Fatigue of Riser

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Assessment of FatigueDamage

Stefan Palm

07.05.2008

Application to risers and umbilicals


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Version Slide 230 April 2009

Objectives

Give introduction to principles for assessment of fatigue damage with

reference to design codes and engineering practice

Give an overview of typical fatigue loads, analysis methodology and fatigue

capacity

Show a few examples for typical riser configurations


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Typical riser fatigue assessment procedureTask Comment

Define fatigue loading. Based on operating limitations including WF, LF and possible VIV

load effects.

Identify locations to be assessed. Structural discontinuities, joints (girth pipe welds, connectors, bolts), anode attachmentwelds, 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 elementanalysis.

Identify fatigue strength data.

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

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

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

Further actions if too short fatigue life. Improve fatigue capacity using:-

more refined stress analysis

-

fracture mechanics analysis

-

change detail geometry

-

change system design

-

weld profiling or grinding

-

improved inspection /replacement programme


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Content

Fatigue loading

Analyses methodologies

Critical hotspots and SN-curves Damage calculation

Combined damage from two different processes

Fatigue considerations for typical riser configurations

-

Steel Catenary

Risers (SCR)

-

Top

Tensioned

Risers (TTR)


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Fatigue load historyH \ T 1.5 4 6 8 10 12 14 16 18 20 22 24 26

0.5 6563543 15665333 11341123 4560268 1584350 569519 226896 97258 44501 21500 10893 5760 7722

1.5 749462 9371771 12490552 6030618 2014398 551039 147376 42996 13543 4293 1370 484 313

2.5 38841 1991649 5671979 3978348 1481485 396341 90799 21387 4966 1136 254 62 243.5 2557 326059 2059422 2267390 989181 275053 61169 12542 2308 397 61 10 1

4.5 220 46297 655471 1161634 639748 183157 40824 7931 1225 175 20 1

5.5 22 6170 187711 547886 398108 121309 27030 5134 706 81 7

6.5 2 776 48968 239758 240142 80648 17611 3327 439 43 3

7.5 90 11805 98656 138934 53372 11415 2153 280 24 1

8.5 8 2589 37831 77371 35332 7446 1362 182 15 1

9.5 1 540 13929 40916 22965 4927 861 119 10

10.5 103 4931 20643 14584 3357 561 80 6

11.5 18 1676 9997 9023 2292 363 51 4

12.5 3 547 4663 5406 1565 239 34 2

13.5 167 2095 3137 1065 160 22 2

14.5 48 903 1760 721 109 14 1

15.5 13 371 955 479 74 9 1

16.5 3 145 501 311 50 6 1

17.5 1 55 252 198 33 4

18.5 20 121 122 22 3

19.5 7 56 73 15 1

20.5 2 24 42 10 1

21.5 1 11 24 6

22.5 4 13 4

23.5 1 7 3

24.5 3 1

25.5 2 126.5 1

27.5

Long-term description of individual waves


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Content

Fatigue loading


Critical hotspots and SN-curves Damage calculation



-

Steel Catenary

Risers (SCR)

-

Top

Tensioned

Risers (TTR)

- Umbilical, Bellmouth area


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Global riser response analysis - Fatigue stressin steel pipe Time histories of fatigue stress calculated for a selected

number of hotspots around the pipe circumference atrelevant locations along the riser

where

r is radius out

to the

location where

the

fatigue

is to

be checked

(inside, outside

or midwall)

steel pipe thickness used in stress calculation is normallyreduced by half of the corrosion/wear allowance

t=tsteel

-0.5*tcorr

( ) ( ) ( ) ( )A

tTr

I

tMr

I

tMt

yx ++= )cos()sin(

r

t

y

x

( )

)(4

64

22

44

IDODA

IDODI

=

=


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Fatigue stress in component of flexible riser orumbilical Simplified method often used where one assume that e.g. pipe in umbilical cross

section is located at the center of the pipe having the same curvature as the

global model:

-

where

is curvature

and r is radius to hotspot

e.g. (OD-t)/2 for midwall stress end E is module of elasticity Calculation of stress in each component in cross section

-

Need

purpose made

software to find

relation

between

the

global responses

and stress

in each

component

(i.e. cross section

analyses)

-

Important

to consider

friction

stress due to contact

pressure

Testing of components and complete cross-sections required for designs outside

previous experience

SCRflexible riserumbilical

)cos()()sin()()( += rEtrEtt yx


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Content

Fatigue loading

Global Load Effect Analyses methodologies

Fatigue analysis and SN-curves Damage calculation



-

Steel Catenary

Risers (SCR)

-

Top

Tensioned

Risers (TTR)

- Umbilical, Bellmouth area


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Method for fatigue analysis Fatigue analysis based on SN-data

