The aim of Structural Integrity Management (SIM) is to reduce to a reasonable level the risk of structural failure throughout the life cycle, including any life extension, of an offshore installation. Structural Integrity is an essential element of the reliability that can only be sustained by proactive management. A program of measures is therefore required to counter threats to the Structural Integrity of any offshore installation.
Structural Integrity Management and Ultrasonic Peening
Structural reliability-based methods can assist in providing a framework for assessing site-specific loading and degradation mechanisms (such as fatigue and corrosion) through a systematic consideration of the uncertainty in each degradation mechanism. One of these degradation mechanisms, namely fatigue, could be strongly deferred by the application of ultrasonic peening to critical weld connections. The treatment will increase the reliability of these weld connections and hence decrease the risk for an early fatigue cracking to occur.
Inspection planning and ultrasonic peening treatment
By applying structural reliability analysis and risk assessment techniques to inspection planning, the operator is given a tool to optimize inspection efforts by which he or she can justify the allocation of resources to those structural components with a higher risk profile and potentially relax inspection activities for lower risk components. It is therefore important to increase the number of lower risk components, increase fatigue strength of critical components, so less inspection activities and therefore resources would need to be allocated to just these components. The structural reliability methodology and its application to the risk-based inspection (RBI) planning of an FPSO structure have been proposed by Ku et al. OMAE 2004 and OTC 2005.
The SMI process identifies which rigs components have the greatest risk and hence request a more focused inspection effort to maintain the platform integrity throughout its life. Structure Integrity Management process (SIM) is based on a matrix likelihood vs. consequence.
Target reliability level, reliability-based fatigue assessment and ultrasonic peening
Selecting a target reliability level is necessary in order to establish reliability based design guidelines for critical structural parts in an offshore installation subjected to fatigue. The targeted reliability level for offshore structures is normally decided on the basis of minimizing the total expected costs over the service life and/or life extension of the same for dealing with failures which results in economic losses and its consequences. So it would be possible and desirable to increase the targeted reliability to include the minimum possible of service disruptions in a producing oil rig. Any increased target reliability will include an increased cost in RBI and therefore it will compromise other critical components for a given RBI budget. Therefore if higher reliability could be achieved by the ultrasonic peening treatment of critical weld connections the general scope, including the targeted reliability, could be enhanced.
Model for use in reliability assessment of welded structures
Fatigue behavior of a structural detail is a function of a variety of factors. These factors include : general configuration and local geometry of the member or detail, the material from which the members are made, welding that is used to produce continuity in the joints and members of a welded structure and the loading conditions to which the detail is subjected. Any of the factors have a probability distribution which in some cases is only partially known. Furthermore there are inherent uncertainties arising from unknown local stress ranges occurred during the previous service life of a converted tanker and/or future fatigue loads which the FPSO installation will be subjected to.
Ultrasonic peening treatment have demonstrated it increases the resistance of a welded connection against premature or unexpected fatigue cracking. The treatment will therefore contribute to ensure a reduction of the risk to premature fatigue failure by increasing the fatigue resistance of any critical welded structural member and by that improving its reliability.
Risk Based Inspections and Ultrasonic Peening
Conversely the increased fatigue resistance will contribute to reduce the risk of failure for a specific critical weld connection. Hence, the application of the ultrasonic peening to critical structural welds will contribute to reduce the extent of any RBI. A possible consequence of this reduced risk is the potential to extend the intervals originally required for a RBI program.
Considering the number of critical structural welds which would need to be incorporated, or at least considered, in any comprehensive RBI program and if these number could be reduced, then the matrix likelihood vs. consequence will contain far fewer critical locations and hence the Structural Integrity Management program will be less extensive and therefore less expensive.
Advantages of Ultrasonic Peening Treatment used to increase fatigue resistance:
- 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.