sýdahl riser and umbilicals - fatigue damage assessment rev0
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Assessment of Fatigue DamageTRANSCRIPT
Assessment of Fatigue Damage
Stefan Palm07.05.2008
Application to risers and umbilicals
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 fatiguecapacity
Show a few examples for typical riser configurations
Version Slide 330 April 2009
Typical
riser fatigue
assessment
procedure
Task 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 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 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
Version Slide 430 April 2009
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)
Version Slide 530 April 2009
Fatigue
load
history
H \ T 1.5 4 6 8 10 12 14 16 18 20 22 24 260.5 6563543 15665333 11341123 4560268 1584350 569519 226896 97258 44501 21500 10893 5760 77221.5 749462 9371771 12490552 6030618 2014398 551039 147376 42996 13543 4293 1370 484 3132.5 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 14.5 220 46297 655471 1161634 639748 183157 40824 7931 1225 175 20 15.5 22 6170 187711 547886 398108 121309 27030 5134 706 81 76.5 2 776 48968 239758 240142 80648 17611 3327 439 43 37.5 90 11805 98656 138934 53372 11415 2153 280 24 18.5 8 2589 37831 77371 35332 7446 1362 182 15 19.5 1 540 13929 40916 22965 4927 861 119 10
10.5 103 4931 20643 14584 3357 561 80 611.5 18 1676 9997 9023 2292 363 51 412.5 3 547 4663 5406 1565 239 34 213.5 167 2095 3137 1065 160 22 214.5 48 903 1760 721 109 14 115.5 13 371 955 479 74 9 116.5 3 145 501 311 50 6 117.5 1 55 252 198 33 418.5 20 121 122 22 319.5 7 56 73 15 120.5 2 24 42 10 121.5 1 11 24 622.5 4 13 423.5 1 7 324.5 3 125.5 2 126.5 127.5
Long-term description of individual waves
Version Slide 630 April 2009
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
Version Slide 730 April 2009
Global riser response
analysis
-
Fatigue
stress in steel
pipe
Time histories of fatigue stress calculated for a selectednumber of hotspots around the pipe circumference at relevant 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
( ) ( ) ( ) ( )AtTr
ItM
rI
tMt yx +⋅+⋅= )cos()sin( φφσ
rt
φ
y
x
( )
)(4
6422
44
IDODA
IDODI
−=
−=
π
π
Version Slide 830 April 2009
Fatigue
stress in component
of
flexible
riser or umbilical
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 theglobal model:
-
where
κ
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
SCR flexible
riserumbilical
)cos()()sin()()( φκφκσ ⋅⋅⋅+⋅⋅⋅= rEtrEtt yx
Version Slide 930 April 2009
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
Version Slide 1030 April 2009
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
Version Slide 1130 April 2009
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:
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+10
Number of cycles, N
Stre
ss ra
nge,
S
log(ā)=intercept of log N-axism= negative inverse slope)
Version Slide 1230 April 2009
Corrosion
fatigue
test set-up
Testing setup
with
4 segment specimens
linked together
Test specimen
with corrosion
chamber
Version Slide 1330 April 2009
Pipieline
girth
weld
test specimen
Version Slide 1430 April 2009
Version Slide 1530 April 2009
Version Slide 1630 April 2009
Version Slide 1730 April 2009
Version Slide 1830 April 2009
10
100
1000
1,E+04 1,E+05 1,E+06 1,E+07 1,E+08
Number of load cycles
Stre
ss ra
nge
(MPa
)
RP C203, original NFailureDesign curve
10
100
1000
1,E+04 1,E+05 1,E+06 1,E+07 1,E+08
Number of load cycles
Stre
ss ra
nge
(MPa
)
