Api 579 - A Comprehensive Fitness-for-service Guide-a 2t663t

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0.2

0.4

0.6

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Fig. 2. Level 2 FAD. which s hows 1ypical cut off values. 8

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959

7:1_ Andersu, , D.A. O.wge I lnrern(lfiOn(l/ Journ"l of Pressure V<ssels """Piping 77 (2(}0()) 953- 'MJ

2.5. Remaining life and rerating The FFS assessment procedures in API 579 cover both the present in tegrity of the component gi ven •• current stare of damage and the projected remaining life. Tf the results of a FFS assessment indicate that the equipment is suitable for the current opemting conditions. the equipment can continue to be oper.ned at these conditions. if a suitable inspection program is established. If !he resultS of the FFS assessment indicate that the equipment is not suitable for the current opemting conditions, calculat ion methods are provided in API 579 to rerate the component. For pressurized components (e.g. pressure vessels and piping) these calculation methods can be used to find a reduced maximum allowable working pressure and/or coincident temperature. For tank components (i.e. shell courses) the ca lculation methods can be used to determine u reduced Maximum Fill Height. The remuining life calcu lation in APT 579 is not intended to prov ide a precise est imate of the actual time to failure. Alternatively, the remui ning life calcu lation is used to establish an approprime inspection interval in conjunction with the governing inspection code and/or inservice monitoring plan, or the need for remediation.

2.6. Relations/tip to other FFS swndards As previously di scussed. of the MPC FFS JTP reviewed existing interm1tional FFS standards to detern1ine !he suitability for use in the refining and petrochemical industry. Although a ~ingle comprehensive standard did not exist. technology contained in these international standards was identified that could be utilized for certain flaw types. Where possible. parts of these methodologies were incorporaled into API 579. and in many cases they were significantly enhanced. In some cases. where the technology was not directly incorpomted. the APT CRE FFS Task Group felt that
3. Overview of API 579 crack-like flaw assessm ent Section 9 of API 579 covers the assessment of cr;1cks and other planar flaws. As is the case with other prominent procedures. such as R6 and OS 79 10, the failure assessment diagr.tm (FAD) methodology fom1s the basis of the flaw evaluation. Fig. l illustrates the FAD concept. The toughness ratio, K.- and the load mtio. /..,. for the structure of interest are plotted on the diagmm. The FAD curve represents the

predicted failure locus. If the assessment point f;•lls within the curve, it is considered acceptable. The roughness mtio is computed from the following expression:

K, = Kf + 4>K~R

(I)

K mao

where Kf is the applied stress intensity factor due to primary loads. K~R is the stress intensity factor due to secondary and residual stress. Krna~ is the fmcture toughnes . and 4> is a plasticity adjustment factor on K15 R. Note that the above fonnulation, whkh was recently suggested by Ainsworth et al. [ 12], differs somewhat from that in the current versions of R6 and BS 79 10, which for secondary and residual stress plasticity effects through the p factor. which is added ro K,. Eq. (I). which has a multiplying factor on KfR . is a more rigorous formu lation . Both the p and cl> formulations were derived from the same analyses. However, the p factor fomlUiation implies a toughness dependence on plastic zone formation, wh ich has no theoretica l bas is. T he more correct form forK, in Eq. ( I) will most likely appear in furure revisions of R6 and BS 7910. The load ratio in API 579 is def'ined ;._.;

L,

= u,.r .

(2)

Uys

where u ref is the reference srress ;md u 1, is the yield strength. Eq. (2) is identical to the 4 definition in R6 and BS 7910. However. API 579 proposes an alternative definition of the reference stress. as discussed later in this article. The main cr.tck-Jike flaw assessment in API 579 is Level 2, which uses !he following FAD equmion:

K,

= fl -

for L, ::::

0.14(4)2](0.3

+ 0.7 expf -

0.65(1...,)6 ])

(3)

4im:uJ·

which is !he same as the R6 Option I FAD, as well as !he one of the available Level 2 FAD expressions in BS 7910. This FAD has a cut-off at~"'"''' which is defined as

Lr(max)

= -I ( 2

I

u,. )'

+ -

(4)

U y,,.

where u,,, is the tensile strength. Fig. 2 shows a plot ofEq. (3) with typical cut-offs for various steels. Level2 util izes partial s afety factors (PSFs) on toughness, flaw size and stress. whereby the can select a t:1rget reliability and perfom1 a deterministic analysi s. If. after adjusting the input va lues by the PSFs. the assessment point lies inside the FAD. one can conclude that the actual probability of fai lure is less than the target value. The PSFs tabulated in Section 9 of API 579 were generated as part of tbe MPC FFS project (13]. The API 579 Level 3 assessment is a more advance analysis !hat gives the a subsmntial ••mount of flexibility. The

