Overview of the European FITNET fitness-for-service procedure

Isabel Hadley, TWI, and Mustafa Koçak, GKSS Research Centre

Paper presented at OMAE 2008, 27th international Conference on Offshore Mechanics and Arctic Engineering, Estoril, Portugal, 15 - 20 June 2008. Paper OMAE 2008 - 57741.

Abstract

The European FITNET consortium was convened in 2002 with the remit of preparing a procedure for fitness-for-service (FFS) evaluation of flawed engineering components. The procedure is intended to be used by a broad range ofindustries across Europe, and can be used at any stage in the life of an engineering component, eg design, fabrication, operation, failure analysis or life extension.

This paper presents an overview of the structure of the procedure. There are four main modules, each covering a particular failure or damage mechanism: fracture, fatigue, creep and corrosion (including environmentally-assistedcracking). These are linked by the use of a common terminology and a single set of reference compendia (annexes), eg for stress intensity, plastic collapse and residual stress, so that a particular flaw can be rapidly analysed for morethan one failure mechanism.

The FITNET fracture assessment procedures in particular represent a significant advance compared with current published FFS procedures such as API 579-1/ASME FFS-1 and BS 7910. There is a hierarchy of different approaches,designated Options 0 to 5, the choice between them depending on the quality of information (in particular, materials property data) available to the user. This could range from Charpy and tensile data only (Option 0) through to theconstraint-dependence of fracture toughness (Option 5). Other Options allow crack driving force to be calculated directly from FEA (Option 4), or permit weld metal strength mismatch to be taken into account (Option 2).

The fatigue analysis module likewise contains several alternative approaches, termed Routes. Some (Routes 1-3) are based on the concept of a nominally flaw-free structure, whilst Route 4 is based on cycle-by-cycle integration of theParis law, and Route 5 addresses non-planar flaws.

FITNET also set itself the goal of providing training in FFS techniques, both through a series of seminars held during the project (2002-2006) and through provision of lasting training material (slides, tutorials, case studies and avalidation document), which are now publicly available.

1. Introduction

Flaws (such as cracks, welding defects and corrosion damage) can arise during the manufacture and/or use of metallic components. For safety-critical items such as pressure vessels and pipelines, the failure of even a singlecomponent due to the presence of a flaw could threaten human life, as well as having severe economic and environmental consequences. On the other hand, some flaws are harmless, as they will not lead to failure during the lifetime ofthe component; replacement and/or repair of such flaws would be economically wasteful. A fitness-for-service (FFS) procedure allows flaws to be evaluated consistently and objectively, using fracture mechanics principles. Althoughseveral reputable fitness-for-service procedures already exist (e.g. BS 7910, API 579-1/ASME FFS-1, SINTAP, R6) ^[1-4], they tend to be aimed at a particular industry sector, or a single failuremode, or are national documents. There is therefore a need for a unified European procedure covering a range of industry sectors.

With this in mind, the European FITNET (FITness-for-service NETwork) consortium was convened in 2002. The vehicle for the project was a Thematic Network, a mechanism which promotes international collaboration on a particular topic,primarily to collate and formalize existing research work. This was part-funded by the European Union (EU), with the balance of the funding provided by the members (representatives of industry, academia, safety bodies and researchorganizations) themselves. Participants outside the EU were also sought, providing self-funded contributions from organizations in Switzerland, Korea, Japan and the USA.

2. Overview of FITNET

2.1 Structure of the Network

The management structure of the network is shown in Figure 1. A series of Working Groups (WGs) and Work Packages (WPs) were set up under the overall management of GKSS (Germany), each led by a different contractor. Whilstthe WGs covered a particular failure/damage mechanism (eg fracture, corrosion), the Work Packages comprised a particular activity cutting across all WGs, for example validation and case studies, training and education. Additionalsub-groups and task groups were set up as the network progressed, to pursue specialist areas of the procedure.

So far as LTA analysis is concerned, FITNET encompasses damage in pressure piping, pipelines and pressure vessels that have been designed to a recognised code such as BS PD5500, EN13445, ASME VIII Divisions I and II or ASME B31.3.The types of geometry covered by the procedure include spheres, cylinders, elbows, ends (hemispherical, torispherical and elliptical) and integrally reinforced nozzles. The principles of the method are that the corrosion-damaged vesselor pipework should be capable of undergoing a hydrotest without failure, and that the stresses in the thinned area of the component should not exceed the yield strength of the material under design pressure. The safe working pressureof the equipment may need to be reduced in order to retain a safety factor similar to that for the undamaged equipment; a summary of the approach is shown in Figure 6.

Various conditions exclude the use of the LTA approach - for example, there must be no crack-like flaws, no mechanical damage combined with corrosion, no cyclic loading and no flaws with depth greater than 80% of the original wallthickness. These exclusions are intended to ensure that the damage is truly LTA, not crack-like, and that the extent of damage lies within the envelope for which experimental validation exists.

Fig.6. Flow chart for LTA analysis

2.7 Validation, case studies and tutorials

FITNET also set itself the goal of providing training in FFS techniques, both through a series of seminars held during the project (2002-2006) and through provision of lasting training material (slides, tutorials, case studies and avalidation document). An additional volume of FITNET brings together information on validation, case studies and tutorials.

