fatigue of riser
TRANSCRIPT
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Assessment of FatigueDamage
Stefan Palm
07.05.2008
Application to risers and umbilicals
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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
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)
- 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
Combined damage from two different processes
Fatigue considerations for typical riser configurations
-
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
Large defect/Unstable
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
Crack growth, Ds=50MPa, a0=2mm
Crack growth, Ds=60MPa, 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
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)
-
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]
Crack growth, Ds=40MPa, a0=2mm
Crack growth, Ds=50MPa, a0=2mm
Crack growth, Ds=60MPa, a0=2mm
mKC
dN
da)(=
Unstable
fracture
Nf
D = (Number of load cycles)/Nf
Damage accumulation
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g Crack
size
Large defect/Unstable
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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http://www.dnv.com/