960

TL Anderson. D.A. Osa..~• I lnttrmuiontll Journal of Pr.ssure Vns<& am/ Piping 77 (2000) 953-963

available options for a Level 3 assessment include: • Method A - Level 2 assessment with -generated partial safety factors or a probabilistic analysis. • Method B - Material-specific FAD. similar to R6 Option 2. • Me thod C - J -based FAD obtained from clastic - pl;tstic finite element analysis, si milar to R6 Option 3. • Method D - Ductil e teari11g assessment. • Method E - Use a recognized assessment procedure. such as R6 orBS 7910. The Level I as e~sment is very simple screening evaluation that can be perfonned by a qualified inspector. Level I consists of a series of allowable flaw size curves. These curves were gcner.ucd using the Level 2 assessment with conservative input assumptions. l'ote that the APT 579 Level I asscssmcm of cracks is completely diffcrcnr than the BS 7910 Level I iiSSessment. The Iauer is u pseudo FAD ana lysis that is intended to mai ntain backward compatibility wi th the 1980 vc1·sion of the BS PD 6493 procedure. Unlike Level I of BS 79 10, the APT 579 Level I assessment requires almost no calcu lations.

tion. which can be used to infer K for an arbitrary throughwall stress field. The procedure for generating weight functions from the uniform and linear crack face pressures is outlined in Appendix C of APT 579. The range of dimensional paramerers for the cylinder 01nd sphere analyses is as follows:

• R;lt = 3, 5, 10. 20. 60, 100, oo. • all = 0.2, 0.4, 0.6. 0.8.

• cia= 0.5, I. 2, 4. 8, 16. 32. where R, is the inside hell radius. 1 is the wall thickness. a is the depth of the surface flaw. and 2c is the surface flaw length. Fig. 3 is a plot o f typical resultS from the recent analyses. Uniform crack face pressure was applied. giving a stress intensity solution of the following form:

(5) where p is the crack face pressure, G0 is a dimension less geometry fuctor, and Q is the flaw shape parameter:

a

Q =I+ 1.464 ( ~

)1.65

(6)

4. New K solu tions in API 579 Appendix C contains an extensive library of stress imensity solutio ns for cracked bodies. Many of these solutions were obtained from the published literature as well as other assessment procedures, including BS 7910. New K solutions were also generated for inclusion in APl579. Tn particular, a comprehensive set of solutions for c racks in cyli ndrical and spherical shells was recently developed [14]. Th is study involved over 2400 finite clemem runs. Of course. there were a number or existing solut ions for cylinders and spheres. bm these tended to cover a limited mnge of radius/thickness and flaw aspect ratios. ln a study commissioned by the MPC FFS project [14), the following geometries and Haw orientations were considered: • • • • • •

Internal axial surface flaws in a cylinder. External axial surface flaws in a cy linder. Interna l circumferential sUJ:face flaws in a cylinder. Ex ternal c ircumferential surface flaws in a cyl inder. Interna l meridiana! surface flaws in a sphere. External meridiana! surface flaws in a sphere. Three load cases were analyzed:

• Uniform crack f01ce pressure. • Linearly varying crack face pressure. • Global bending moment (circwnfcrcntial cr.tck.~ in cyl inders). The first 2 load cases can be used to derive a weight func-

Note that there is a significant R/1 efl'ect on the nondimcnsional stress intensity factor, G0• Consequently. using a K solution for a surt'<~ce cmck in a flat plate when assessing a curved shell could lead to significam errors. The K solurion libmry in APT 579 will be expanded as new cases become avai Iable. Currently. solutions for cy linders with R/1 = I arc being computed. ln the ncar future. K solutions for cracks at structural discontinuities such as noz2eles and stiffening rings wi ll be generated.

5. Fra cture toughness estimation Appendix F of API 579 eomains information on material properties. including 10ughness. This appeodix does not contain a database of toughness values, however. Rather. it provides correlations and estimation methods. For ferritic steels, there are lower-bound correlations of toughness to Charpy transition temperature. These correlations were adapted from Sections lTl and Xl of the ASME boiler und pressure vessel code. For static loading in the <~bsence of dissolved hydrogen. the lower-bound toughness correla tion is as follows:

K1c

= 36.5 + 3.084 exp[0.036(T- T,cr + 56))

(MP-a../iii.

(7a)

oq.

K1c = 33.2

+ 2.806 cxpt0.02(T- T,.r +

100)] (7b)

961

1:1-. Amlcrson, D.A. Os<1ge I lnrern(l(iona/ Journal of Pressure v,ssels tmd Piping 77 (2000) 953- 963

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0

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---6.

...

:· . - .. t.

