Fad api 579.pdf

API 579: a comprehensive ®tness-for-service guide

Ted L. Andersona,*, David A. Osageb

aStructural Reliability Technology, 1898 S Flatiron Court, Suite 235, Boulder, CO 80301, USAbM & M Engineering, Shaker Heights, OH, USA

Received 4 August 2000; revised 11 December 2000; accepted 13 December 2000

Abstract

This article presents an overview of the recently published American Petroleum Institute (API) Recommended Practice 579, which covers

®tness-for-service assessment of pressure equipment in petrochemical and other industries. Although API 579 covers a wide range of ¯aws

and damage mechanisms, including local metal loss, pitting corrosion, blisters, weld misalignment, and ®re damage, the emphasis of the

present article is on the assessment of crack-like ¯aws. The API 579 procedure for evaluating cracks incorporates a failure assessment

diagram (FAD) methodology very similar to that in other documents, such as the British Energy R6 approach and the BS 7910 method. The

API document contains an extensive compendium of K solutions, including a number of new cases generated speci®cally for API 579. In the

initial release of the document, API has adopted existing reference stress solutions for the calculation of Lr in the FAD procedure. In a future

release, however, API plans to replace these solutions with values based on a more rational de®nition of reference stress. These revised

reference stress solutions will incorporate the effect of weld mismatch. In addition to the Appendices of K and reference stress solutions, API

579 includes appendices that provide guidance on estimating fracture toughness and weld residual stress distributions. Over the next few

years these appendices will be enhanced with advances in technology. Recently, API has entered into discussions with the American Society

of Mechanical Engineers (ASME) to convert API 579 into a joint API/ASME ®tness-for-service guide. q 2001 Published by Elsevier

Science Ltd.

Keywords: American Petroleum Institute; Failure assessment diagram; Flaw assessment; Fitness for service; Fracture toughness; Reference stress; Residual

stress; Stress intensity factor

1. Background

Existing US design codes and standards for pressurized

equipment provide rules for the design, fabrication, inspec-

tion and testing of new pressure vessels, piping systems, and

storage tanks. These codes do not address the fact that

equipment degrades while in-service and de®ciencies due

to degradation or from original fabrication may be found

during subsequent inspections. Fitness-for-service (FFS)

assessments are quantitative engineering evaluations,

which are performed to demonstrate the structural integrity

of an in-service component containing a ¯aw or damage.

The American Petroleum Institute (API) Recommended

Practice 579 [1] has been developed to provide guidance

for conducting FFS assessments of ¯aws commonly encoun-

tered in the re®ning and petrochemical industry which occur

in pressure vessels, piping, and tankage. However, the

assessment procedures can also be applied to ¯aws encoun-

tered in other industries such as the pulp and paper industry,

fossil fuel utility industry, and nuclear industry. The guide-

lines provided in API 579 can be used to make run-repair-

replace decisions to ensure that pressurized equipment

containing ¯aws that has been identi®ed during an inspec-

tion can continue to be operated safely.

API 579 is intended to supplement and augment the

requirements in API 510 [2], API 570 [3], and API 653

[4]: to ensure safety of plant personnel and the public

while older equipment continues to operate; to provide tech-

nically sound FFS assessment procedures: to ensure that

different service providers furnish consistent remaining

life predictions; and to help optimize maintenance and

operation of existing facilities to maintain availability of

older plants and enhance long-term economic viability. In

addition, API 579 will also be used in conjunction with API

580 Recommended Practice For Risk-Based Inspection [5]

that is being developed to provide guidelines for risk assess-

ment, and prioritization for inspection and maintenance

planning for pressure-containing equipment.

The initial impetus to develop an FFS standard that could

be referenced from the API inspection codes was provided

by a Joint Industry Project (JIP) administered by the

International Journal of Pressure Vessels and Piping 77 (2000) 953±963

0308-0161/00/$ - see front matter q 2001 Published by Elsevier Science Ltd.

PII: S0308-0161(01)00018-7

www.elsevier.com/locate/ijpvp

* Corresponding author. Tel.: 11-303-415-1475; fax: 11-303-415-1847.

E-mail address: [email protected] (T.L. Anderson).

Material Properties Council (MPC). The driving force

behind this development was plant safety. The methodology

provided for in this document, together with the appropriate

API inspection code, had to ensure that equipment integrity

could be safely maintained when operating equipment with

¯aws or damage, and could also be used to demonstrate

compliance with US Occupational Safety and Health

Administration (OSHA) 1910 Process Safety Management

(PSM) Legislation.

A review of the existing international FFS standards by

the members of the MPC JIP was undertaken in 1991 as the

starting point for the development of a new FFS standard.

