Forskningsstatus på utmatting og bruddmekanikk ISSC August 2010

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1 Forskningsstatus på utmatting og bruddmekanikk ISSC August 2010

2 Innledning Hva er ISSC Oppsummering av ISSC fatigue and fracture rapporten som ble presentert i Seoul i 2009 Forskningsprosjekter som kjøres og skal starte opp i DNV 02 September 2009 Det Norske Veritas AS. All rights reserved. 2

3 Aims of ISSC Den internasjonale Skip og Offshore Structures Congress (ISSC) er et forum for utveksling av informasjon og kunnskap innen Offshore og skipsdesign. Målet for ISSC er å få en dypere forståelse innen de ulike disiplinene som omhandler marine design, produksjon og drift gjennom et internasjonalt samarbeid. 3

4 Spesifikke mål er; anmelde forskningsresultater samt evaluere og drøfte resultatene; analysere resultater fra nyere undersøkelser; identifisere områder som krever fremtidig forskning, og foreslå forbedringer i design, produksjon og operasjons prosedyrer for på den måten å kunne sikre et bedre og sikrere produkt i fremtiden 4

5 ISSC2012 Working groups I.1 Environment I.2 Loads II.I Quasi Static Response II.2 Dynamic Response III.1 Ultimate Strength III.2 Fatigue and Fracture IV.1 Design Principles and Criteria IV.2 Design Methods V.1 Damage Assessment after Accidental Events V.2 Natural Gas Storage and Transportation V.3 Materials and Fabrication Technology V.4 Ocean Wave & Wind Energy Utilization V.5 Naval Vessels V.6 Arctic Technology V.7 Impulsive Pressure Loading and Response Assessment V.8 Yacht Design Working Group: Trends in Offshore Research and Technology 5

6 Komiteens mandat Mandatet skal legge vekt på sprekk initiering og vekst under syklisk last samt ustabil sprekk vekst og kritiske sprekker i skip og offshore konstruksjoner. Oppmerksomhet skal rettes til praktisk anvendelse og statistisk beskrivelse av ulike bruddmekaniske metoder i design, fabrikasjon og opperasjon. Det skal legges vekt på vurderinger av passende fysiske modeller og usikkerheten til disse 6

7 COMMITTEE MEMBERS III.2 A. M Horn, Norway M R Andersen, Denmark M Biot, Italy B Bohlmann, Germany S Mahérault-Mougin, France J Kozak, Poland N Osawa, Japan Young-Sik Jang, Korea H Remes, Finland J Ringsberg, Sweden J van der Cammen, The Netherlands 7

8 CONTENTS III.2 CH. 2: State of the art of Life Prediction Methods CH. 3: Probabilistic approaches and mulitiaxial fatigue CH. 4: Factors Influencing Fatigue CH. 5: Developments in Materials and new structures CH. 6: Fatigue Design Methods for Ship and Offshore Structures CH. 7: Practical examples and Benchmark study CH. 8: Operating experience and life extension of ships and offshore structure 8

9 CH. 2: Stress Approaches NOMINAL STRESS APPROACH; is the stress calculated in the sectional area under consideration HOT SPOT STRESS APPROACH defines the fatigue strengths directly by the local stress at the crack initiation point EFFECTIVE NOTCH STRESS APPROACH NOTCH STAIN APPROACH predicts the life for a crack at the root of a notch The uncertainties in calculation and measurement of structural stress concentration factors are stressed by Lotsberg et al (2007c), who compared the results obtained by FE analyses with those derived by on-site measurement. (standard strain gauges & detections of the stress field based on the magnetostriction effect) real geometry of a welded toe can be capture by three-dimension laser scan technique is a useful tool, Hou (2007). A round robin was carried out within the Network of Excellence MARSTRUCT (Fricke et al, 2007a) with the objective to quantify the uncertainties related to modelling, stress evaluation and identification of sources of scatter of weld root cracking of fillet-welded structures. They provided design recommendations for effective notch stress calculation. 9

10 CH. 2: State of the art of Life Prediction Methods Relation ship between S-N and Fracture Mechanics Approaches The S-N approach is a suitable method for fatigue assessment of welded components or structures. S-N curves are derived from tests on samples of the material to be characterized. Miners rule D k = i = 1 n N i i Fracture mechanics study the formation of cracks in materials. Paris law da dn = C K m harmonization of obtained life based on fracture mechanics and S-N approaches are welcome, especially in order to define a common safety level approach between the two methods 10

