fatigue lecture 3 2012

7/29/2019 Fatigue Lecture 3 2012

1/18

Risk Based Fatigue Analysis andInspection Planning

Rod Pinna

Platform, Pipeline and

Subsea Technology


2/18

There are 3 importantsources of uncertainty in

fatigue analysis:

1. Fatigue Loads

2. S-N Curve

3. Miners Sum

Uncertainties

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Miner's Sum

FrequencyofOccurrence

0 1 2 3 4 5 6 7 8 9

Corresponding lognormal distribution


3/18

These uncertainties mean there is uncertainty inestimates of fatigue life

Therefore the fatigue life may be describedstatistically

Uncertainties

Fatigue Life (Years)

Probability

Density

sTT~

T

Probabilityof Failure

ServiceLife

MedianFatigueLife


4/18

The Probability of fatigue failure canbe calculated using Fatigue Reliabilitytheory

(See Appendix B of notes)

Fatigue Reliability


5/18

Fatigue Reliability

T

s

F

TT

ln

ln-~

ln

mB

KT ~

~~~

21

2

)1)(1)(1(ln222

ln

m

BKTCCC

)(FFP

Best estimate of fatigue life

Estimate of variability

Fatigue Reliability Index

Probability of fatigue failure in service lifetime

sT Service Life

Actually, maths is fairly simple but uncertainty

estimates and calculation of can be a challenge


6/18

Probability of fatigue failure is afunction of time (T

S)

It needs to be compared with someTarget what is acceptable?

Fatigue Reliability


7/18

Probability of failure increases with time

Probability of Fatigue

Failure

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0 5 10 15 20

Service Life (years)

P

robabilityofF

atigueFailure,

PF


8/18

The basis of Risk Based Inspection (RBI) Planning

Effect of Inspection

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0 5 10 15 20


P

robabilityofF

atigueFailure,

PF

P F = 2.2 x 10-4


at 10 years -no crack detected

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0 5 10 15 20


P

robabilityofF

atigueFailure,

PF

P F = 2.2 x 10-4


at 10 years -no crack detected

Inspectioncan beused toUpdate

the Probof failure


9/18

How do we decide what an appropriateTarget Probability of Failure is?

Target Reliability

Depends on whatthe consequence isand what level of

risk we are preparedto accept

These issues will bediscussed in moredepth later


10/18

Through-Life CostOptimisation

Prob of Failure Failure Cost

Total Cost = CAPEX + OPEX + RISKEX

InspectionCost

Fabrication +Installation Cost

COST

SIZE

Capex

Opex

Riskex

TOTAL


11/18

Tubular Joint CostOptimisation

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

0 5 10 15 20


Pr

obabilityofF

atigueFailure

41mm

44mm47mm

50mm

Minimum J oint

Design Thickness

Increasing J oint

Thickness =

Increasing CAPEX

Increasing Inspection Interval

= Decreasing OPEX

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

0 5 10 15 20


Pr

obabilityofF

atigueFailure

41mm

44mm47mm

50mm

Minimum J oint

Design Thickness

Increasing J oint

Thickness =

Increasing CAPEX

Increasing Inspection Interval

= Decreasing OPEX


12/18

There are a range of techniques usedto inspect for weld defectsonshore, e.g.

Radiography (X-ray)

Ultrasonics

Magnetic particle

Not all are suitable forsubsea application

Fatigue Crack Inspection


13/18

Some techniques are suitable for detecting cracksin offshore structures, e.g.

ACFM = Alternating CurrentField Measurement



14/18

ACFM = Alternating Current Field Measurement



15/18

FMD = Flooded Member Detection



16/18

FMD = Flooded Member Detection

Detector can be mounted on ROV



17/18

ROV = Remotely Operated Vehicle - Movie



18/18

Questions and Revision

?

fatigue lecture 3 2012

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