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“Fatigue Analysis of a Harsh Environment FPSO using SESAM”
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Scope of Presentation
• Typical harsh environment FPSO
• Identification & classification of fatigue sensitive locations
• Outline of analysis procedure
• Example using SESAM
• Performance
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Terra Nova – Harsh Environment FPSO
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Key Questions!
• What locations ?
• What analysis procedure ?
• What software ?
• What model ?
• What vessel condition ?
• How is procedure implemented ?
• How can performance be improved ?
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Fatigue Sensitive Locations
Main hull
Mooring system
Flare tower Crane pedestals
Openings Module supports
Hopper knuckles
Turret structure
Main bearing points Mooring line connection
Shell connections
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DnV Classification Note CN30.7
Design Wave Approach Sec. 5.5
Equivalent Long Term Stress Distribution (Weibul
param.), Sec. 5.2
Long Term Stress Distribution, Sec. 3.2
FE model of detail, Sec. 6.3-6.6
Stress Component based Stochastic Fatigue Analysis
Sec. 5.6
Simplified Analysis Direct Analysis
Load Response Sec. 4.2-4.4 Load Response Sec.5.2 Load Transfer Function. Sec. 5.3
2.2 Stress Components Interchangeable Results
FE model of ship, Ch.6
SCF: K-factors,Ch.7 Interchangeable Results
Combination of Stresses, Sec. 3.4-3.4 Local Stress Transfer Functions
for stress components Sec. 5.4
Full Stochastic Fatigue Analysis Sec. 5.7
Fatigue Damage Summation:
Summation of damage contributions from each wave period/ship heading combination for each sea state in the
wave scatter diagram
Fatigue Damage Calculation, Sec. 2.1
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Software
• Repetitive hull geometry makes it ideally suited to superelement approach.
• Sub-modelling can be used where necessary. Hot spots do not have to be known a priori.
• Totally integrated solution. All pre- & post- processors necessary for complete analysis are available.
• Committed software support
SESAM is the Preferred Tool for Fatigue Analysis of FPSO’s because . . .
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Hydrodynamic Analysis Model Hull Form Example
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Structural Analysis Model Moonpool Fatigue
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Structural Analysis Model Mid-ship and Wingtank Superelements
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Structural Analysis Model Moonpool Superelements
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Structural Analysis Model Turret Superelement
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Structural Analysis Model Typical Mid-ship Section Showing Scantlings
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Implementation
POSTRESP Calculate fatigue lives
POSTFEM Extract stresses Review behaviour Select SN curves
Stage 1
Obtain vessel mass and geometry data
Obtain moonpool geometry data
Establish locations to be analysed & level of modeling detail
WADAM Create motions model
Stage 3
PREFEM Create Geometry model
WADAM Create RAO’s
PREFEM Apply loads & boundary conditions
SESTRA Obtain Unit Stresses
Stage 4 Stage 2
PRESEL Assemble superelements
PREPOST Create results database
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Implementation
1 2 3 4 5 6 7 8
Apply Unit Load Cases
Multiply by ‘Wadam’ Transfer Functions H(ϖ/υ)
Interpolate to Obtain Principal Stresses Pmin & Pmax
Combine to Obtain Principal Stress Transfer Functions
MO
OR
ING
, Fx
MO
OR
ING
, Fz
MO
OR
ING
, My
VER
T A
CC
,
N
a z
. .
HO
RZ
AC
C N
a
x . .
BEN
DIN
G M
OM
ENT,
M B
mt
EXT,
PR
ESSU
RE,
P ex
t
INT,
PR
ESSU
RE,
P in
t
A1 A2 A3 A4 A5 A6 A7 A8
A H (ϖ/υ) A H (ϖ/υ) 2 2 A H (ϖ/υ) 3 3 A H (ϖ/υ) 4 4 A H (ϖ/υ) 5 5 A H (ϖ/υ) 6 6 A H (ϖ/υ) 7 7 A H (ϖ/υ) 8 8
H (ϖ/υ)
SEST
RA
STA
GE
3
PO
STF
EM
S
TAG
E 3
P
OS
TRE
SP
S
TAG
E 4
1 1
σ
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Implementation P
OS
TRE
SP
S
TAG
E 4
Tz (secs) X
Stress
Spreading Function
Principal Stress Response Spectrum
Assume Rayleigh Distribution
Establish Probability of Occurrence
Σ for all points in scatter diagram
Total Distribution Average Cross Rate = T z
Select S-N Curve and Calculate Fatigue Life
No of Cycles
Total No of Cycles
σ
σ σ
σ
Rep
eat f
or a
ll P
oint
s in
Sca
tter D
iagr
am H (ϖ/υ)
2 σ
ϖ
HS
(m)
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Fatigue Analysis
• Selection of S-N curve dependent on:
– Direction of principal stress relative to weld
– Mesh size
– Weld type
– CP protection
• SCF’s due to weld notch effect and local geometry
• Weibull or Rayleigh calculation
• Fracture mechanics
• Safety factors
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Fatigue Analysis Example
R60 Cope hole
R60 Cope hole
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Fatigue Analysis Example
Detailed Mesh at Location F
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Fatigue Analysis Example
Location F Maximum Principal Stress S2
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Fatigue Analysis Example
Location F Maximum Principal Stress S2
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Fatigue Damage Calculations S-N curve: DEn-C-29
Fatigue life:= 1 / [8.499 E-3] = 115 years Fatigue Safety factor = 2 [dry, critical, inspectable & repairable] Target fatigue life = service life x safety factor = 25 x 2 = 50 years Hence, OK
No Description Damage per annum Ranking 1 Heave acceleration 4.324 E-16 3 2 Surge acceleration 7.182 E-18 5 3 Mooring force, Fx 3.385 E-16 4 4 Mooring Force, Fz 1.555 E-22 6 5 Mooring moment, My - - 6 Bending 7.741 E-3 1 7 External pressure 2.848 E-10 2 8 Internal pressure - -
Total 8.499 E-3
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Performance
• Structural FE model 400,000 D.O.F.
• Analysis run on UNIX platform took 1.5 hours C.P.U. using new solver. Typically, SESTRA results file was 0.2 GBytes, POSTFEM database was 2 Gbytes.
• Analysis optimised by varying superelement hierarchy. Best performance achieved when minimum number of supernodes were carried forward to higher levels of hierarchy.
• Superelement approach ideally suited to FPSO fatigue problem. Solution times are faster (compared with analysis of one large model) and model can be built by team.
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Use of F.E.A. at Irvine Engineering
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Thank You
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