“Fatigue Analysis of a Harsh Environment FPSO using...

“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


Mid-ship and Wingtank Superelements


MoonpoolSuperelements


TurretSuperelement


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

OR

ING

,Fx

MO

OR

ING

,Fz

MO

OR

ING

,My

VE

RT

AC

C,N

az

..

HO

RZ

AC

CN

ax

..

BE

ND

ING

MO

ME

NT,M

Bm

t

EX

T,P

RE

SSU

RE

,P

ext

INT

,PR

ESSU

RE

,P

int

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

RA

ST

AG

E3

PO

ST

FE

MS

TA

GE

3P

OS

TR

ES

PS

TA

GE

4

11

ImplementationP

OS

TR

ES

PS

TA

GE

4

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


Detailed Mesh at Location F


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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