CSR Harmonisation
Fatigue Assessment
Industry Presentation
September 2012
Philippe Baumans & Åge Bøe
Project Management Team (PMT)
Content
• Basis – acceptance criteria
• Fatigue loads
• Loading condition for fatigue assessment
• Fatigue strength criteria, hot spot stress approach
• Simplified method for longitudinal stiffeners
• Very fine mesh finite element analysis (tn50 x tn50 mesh)
• Fatigue assessment by screening (50x50 mesh)
• Design standard
September 2012
Fatigue Assessment
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Fatigue evaluation - Acceptance criteria
• Calculated fatigue life should equal or exceed the design
fatigue life:
TF ≥ TDF
• Design fatigue life TDF is not to be taken less than 25 years.
• Acceptance criteria expressed as life and not Miner sum,
D ≤ 1.0, since Miner sum D is to be based on design life
TD = 25 years independent on design fatigue life.
September 2012
CSR-H Buckling
3
Fatigue evaluation - Acceptance criteria
Acceptance criteria, same as CSR-OT and CSR-BC:
• Design life of 25 years in North Atlantic environment.
• Ship speed: ¾ Vdesign for fatigue loads = average speed in 25 years
(5 knots for extreme loads = minimum speed to maintain manoeuvring)
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Fatigeu Assessment
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Selected dynamic load cases for fatigue
• Selection of 5 EDWs for fatigue loads at 10-2
level:
OSA
OST
BSP
• HSM: head sea EDW maximizing VBM
amidships
BSR
HSM
FSM
• HSA: head sea EDW maximizing AZ at FP
• FSM: following sea EDW maximizing VBM
HSA
amidships
• BSR: beam sea EDW maximizing roll motion
BSP
OST
BSR
OSA
• BSP: beam sea EDW maximizing waterline
Az
pressure at amidships
• OST: oblique sea EDW maximizing torsional
moment at ¼L
• OSA: oblique sea EDW maximizing pitch acceleration
HSA and OSA are covered by HSM.
ƒ Stress range computed for each EDW
ƒ Damage is evaluated based in the highest
stress range:
Ps
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Std
Fatigue Assessment
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Weibull distributed stress range
• Damage calculation is based on the
assumption of long term Weibull
distributed stress ranges.
• Stress range is taken at 10-2
probability level with shape
parameter equal to 1.0.
• f0 = 0.85 : Factor taking into account
time in seagoing operations.
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Fatigue Assessment
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Loading conditions for fatigue
assessment – Oil tankers
Fraction of time in each loading
condition for oil tankers
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Fatigue Assessment
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Loading conditions for fatigue
assessment – Bulk carriers
BC-A, empty hold
BC-A, loaded cargo hold
September 2012
Fraction of time in each loading condition
for bulk carriers
BC-B and BC-C
CSR-H Buckling
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Fatigue strength - Failure modes
Included: cracks at weld toe and free plate edges
Excluded: crack at weld root is covered by design standards
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Fatigue Assessment
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Hot spot at weld toe and at free edge
Hot spot in way of
free edge (base material)
Hot spot in way of weld toe
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Fatigue Assessment
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Hot spot stress
Nominal stress
Hot spot stress
Notch stress
Stress
range
Hot spot stress: σ HS = K g ⋅ σ nom
Notch stress
Hot spot stress
S-N curves
Nominal stress
Number of cycles
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Fatigue Assessment
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Fatigue evaluation
S-N curves
• CSR-H applies the DEn S-N curves “B”, “C” and “D”, with
modification on slope value for B and C curve.
