OIL & GAS. Fatigue. Seminar on revision of Eurocodes. Gunnar Solland 02 May DNV GL May 2018 SAFER, SMARTER, GREENER

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1 OIL & GAS Fatigue Seminar on revision of Eurocodes Gunnar Solland 02 May 2018 SAFER, SMARTER, GREENER

2 Increase in the research activity within the field 2

3 Development of fatigue design methods 3

4 Structures on land and offshore Fatigue is the decisive check for structural dimensions for many offshore structures Fatigue is rarely governing the structural design for structures on land This is reflected in the current design philosophy and code recommendations However, new type of structures will increase the need of adequate fatigue design recommendations also for structures on land 4

5 Floating bridge; dynamically loaded by wind and waves 5

6 Tower structures for wind generators Fatigue govern the design both for towers on land and offshore 6

7 Fatigue in traditional structures on land Even with moderate fatigue loading fatigue failures occur in e.g. bridges, The checks are often formulated to eliminate fatigue as a possible failure mode rather than giving estimates of the design life From: Fatigue and fracture in steel bridges case studies. J.W. Fisher. 7

8 Characteristics of fatigue capacity Structural failure due to fatigue is different from other failure modes: Larger uncertainty Develops over time These aspects will affect recommendations in design codes 8

9 Uncertainty in fatigue resistance 9

10 Test data plotted in linear scale 10

11 Importance of in-service inspection The time needed for a fatigue crack to grow to a size that will lead to failure makes it possible to control the safety of the structure by inspection of critical details The inspection can be made by several methods and principles for where to inspect Traditionally, owners of structures on land are negative to rely on inspection to ascertain safe structures This is probably reflected in the design codes 11

12 Requirements in NS-EN Table 3.1 Recommended values for γ Mf Assessment method Consequence of failure Low High Damage tolerant 1,0 1,15 Safe life 1,15 1,35 The small difference in safety factor between Safe life (no inspection) and Damage tolerant is driving the design against Safe life The preferred safety regime for offshore structures is to design details where cracks will be detected before failure of large consequences may occur and to require adequate inspection to be carried out Details where failure can lead to large consequences need significant larger safety factors than recommended in NS-EN

13 Important terms What is large consequences? What is necessary inspection? What is characteristic load? 13

14 Large consequences In offshore practise and in the Norwegian NA to NS-EN the definition of large consequences is that the residual capacity given a fatigue failure of the considered detail is less than the structure checked with safety factors equal to 1.0 From Tabell NA.3.1: iii) levetidsmetode dersom utmattingsbrudd vil bli oppdaget av ordinær tilstandskontroll. Ved fullt utviklet utmattingsbrudd i en detalj (sprekk gjennom tykkelsen) skal konstruksjonen ha tilstrekkelig restkapasitet i bruddgrensetilstanden med partialfaktor både for materiale og påvirkninger lik 1,0. 14

15 Inspection requirements Fatigue cracks are often detected by ordinary survey of the structure before any catastrophic failure Examples Fatigue in orthotropic steel decks Fatigue of hangers of a suspension bridge Loose bolts in the foundation of a tower 15

16 Characteristic load for fatigue checks Definition of the characteristic fatigue design is presently not clear The definition of the characteristic load is ambiguous The characteristic load in NS-EN (Annex A) are defined as a credible upper bound estimate while in EN 1990 it is defined as expected action spectra The loads for fatigue of bridges is defined in , but is not clear (at least not to me) what is the intended probability level 16

17 Use of FE-analysis in fatigue checks An important deficiency in NS-EN is the lack of design recommendation on how to apply FE-analysis results There is presently no definite plans to include this in the next revision of the code As this has been part of the recommendations for check of offshore structures since 1999 the code should either give sufficient recommendations or be generally harmonized with offshore practise so the methods used there can be utilized 17

18 Thanks for your attention SAFER, SMARTER, GREENER 18