----- ----------------e

1.5 Uniform Crack Face Pressure alt • 0.6 cJa s "12

= +=

Cylinder-Circumferential-lntemal - - ·~ - - Cyllnder-Circumferential-Extemal Cylincler-Axial-lntemal • Cylincler-Axial-Extemal ---.!.-- Sphere-Meridianal-lntemal .u Sphere-Meridianai-Extemal

0.5

0

0.05

0.1

0.25

0.2

0.15

0.3

0.35

t/R.I Fig. 3. Nondimensiona.l stress intensity factot· at the deepest point of a surface crack ( = 7rl2) as a funct ion of thickness/radius mtio in cylinders and spheres.

where Trer is the 20 J ( 15 ft-lb) tr-ansition temperature in the case of carbon steels. For dynamic loading or for hydrogen charged steels, the foll owing lower-bound correlati on can be used: 1

Km

= 29.5 +

1.344 exp[0.0260(T-

7~.r

+ 89)) (8a)

0

(MPaJffi, C),

K1R = 26.8 + 1.223 exp[0.0144(T-

7~cr

+ 160)) (8b)

(ksi.Jin.. 0 F}. An upper-shelf cut-off must be imposed on the above expressions. For older, high-su lfur steels. a cut-off of 11 0 MPaJffi (100 ksi.Jin.) is recommended. For newer, low-sulfur steels, a cut-off of 220 MPa.JiTI (200 ksi.Jin.) may be assu med. For probabil istic fracture analyses of steel structures, API 579 endorses the use of the fracture toughness Master Curve, as implemented in ASTM Standard E 1921-97 l l5]. The Master Curve quantifies the temperature dependence of steels in the transition range, as well as the statistical d istri bution of toughness at a given temperature. The lat.ter is characterized by a three-parameter We ibull di stri -

bution with two of the three parameters specified: F

=I-

K)c - 18.2 ) F = I - exp [ -8 ( Ko _ _ 18 2

4

]

4 ] (lllJll,

.

MPaJffi), (9a)

.

~

(m., ks1v111.),

(9b)

where F is the cumulative probability. 8 the specimen thickness (crack front length), and K0 is the Weibullmean toughness, wh.ich corresponds to the 63rd perceotile value. The temperature dependence of the median (50th percentile) toughness is given by KJc{median)

= 30 +

KJc{rncdiun1

= 27 + 64 exp[0.0106(T - To)] (ksi.Jin., "F),

70 exp[O.OI90(T- To)] (MPaJffi, "C), ( lOa)

( lOb) where To is the index transition temperature material for the material of interest. It corresponds to the temperature at which the median tOughness for a 25 mm ( l in.) th ick specimen is I00 MPaJiil (9 1 ks i.Jin.). The median and Wei bull mean are related as follows:

Ko= 1 The rationale for using a dynamic cra.ck iliTcst fracture toughness com> lalion for hydrogen charged steels is as follows: If dissoh•e d hydrogen is present, it may degrade the material's ahility to resisl brittle fracture i ni t ia~ tion. Once mpid crack propagation begins, however, lhc hydrogen can no longer inlluencc the 111atcrial behavior. Therefore, the cr•ck arre~t toughness should be a reasonable lower-bound estitnate of the mate1iaPs ability to resist unstable crack propagation.

[ B (K~c0-- 2020)

exp - - -2;,.4

Ko=

K Jc(mcdi.m) -

[ln(2)]0.25

20

+20

Kk(median) - 18.2 [In (2)]025

+ L8.2

(MPaJiU),

(ksiJin.).

( !Ia)

{II b)

By combining Eqs. (9a), (9b), (lOa), ( lOb) and ( II a), ( lib), we see that once T0 is known , the toughness in the transition

TL Anderson. D.A. Osa..~• I lnttrmuiontll Journal of Pr.ssure Vns<& am/ Piping 77 (2000) 953-963

962

region is completely described. ASTM E 1921 -98 outlines the procedure for determining To from fmcrure 10ughness test ing in the transition region. When fracture 10ughness testing is not feasible. To can be esti mated from the 27 J (20 ft-lb) rransition temperature:

To= T21 J

(12a)

18°C.

-

To = T2o 11 ·1~ - 32.4oF.

( 12b)

The above correlation has a standard deviation of approx imate! y I5°C (2'PF).

6. Refer ence strcs.~ and weld mismatch Appendix D of API 579 contains reference stress solutions for a variety of cracked bodies. Fo r the most part, these solutions were adopted directly from R6 and BS 79 10 and are bascu on lim it loau solutions. The authors believe that the current uefinition of refe re nce stress based o n li mit load is inappropriate and should be replaced in the long run. When rigorous c lastic- plastic J solution~ for cr;1cked bodies are plolted in of FADs. the resulting curves exhibit a strong geometry dependence when 4 is compute
~

( 13)

K, = v ---:~·

This is ploned against the load ratio. a.~ defined in Eq. (2). The potential geometry dependence of the FAD curve arises in the uefinition ofreference stress. A self-consistent defini tion of 1 can be derived from the R6 Option 2 FAD equation , wh ich is material-specific but is assumed to be geometry-independent. Setting L, = 1 in this expression leads to

u,.

