“Fatigue Analysis of aHarsh EnvironmentFPSO using SESAM”
Scope of Presentation
• Typical harsh environment FPSO
• Identification & classification of fatigue sensitive locations
• Outline of analysis procedure
• Example using SESAM
• Performance
Terra Nova – Harsh Environment FPSO
Key Questions!
• What locations ?
• What analysis procedure ?
• What software ?
• What model ?
• What vessel condition ?
• How is procedure implemented ?
• How can performance be improved ?
Fatigue Sensitive Locations
Main hull
Mooring system
Flare towerCrane pedestals
OpeningsModule supports
Hopper knuckles
Turret
structure
Main bearing points Mooring line connection
Shell connections
DnV Classification Note CN30.7
Design WaveApproach Sec. 5.5
EquivalentLong TermStress Distribution (Weibul
param.), Sec. 5.2
Long Term StressDistribution, Sec.3.2
FE model of detail, Sec.6.3-6.6
Stress Component basedStochastic Fatigue Analysis
Sec. 5.6
Simplified Analysis Direct Analysis
Load Response Sec. 4.2-4.4Load Response
Sec.5.2Load Transfer Function. Sec. 5.3
2.2 Stress Components InterchangeableResults
FE model of ship, Ch.6
SCF: K-factors,Ch.7 InterchangeableResults
Combination ofStresses, Sec. 3.4-3.4 Local Stress Transfer Functions
for stress componentsSec. 5.4
Full Stochastic FatigueAnalysis Sec. 5.7
Fatigue DamageSummation:
Summation of damage contributionsfrom each wave period/ship headingcombination for each sea state in the
wave scatter diagram
Fatigue DamageCalculation, Sec. 2.1
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- processorsnecessary for complete analysis are available.
• Committed software support
SESAM is the Preferred Tool for Fatigue Analysisof FPSO’s because . . .
Hydrodynamic Analysis Model
Hull Form Example
Structural Analysis Model
Moonpool Fatigue
Structural Analysis Model
Mid-ship and Wingtank Superelements
Structural Analysis Model
MoonpoolSuperelements
Structural Analysis Model
TurretSuperelement
Structural Analysis Model
Typical Mid-shipSection ShowingScantlings
Implementation
POSTRESPCalculatefatiguelives
POSTFEMExtract stressesReviewbehaviourSelect SN curves
Stage 1
Obtain vessel massand geometry data
Obtain moonpoolgeometry data
Establish locationsto be analysed & levelof modeling detail
WADAMCreatemotions model
Stage 3
PREFEMCreateGeometrymodel
WADAMCreateRAO’s
PREFEMApply loads& boundaryconditions
SESTRAObtainUnitStresses
Stage 4Stage 2
PRESELAssemblesuperelements
PREPOSTCreate resultsdatabase
Implementation
1 2 3 4 5 6 7 8
Apply Unit Load Cases
Multiply by ‘Wadam’ Transfer Functions H(/)
Interpolate to Obtain Principal Stresses Pmin & Pmax
Combine to Obtain Principal Stress Transfer Functions
MO
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A1 A2 A3 A4 A5 A6 A7 A8
A H (/) A H (/)22
A H (/)33
A H (/)44
A H (/)55
A H (/)66
A H (/)77
A H (/)88
H (/)
SE
ST
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OS
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Tz (secs)X
Stress
Spreading Function
Principal StressResponse Spectrum
Assume Rayleigh Distribution
Establish Probabilityof Occurrence
for all points in scatter diagram
Total DistributionAverage Cross Rate = T z
Select S-N Curve andCalculate Fatigue Life
NoofCycles
Total NoofCycles
Re
pe
atfo
ra
llP
oin
tsin
Sca
tte
rD
iag
ram
H (/)2
HS
(m)
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
Fatigue Analysis Example
R60Cope hole
R60Cope hole
Fatigue Analysis Example
Detailed Mesh at Location F
Fatigue Analysis Example
Location F Maximum Principal Stress S2
Fatigue Analysis Example
Location F Maximum Principal Stress S2
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 32 Surge acceleration 7.182 E-18 53 Mooring force, Fx 3.385 E-16 44 Mooring Force, Fz 1.555 E-22 65 Mooring moment, My - -6 Bending 7.741 E-3 17 External pressure 2.848 E-10 28 Internal pressure - -
Total 8.499 E-3
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.
Use of F.E.A. at Irvine Engineering
“Fatigue Analysis of aHarsh EnvironmentFPSO using SESAM”
Thank You
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