Based on the results of this review, it was determined that a

comprehensive FFS standard covering many of the typical

¯aw types and damage mechanisms found in the re®ning

and petrochemical industry did not exist. In addition, the

existence of many company-based FFS methods, the

complexity of the technology that no single company can

solve on its own, and the need to gain acceptance by local

jurisdictions in the US further indicated the need for a new

standard. Therefore, the JIP decided to start the develop-

ment of the required FFS technology that would be needed

to write a comprehensive FFS standard for the re®ning and

petrochemical industry. The results of this work were docu-

mented in a MPC FFS JIP Consultant's Report [6], and this

document was subsequently turned over to the API Commit-

tee on Re®nery Equipment (CRE) FFS Task Force charged

with development of the FFS standard.

In terms adopted by the API CRE FFS Task Group devel-

oping API 579, an FFS assessment is an engineering analy-

sis of equipment to determine whether it is ®t for continued

service. The equipment may contain ¯aws, may not meet

current design standards, or may be subjected to more

severe operating conditions than the original or current

design. The product of a FFS assessment is a decision to

operate the equipment as is, alter, repair, monitor, or

replace; guidance on an inspection interval is also provided.

FFS assessments consist of analytical methods to assess

¯aws and damage and usually require an interdisciplinary

approach consisting of the following:

² Knowledge of damage mechanisms/material behavior.

² Knowledge of past and future operating conditions and

interaction with operations personnel.

² NDE (¯aw location and sizing).

² Material properties (environmental effects).

² Stress analysis (often ®nite element analysis).

² Data analysis (engineering reliability models).

T.L. Anderson, D.A. Osage / International Journal of Pressure Vessels and Piping 77 (2000) 953±963954

Table 1

Organization of each section in API 579

Section

subparagraph

number

Title Overview

1 General The scope and overall requirements for an FFS assessment are provided

2 Applicability and limitations of

the FFS assessment procedures

The applicability and limitations for each FFS assessment procedure are clearly indicated; these

limitations are stated in the front of each section for quick reference

3 Data requirements The data requirements required for the FFS assessment are clearly outlined; these data requirements

include:

Original equipment design data

Maintenance and operational history

Required data/measurements for a FFS assessment

Recommendations for inspection technique and sizing requirements

4 Assessment techniques and

acceptance criteria

Detailed assessment rules are provided for three levels of assessment: Level 1, Level 2,

and Level 3. A discussion of these assessment levels is covered in the body of this paper

5 Remaining life evaluation Guidelines for performing a remaining life estimate are provided for the purpose of establishing an

inspection interval in conjunction with the governing inspection code

6 Remediation Guidelines are presented on methods to mitigate and/or control future damage. In many cases,

changes can be made to the component or to the operating conditions to mitigate the progression of

damage

7 In-service monitoring Guidelines for monitoring damage while the component is in-service are provided, these guidelines

are useful if a future damage rate cannot be estimated easily or the estimated remaining life is short.

In-service monitoring is one method whereby future damage or conditions leading to future damage

can be assessed or con®dence in the remaining life estimate can be increased.

8 Documentation Guidelines for documentation for an assessment are provided; the general rule is Ð A practitioner

should be able to repeat the analysis from the documentation without consulting an individual

originally involved in the FFS assessment

9 References A comprehensive list of technical references used in the development of the FFS assessment

procedures is provided; references to codes and standards are provided in this section

10 Tables and ®gures Tables and ®gures including logic diagrams are used extensively in each section to clarify assessment

rules and procedures

11 Example problems A number of example problems are provided, which demonstrate the application of the FFS

assessment procedures

Based on this de®nition, the API CRE FFS Task Group

modi®ed and greatly enhanced the initial efforts of the

MPC JIP to produce the ®rst edition of API 579. The

MPC JIP continued to provide valuable technical contribu-

tions throughout this development effort and essentially

became the technical development arm of the API Task

Group. The MPC FFS JIP is still in existence and continues

to provide FFS technology development while working

closely with the needs of the API CRE FFS Task Group.

The overall organization and assessment procedures

in API 579 are reviewed below. This is followed by a

more detailed discussion of the API 579 assessment of

cracks.

2. Overview of API 579

2.1. Applicable codes

API 579 provides guidelines for performing FFS assess-

ments that can be used in conjunction with the API Inspec-

tion codes (API 510, API 570 and API 653) to determine the

suitability for continued operation. The assessment proce-

dures in this recommended practice could be used for FFS

assessments and/or rerating of components designed and

constructed to the following codes:

² ASME B and PV code, Section VIII, Division 1

² ASME B and PV code, Section VIII, Division 2

² ASME B and PV code, Section I

² ASME B31.3 Piping code

² ASME B31.1 Piping code

² API 650

² API 620.

