Fatigue of Riser

Assessment of Fatigue Damage Application Applicati on to risers and umbilicals umbilicals Stefan Palm 07.05.2008

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Objectives 

Give introducti introduction on to to princip principles les for assessme assessment nt of of fatigue fatigue damage damage with reference to design codes and engineering practice

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Give an over Give overvi vie ew of typi typic cal fati fatigu gue e load loads, s, analy nalysi sis s meth method odol olog ogy y and and fat fatig igue ue capacity

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Show Show a few few exam exampl ples es for for typi typica call rise riserr con confi figu gura ratio tions ns















Typical riser fatigue assessment assessment procedure procedure Task

Comment

Define fatigue loading.

Based on operating limitations including WF, LF and possible VIV load effects.

Identify locations locations to be assessed. assessed .

Structural discontinuities, joints (girth pipe welds, connectors, bolts), anode attachment welds, repairs, etc.

Global riser fatigue analysis.

Calculate short-term nominal stress range distribution at each identified location.

Local joint stress analysis.

Determination of the hot-spot SCF from parametric equations or detailed finite element analysis.

Identify fatigue strength data.

S-N curve depends on environment, construction detail and fabrication among others.

Identify thickness correction factor.

Apply thickness thickness correction correction factor to compute compute resulting resulting fatigue fatigue stresses.

Calculate accumulated fatigue damage from weighted short-term fatigue damage.















Content  

Fati Fatigu gue e load loadin ing g Analyses methodologies

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Criti Cr itica call ho hots tspo pots ts an and d SN SN-c -cur urve ves s

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Dama Da mage ge ca calc lcul ulati ation on

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Com Co mbi bine ned d da dama mage ge fr fro om tw two o di diff ffe ere ren nt pr proc oces esse ses s

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Fatig Fa tigue ue co cons nsid ider erat atio ions ns for typi typica call ri rise serr config configur urat atio ions ns - Steel Catenary Risers (SCR) - Top Tensioned Risers (TTR)















Fatigue load history Long-term description of individual waves H \ T 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5

1.5 4 6563543 15665333 749462 9371771 38841 1991649 2557 326059 220 46297 22 6170 2 776 90 8 1

6 11341123 12490552 5671979 2059422 655471 187711 48968 11805 2589 540 103 18 3

8 4560268 6030618 3978348 2267390 1161634 547886 239758 98 9 8656 37831 13929 4931 1676 547 167 48 13

10 15 1584350 2014398 1481485 989181 639748 398108 240142 138934 77371 40916 20643 9997 46 4663 2095 903 371

12 569519 551039 39 396341 275053 18 183157 121309 80 8 0648 53372 35332 22965 14584 9023 5406 3137 1760 955

14 226896 147376 90799 61169 40824 27030 17611 11 1 1415 7446 4927 3357 2292 1565 1065 721 479

16 97258 42996 21387 12542 7931 5134 3327 2153 1362 861 561 363 239 160 109 74

18 44501 13543 4966 2308 1225 706 439 280 182 119 80 51 34 22 22 14 9

20 21500 4293 1136 397 175 81 43 24 15 10 6 4 2 2 1 1

22 10893 1370 254 61 20 7 3 1 1

24 5760 484 62 10 1

26 7722 313 24 1















Content  

Fati Fa tigu gue e lo load adin ing g Analyses methodologies



Criti Cr itica call ho hots tspo pots ts an and d SN SN-c -cur urve ves s



Dama Da mage ge ca calc lcul ulati ation on



Com Co mbi bine ned d da dama mage ge fr fro om tw two o di diff ffe ere ren nt pr proc oces esse ses s



Fatig Fa tigue ue co cons nsid ider erat atio ions ns for typi typica call ri rise serr config configur urat atio ions ns - Steel Catenary Risers (SCR) - Top Tensioned Risers (TTR)

















Global riser response analysis - Fatigue stress in steel pipe steel pipe 

Time Time hist histori ories es of fatig fatigue ue stress stress calcul calculate ated d for a selec selected ted num number of hots otspots ots arou around nd the the pipe pipe circ circum umfe fere ren nce at releva relevant nt locati locations ons along along the riser riser

()

M (t )

M (t )