11 CH. 2: Fracture Mechanics Approach Fracture mechanics evaluation requires detailed knowledge of the material in question like: yield and tensile stress, residual stress, material toughness. For designers, the goal is to select material with sufficient toughness measured by e.g. (CvN, CTOD, SENT, J-R, Kmat) when subjected to service loads and environments in order to ensure that the component does not fail in a brittle behaviour. PIPE SENT FRACTURE TOUGHNESS [J, K, CTOD] SENB (a/w = 0.3) SENB (a/w = 0.5) CT (a/w = 0.5) 300mm mm 400 mm GEOMETRY / CONSTRAINT [T,Q,M] Facture toughness compared by the constraint level for different specimens, Nyhus et al (2001) 900mm mm CWPT specimens geometry and test machine (Fairchild et al 2007) 11

12 CH. 2: Strain based fracture mechanics Current fracture mechanic assessment procedures, e.g. BS 7910, R6, API 579 and API 1104 are not specifically developed to handle strain > 0.5% pipelines have been designed for strain in the magnitude of up to 4 % due to reeling or environmental loads Two JIP s are currently running; Fracture Control JIP: Sintef and DNV, DNV-RP-F108 PRCI (Pipeline Research Council International) 12

13 CH. 3: Probabilistic Approaches and Mulitiaxial fatigue The fatigue damage in marine structures is a stochastic process with large uncertainties associated with both the loading and the resistance side. Muhammed and Stacey (2008) examined the factors affecting the uncertainties in the fatigue life prediction of welded joints and showed that fatigue life predictions for offshore structures are mainly dominated by uncertainties in the estimation of nominal stress and stress concentration factor. Ayala-Uraga and Moan (2007) studied various alternative S N and fracture mechanics formulations for welded joints in ship and offshore structures. Multiaxial fatigue procedures are not used to a large extent in ship design Ninic et al (2007) present a multi-axial fatigue damage function that search of the most critical plane where crack occurrence and initial growth is assumed to take place. Despite that most structures are subjected to multiaxial stresses most design rules only consider the first principal stress. 13

14 4 FACTORS INFLUENCING FATIGUE Mean stresses and their redistribution and Residual stresses The design against fatigue for pile driving scenario is however normally based on fatigue data related to external tensile loading. Lotsberg et al (2008) therefore decided to investigate possible positive effect of the loading being mainly in compressive or if shakedown of residual stresses arising from manufacturing can be observed. Loading of the structure including load sequences Many models have been developed for the analysis of fatigue lives of structures exposed to variable amplitude loading. However, no universal models have yet been generally agreed on. Xiaoping et al (2008) proposed a concept for the modelling of crack growth based on a Paris equation with equivalent stress intensity factor and modified Wheeler model taking into account the effect of overloads and underloads. The model makes it necessary to obtain material data for R=0 only (see also Huang and Moan (2007)) which reduces the required experimental work. 14

15 4 FACTORS INFLUENCING FATIGUE Thickness of the structural joints The thickness and size effect is related to the decrease in fatigue strength of welded structures as the wall thickness of the member increases. ks = (tref/t)k where t is the relevant plate thickness, tref is a reference thickness and k is an empirical constant. T An overview over the available technical literature and approaches proposed by codes for the description of the thickness effect is given by Mashiri and Zhao (2005) including tref and k for a large number of design codes. Corrosive environments and temperature of the surroundings McMaster et al (2008) measured the fatigue knockdown factor of sour/brine environments compared to that of laboratory air. It was found that the measured fatigue life decreased in the range of 3-12 times when tested under sour brine environmental conditions compared to lab air. See also Buitrago et al (2008) and Pargeter et al (2008) for influence of testing frequency on fatigue crack growth in various corrosive environments and Fourozan et al (2008) for simulation of stress corrosion crack growth in pipelines under the influence of residual stresses. 15

16 CH. 5 New Steel with Improved Crack Growth Properties (FCA, FMDP Steels) It is often assumed that the fatigue crack growth properties of structural steels fall in a common scatterband like in BS 7910 Studies have shown that improvement of microstructure of steel can leads to an increased life for the component in question Katsumoto et al (2005) and Konda et al (2007) showed that fatigue crack growth rates of fracture crack arrester (FCA) steel were reduced compared to the conventional steel It is recommended that more details need to be introduced in the ECA procedures for fatigue crack growth with the advent of steels with improved crack growth properties Fracture crack growth test results in atmospheric environment, Katsumoto et al (2005) 16

17 CH. 7 PRACTICAL EXAMPLES AND A BENCHMARK STUDY As more economic and reliable design is required, benchmark studies have been performed to justify upgrading of current fatigue design S-N curves, to provide a better basis for design, or to propose improved methods. Girth welds in pipelines; usually UK Class F2. Maddox et al (2006) performed full scale fatigue tests on a range of girthwelded pipes, and results supported the use of UK Class C. Haagensen et al (2007) introduced artificial welding defects of approximately 2ⅹ50mm on the pipe, and the results supported the use of the E design curve 17

