Case Study:RBI and its Application toInspection of DHTU Flare Knockout Drums

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.

Notional risk increase with extended inspection date

1C-1

1C-2

1D-1

1D-2

1D-3

1D-4

1D-5

1D-6

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Ris

k R

anki

ng

0

25

50

75

100Risk @ 2008Risk @ 2006 Acceptable Risk Level (ALARP)

Low

Medium

Medium-High

High

Statutory inspection v. Risk based inspection

0

25

50

75

100

1C-1

1C-2

1D-1

1D-2

1D-3

1D-4

1D-5

1D-6

1D-7

1D-8

Rel

ativ

e R

isk

Relative Risk

Statutory Inspection Level

Optimum (Risk Based) Inspection Level

Statutory inspection v. Risk based inspection

n AS3788 advocates regular visual inspection, which may not adequately address all deterioration modes. It does not quantifythe 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

Risk Based Inspection

“RBI may … be validly used … to modify some of [the AS3788] prescriptive requirements…”

(AS3788 App.B)

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)

RBI principles – API 581

nConsequence (loss of containment)n Flammability

n Toxicity

n Production loss

nProbability – deterioration modesn Internal corrosion

n External corrosion

n Environmental cracking (eg. stress corrosion, hydrogen induced)

Not influenced by inspection!

Can reduce by effective inspection

Typical risk matrix

Uninspectable risk

nCannot 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

Can inspection extend the life of an asset?

nCan’t inspect good condition back into equipment at end of life! (API581 9.3.3.3d)

nHowever…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

45C-612 & 45C-612A – inspection history

n C-612– in service Oct 1978

– Feb 1980: visual

– May 1983: visual

– Sep 1987: visual (internally coated)

– Sep 1991: visual

– Sep 1995: visual

– Feb 2002: visual

n C-612A

– Feb 1980: visual

– Aug 1982: visual

– Jun 1985: visual

– Jan 1989: visual (internally coated)

– Jan 1992: visual, MPI

– Apr 1996: visual, MPI

45C-612A is the vessel due for inspection based on time

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

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

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 Lowsusceptibility 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

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

45C-612 & 45C-612A – application of RBI

n45C-612 is the vessel due for inspection based on risk

n45C-612A is the vessel due for inspection based on time

nCracking, rather than corrosion, is the highest risk for both vessels

nVisual inspection does not effectively address cracking

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

45C-612 45C-612A

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 gaugingè Fairly Effective inspection for Thinning per API 581 Table G-6A

è confirms low corrosion rate

Ø credit claimed for representative cracking inspectionè 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)

45C-612 & 45C-612A - results

n Reduction in OH&S risk (H2S 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)

45C-612 & C-612A risk @ 2011

45C-612 & 45C-612A - lessons

nCorrosion is not the only, or even the most important, deterioration mechanism – this leads to some (initially) surprising results

nVisual inspection may not be the best way to mitigate all deterioration risks – can do an “effective” inspection without entry

nPWHT may not eliminate the risk of environmental cracking!

nAPI 581 allows quantification & comparison of cracking risk with other risks

45C-612 & 45C-612A - postscript

n Internal inspection 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

nCapstone RBMI has an Inspection Planning tool (Last Date, Frequency, calculated Next Date) & reporting functions to ensure appropriate testing is done at suitable intervals

Benefits of Risk Based Inspection

n 5 year plant T&I schedule for 2004 on based on RBI

nVarious plant runlength extensions granted

nEstimated saving…