T (t )

















Fatigue stress in component of of flexible flexible riser or umbilical 

Sim Simpli plified fied meth ethod ofte ften used used whe where re one one assume ume tha that e.g e.g. pipe pipe in in umbi umbillica ical cros ross section is lo located at th the center of the pipe ha having the same cu curvature as th the global model: - σ (t ) = κ x (t ) ⋅ E ⋅ r ⋅ sin(φ ) + κ y (t ) ⋅ E ⋅ r ⋅ cos(φ ) where κ is curvature and r is radius to hotspot e.g. (OD-t)/2 for midwall stress end E is module of elasticity of elasticity

















Content 

Fati Fa tigu gue e lo load adin ing g



Global Glo bal Loa Load d Effe Effect ct Ana Analys lyses es met method hodolo ologie gies s

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Fatig Fatigue ue anal analys ysis is and and SN-c SN-cur urve ves s

















Method for fatigue analysis 

Fatig tigue analysis based on SNSN-data - SN-data determined by fatigue testing of considered of considered weld detail - based on linear cumulative damage - most commonly used for risers

















Fatigue capacity for constant stress range The basic fatigue capacity is given in terms of S-N curves expressing the number of stress cycles to failure, N, for a given constant stress range, S:



















Corrosion fatigue test set-up

















Pipieline girth weld test specimen













































































































































































































































SN-curves















SN-curves (DNV RP-C203) Non-welded sections: B1 SN-curve Longitudinal seam weld: B2 SN-curve Cast nodes: C SN-curve Forged nodes: An SCF is used that accounts for

Eq. (2.9.1)

B1 SN-curve if if DFF=10 DFF=10 C SN-curve if if DFF DFF < 10















Fabrication tolerances Stress concentration factor due factor due to fabrication tolerance:

SCF = 1 +

3(δ − δ 0 ) t

e

−

t D

Eq. (2.9.1)



δ is total total ecc eccent entric ricity ity (δthickness + δovality)



δ0 is eccentri eccentricity city inhe inherent rent in SN data data (=0.1t) (=0.1t)

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t is pipe thickness



D is pipe pipe outer outer dia diamet meter er

Total eccentricit eccentricity y is sum of fabrication of fabrication tolerance of of thickness thickness and ovality:

δ

(

)/2















Eccentricity















Thickness effect Fatigue strength of welded of welded joints joints to some some extent dependent on thickness Re Redu duce ced d

capaci capa city ty du due e to to inc incre reas ased ed lo loca call st stre ress ss in to toe e for for increa inc reased sed thi thick cknes ness s Th Thic ickn knes ess s

effe ef fect ct ac acco coun unte ted d fo forr by mo modi difi fica cati tion on of th the e st stre ress ss 1.15

1.10

1.05

















S-N curves for different environment (media) 1000

DNV F1-curve CP DNV F1-curve in air DNV F1-curve free corrosion

















Bi-linear S-N curves 1000

(a1;m1) S

100

SSW

⎧ a1 ⋅ S− m1 S > Ssw N = ⎨ −m2 ⋅ a S S ≤ Ssw ⎩ 2

















SN-curves – SN-curves – Umbilicals/flexible Umbilicals/flexible risers 

Proj Projec ectt spe speci cifi fic c data data base based d on test testin ing g appl applie ied d for: for: - armour wires armour wires (flexible risers, umbilical) - copper conductors copper conductors (umbilicals) - super duplex pipes (umbilicals)

















When to use SN-curves and da/dN ? 

SN-curves:

¾

The The deta detailil has has to to be be spe speci cifi fied ed and and pos possi sibl ble e to be repr repres esen ente ted d by one one of the the clas classe ses. s.

¾

Alterantively, component specific design curve can be established by

















Content   

Fati Fa tigu gue e lo load adin ing g Analyses methodologies Criti Cr itica call ho hots tspo pots ts an and d SN SN-c -cur urve ves s



















Fatigue capacity for variable stress range

















Fatigue analysis - Short term fatigue damage

















”Damage accumulation” accumulation” – – fatigue fatigue crack crack growth growth calculation

















”Damage accumulation”

















Detailed fatigue analysis necessary?

















































































































Fatigue of Riser

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