Version Slide 1930 April 2009
Fatigue
cracking
failure
modesfatigue 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
Version Slide 2030 April 2009
Fatigue
crack
growth
Base material
Large defect/Unstable
fracture
Ni
Crack
size
Initiation
period Propagation
period
Version Slide 2130 April 2009
Weld Base material
Ni (weld) Ni
Crack
size
Large defect/Unstable
fracture
Version Slide 2230 April 2009
Fatigue
crack
growth
–
Paris law
mKCdNda )(Δ⋅=
agK ⋅⋅⋅= πσ
Paris law:
σ 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)
Version Slide 2330 April 2009
Fatigue
crack
growth
testing
Testing arrangement showing corrosion
chambers
2 off compact
tension crack
growth
test
specimens
instrumented with
strain
gauges
Version Slide 2430 April 2009
Compact
tension
specimen
–
fatigue
crack growth
Version Slide 2530 April 2009
Measurement
of
crack
growth
rate
Base 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/d
N m
m/c
ycle
Base Material 5+6 Regres BS-B BS-B +2 sd BS-A BS-A +2sd
Version Slide 2630 April 2009
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+06Number of cycles, N
Cra
ck h
eigh
t, a [m
m]
Crack growth, Ds=40MPa, a0=2mmCrack growth, Ds=50MPa, a0=2mmCrack growth, Ds=60MPa, a0=2mm
mKCdNda )(Δ⋅=
Version Slide 2730 April 2009
Fatigue
crack
growth
–
intiation
period
Ni
Crack
size
Initiation
periodPropagation
period
Macroscopic
defect
Version Slide 2830 April 2009
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
Version Slide 2930 April 2009
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.
Version Slide 3030 April 2009
Weld
classes
–
DNV RP C203
Version Slide 3130 April 2009
Weld
classes
–
DNV RP C203
Version Slide 3230 April 2009
SN-curves
Version Slide 3330 April 2009
SN-curves
(DNV RP-C203)Non-welded
sections:
B1 SN-curve
Longitudinal 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 actual fabrication 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
hotspots
The
nominal stress on
the
inside
of
the
pipe to be used for fatigue
assessment
of
the
inside
hotspots
Version Slide 3430 April 2009
Stress concentration
factor
due to fabrication
tolerance:
δ is total eccentricity (δthickness + δovality)
δ0 is eccentricity inherent in SN data (=0.1t)
t is pipe thickness
D is pipe outer diameter
Fabrication
tolerances
Dt
et
SCF−−
+= )0(31
δδEq. (2.9.1)
Total eccentricity
is sum of
fabrication
tolerance
of
thickness
and ovality:
4/)(
2/)(
2/)(
minmax
minmax
minmax
minmax
DD
DD
DDtt
ovality
ovality
ovality
thickness
−=
−=
−=−=
δ
δ
δδ
(no
pipe centralisation)
(pipe centralisation
during contruction)
(pipe centralisation
during contruction and rotated
until
good
fit)
Version Slide 3530 April 2009
Eccentricity
Version Slide 3630 April 2009
Thickness
effect
Fatigue
strength
of
welded
joints to some
extent
dependent on thickness
Reduced 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 0.060pipe thickness t (m)
(t/t re
f)k
tref=0.025, k=0tref=0.025, k=0.15
Version Slide 3730 April 2009
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
Stre
ss ra
nge
(MPa
)
DNV F1-curve CP
DNV F1-curve in airDNV F1-curve free corrosion
factor 1.2
factor 4.5
factor 3
Version Slide 3830 April 2009
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
Stre
ss R
ange
, S
NSW
SSW
(a1;m1)
(a2;m2)⎟⎟⎠
⎞⎜⎜⎝
⎛ −
= 1
sw1m
)Nlog()alog(
sw 10S
⎩⎨⎧
≤⋅>⋅
=−
−
swm
2
swm
1
SSSaSSSa
N2
1
Log(Nsw) is typically 6-7
Version Slide 3930 April 2009
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%
Version Slide 4030 April 2009
When
to use
SN-curves
and da/dN
?
