations on fatigue sensitive weld connections with the objective to extend the service life of the these. 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 reach 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 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. 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. 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 among others. Therefore the Life Extension of Offshore Installations is a subject of current interest for the upstream industry.
11th International Conference on the Mechanical Behavior of Materials (ICM11)
June 5- 9, Villa Erba, Como, Italy
Fatigue life extension has been achieved by the application of ultrasonic peening to high stressed areas on pallet stool to deck and longitudinal to frames weld details on FPSO installations. High stressed welds showed too short fatigue lives in as-welded condition. The aim with the ultrasonic peening treatment was to avoid any further weld repairs during the remaining service life of these installations. Fatigue test results of treated relevant weld details have been used to assess the potential life extension. The results showed four to six times fatigue life extension. The fatigue test was designed to confirm that relaxation by external loads of the induced compressive stresses during treatment would not diminish the benefit. The fatigue lives for the treated welds were extended up to twenty years which often is the targeted service life for these installations. 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.
Fatigue Life Extension by Ultrasonic Peening for Offshore Structures: As-built Weld Quality and Overloads during Remaining Service Life
Offshore Technology Conference 2011 (OTC2011)
2-5 May 2011, Houston, Texas, USA
The fatigue life extension procedure by ultrasonic peening uses FEA to assess the critical weld details and to define the location and extension of the areas to be treated. However, FEA models do not take into account variations in as-built weld quality. Areas with moderate level of stresses but showing varying weld quality might benefit of ultrasonic peening treatment as much as areas with the highest stresses. The ultrasonic peening procedure takes into account certain weld parameters originating in the as-build quality so these can be improved during the application of the treatment in order to achieve full benefit.
As-build weld quality has a great influence on the fatigue life of offshore structures. Hence the fatigue life extension procedure needs to address the implications of a low as-build weld quality previous to the application of the ultrasonic peening procedure. When as-built weld quality limits the targeted fatigue life, ultrasonic peening could enhance the fatigue strength up to a desired level. For weld details which have already consumed half of their fatigue life ultrasonic peening can reset the clock to zero.
The increased fatigue resistance due to ultrasonic peening treatment is partly based on the introduction of compressive stresses. Extreme load conditions could redistribute the compressive stresses and therefore it is necessary to verify if the improvement would be still present during the remaining service life of the installation. Therefore fatigue testing with compressive preloads has been carried out on ultrasonic peening treated specimens.
Despite the extreme low weld quality, the ultrasonic peening improved weld details, showed a factor 2 in fatigue life extension which is a value presented in literature as a lower band for the improvement. This improvement is estimated to be unchanged even if overloads could relax the induced compressive stresses.
The poor weld quality have reduced the level of improvement to a factor 2 in life and therefore it is recommended an improvement of weld angle by machining before the application of ultrasonic peening when required. For structural details with low as-built weld quality ultrasonic peening treatment could be the only solution left to enhance the fatigue life avoiding weld repair.
The economical benefits due to reduced weld repair and/or maintenance on offshore installations as a result of the ultrasonic peening treatment include:
- Restore the fatigue strength for structural details with poor weld quality
- Avoidance of a long term plan for extensive weld repair or hot work
- Avoidance of long and unscheduled operational disruptions
- Increased structural safety for the installation during the remaining service life
Offshore Technology Conference 2010 (OTC 2010)
2-5 May 2010, Houston, Texas, USA
The service life of a FPSO is limited by the structural integrity of the ship’s hull. 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 stool brackets welds and longitudinal weld connections to web frames and to bulkheads in ballast tanks. Ultrasonic Peening can improve the fatigue resistance of welded joints. Fatigue test results show a life extension of four times for high stress ranges and up to ten times for high cycle fatigue. For weld details which have already consumed half of their fatigue life ultrasonic peening treatment resets the clock to zero, as a minimum value. Consequently ultrasonic peening treatment was applied on FPSO Triton on fatigue sensitive weld connections with the objective to extend the service life of this offshore installation. The economical benefits due to reduced maintenance as a result of the ultrasonic peening treatment include:
Avoidance of a long term plan for extensive hot work
Avoidance of long and unscheduled operational disruptions
Increased structural safety for the installation during remaining service life
Brasil Offshore Conference and Exhibition 2011
14–17 June 2011 Macaé, Brazil.