Guidelines are also provided for applying API 579 to pres-

sure-containing equipment constructed to other recognized

codes and standards, including international and internal

corporate standards.

2.2. Organization

API 579 is a highly structured document designed to

facilitate use by practitioners and to facilitate future

enhancements and modi®cations by the API CRE FFS

Task Group. Section 1 of the document covers: introduction

and scope; responsibilities of the owner-user, inspector, and

engineer; quali®cation requirements for the inspector and

engineer; and references to other codes and standards. An

outline of the overall FFS assessment methodology that is

T.L. Anderson, D.A. Osage / International Journal of Pressure Vessels and Piping 77 (2000) 953±963 955

Table 2

Overview of ¯aw and damage assessment procedures

Section in

API 579

Flaw or damage mechanism Overview

3 Brittle fracture Assessment procedures are provided to evaluate the resistance to brittle fracture of in-service

carbon and low alloy steel pressure vessels, piping, and storage tanks. Criteria are provided to

evaluate normal operating, start-up, upset, and shutdown conditions

4 General metal loss Assessment procedures are provided to evaluate general corrosion. Thickness data used for the

assessment can be either point thickness readings or detailed thickness pro®les. A

methodology is provided to guide the practitioner to the local metal loss assessment procedures

based on the type and variability of thickness data recorded during an inspection

5 Local metal loss Assessment techniques are provided to evaluate single and networks of Local Thin Areas

(LTAs), and groove-like ¯aws in pressurized components. Detailed thickness pro®les are

required for the assessment. The assessment procedures can also be utilized to evaluate blisters

6 Pitting corrosion Assessment procedures are provided to evaluate widely scattered pitting, localized pitting,

pitting which occurs within a region of local metal loss, and a region of localized metal loss

located within a region of widely scattered pitting. The assessment procedures can also be

utilized to evaluate a network of closely spaced blisters. The assessment procedures utilize the

methodology developed for local metal loss

7 Blisters and laminations Assessment procedures are provided to evaluate either isolated, or networks of blisters and

laminations. The assessment guidelines include provisions for blisters located at weld joints

and structural discontinuities such as shell transitions, stiffening rings, and nozzles

8 Weld misalignment and

shell distortions

Assessment procedures are provided to evaluate stresses resulting from geometric

discontinuities in shell type structures including weld misalignment and shell distortions

(e.g. out-of-roundness, bulges, and dents)

9 Crack-like ¯aws Assessment procedures are provided to evaluate crack-like ¯aws. Recommendations for

evaluating crack growth including environmental concerns are also covered

10 High temperature operation

and creep

Assessment procedures are provided to determine the remaining life of a component operating

in the creep regime. The remaining life procedures are limited to the initiation of a crack

11 Fire damage Assessment procedures are provided to evaluate equipment subject to ®re damage. A

methodology is provided to rank and screen components for evaluation based on the heat

exposure experienced during the ®re. The assessment procedures of the other sections of this

publication are utilized to evaluate component damage

common to all assessment procedures included in API 579 is

provided in Section 2 of the document. The organization of

Section 2 is shown in Table 1. This same organization is

utilized in all subsequent sections that contain FFS assess-

ment procedures.

Starting with Section 3, a catalogue of FFS assessment

procedures organized by damage mechanism is provided in

API 579. A complete listing of the ¯aw and damage assess-

ment procedures currently covered is shown in Table 2.

These damage mechanisms can be grouped at a higher

level to form a degradation class (see Fig. 1). This higher

level of organization is useful in that it provides insight into

how the assessment procedures of different sections may be

combined to address complex ¯aws in a component. As

shown in Fig. 1, several ¯aw types and damage mechanisms

may need to be evaluated to determine the FFS of a compo-

nent. Each section in API 579 referenced within a degrada-

tion class includes guidance on how to perform an

assessment when multiple damage mechanisms are present.

When assessment procedures are developed for a new

damage mechanism, they will be added as a self-contained

section to maintain the structure of API 579. Currently, new

sections are being developed to address hydrogen induced

cracking (HIC) and stress-oriented hydrogen induced crack-

ing (SOHIC) damage, local hot spots, assessment proce-

dures for riveted components, and creep crack growth.

A series of appendices are provided which contain tech-

nical information that can be use with all sections of API


Fig. 1. Schematic overview of the FAD procedure.