18 CH. 7 Case Study Structural Hot Spot Stress (HSS) Determined by extrapolating the structural stress outside the localized notch-affected zone to the weld toe. Classification society rules HSS is evaluated with shell FE models where the welds are not included. The accuracy of shell-based HSS determination techniques can be examined by comparing shell stress and solid stress. Newly developed shell-based HSS determination techniques for HSSs of web stiffened cruciform connections CSR-B's modification factor Lotsberg et al., 2007 (DnV C.N. 30.7) Osawa et al., 2007 The validity and applicability of these new techniques has been examined 18

19 CH. 7 Case Study Surface stress σ sx [MPa] hopper corner model perpendicular corner joint bilge knuckle model 0 HHI Hopper corner (135deg.) d or ξ [mm] 8Solid4w_OU Shell4_OU Shell4_LR Shell4_HHl Shell4_BVl Shell4_BV2 Shell8_BV Shell results show fairy good agreement with the reference solid stress (1) SR202B method tends to overestimate the target HSS. The estimation error becomes larger when the bevel angle approaches 90 deg. (2) DNV CN 30.7 and Osawa s proposal derive almost identical HSS for all models examined, and the accuracy of the derived HSS is good. (3) CSR-B s modification factor method derives HSS approximately equals to those derived by DNV CN 30.7 and Osawa s proposal, but the scatter of the derived stress is a little larger in some cases. 19

20 CH. 8: OPERATING EXPERIENCE AND LIFE EXTENSION OF SHIPS AND OFFSHORE STRUCTURES Several units have been on location for a significant part of their intended service life or beyond, inspection, maintenance and repair strategies are necessary to ensure safe operation, Van der Cammen (2008) Johansen et al (2006) discuss aspects regarding life cycle management. Fast development of monitoring technique opens new possibilities to ensure that the unit operates within design conditions, May et al (2008); acoustic emission, on-line monitoring and sensors The tremendous amount of monitoring information gives a challenge to analyse this data and to arrive at meaningful conclusions. 20

21 SUMMARY (1) 280 publikasjoner har blitt referert Ulike bruddmekaniske modeller har vært vurdert, og det meste av ny utvikling og forskning baserer seg på å forbedre det praktiske aspekt og videreutvikling av eksisterende modeller og metoder. En harmonisering av levetidsberegninger basert på bruddmekanikk og SN er velkommen, særlig for å definere et felles sikkerhetsnivå mellom de to metodene. Tøyningsbasert bruddmekanikk er et ganske nytt felt innen offshore og shipping, er det av stor betydning at en felles anvendbarhet standardiseres i en praktisk design prosedyre. Spennings tilstanden i utmattinspåkjente strukturer er typisk multiaxial og en bedre forståelse av disse spenninger og deres utmattingsinnflytelse er av stor betydning og bør undersøkes nærmere. 21

22 SUMMARY (2) Materialer må ha egenskaper som kan motstå høyt trykk, høye og lave temperaturer og sur service over tid samt er kostnaddseffektive for å kunne utvinne olje og gass i krevende miljøer og på dypt vann. Verftsindustrien har vist økende interesse for lasersveising for å øke effektiviteten og forbedre dimensjons nøyaktighet. Denne nye sveisemetoden åpner muligheter for design av lettvekt strukturer. Revurdering av utmattingslaster i kritiske hotspots på offshore konstruksjoner er viktig for å sikre sikker drift, særlig hvis det er ønskelig at konstruksjonens driftstid økes fra opprinnelig designlevetid. Benchmark tester er likevel nødvendig. 22

23 DNV utmattingrelaterte JIP er som kjøres /starter opp høsten 2010 DNV deltar i en JIP i Korea som er i ferd med å avsluttes med å etablere utmattingsdata ved lave temperaturer (RT, og -163 grader C) for SUS 304, 9% Nickel og Al legering. DNV arbeider sammen med Statoil om å gi bedre veiledning om forbedringsfaktorer ved sliping av sveisetå og UIT. Fullskala laboratorieforsøk av doblingsplater som vil bli avsluttet i løpet av året. DNV er avslutter i høst en JIP på groutforbindelser i vindmølletårn som har vist seg å ha en langt mindre styrke enn tidligere antatt. Resultatene herfra har gitt grunnlag for bekymring av dynamisk kapasitet også av grouta forbindelser med "shear keys" som er påkjent av reversibel dynamisk belastning. DNV lanserer (høst 2010) en JIP på groutforbindelser med skjærkile som er utsatt for reversibel dynamisk last, kontakt; Rune.Torhaug@dnv.com DNV lanserer (høsten 2010) en JIP på sur service ECA for bestemmelse av akseptkriterier 02 September 2009 Det Norske Veritas AS. All rights reserved. 23