-

SN-data

determined

by fatigue

testing of

considered

weld

detail

-

based

on

linear cumulative

damage

- most commonly used for risers Fatigue analysis based on Fracture Mechanics

-

used as supplement to SN data

-

document

sufficient

time interval

from crack

detection

during inspection

and

time of

unstable

fracture

-

document

that

fatigue

cracks

occuring

during operation

will

not exceed

the

crack

size

corresponding

to unstable

fracture


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Fatigue capacity for constant stress range

The basic fatigue capacity is given in terms of S-N curvesexpressing the number of stress cycles to failure, N, for a given

constant stress range, S:

mSaN =

)Slog(m)alog()Nlog( =

where a and m are empirical constants established

by experiments.

Equivalently:

1

10

100

1000

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09 1.00E+1

Numbe r of cycles, N

Stress

range,

S

log()=intercept of log N-axis

m= negative inverse slope)


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Corrosion fatigue test set-up

Testing setup

with

4 segment specimens

linked

together

Test specimen

with

corrosion chamber


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Pipieline girth weld test specimen


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10

100

1000

1,E+04 1,E+05 1,E+06 1,E+07 1,E+08Number of load cycles

Str

essrange(MPa)

RP C203, original N

Failure

Design curve

10

100

1000

1,E+04 1,E+05 1,E+06 1,E+07 1,E+08Number of load cycles

Str

essrange(MPa)


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Fatigue cracking failure modes fatigue cracking from weld toes/roots into the base material

-

frequent

failure

mode

-

most common

weld

in risers

is symmetric, single sided

with

welding

from outside

-

more difficult

to inspect/have control

of

the

root

- weld toe discontinuities generally present and behave like pre-excisting crack- crack

initiation

time short

fatigue cracking from a surface irregularity or notch into the base material (e.gcorrosion)

-

concern

for components

with

stress cycles

of

high

magnitude

-

crack

initiation

time is long, crack

propagation

time is short


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Fatigue crack growth

Base material

Large defect/Unstable

fracture

Ni

Crack

size

Initiation

period Propagation

period


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Weld Base material

Ni (weld) Ni

Crack

size


fracture


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Fatigue crack growth Paris lawmKC

dN

da)(=

agK =

Paris law:

stress

MPa

K

stress intensity

factor

MPam-1/2

a

crack

length/size

m

g

function

dependent on

crack

size

and geometry

(e.g. presence

of

stress concentrations)

C

dimensionless

constant

m

dimensionless

constant

(typically

in the

range 3-5)


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Fatigue crack growth testing

Testing arrangement showing corrosion chambers

2 off compact

tension

crack growth

test

specimens

instrumented

with strain gauges

Compact tension specimenfatigue crack


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p p g growth


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Measurement of crack growth rateBase Material - Sea Water

1,E-06

1,E-05

1,E-04

1,E-03

1,E-02

1 10 100

dK MPam1/2

da/dNm

m/cycl

Base Material 5+6 Regres BS-B BS-B +2 sd BS-A BS-A +2sd


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Fatigue crack growth

0

2

4

6

8

10

12

14

16

18

20

0,E+00 5,E+05 1,E+06 2,E+06 2,E+06

Number of cycles, N

Crackheight,a[mm]

Crack growth, Ds=40MPa, a0=2mm



mKC

dN

da)(=


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Fatigue crack growth intiation period

Ni

Crack

size

Initiation

period

Propagation

period

Macroscopic

defect


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Examples of Riser fatigue critical hotspots Threaded connectors

-

example

of

use: coupling

between

riser joints in C/WO and drilling risers

-

critical

location: hotspot

with

SCF>1 at transition

between

pipe and connection

Bolted flanges-

example

of

use: coupling

between

riser joints in permanent TTR

-

critical

location: weld

between

flange and pipe, flange w/bolts

Welds-

example

of

use: SCR

-

critical

location: weld

root

and cap

Base material in the pipe-

critical

location: areas with

large

responses

S l i f SN


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Selection of SN curves construction details;

fabrication process welded, clad, forged, machined, etc;

base material or weld;

welds - hotspots on the inner surface and outer surface

weld details and tolerances, weld type (welding with or without backing,

double sided weld); stress concentration factors from concentricity, thickness variations, out

of roundness and eccentricity; angularity;

environment - air, free corrosion or cathodic protection in sea water.

W ld l DNV RP C203


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Weld classes DNV RP C203

W ld l DNV RP C203


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Weld classes DNV RP C203

SN


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SN-curves

SN curves (DNV RP C203)


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SN-curves (DNV RP-C203)Non-welded sections:B1 SN-curveLongitudinal seam

weld:

B2 SN-curve

Cast

nodes:

C SN-curve

Forged

nodes:

B1 SN-curve

if

DFF=10

C SN-curve

if

DFF < 10

An SCF is

used that

accounts for

the actualfabrication

tolerances.