579, which cover FFS assessment procedures. The majority

of the information in the appendices covers stress analysis

techniques, material property data, and other pertinent infor-

mation that is required when performing a FFS assessment.

An overview of the appendices is provided in Table 3.

2.3. Assessment methodology

The API 579 FFS assessment methodology used for all

damage types is provided in Table 4. The organization of

each section of API 579 that covers an assessment procedure

is consistent with this methodology. This consistent

approach to the treatment of damage and the associated

FFS assessment procedures facilitates use of the document

in that, if a practitioner is familiar with one section of the

document, it is not dif®cult to utilize another section

because of the common structure. This assessment metho-

dology has proven to be robust for all ¯aw and damage types

that have been incorporated into API 579. Because of this

success, when new sections are added to API 579, the

template used for the development will be based on this

assessment methodology.

2.4. Assessment levels

Three levels of assessment are provided in API 579 for

each ¯aw and damage type. A logic diagram is included in

each section to illustrate how these assessment levels are

interrelated. As an example, the logic diagram for evaluat-

ing crack-like ¯aws is shown in Fig. 2. In general, each

assessment level provides a balance between conservatism,

the amount of information required for the evaluation, the

skill of the practitioner performing the assessment, and the

complexity of analysis being performed. Level 1 is the most

conservative, but is easiest to use. Practitioners usually

proceed sequentially from a Level 1 to a Level 3 assessment

(unless otherwise directed by the assessment techniques) if

the current assessment level does not provide an acceptable

result or a clear course of action cannot be determined.

A general overview of each assessment level and its

intended use are described below.

² Level 1 Ð The assessment procedures included in this

level are intended to provide conservative screening

criteria that can be utilized with a minimum amount of

inspection or component information. The Level 1

assessment procedures may be used by either plant

inspection or engineering personnel.


level are intended to provide a more detailed evaluation

that produces results that are less conservative than those


Table 3

API 579 appendices

Appendix Title Overview

A Thickness, MAWP and membrane stress

equations for a FFS assessment

Equations for the thickness, MAWP, and membrane stress are given for most of the common

pressurized components. These equations are provided to assist international practitioners who

may not have access to the ASME code and who need to determine if the local design code is

similar to the ASME code for which the FFS assessment procedures were primarily designed for

B Stress analysis overview for a FFS

assessment

Recommendations for stress analysis techniques that can be used to perform an FFS assessment

are provided including guidelines for ®nite element analysis

C Compendium of stress intensity factor

solutions

A compendium of stress intensity factor solutions for common pressurized components (i.e.

cylinders, spheres, nozzle, etc.) are given. These solutions are used for the assessment of crack

like ¯aws. The solutions presented represent the latest technology and have been re-derived using

the ®nite element method in conjunction with weight functions

D Compendium of reference stress solutions A compendium of reference stress solutions for common pressurized components (i.e. cylinders,

spheres, nozzle, etc.) are given. These solutions are used for the assessment of crack-like ¯aws

E Residual stresses in a FFS evaluation Procedures to estimate the through-wall residual stress ®elds for different weld geometries are

provided; this information is required for the assessment of crack like ¯aws

F Material properties for a FFS assessment Material properties required for all FFS assessments are provided including:

Strength parameters (yield and tensile stress)

Physical properties (i.e. Young's Modulus, etc.)

Fracture toughness

Data for fatigue crack growth calculations

Fatigue curves (Initiation)

Material data for creep analysis including remaining life and creep crack growth

G Deterioration and failure modes An overview of the types of ¯aws and damage mechanisms that can occur is provided,

concentrating on service-induced degradation mechanisms. This appendix only provides an

abridged overview on damage mechanisms; API 571 is currently being developed to provide a

de®nitive reference for damage mechanisms that can be used with API 579 and API 580

H Validation An overview of the studies used to validate the general and local metal loss, and the crack-like

¯aw assessment procedures are provided

I Glossary of terms and de®nitions De®nitions for common terms used throughout the sections and appendices of API 579 are given

J Technical inquiries Guidelines for submitting a technical inquiry to API are provided. Technical inquires will be

forwarded to the API CRE FFS task group for resolution

from a Level 1 assessment. In a Level 2 assessment,

inspection information similar to that required for a

Level 1 assessment are required; however, more detailed

calculations are used in the evaluation. Level 2 assess-

ments are typically conducted by plant engineers or engi-

neering specialists experienced and knowledgeable in

performing FFS assessments.


level are intended to provide the most detailed evaluation

that produces results that are less conservative than those

from a Level 2 assessment. In a Level 3 assessment the

most detailed inspection and component information is

typically required, and the recommended analysis is

based on numerical techniques such as the ®nite element

method. The Level 3 assessment procedures are primar-

ily intended to be used by engineering specialists experi-

enced and knowledgeable in performing FFS

evaluations.