CSR-H curves
DEn curves
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Fatigue Assessment
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S-N curve - Corrosive environment
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Fatigue Assessment
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Stress corrections – Welded joints
Mean stress correction factor, fmean, i(j)
September 2012
Thickness effect, fthick
Fatigue Assessment
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Fatigue assessment
Fatigue damage calculation
– Elementary damage: (all loading conditions, both environment)
DE ( j ) =
m
α ( j ) ⋅ N D Δσ FS
,( j )
K2
(ln N R )
m/ξ
⎛ m⎞
⋅ μ( j ) ⋅ Γ⎜⎜1 + ⎟⎟
⎝ ξ⎠
– Combined damage (all loading conditions)
– Total damage:
Sum of all loading conditions
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Fatigue Assessment
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Fatigue assessment
Fatigue analysis methodology
• Miner-Palmgren damage summation used to calculate
damage ratio in all relevant loading conditions for the
predominate dynamic load case. Total fatigue damage, sum
of all loading conditions:
• Fatigue life calculation (design life TD =25 years), TF ≥TDF :
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Fatigue Assessment
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Hot spot stress range
Fatigue assessment
Hot spot stress concept
Simplified stress analysis
Nominal stress (beam theory) x Tabulated “SCF”
Very fine mesh FE stress analysis (tn50 x tn50)
Very fine mesh FE model
Screening Fatigue Assessment (50 x 50 mesh)
Nominal stress (Fine mesh )FE model) x Tabulated “η”
Hot spot S-N curve
Fatigue life calculation
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Fatigue Assessment
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Fatigue assessment
Hot spot stress concept
Simplified stress
analysis
d
Very fine mesh
FE stress analysis
Stresses are based on:
• tn50
• Correction factor for HG
stresses and FE stresses:
fc = 0.95
Screening fatigue
assessment
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Fatigue Assessment
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Simplified stress analysis for
longitudinal stiffeners
Mandatory for all longitudinal stiffener end connection for all cargo
area, at web-frame, at transverse bulkhead and at swash bulkhead:
•The hot spots stress is a combination of:
•Hull girder bending:
•Relative deflections in way of transv. bhd:
•Local stiffener bending:
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Fatigue Assessment
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Simplified stress analysis for
longitudinal stiffeners
Stress concentration factors at top of longitudinal stiffener:
• Stress concentration factors ( Kg ): σHS = Kg· σnom
• Where
Kg = Ka is given for axial loads due to global bending
Kg = Kb Kn is given for lateral loads due to stiffener bending
Kb : stress concentration factor for local bending
Kn : additional factor for unsymmetrical stiffener profile (warping of stiffener)
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Fatigue Assessment
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Very fine mesh finite element analysis
• Mandatory for specific details, OT and BC
• Required if fatigue screening fails
• Required if detail design standard is not followed
Modeling requirement:
•
tn50 x tn50 mesh
•
4 node elements with improved
in-plane bending
•
8 node recommended at steep
stress gradients
•
Model extension in tn50 x tn50 : at
least 10 elements in all directions
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Fatigue Assessment
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Details to be assessed
Very fine mesh FE analysis
•Mandatory details:
Oil tanker
Bulk carrier
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Fatigue Assessment
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Screening fatigue assessment
Fine mesh FE model (50x50 mm)
“Nominal” membrane stress
Tabulated “η”
Hot spot stress
Stress correction factors:
Mean stress effect
Thickness effect
Fatigue damage calculation
Damage criteria, TF ≥ TDF
OK
Damage criteria, TF < TDF
Very fine mesh fatigue analysis (tn50 x tn50 )
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Fatigue Assessment
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Details to be assessed
Screening fatigue assessment
•Mandatory details, fatigue screening assessment based on 50x50 mesh:
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Fatigue Assessment
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Detail design standard
• Stiffener-frame connections
• Scallops in way of block
joints
• Hopper knuckle connection
• Horizontal stringer heel
• Bulkhead connection to
lower/upper stool
• Bulkhead connection to inner
bottom
• Toe of hold frame
• Hatch corner
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Fatigue Assessment
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Weld improvement - Post weld treatment
• Benefit factor 2.2 on fatigue life; a stress reduction factor of 1.3,
applicable for transverse butt joints and cruciforms, not longitudinal
stiffener end connections
• Minimum requirement without treatment:
• TF ≥ TDF / 1.47 corresponding to 17 years for TDF = 25 years; same as
CSR-OT
• Inside bulk cargo holds TF ≥ 25 years
• Reduced thickness effect exponent ( n ) given for improved welds
TIG dressing
September 2012
Peening
Burr grinding
Fatigue Assessment
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Fatigue - Summary
Changes from current CSR:
• S-N curve for corrosive environment introduced, damage
calculated for both environments, in air and corrosive
• Surface finishing factors for base material are introduced
• Reference stresses are calculated at a probability level of 10-2
with shape parameter fixed to 1.0
• Thickness effect exponent depends on structural detail and is in
the range of 0.0 – 0.25
• Mean stress effect is revised with
lower bound in compression = 0.3, at zero mean stress = 0.9
• Definition of hot spot stress in very fine mesh FE analysis is
improved
• Screening criteria based on fine mesh (50x50) introduced
• Design standard for critical areas given
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Fatigue Assessment
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