- J-

I

_ £) -l = I+ 0.002£ +-I ( I + _0._002

}
CT) S

2

( 14)

fF) >

The above expression a.~sumes that u 1, is the 0.2% offset yield strength. The reference stress is linearly relate
H CTnuminal •

( 15)

where H is inferred from the nominal stress at

4 = I:

H = _ _u..!).:. • --

( 16)

Unomina:JIL,- 1 .

Thus. the refcrenuc stress is chosen in such a way that the Optj~n 2 FAD will nearly match a rigorous elastic-plastic J analysis. That is. give n the above definition of reference srress, Option 2 unci Option 3 FADs will be virtua lly identical. The forgoing begs the question: if an elastic- plastic J analysis is ncquircd to determi ne 4- wh
7. Residual stress distributions for FFS assessment One of the key assumptions in fracture assessments of welded structures is the residual stress distribution. Earlier assessment procedures. uch as PO 6493 (both the 1980 and 1991 versions). made the very conservative assumption of yield-magnituue membrane residual stresses in as wclue
1:1.. Arukmm. V.A. Osu,qe l lfllenwtionfll Jounwl of Pressure Vessels und f>ipi11g 77 (2000) 9SJ- 96J

project will ;tddress the following issues: • Confirmation of some of the parametric distributions in Appendix E. • A c lear criterion for electing 'bending' and 'self-equilibrating' type· of residual stress distributions in pipe/ vessel welds. • Development o f improved residual stress distributions for fillet welds at comer ts. nozzle welds. and repai r welds. • Tncorpomti on of local post-weld heat treatment e lfects.

963

as requ ired by the standards committee. Discussions are already in progress. and suggest ions have been made to have the new standards com mittee meet ings in conjunction with PVRC. This would help to create a focal point for FFS technology development in that the PVRC COE and MPC FFS JlP have previously met at this time. In addition. the of the standard~ committee could directly interface with of these groups to define technology needs and help arrange for appropriate fu nding levels.

References Appendix E will continually be expanded and revised as new results become available.

[I) API. Re<:onunended practice for litnc«·for·<el\ ice. API 579. \Vashington, DC: American Peunleum Jn,.l itule, 2000. 121 API. Pressure ves.~>el inspec1ion code: mainlenance in~peclion.

8. API and ASME FFS activities T he American Society of Mechanical Engineers (ASME) has formed a new main comm ittee, the Post ConsmJCtion Mai.n Committee. with a charter to develop codes and standards for in-service pressure containing equ ipment covering all industries. Curre ntly. standards development activity is underway in the areas of Risk-Based Inspection (RBI) and repair methods (e.g. leak seal ing. boxes, patches. etc.). Tn the area of FFS. API and ASME are working to create a new standards comminee that will tly produce a s ingle FFS standard in the US thai c;m be used for pressure containing equipment. It is envis ioned that once the negotiations and opemti ng procedures for the new committee srrucrure arc complete. API 579 will form the basis of the t APIJASME stand;trd that will be produced by this committee. The inili
rerating. repair and alteration. API 51 0. Vl u.;.hinglOn. DC: American Petroleum Institute. 1999. [3] API. Piping inspection code: in
[5) API. Recommended practice i'o•· risk· bascd inspcctitlll. API 580 (in development). \Vashing1on, DC: American Petroleum fno,; tilutc. PI'OCctlure~ rur operating pre~ surc vessels, tank.-;, and piping in renncl'y und chemical .~rvicc. FFS-26. New York. NY: The Material> Pt"l>pct·tic< Council. Ocwlx:r, 1995. British Energy. Assessment of the integrity of <tructure< containmg defects. Britislt Energy R-6. 1999. BSI. Guide on method< for O'<e"lng the acceptahiluy of flaw< 10 sttuc1ures. BS 7910. Briti

. t93J. l'alo Aho. CA: EPRI. 1981. Ainsworth RA. Sharples JK. Smith SO. EfTccts of rc:' l
16l MPC. F'itncss-ror-servic.:c cvaluutiun

(7]

[81 (91 110]

1111 I t2]

[ 13]

2000. [ 14) Anderson TL, ·n-.orwaltl GV. Revelle 0 .1 . Strc« intcn
surface cracks and bw·ied t racks in cylinders, sphere~. and nmplmes. Presented at the 2000 ASME l'ressu1'C Vessel and Piping Conference,

Seattle. July. 2()()(). [ 15] ASTM E 192 J -97 St:111dard 1<.>1 method for detct'lllination of reference 1emperatu1'e. T0 • liJt fe•·ritic steels In the trnn

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