Table 4

API 579 FFS assessment methodology for all damage types

Step Description

1 Flaw and damage mechanism identi®cation Ð The ®rst step in a FFS assessment is to identify the ¯aw type and cause of damage. FFS assessments

should not be performed unless the cause of the damage can be identi®ed. The original design and fabrication practices, materials of construction,

service history, and environmental conditions can be used to ascertain the likely cause of the damage. Once the ¯aw type is identi®ed, the appropriate

section of this document can be selected for the assessment

2 Applicability and limitations of the FFS assessment procedures Ð The applicability and limitations of the assessment procedure are described in each

section, and a decision on whether to proceed with an assessment can be made

3 Data requirements Ð The data required for FFS assessments depend on the ¯aw type or damage mechanism being evaluated. Data requirements may

include: original equipment design data; information pertaining to maintenance and operational history; expected future service; and data speci®c to the

FFS assessment such as ¯aw size, state of stress in the component at the location of the ¯aw, and material properties. Data requirements common to all

FFS assessment procedures are covered in Section 1. Data requirements speci®c to a damage mechanism or ¯aw type are covered in the section

containing the corresponding assessment procedures

4 Assessment techniques and acceptance criteria Ð Assessment techniques and acceptance criteria are provided in each section. If multiple damage

mechanisms are present, more than one section may have to be used for the evaluation

5 Remaining life evaluation Ð An estimate of the remaining life or limiting ¯aw size should be made. The remaining life is established using the FFS

assessment procedures with an estimate of future damage rate (i.e. corrosion allowance). The remaining life can be used in conjunction with an

inspection code to establish an inspection interval

6 Remediation Ð Remediation methods are provided in each section based on the damage mechanism or ¯aw type. In some cases, remediation

techniques may be used to control future damage associated with ¯aw growth and/or material degradation

7 In-service monitoring Ð Methods for in-service monitoring are provided in each section based on the damage mechanism or ¯aw type. In-service

monitoring may be used for those cases where, a remaining life and inspection interval cannot be adequately established because of the complexities

associated damage mechanism and service environment

8 Documentation Ð The documentation of an FFS assessment should include a record of all data and decisions made in each of the previous steps to

qualify the component for continued operation. Documentation requirements common to all FFS assessment procedures are given in Section 2 of API

579. Speci®c documentation requirements for a particular damage mechanism or ¯aw type are covered in the section containing the corresponding

assessment procedures

Fig. 2. Level 2 FAD, which shows typical cut-off values.

2.5. Remaining life and rerating

The FFS assessment procedures in API 579 cover both

the present integrity of the component given a current state

of damage and the projected remaining life. If the results of

a FFS assessment indicate that the equipment is suitable for

the current operating conditions, the equipment can

continue to be operated at these conditions, if a suitable

inspection program is established. If the results of the FFS

assessment indicate that the equipment is not suitable for the

current operating conditions, calculation methods are

provided in API 579 to rerate the component. For pressur-

ized components (e.g. pressure vessels and piping) these

calculation methods can be used to ®nd a reduced maximum

allowable working pressure and/or coincident temperature.

For tank components (i.e. shell courses) the calculation

methods can be used to determine a reduced Maximum

Fill Height. The remaining life calculation in API 579 is

not intended to provide a precise estimate of the actual

time to failure. Alternatively, the remaining life calculation

is used to establish an appropriate inspection interval in

conjunction with the governing inspection code and/or in-

service monitoring plan, or the need for remediation.

2.6. Relationship to other FFS standards

As previously discussed, members of the MPC FFS JIP

reviewed existing international FFS standards to determine

the suitability for use in the re®ning and petrochemical

industry. Although a single comprehensive standard did

not exist, technology contained in these international stan-

dards was identi®ed that could be utilized for certain ¯aw

types. Where possible, parts of these methodologies were

incorporated into API 579, and in many cases they were

signi®cantly enhanced. In some cases, where the technology

was not directly incorporated, the API CRE FFS Task

Group members felt that alternate approaches may be desir-

able for use by more advanced practitioners. Therefore, the

Level 3 assessment in API 579 permits the use of alternative

FFS assessment methodologies. For example, the Level 3

assessment in Section 9 of API 579 covering crack-like

¯aws provides references to British Energy R6 [7], BS

7910 [8], SAQ/FoU-Report 96/08 [9], WES 2805 [10],

and EPRI J-Integral methodology [11].

