1. RBI Intro & some activities at DNV
19 April 2017
RBI Intro & some activities at DNV
Fatigue Workshop

2. 19 April 2017
Contents - tentative
Risk-based inspection planning intro, with emphasis on use of stress processes --- RBI
Flow-induced vibration --- FIV
Low and high cycle fatigue in ships --- LC+HC

3. RBI - principle Plan inspection such that, either
the probability of fatigue failure is kept below a target level, or
the expected, combined cost of inspections and repairs is minimised.

4. RBI - Quantitative
Non-destructive testing inspection results typically, either
No crack was detected (with a certain probability of detection PoD), or
A crack was detected with an estimated, uncertain size.
Requires fracture mechanics to handle information about crack sizes
S-N approach is not detailed enough
Probabilistic modelling is important to handle uncertainties in
Inspection method, load model, crack growth model, crack initiation or initial size
Reference
Sigurdsson, G., Lotsberg, I. & Landet, E., (2000), “Risk Based Inspection of FPSOs”, Int. Conf. on Offshore Mechanics and Arctic Engineering, OMAE'2000, New Orleans.

5. Probabilistic crack growth
Time of
failure
Time for
crack growth
to critical depth
crack
initiation
Critical
depth
Crack depth
at time t
Specified
time

6. From API RP 579:
Crack depth increment per cycle
as a function of log stress intensity factor range

7. RBI – fracture mechanics
Lacks convenient model for crack initiation or initial crack size
S-N data includes crack initiation
Calibrate probabilistic FM model against probabilistic S-N model
Fatigue design standards can be used to imply a target probability of failure from the probabilistic S-N model
Paris equation models crack growth
From initial to critical crack size
Failure assessment diagram models rupture in the presence of a crack
Can give a critical crack size as a function of rupture load
References
BSI, “Guide to methods for assessing the acceptability of flaws in metallic structures,” BS7910:2005.
API 579-1/ASME FFS Fitness-For-Service

8. RBI - Deterministic crack growth – 2-D Paris law
Slope
parameter
for crack
growth
Increments in
crack depth a &
half-length c
per stress cycle
Initial depth
Initial length/2
Intercept
for growth
in depth
Stress intensity
factor range at
crack tip on
surface
in length
factor threshold
for crack growth
deepest point

9. RBI - Stress intensity factor range
magnification
factor for
bending stress
Newman-Raju
geometry factor for
membrane stress
Stress intensity
factor range
Separate geom.
factors at the
deepest point &
the surface tip
Membrane
stress range
Bending
Dependent on crack size, can be determined from FEM

10. Point location and stress components
Cross-section through hot-spot
Definition of stress components:
membrane stress
bending stress
outer fibre stress

11. RBI – handling load process
Assume load-sequence effects negligible
Good if crack growth rate is slow compared to load variability
Then expected crack increment can be expressed in terms of distribution of stress cycles
rM and rB are dependent on crack size but independent of stress processes
As written, assumes threshold stress intensity factor range = 0

12. RBI –If membrane and bending stresses are linearly dependent
Familiar expectation from S-N analysis
A detail --- If the threshold stress intensity factor range is non-zero - then use conditional expectation - with a corresponding stress threshold - but this stress threshold will be dependent on crack size - and will introduce numerical noise if an empirical stress distribution is used - hence a smooth stress distribution function is desirable to ensure convergence in the reliability analysis

13. RBI – Not linearly dependent membrane & bending stresses
Suggest to: - Identify range in outer fibre stress by RFC
Pick off membrane &
bending stress ranges from peak & trough - Develop a 2-D histogram for use in crack growth
Maybe a problem worth pursuing!

14. Flow-induced vibration (FIV) – physical context
19 April 2017
Flow-induced vibration (FIV) – physical context
Well fluid:
Flow rate
Pressure
Sub-sea
Processing:
Bends
chokes
Flow-meters
MEG injection
Unsteady
pressure
distribution
Vibration of
(flexible)
piping
system
Oscillatory
stresses
Fatigue

15. FIV – sample stress time history (A)

16. FIV – sample stress time history (B)

17. FIV – sample spectrum (A)

18. FIV – sample spectrum (B)

19. FIV – sample probability density – (A)

20. FIV – sample probability density – (B)

21. FIV - Comments
Novel application, combining computational fluid dynamics (CFD) and dynamic finite element stress analysis
CFD part is CPU-intensive, only short time series practicable at present
Is the response stationary?
Needs verification
Stochastic stress response
Dominated by some of the many natural frequencies of piping system
Damping is light and uncertain in magnitude
Might tend towards harmonic response, might tend towards Gaussian response
Frequencies around 8 Hz, period of 1/8 s
Fatigue assessment
Rainflow counting applied
High cycle
Low stress ranges
Validity of S-N curves?

22. Low and high cycle fatigue in ships
From presentation by Inge Lotsberg
Fatigue Methodology of Offshore Ships
Part 15 Combination of low cycle and high cycle fatigue
17 July 2009
Some discussion to be given by Inge Lotsberg in
“Background for new revision of DNV-RP-C203 fatigue design of offshore steel structures,” OMAE
See also:
“Fatigue Assessment of Ship Structures,” DNV Classification Notes, No. 30.7, Oct
Joo-Ho Heo, Joong-Kyoo Kang, Yooil Kim, In-Sang Yoo, Kyung-Su Kim, Hang-Sub Urm: “A Study on the Design Guidance for Low Cycle Fatigue in Ship Structure.”
Urm, H. S., Yoo, I. S., Heo, J. H., Kim, S. C. and Lotsberg, I.: “Low Cycle Fatigue Strength Assessment for Ship Structures.” PRADS 2004.

23. LC+HC - Vessel with one longitudinal bulkhead

24. LC+HC - Operation: Ballast – Full last

25. LC+HC - Operation: Alternating
Half cycles

26. LC+HC - Non-linear analysis
Transverse frame in double bottom
LC+HC - Non-linear analysis

27. LC+HC - Stress range from wave loading
Weibull dstn.
nLCF number of loading/unloading cycles during lifetime

28. LC+HC - Low cycle fatigue loading/unloading
Eurocode plasticity correction EN

29. Safeguarding life, property and the environment
19 April 2017
Safeguarding life, property and the environment