Eq. (2.9.1)

Eq. (2.9.1)

The

nominal stress on

the

outside

of

the

pipe to be used for

fatigue assessment of outside hotspotsThe

nominal stress on

the

inside

of

the

pipe to be used for

fatigue assessment of the inside hotspots

Fabrication tolerances


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Stress concentration factor due to fabrication tolerance:

is total eccentricity (thickness + ovality) 0 is eccentricity inherent in SN data (=0.1t)

t is pipe thickness

D is pipe outer diameter

Fabrication tolerancesD

t

et

SCF

+= )0(3

1

Eq. (2.9.1)

Total eccentricity

is sum of

fabrication

tolerance

of

thickness

and

ovality:

4/)(

2/)(

2/)(

minmax

minmax

minmax

minmax

DD

DD

DD

tt

ovality

ovality

ovality

thickness

=

=

=

=

(no

pipe centralisation)

(pipe centralisation

during contruction)

(pipe centralisation during contruction and rotated until

good

fit)

Eccentricity


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Eccentricity

Thickness effect


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Thickness effectFatigue

strength

of

welded

joints to some

extent

dependent on

thicknessReduced capacity due to increased local stress in toe for

increased thickness

Thickness effect accounted for by modification of the stress

Reference thickness tref=25mm

k is thickness exponent

(recommended

k=0.15 for pipes)

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.055 0.060

pipe thickness t (m )

(t/t

ref)

k

tref=0.025, k=0

tref=0.025, k=0.15

S N curves for different environment (media)


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S-N curves for different environment (media)

10

100

1000

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09

Number of cycles

Stressrange(MPa)

DNV F1-curve CP

DNV F1-curve in air

DNV F1-curve free corrosion

factor 1.2

factor 4.5

factor 3

Bi-linear S-N curves


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Bi-linear S-N curves

1

10

100

1000

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+10

No of cycles, N

StressRange,

S

NSW

SSW

(a1;m1)

(a2;m2)

= 1sw1

m

)Nlog()alog(

sw 10S

>=

swm2

sw

m

1

SSSa

SSSaN

2

1

Log(Nsw) is typically 6-7

SN-curves Umbilicals/flexible risers


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SN curves Umbilicals/flexible risers Project specific data based on testing applied for:

-

armour

wires (flexible

risers, umbilical)

-

copper

conductors

(umbilicals)

-

super duplex

pipes (umbilicals)

-

DNV-RP-C203 => SN-curve

for small diameter super duplex

steel

pipe (pipe

OD=10-100 mm)

Sn-curve

applicable

for umbilicals

that

have been reeled:

number of cycles under reeling < 10

strain range during reeling < 2%

When to use SN-curves and da/dN ?


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When to use SN curves and da/dN ? SN-curves:

The detail has to be specified and possible to be represented by one of

the classes.

Alterantively, component specific design curve can be established by

testing.

Fatigue crack growth caclulcations (da/dN):

The initial and final crack sizes have to be known.

Crack growth parameters in Paris law, m and C, has to be known. Somestandardised m/C values given in BS 7910. Otherwise, have to be

determined by testing.

Detailed stress distribution has to be known

Content


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Content

Fatigue loading


Critical hotspots and SN-curves

Damage calculation


Fatigue considerations for typical riser configurations-

Steel Catenary

Risers (SCR)

-

Top

Tensioned

Risers (TTR)

-

Umbilical, Bellmouth

area

Fatigue capacity for variable stress range


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Fatigue capacity for variable stress range

n(Si) : Number of stress cycles with range Si

N(Si) : Number of stress cycles to failure given by S-N curveD : Fatigue damage

: Usage factor (0.1-0.3)

The Miner-Palmgren

rule is adopted for accumulation of fatigue

damage from stress cycles with variable range:

)()( = i i

i

SNSnD

m

i

i

i SSn

a

D )(1

= Single slope S-N curve

Equivalently:

Fatigue analysis - Short term fatigue damage


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Fatigue analysis Short term fatigue damageLong term stress range distribution: Number of stress blocks (Nb) and each block stress

range () calculated from the analysis. Number ofstress cycles (ni) with range is counted

Number of stress blocks (Nb) should not be less

than 20

Total fatigue damage for the short term sea state

found by summation (Palmgren-Miner):

( )

==

bN

i

m

iitermshort SCFn

a

D1

_

1

Block

no. Stress

range

()

Number

of

cycles

(ni

)

1 0-10 1928372

2 10-20 2342732

3 20-30 1338753

4 30-40 453132

5 40-50 34321

6 50-60 4332

7 60-70 433

8 70-80 223

:

::

Nb 120-130 3

Example

of

stress histogram for

one

seastate

Damage accumulationfatigue crackgrowthcalculation


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calculation

0

2

4

6

8

10

12

14

16

18

20

0,E+00 5,E+05 1,E+06 2,E+06 2,E+06

Number of cycles, N

Crackheight,a[mm]




mKC

dN

da)(=

Unstable

fracture

Nf

D = (Number of load cycles)/Nf

Damage accumulation


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g Crack

size


fracture

Detailed fatigue analysis necessary?