Extensive life extension studies for a FPSO installation have been carried out. High stressed welds showed too short fatigue lives in as-welded condition. Ultrasonic peening has been selected to extend the fatigue lives of the concerned weld details. 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. Despite the variable weld quality encountered on the pallet stool welds the treatment was carried out at perfection and it showed to be relatively easy and straight forward application even in locations of difficult access.
The economical benefits due to reduced maintenance as a result of the ultrasonic peening treatment include: Avoidance of long term plan for extensive hot work, Avoidance of long and unscheduled operational disruptions, Increased structural safety for the installation during remaining service life, Ultrasonic peening treatment can be applied while the installation is in operation.
Brasil Offshore Conference and Exhibition 2011
14–17 June 2011 Macaé, Brazil.
By using Ultrasonic Peening the service-life of an offshore installation can be extended. Every floating installation will have weak spots in its structure, this is almost inevitable. We call these weak areas “fatigue hot-spots”. In practice the fatigue hot-spots are the locations were the first cracks start to initiate in the structure; in affect these cracks will threaten the structural integrity of the structure. Ultrasonic peening is a pro-active solution to prevent fatigue cracks to initiate at these locations. The method is a practical and efficient solution for structural integrity challenges related to fatigue cracking. It can extend the service life of high stressed weld joints in an offshore platform by four times if the weld quality is adequate. Even though Ultrasonic Peening is a very efficient way to strengthen the integrity of offshore installation, the method will always be dependent on the existing weld-quality. As ultrasonic peening is a post weld treatment it can not magically transform a bad weld into a good weld, it can only enhance a good weld to an even better weld. Occasionally Ultrasonic Peening Technicians comes across fatigue hot-spots with bad weld quality. In order to be able to estimate the benefit of Ultrasonic Peening even if the weld quality is inadequate, a couple of fatigue trails were designed to investigate this further.
Two separate batches of F Specimens were manufactured.
-As- welded; used for reference purpose
-As welded + UP treated; Specimens were first put under fatigue cycles until ½ of the predicted fatigue life had been consumed, the specimens were then treated with the LETS Global Ultrasonic Peening Procedure, after these two steps the specimens were put back on the fatigue trail until failure. It was made sure that all the specimens had a relatively bad weld quality. For example the weld toe angle was too high and the weld toe was filled with irregularities.
The results show that Ultrasonic peening doubles the fatigue life of a welded attachment even though; the weld quality is poor and even if the welded attachments have already consumed half of its fatigue life. This corresponds with American Bureau of Shipping’s official guidelines
This document focuses on the benefit of Ultrasonic peening on offshore installations which have already consumed a big part of their service life and have locations with bad weld quality. Since operators tend to focus on life extension of offshore installations beyond the original designed service life at a relatively late stage, ultrasonic peening is often used when the welded attachments have used half or more of their intended service life.
Fatigue Design 2009
25–26 November 2009 Cetim, Senlis, France.
LETS Global® has during five years developed its own procedure and equipment for fatigue life extension of offshore structures by ultrasonic peening treatment. The general validation of the procedure requires specific knowledge about the consequences on the UP improvement when compressive stresses are relaxed during the remaining service life of the installation. Fatigue testing has been carried out in Norway and Sweden where fatigue loading sequences have been specifically designed to comply with the requirements of Classification Societies. The fatigue test specimens have been subjected to a sequence of static loads previous to fatigue testing to ensure almost complete relaxation of compressive residual stresses induced during ultrasonic peening treatment. The results demonstrated that the degree of life extension could be retained even if the treated structure would suffer the most severe loads conditions during its remaining service life after ultrasonic peening treatment was applied. Subsequent fatigue test series consisted of welded and UP-treated cruciform specimens where compressive stresses have been drastically decreased by cutting them to a reduced width. These fatigue test results confirms the previous UP test results where compressive residual stresses were relaxed and the improvement was retained. The fatigue life extension achieved for improved welds is four times calculated on the basis of FAT curves when compared to its endurance in as-welded condition for a given welded detail. The degree of improvement is up to ten times when high cycle regime is considered.