3. Overview of API 579 crack-like ¯aw assessment

Section 9 of API 579 covers the assessment of cracks and

other planar ¯aws. As is the case with other prominent

procedures, such as R6 and BS 7910, the failure assessment

diagram (FAD) methodology forms the basis of the ¯aw

evaluation.

Fig. 1 illustrates the FAD concept. The toughness ratio,

Kr, and the load ratio, Lr, for the structure of interest are

plotted on the diagram. The FAD curve represents the

predicted failure locus. If the assessment point falls within

the curve, it is considered acceptable.

The toughness ratio is computed from the following

expression:

Kr � KPI 1 FKSR

I

Kmat

; �1�

where KPI is the applied stress intensity factor due to primary

loads, KSRI is the stress intensity factor due to secondary and

residual stress, Kmat is the fracture toughness, and F is a

plasticity adjustment factor on K SRI : Note that the above

formulation, which was recently suggested by Ainsworth

et al. [12], differs somewhat from that in the current versions

of R6 and BS 7910, which account for secondary and resi-

dual stress plasticity effects through the r factor, which is

added to Kr. Eq. (1), which has a multiplying factor on KSRI ;

is a more rigorous formulation. Both the r and F formula-

tions were derived from the same analyses. However, the rfactor formulation implies a toughness dependence on plas-

tic zone formation, which has no theoretical basis. The more

correct form for Kr in Eq. (1) will most likely appear in

future revisions of R6 and BS 7910.

The load ratio in API 579 is de®ned as

Lr � sref

sys

; �2�

where s ref is the reference stress and s ys is the yield

strength. Eq. (2) is identical to the Lr de®nition in R6 and

BS 7910. However, API 579 proposes an alternative de®ni-

tion of the reference stress, as discussed later in this article.

The main crack-like ¯aw assessment in API 579 is Level

2, which uses the following FAD equation:

Kr � �1 2 0:14�Lr�2�{0:3 1 0:7 exp�20:65�Lr�6�}for Lr # Lr�max�;

�3�

which is the same as the R6 Option 1 FAD, as well as the

one of the available Level 2 FAD expressions in BS 7910.

This FAD has a cut-off at Lr(max), which is de®ned as

Lr�max� � 1

21 1

s ts

sys

!; �4�

where s ts is the tensile strength. Fig. 2 shows a plot of Eq. (3)

with typical cut-offs for various steels.

Level 2 utilizes partial safety factors (PSFs) on toughness,

¯aw size and stress, whereby the user can select a target

reliability and perform a deterministic analysis. If, after

adjusting the input values by the PSFs, the assessment

point lies inside the FAD, one can conclude that the actual

probability of failure is less than the target value. The PSFs

tabulated in Section 9 of API 579 were generated as part of

the MPC FFS project [13].

The API 579 Level 3 assessment is a more advance analy-

sis that gives the user a substantial amount of ¯exibility. The


available options for a Level 3 assessment include:

² Method A Ð Level 2 assessment with user-generated

partial safety factors or a probabilistic analysis.

² Method B Ð Material-speci®c FAD, similar to R6

Option 2.

² Method C Ð J-based FAD obtained from elastic±plastic

®nite element analysis, similar to R6 Option 3.

² Method D Ð Ductile tearing assessment.

² Method E Ð Use a recognized assessment procedure,

such as R6 or BS 7910.

The Level 1 assessment is very simple screening evaluation

that can be performed by a quali®ed inspector. Level 1

consists of a series of allowable ¯aw size curves. These

curves were generated using the Level 2 assessment with

conservative input assumptions. Note that the API 579

Level 1 assessment of cracks is completely different than

the BS 7910 Level 1 assessment. The latter is a pseudo FAD

analysis that is intended to maintain backward compatibility

with the 1980 version of the BS PD 6493 procedure. Unlike

Level 1 of BS 7910, the API 579 Level 1 assessment

requires almost no calculations.

4. New K solutions in API 579

Appendix C contains an extensive library of stress inten-

sity solutions 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 API 579. In particular, a

comprehensive set of solutions for cracks in cylindrical and

spherical shells was recently developed [14]. This study

involved over 2400 ®nite element runs. Of course, there

were a number of existing solutions for cylinders and

spheres, but these tended to cover a limited range of

radius/thickness and ¯aw aspect ratios.

In a study commissioned by the MPC FFS project [14],

the following geometries and ¯aw orientations were

considered:

² Internal axial surface ¯aws in a cylinder.

² External axial surface ¯aws in a cylinder.

² Internal circumferential surface ¯aws in a cylinder.

² External circumferential surface ¯aws in a cylinder.

² Internal meridianal surface ¯aws in a sphere.