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g y y A detailed fatigue analyses can be omitted if the largest local stress

range is less than the stress range at 1.107 cycles (i.e. fatigue limit)

Guidance applicable for air and seawater with cathodic protection (i.e.

two sloped curves) In case of DFF > 1.0, the allowable fatigue limit needs to be reduced by a

factor (DFF)-1/3

If one cycle is above the fatigue limit, fatigue damage from all stresscycles has to be included

Detailed

fatigue

assessment

can

be omitted Detailed

fatigue

assessment

required

Fatigue analysis - Short term fatigue damage


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g y g gRainflow counting: Number of cycles (Nc) in stress time series and

stress ranges () calculated by Rainflowcounting

Fatigue damage calculated for each cycle and

total fatigue damage for the short term sea state

found by summation (Palmgren-Miner):

( )=

=cN

i

m

itermshort SCFa

D

1

_

1

Fatigue analysis - Long term fatigue damage


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g y g g g Long term fatigue damage as a weighted sum of short term fatigue damages:

where

-

DL

accumulated

long-term

fatigue

damage

at given location

- Dij Short term fatigue damage for seastate i in direction j- Pij

Probability

of

occurrence

for seastate

i in direction

j

-

Nd

number

of

wave

directions

-

Ns

number

of

sea-states

in the

wave

scatter

diagram

= =

=d sN

j

ij

N

i

ijL PDD

1 1

Design Fatigue factors


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Design fatigue factors (DFF) versus

Safety Class (DNV OS F201)Low (API-RP-2RD) Normal High(API-RP-2RD)

3.0 6.0 10.0

1DFFDL

Fatigue

criterion:

A risk based

fatigue

criterion

benchmarked

against

reliability

analyses is outlined

in DNV RP-F204 Riser Fatigue. Relevant for novel

concepts

to evaluate

the

standard DFF and relative importance of

each

parameter.

Steel risers:

Flexible risers and umbilicals => DFF=10

Reflection : Desired properties of integration scheme


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Need good physical understanding of the system to select properanalysis methodology

Simplified analysis methods need validation

Three important contributions to fatigue damage are wave-induced,low-frequency and vortex-induced stress cycles

Recommended SN-curves and SCFs for relevant riser/pipeline

geometries is given in DNV-RP-C203

Methods for improving fatigue capacity.

Improving fatigueperformance


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Reduce stress concentrations

Change geometry: tapering, increase fillet radius,

Grinding

Remove defectsGrinding

NDT - repair

Reduce stress levelReduce global response

Reduce stress concentrations

Increase dimensions

Reduce number of load cycles

Use a bend stiffener instead of a bellmouth

References


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Dynamic Risers. Offshore Standard DNV-OS-F201. October 2003

Submarine Pipeline Systems. Offshore Standard DNV-OS-F101.October 2007

Riser Fatigue. Recommended Practice DNV-RP-F204. July 2005 Fatigue Design of Offshore Steel Structures. Recommended Practice

DNV-RP-C203. October 2006

Environmental Conditions and Environmental Loads. RecommendedPractice DNV-RP-C205. October 2007

References


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Faltinsen, O.M. Sea Loads on Offshore Structures. CambridgeUniversity Press

Tucker, M.J. & Pitt, E.G. (2001) Waves in Ocean Engineering. Elsevier

Ocean Engineering Book Series. Vol. 5 Ochi, M. (1998) Ocean Waves The stochastic approach. Cambridge

Ocean Technology Series 6. Cambridge University Press.

Sarpkaya, T. and Isaacson, M. (1981) Mechanics of wave forces onoffshore structures. Van Nostrand Reinhold Co.

References


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Sparks, C.S. The Influence of Tension, Pressure and Weight on Pipeand Riser Deformations and Stresses. Transactions of the ASME. Vol.

106. March 1984. pp.46-54

Newland, D.E. An Introductin to Random Vibrations and SpectralAnalysis. Longman Scientific and Technical

Blevins, R.D. Flow-Induced Vibration. Krieger Publishing Company


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