Ship Reliability Analysis
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Transcript Ship Reliability Analysis
Reliability analysis of Ship Structures
Fatigue and Ultimate Strength
Fabrice Jancart
François Besnier
PRINCIPIA MARINE
[email protected]
ASRANet Colloquium 2002
Summary
Uncertainties identification
Rule based design and rational design
Industrial applications using PERMAS reliability
capabilities
Optimisation and reliability
Fatigue
Ultimate strength
Conclusions
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A major concern: safety
On a competitive market
New ship concepts
Cost / Weight reduction
Considerations on sea safety are
increasing
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Designing in an uncertain world:
from models…
Modelling uncertainties: due to imperfect knowledge of
phenomena and idealization and simplification in
analysis procedure
Loading
Hydrodynamic forces (physical and mathematical models)
Damage evaluation
Structural response
Finite element model
Approximations, simplifications
From global to local:
Uncertainties on fabrication effects
Fabrication tolerance, residual stresses
“ Natural” uncertainties
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Load modelling
MODIFIED HULL, 0 knots
2,00E+04
0,00E+00
-100
-80
-60
-40
-20
0
20
40
60
80
100
Wave bending moment (t.m)
-2,00E+04
L1
-4,00E+04
L1(bis)
L2
L3
-6,00E+04
L4
L5
L6
-8,00E+04
L7
L8
-1,00E+05
-1,20E+05
-1,40E+05
X (m)
Numerical wave bending moment scatter according to
the same hypothesis
from 5.104 T*m to 12 104 T*m
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From global to local
50 000 dof
300 000 dof
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Designing in an uncertain world:
From material stochastic properties
Material properties scatter
True or nominal values
S-N curves approximated by
log10 ( N ) log10 (C) m. log10 ( )
P(f)=50%
N
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Designing in an uncertain world:
From “natural” stochastic properties
Natural uncertainties: due to statistical nature of ship mission
Environmental loading
Short term sea states
Long term sea states distribution
Missions and routes
Scatter Diagram
250
100
m(
150
s)
Occurence
200
dT
50
Signi 0
ficativ 1
e Heig 2
ht...
Pe
rio
0
16-18
Example of block decomposition
10-12
4-6
introduce scatter in prediction
3
Wave scatter diagram for one block
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Rule based design:
method and limits
Rule based approach with
Historical hidden safety margins
Calibrated by experience on large conventional ships
Incompatible with innovative ship or structural concepts
Cannot be applied on structural optimisation process
Incompatible with uncertainties on the complex ship
environment and structural behavior
Difficulty to determine the safety margins and their evolution
Conflicting with first principal or rational design
Need to update the safety partial coefficients with first principles
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Reliability approach:
risk quantification
Stochastic definition of the problem:
Closer to reality
Computes the probability that solicitations L exceed
strength of the structure R
Deterministic
LD
LR
R
L
R
RD
RR
R
LL
R
Pf ( R L) Pf ,t arget
Probabilistic
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Use of PERMAS
reliability capabilities
Work mainly done during EC supported ASRA Esprit project
Objective : Optimisation under reliability constraints with
Permas software
Numerical calculation of failure probability
Comparison of various methods:
FORM/SORM gradient based methods
Response surface methods (RSM)
Crude and adaptive Monte Carlo
Stochastic calibration of partial safety factors
Sequences of reliability - optimisation – reliability
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Industrial Application:
reinforced opening
Optimisation of reinforced passengers ship doors
Many occurrences of this costly detail
Submitted to alternate shear forces
Reinforced for fatigue criteria
F
Door
-F
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Gangway
12
Industrial Application:
reinforced opening
Limit stress
Scantling Load
Maximum shear stress criterion
Evolution of reliability with optimisation
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Industrial Application
reinforced opening
Optimisation:
Mass decreases by 10%
Reliability of initial and optimised designs
Stochastic loading, normal distribution
Failure function G = lim - FE
lim stochastic variable, normal distribution
Failure probability increases from 1.7 10-5 to 2.8 10-3
Optimisation without reliability constraints
jeopardises safety