SN-curves:
The detail has to be specified and possible to be represented by one ofthe 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
Version Slide 4130 April 2009
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
Version Slide 4230 April 2009
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 curve D : Fatigue damage η : Usage factor (0.1-0.3)
The Miner-Palmgren
rule is adopted for accumulation of fatigue damage from stress cycles with variable range:
η )()(∑ ≤=
i i
i
SNSnD
mi
ii SSn
aD )(1 ∑= Single slope S-N curve
Equivalently:
Version Slide 4330 April 2009
Fatigue
analysis
-
Short term fatigue
damage
Long
term stress range distribution:
Number of stress blocks (Nb) and each block stress range (Δσ) calculated from the analysis. Number ofstress cycles (ni) with range Δσ is counted
Number of stress blocks (Nb) should not be less than 20
Total fatigue damage for the short term sea statefound by summation (Palmgren-Miner):
( )∑=
Δ⋅⋅=bN
i
miitermshort SCFn
aD
1_
1 σ
Block
no. Stress range (Δσ)
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
Version Slide 4430 April 2009
”Damage
accumulation”
–
fatigue
crack
growth 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+06Number of cycles, N
Cra
ck h
eigh
t, a [m
m]
Crack growth, Ds=40MPa, a0=2mmCrack growth, Ds=50MPa, a0=2mmCrack growth, Ds=60MPa, a0=2mm
mKCdNda )(Δ⋅=
Unstable
fracture
Nf
D = (Number
of
load
cycles)/Nf
Version Slide 4530 April 2009
”Damage
accumulation”
Crack
size
Large defect/Unstable
fracture
Version Slide 4630 April 2009
Detailed
fatigue
analysis
necessary?
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 stress cycles has to be includedDetailed
fatigue
assessment
can
be omitted Detailed
fatigue
assessment
required
Version Slide 4730 April 2009
Fatigue
analysis
-
Short term fatigue
damage
Rainflow
counting:
Number of cycles (Nc) in stress time series and stress ranges (Δσ) calculated by Rainflowcounting
Fatigue damage calculated for each cycle and total fatigue damage for the short term sea statefound by summation (Palmgren-Miner):
( )∑=
Δ⋅=cN
i
mitermshort SCF
aD
1_
1 σ
Version Slide 4830 April 2009
Fatigue
analysis
-
Long
term fatigue
damage
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
jij
N
iijL PDD
1 1
Version Slide 4930 April 2009
Design Fatigue
factors
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
1≤⋅ DFFDL
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
Version Slide 5030 April 2009
Reflection : Desired properties of integration scheme
Need good physical understanding of the system to select proper analysis 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 SCF’s for relevant riser/pipeline geometries is given in DNV-RP-C203
Methods for improving fatigue capacity.
Version Slide 5130 April 2009
Improving
fatigue
performance
Reduce stress concentrationsChange geometry: tapering, increase fillet radius,Grinding
Remove defectsGrinding NDT - repair
Reduce stress levelReduce global responseReduce stress concentrationsIncrease dimensions
Reduce number of load cyclesUse a bend stiffener instead of a bellmouth
Version Slide 5230 April 2009
References
“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”. Recommended Practice DNV-RP-C205. October 2007
Version Slide 5330 April 2009
References
Faltinsen, O.M. “Sea Loads on Offshore Structures”. Cambridge University 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 on offshore structures”. Van Nostrand Reinhold Co.
Version Slide 5430 April 2009
References
Sparks, C.S. “The Influence of Tension, Pressure and Weight on Pipe and Riser Deformations and Stresses”. Transactions of the ASME. Vol. 106. March 1984. pp.46-54
Newland, D.E. “An Introductin to Random Vibrations and Spectral Analysis”. Longman Scientific and Technical
Blevins, R.D. “Flow-Induced Vibration”. Krieger Publishing Company
Version Slide 5530 April 2009