Validation typically consist of the application of FITNET principles to a set of experimental data, for example a large-scale laboratory test or a real structural failure. A failure event is unambiguous; for example, if failure of alaboratory specimen has in practice occurred, then analysis of the failure conditions in accordance with FITNET should predict failure conservatively - if it predicts survival, then the method is non-conservative. Extensive validationof all four major failure/damage modes is documented in this additional volume, in the FITNET final conference proceedings and elsewhere. An example of the validation of the fracture procedure is shown in Figure 7. Thissummarises the results of over 300 full-scale and large-scale tests, including wide plate, pressure and bend tests on welded and plain materials.^[11] A range of materials (structural, linepipeand stainless steels and aluminium alloys) is included in the database. All tests ended in failure; consequently, all analysis points fall outside the Failure Analysis Line, in the 'potentially unsafe' area of the FAD. All the analysesshown in Figure 7 are based on Option 1, ie the simplest fracture mechanics-based analysis route, although selected cases have subsequently been re-analysed using more advanced Options. ^[16,17]

Fig.7. Validation of the FITNET fracture assessment procedure

Case studies and tutorials consist of the application of FITNET principles either to equipment that has not failed in service, or to hypothetical situations devised for teaching and illustration. As such, these examples illustratecertain features of the procedure (for example, how to analyse creep-fatigue interaction) but do not necessarily test the procedure critically. The document includes examples of the application of FITNET to a broken forklift truck, tostrength mismatched welded components, to ship structures and to a hip implant, amongst other things.

2.8 Current status of the FITNET procedure

The FITNET procedure is currently available to interested parties (members of the consortium and selected standards bodies) in the form of a final draft, MK8.^[18,19] The ultimate aimremains to publish the procedure (Volumes 1 and 2) as a CEN document, via a CEN workshop agreement (CWA). It is likely that the volume containing validation, case studies and tutorials will remain the intellectual property of theFITNET consortium, and will be published separately by them. In the meantime, plans are underway to adopt relevant parts of FITNET into a future edition of the BS 7910, the UK national procedure.

3. References

1. BSI, 2005: BS 7910:2005 (incorporating Amendment 1); 'Guide to methods for assessing the acceptability of flaws in metallic structures'.
2. API 579-1/ASME FFS-1 2007, 'Fitness-for-service'.
3. SINTAP: www.eurofitnet.org/sintap_index.html
4. R6 - Assessment of the Integrity of Structures containing Defects, Revision 4.
5. Koçak, M.; 'FITNET fitness-for-service procedure: an overview', International Institute of Welding, Welding in the World, vol.51, no.5-6. May-June 2007. pp.94-105.
6. Koçak, M.; 'FITNET fitness-for-service procedure: an overview', FITNET 06-04, in Proceedings of the International Conference on Fitness-for-service (FITNET 2006): 17-19 May, Shell Global Solutions, Amsterdam, The Netherlands (ISBN 978-3-00-021084-6).
7. Koçak, M.: 'Fitness for service analysis of structures using the FITNET procedure: an overview'. In: Offshore Mechanics and Arctic Engineering (OMAE 2005). Proceedings, 24th International Conference, Halkidiki, Greece, 12-17 June 2005. Publ: New York, NY 10016, USA; American Society of Mechanical Engineers; 2005.
8. Koçak, M.; 'FITNET fitness-for-service procedure: an overview', European Seminar on Pressure Equipement (ESOPE), Paris, 9-11 Oct, 2007
9. Schwalbe, K-H. et al, 'EFAM ETM 97 - The ETM method for assessing the significance of crack-like defects in engineering structures', comprising versions ETM 97/1 and ETM 97/2. GKSS report 98/E/6.
10. Koçak, M., Seib, E. and Motarjemi, A., 2006, 'Treatments of structural welds using FITNET fitness-for-service procedure: FITNET 06-013', as Ref.^[6]
11. Hadley, I. and Moore, P., 'Validation of fracture assessment procedures through full-scale testing: FITNET 06-018', as Ref.^[6]
12. Seib, E., Volkan Uz, M. and Koçak. M., 'Fracture analysis of thin-walled laser beam and friction stir welded Al-alloys using the FITNET procedure', FITNET 06-019, as Ref.^[6]
13. Hobbacher, A., 2004, Recommendations for fatigue design of welded joints and components, IIW document XIII-1965-03/XV-1127-03, February.
14. BS 7608:1993, 'Code of practice for fatigue design and assessment of steel structures'.
15. R5 - 'Assessment procedure for the high temperature response of structures, Issue 3'.
16. Hadley, I., 2007, 'Validation of the European FITNET fitness-for-service procedure: incorporation of weld strength mismatch into fracture assessment (Options 2 and 3)'. TWI Industrial Members' report 890/2007.
17. Hadley, I., 2008, 'Validation of the European FITNET fitness-for-service procedure: Use of fracture assessment Option 4'. TWI Industrial Members' report 893/2008.
18. FITNET Fitness-for-Service (FFS) - Procedure (Volume 1) ISBN 978-3-940923-00-4, Koçak, M., Webster, S., Janosch, J.J., Ainsworth, R.A., Koers, R., printed by GKSS Research Center, Geesthacht, 2008
19. FITNET Fitness-for-Service (FFS) - Annex (Volume 2) ISBN 978-3-940923-01-1, Koçak, M., Hadley, I., Szavai, S., Tkach, Y., Taylor, N., printed by GKSS Research Center, Geesthacht, 2008