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Industrial Application:
High speed craft
Exploitation of high speed crafts (fast mono hulls) reveals:
Fatigue problems under alternate bending and repeated slamming
Ultimate strength problems (local and deck buckling )
Impact (slamming)
sagging
First principle design reliability based approach compared
to traditional (rule based) approach
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Industrial Application:
High speed craft
Fatigue failure &
buckling collapse
Confirmed to be very critical design criteria
and subjected to significant uncertainties
Loading uncertainties (models and stochastic nature)
Structural strength uncertainties
Fatigue limit
Ultimate buckling stress
Missions, routes and service life
Heavy weather countermeasures
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High speed craft
Buckling
High speed vessel on large
wave crest
Significant bending
moment inducing buckling
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High speed craft
Buckling
Buckling reliability at mid-ship section
Failure state function
G u (Mextr )
Uncertainties on
Ultimate buckling stress u due to scatter on in-yard fabrication
tolerances, built in stresses, described by a log-normal
distribution
Extreme value of wave bending moment Mextr, with a Gumbel
max probability density law depending on ship service time T
: load modelling effect due to FEM approximations, with a
normal distribution
u
(Mextr)
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T
18
Fatigue
Reliability analysis
Large number of welded
connections, where cracks
may initiate
Typical welded structural
detail, fatigue prone
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Fatigue
Reliability analysis
Historic S
K (S-N curve)
Loading
N
T
S
Detail loaded by displacements of
global model
2
1
Local mesh for stress
extrapolation (hot spot)
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Fatigue
Reliability analysis
Fatigue reliability due to global wave loads
Failure state function
Uncertainties on
C( T ) m
G Dc
S
K
Critical damage Dc with a log-normal distribution
S-N curve (K) due to variable fabrication conditions described by
a log-normal distribution
Load modelling S
due to hydrodynamic numerical and navigation condition
hypothesis
due to effort in avoiding numerical singularities with the
extrapolation near the weld
described by log-normal distributions
C(T): function of service time T
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Fatigue
Reliability analysis
m
More complex failure function:
C1
m
G Dc
.( 1).C 2 . K Lm . S m
Kp
Dc:critical damage, taken from Classification Society recommendation and
defined by a lognormal law,
Kp associated to the S-N curve definition Sm.N=Kp,and defined by a lognormal
law
m parameter of the S-N curve
w,
parameters of the Weibull distribution
S
f (S)
ww
1
S
e xp
w
C1 deterministic coefficient associated to the time at sea considered,
C2 deterministic coefficient used in the long term loading distribution
KL associated to the local stress effect
S is the stress variation during wave loading.
gamma function : S
a 1 t
S
e dt
0
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Fatigue and buckling
Reliability analysis
Buckling reliability for 1 year of exploitation
FORM
SORM
RSM_LIN
RSM_AXIAL
- index
Pf
Tps CPU
0,947
0.89
17,2%
18,7%
29 mn
29 mn
0,95
0.95/0.89
17,1%
0.17/0.187
60 mn
72 mn
Fatigue reliability for 15 years of exploitation
- index
Pf
Tps CPU
Rule (SN curve)
2,05
2%
-
SORM
RSM_LIN
1,02
0.976
15,3%
16.45%
26 mn
50 mn
RSM_CCD
1,01
15,7%
84 mn
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Fatigue and buckling
Elasticity
Ultimate strength
Variable
Vs Mean value
Vs Std dev.
Loading
-5.88
-0.24
9
0.69
u
Fatigue
Variable
Vs Mean value
Vs Std dev
K (S-N curve)
Sollicitation S
1.75
3.29
-0.47
-0.58
Critical damage Dc
1.525
-0.24
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Fatigue and service time
Introduction of time-variant effects in the reliability
approach :
Fatigue strength evolution
Effects of aging and corrosion
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Conclusions
« Considering alea in the design process introduces an
additional accuracy» Hasofer
Rule based design is not always conservative
Reliability approach can lead to an optimised and robust design.
Simulation methods (Monte Carlo) are too costly for industrial
applications.
Use of an existing tool coupling structural and reliability calculations
Gradient based and RSM methods efficient
Application on innovative ship structural concepts
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Thank you for your attention
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