² External meridianal surface ¯aws in a sphere.

Three load cases were analyzed:

² Uniform crack face pressure.

² Linearly varying crack face pressure.

² Global bending moment (circumferential cracks in

cylinders).

The ®rst 2 load cases can be used to derive a weight func-

tion, which can be used to infer K for an arbitrary through-

wall stress ®eld. The procedure for generating weight func-

tions from the uniform and linear crack face pressures is

outlined in Appendix C of API 579.

The range of dimensional parameters for the cylinder and

sphere analyses is as follows:

² Ri=t � 3; 5, 10, 20, 60, 100, 1.

² a=t � 0:2; 0.4, 0.6, 0.8.

² c=a � 0:5; 1, 2, 4, 8, 16, 32.

where Ri is the inside shell radius, t is the wall thickness, a is

the depth of the surface ¯aw, and 2c is the surface ¯aw

length.

Fig. 3 is a plot of typical results from the recent analyses.

Uniform crack face pressure was applied, giving a stress

intensity solution of the following form:

KI � pG0

��pa

Q

r; �5�

where p is the crack face pressure, G0 is a dimensionless

geometry factor, and Q is the ¯aw shape parameter:

Q � 1 1 1:464a

c

� �1:65

: �6�

Note that there is a signi®cant Ri/t effect on the nondimen-

sional stress intensity factor, G0. Consequently, using a K

solution for a surface crack in a ¯at plate when assessing a

curved shell could lead to signi®cant errors.

The K solution library in API 579 will be expanded as

new cases become available. Currently, solutions for cylin-

ders with Ri=t � 1 are being computed. In the near future, K

solutions for cracks at structural discontinuities such as

nozzles and stiffening rings will be generated.

5. Fracture toughness estimation

Appendix F of API 579 contains information on material

properties, including toughness. This appendix 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 III and XI of the ASME boiler and

pressure vessel code. For static loading in the absence of

dissolved hydrogen, the lower-bound toughness correlation

is as follows:

KIC � 36:5 1 3:084 exp�0:036�T 2 Tref 1 56��MPa

��mp

; 8C�;�7a�

KIC � 33:2 1 2:806 exp�0:02�T 2 Tref 1 100��

�ksi��in:p

; 8F�;�7b�


where Tref is the 20 J (15 ft-lb) transition temperature in the

case of carbon steels. For dynamic loading or for hydrogen-

charged steels, the following lower-bound correlation can

be used:1

KIR � 29:5 1 1:344 exp�0:0260�T 2 Tref 1 89��MPa

��mp

; 8C�;�8a�

KIR � 26:8 1 1:223 exp�0:0144�T 2 Tref 1 160��

�ksi��in:p

; 8F�:�8b�

An upper-shelf cut-off must be imposed on the above

expressions. For older, high-sulfur steels, a cut-off of

110 MPa��mp

(100 ksi��in:p

) is recommended. For newer,

low-sulfur steels, a cut-off of 220 MPa��mp

(200 ksi��in:p

)

may be assumed.

For probabilistic fracture analyses of steel structures, API

579 endorses the use of the fracture toughness Master

Curve, as implemented in ASTM Standard E 1921-97

[15]. The Master Curve quanti®es the temperature depen-

dence of steels in the transition range, as well as the statis-

tical distribution of toughness at a given temperature. The

latter is characterized by a three-parameter Weibull distri-

bution with two of the three parameters speci®ed:

F � 1 2 exp 2B

25:4

KJc 2 20

K0 2 20

� �4" #

�mm; MPa��mp �; �9a�

F � 1 2 exp 2BKJc 2 18:2

K0 2 18:2

� �4" #

�in:; ksi��in:p �; �9b�

where F is the cumulative probability, B the specimen thick-

ness (crack front length), and K0 is the Weibull mean tough-

ness, which corresponds to the 63rd percentile value. The

temperature dependence of the median (50th percentile)

toughness is given by

KJc�median� � 30 1 70 exp�0:0190�T 2 T0�� MPa��mp

; 8C�;�10a�

KJc�median� � 27 1 64 exp�0:0106�T 2 T0�� ksi��in:p

; 8F�;�10b�

where T0 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 (1 in.) thick speci-

men is 100 MPa��mp

(91 ksi��in:p

). The median and Weibull

mean are related as follows:

K0 �KJc�median� 2 20

�ln�2��0:251 20 �MPa

��mp �; �11a�

K0 �KJc�median� 2 18:2

�ln�2��0:251 18:2 �ksi

��in:p �: �11b�

By combining Eqs. (9a), (9b), (10a), (10b) and (11a), (11b),

we see that once T0 is known, the toughness in the transition


1 The rationale for using a dynamic crack arrest fracture toughness corre-

lation for hydrogen charged steels is as follows: If dissolved hydrogen is

present, it may degrade the material's ability to resist brittle fracture initia-

tion. Once rapid crack propagation begins, however, the hydrogen can no

longer in¯uence the material behavior. Therefore, the crack arrest tough-

ness should be a reasonable lower-bound estimate of the material's ability

to resist unstable crack propagation.

