RBI implementation

1. Case Study:
RBI and its Application to
Inspection of
DHTU Flare Knockout Drums

2. AS3788 Pressure equipment – In-service inspection
n Mandatory under NSW Occupational Health & Safety Regulation 2001
n Mandates maximum periods between internal inspections (Table 4.1)
eg.

3. Notional risk increase with extended inspection date
100
Risk @ 2008
Risk @ 2006
High
Acceptable Risk Level (ALARP)
75
Medium-High
Risk Ranking
50
Medium
25
Low
0
1C-1
1C-2
1D-1
1D-2
1D-3
1D-4
1D-5
1D-6
1D-7
1D-8

4. Statutory inspection v. Risk based inspection
100
Relative Risk
Statutory Inspection Level
Optimum (Risk Based) Inspection Level
75
Relative Risk
50
25
0
1C-1
1C-2
1D-1
1D-2
1D-3
1D-4
1D-5
1D-6
1D-7
1D-8

5. Statutory inspection v. Risk based inspection
n AS3788 advocates regular visual inspection, which may not
adequately address all deterioration modes. It does not quantify
the value of inspection techniques for mitigating risks other than
thickness loss.
eg. for Inspection Periods & Remaining Life Assessment, AS3788
is quantitative with regard to “Wastage” (para. 4.4.4.4,
Appendix N), but qualitative with regard to environmental
cracking & other deterioration modes (Appendix O concerns
Fitness for Service, not Remaining Life)
n API581 quantifies most available inspection techniques in
terms of their effectiveness at reducing a range of
deterioration modes

6. Risk Based Inspection
“RBI may … be validly
used … to modify some
of [the AS3788]
prescriptive
requirements…”
(AS3788 App.B)

7. What is Risk Based Inspection (RBI)?
n “A way to extend runlengths/save maintenance $$$”
n A method of using equipment history and the likely consequences
of equipment failure to determine Inspection regimes focused on
actual risks, so as to prevent unsafe incidents occurring
n The RBI method is based on the API 581 base resource document
– 23 major oil & petrochemical companies
– massive statistical analysis of petrochem facilities over a number of years
– “complex and … best suited for use in a computerized form”
n Each piece of equipment is assigned a risk ranking based on the
Probability and Consequence of a failure (loss of containment)

8. RBI principles – API 581
n Consequence (loss of containment)
n Flammability
Not influenced by inspection!
n Toxicity
n Production loss
n Probability – deterioration modes
n Internal corrosion
n External corrosion Can reduce by effective inspection
n Environmental cracking (eg. stress corrosion, hydrogen induced)

9. Typical risk matrix

10. Uninspectable risk
n Cannot reduce risk to zero!
– Human error
– Natural disasters
– External events (eg. falling objects)
– Secondary effects from other nearby failures
– Deliberate acts, sabotage
– Fundamental limitations of inspection methods
– Design errors
– Previously unknown mechanisms of deterioration

11. Can inspection extend the life of an asset?
n Can’tinspect good condition back into equipment at
end of life! (API581 9.3.3.3d)
n However…we don’t arbitrarily retire equipment at the
end of its design life
n Inspection and recertification are crucial to maximising
the service life of equipment

14. 45C-612 & 45C-612A – inspection history
n C-612 n C-612A
– in service Oct 1978 – Feb 1980: visual
– Feb 1980: visual – Aug 1982: visual
– May 1983: visual – Jun 1985: visual
– Sep 1987: visual (internally – Jan 1989: visual (internally
coated) coated)
– Sep 1991: visual – Jan 1992: visual, MPI
– Sep 1995: visual – Apr 1996: visual, MPI
– Feb 2002: visual
45C-612A is the vessel due for inspection based on time

15. 45C-612A – history of incidents
H2S release on attempting to open for inspection
n Mar 2002
n Sep 2003
Inspection requested to assess possibility of extending run via
external inspection techniques

16. 45C-612 & 45C-612A – base data
n Process: flare gas – water + ~2% H2S, ie. sour service
n Post Weld Heat Treated (stress relieved)
n Design: 350kPa & 450oC
n Operating: 55kPa & ~75oC
n Uninsulated
n Internally coated from ~10 years into service life
n Measured average general Corrosion Rate ~0.05mm/yr
n Remaining Life from general corrosion (“wastage”) >>20yrs
n Pitting Corrosion Rate ~0.5mm/yr prior to internal coating, now largely
mitigated

17. 45C-612 & 45C-612A – application of RBI
Via use of Capstone RBMI program:
n External corrosion risk low – uninsulated, external coating good
n Internal corrosion risk low – reflects in CR & RL calculation
n For water + 20,000ppm H2S & PWHT, API 581 App.H assigns Low
susceptibility to Wet H2S cracking è entered into Capstone RBMI
n “Low” is not zero! Capstone RBMI calculates:
– Medium risk of cracking for C-612A, because of MPI testing in April
1996
– Medium-High risk of cracking for C-612, because it had never been
inspected for cracking

18. Aside – effectiveness of stress relief (PWHT)
US experience (large statistical sample):
n History of cracking in SR/PWHT vessels
n Loosely specified
n Inadequate QA

19. 45C-612 & 45C-612A – application of RBI
n 45C-612 is the vessel due for inspection based
on risk
n 45C-612A is the vessel due for inspection based
on time
n Cracking, rather than corrosion, is the highest
risk for both vessels
n Visualinspection does not effectively address
cracking

20. 45C-612 & 45C-612A – calculated risk levels @ 2003
45C-612 45C-612A

21. 45C-612 & 45C-612A – 2003 inspection regime
n 45C-612:
Ø external shearwave UT of 100% of welds
è Highly Effective inspection for Wet H2S cracking per API 581 Table H-4C
è no defects
n 45C-612A:
Ø spot external thickness gauging
è Fairly Effective inspection for Thinning per API 581 Table G-6A
è confirms low corrosion rate
Ø credit claimed for representative cracking inspection
è AS3788 4.4.4.6 allows representative inspection of parallel equipment
è Usually Effective inspection for Wet H2S cracking per API 581 Table G-6A (one
level lower than for vessel inspected)

22. 45C-612 & 45C-612A - results
n Reduction in OH&S risk (H2S 45C-612 & C-612A risk @ 2011
exposure)
n Saving & deferral of
estimated $60K expenditure
n Inspection period for both
vessels extended to 2011
with no significant increase in
risk
(Capstone RBMI “What if?”
analysis - lookahead)

23. 45C-612 & 45C-612A - lessons
n Corrosion is not the only, or even the most important,
deterioration mechanism – this leads to some (initially)
surprising results
n Visualinspection may not be the best way to mitigate all
deterioration risks – can do an “effective” inspection
without entry
n PWHT may not eliminate the risk of environmental
cracking!
n API 581 allows quantification & comparison of cracking
risk with other risks

24. 45C-612 & 45C-612A - postscript
n Internalinspection of 45C-612 & 45C-612A as part of
Kurnell Clean Fuels upgrade
n Found cracking in 45C-612
– Fabrication defects?
– Missed by shearwave, or appeared since 2003?
n Justifies requirement for crack testing
n Capstone RBMI has an Inspection Planning tool (Last
Date, Frequency, calculated Next Date) & reporting
functions to ensure appropriate testing is done at
suitable intervals

25. Benefits of Risk Based Inspection
n5 year plant T&I schedule for 2004 on based on RBI
n Various plant runlength extensions granted
n Estimated saving…