Fig. 3. Nondimensional stress intensity factor at the deepest point of a surface crack �f � p=2� as a function of thickness/radius ratio in cylinders and spheres.

region is completely described. ASTM E 1921-98 outlines

the procedure for determining T0 from fracture toughness

testing in the transition region.

When fracture toughness testing is not feasible, T0 can be

estimated from the 27 J (20 ft-lb) transition temperature:

T0 � T27 J 2 188C; �12a�

T0 � T20 ft-lb 2 32:48F: �12b�The above correlation has a standard deviation of approxi-

mately 158C (278F).

6. Reference stress and weld mismatch

Appendix D of API 579 contains reference stress solu-

tions for a variety of cracked bodies. For the most part, these

solutions were adopted directly from R6 and BS 7910 and

are based on limit load solutions.

The authors believe that the current de®nition of refer-

ence stress based on limit load is inappropriate and should

be replaced in the long run. When rigorous elastic±plastic J

solutions for cracked bodies are plotted in terms of FADs,

the resulting curves exhibit a strong geometry dependence

when Lr is computed based on the limit load solutions. This

apparent geometry dependence has led some to criticize the

FAD methodology as being inaccurate.

Appendix B of API 579 outlines a procedure to obtain a

self-consistent reference stress de®nition from the elastic±

plastic J solution. This alternative de®nition removes

virtually all of the geometry dependence in the FAD. This

approach also provides an effective means to account for

weld metal mismatch through Lr.

Of course, there is no ambiguity in the de®nition of the

vertical ordinate (y axis) of the FAD from an elastic±plastic

J solution:

Kr ��Jelastic

J:

r�13�

This is plotted against the load ratio, as de®ned in Eq. (2).

The potential geometry dependence of the FAD curve arises

in the de®nition of reference stress. A self-consistent de®ni-

tion of s ref can be derived from the R6 Option 2 FAD

equation, which is material-speci®c but is assumed to be

geometry-independent. Setting Lr � 1 in this expression

leads to

J

Jelastic

��Lr�1� 1 1

0:002E

sys

11

21 1

0:002E

sys

!21

: �14�

The above expression assumes that s ys is the 0.2% offset

yield strength. The reference stress is linearly related to the

nominally applied stress through a geometry factor, H:

sref � Hsnominal; �15�

where H is inferred from the nominal stress at Lr � 1:

H � sys

snominaljLr�1

: �16�

Thus, the reference stress is chosen in such a way that the

Option 2 FAD will nearly match a rigorous elastic±plastic J

analysis. That is, given the above de®nition of reference

stress, Option 2 and Option 3 FADs will be virtually

identical.

The forgoing begs the question: if an elastic±plastic J

analysis is required to determine Lr, what is the point in

using the FAD methodology? Traditionally one of the

advantages of the FAD approach has been that it is consid-

erably simpler than a rigorous elastic±plastic analysis. At

®rst glance, the proposed de®nition of s ref would seem to

eliminate this advantage. Such is not the case, however, as

discussed below.

When Lr is set to unity in the Option 2 FAD expression,

the strain hardening dependence disappears. Consequently,

the geometry factor H, de®ned above, should be insensitive

to the shape of the stress±strain curve. If an elastic±plastic

analysis is performed once for a given cracked body, it

should not have to be repeated for other stress±strain curves.

Non-dimensional reference stress solutions can be

computed and tabulated for use in standard FAD analyses,

much like compendia of K solutions are currently published.

As part of the ongoing efforts to enhance the technology

in API 579, a project is planned in which reference stress

solutions (based on the above de®nition) will be generated

for a range of cracked bodies. This project will also address

weld mismatch effects.

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, such as PD 6493 (both the 1980 and

1991 versions), made the very conservative assumption of

yield-magnitude membrane residual stresses in as welded

components. More recent assessment procedures, including

API 579, have removed much of this conservatism.

Appendix E of API 579 contains a compendium of resi-

dual stress distributions for various weld geometries. These

distributions are based on ®nite element analyses of weld

residual stresses in a series of pipe girth welds, seam welds,

and nozzle-to-head attachment welds performed under MPC

sponsorship. Based on these results, a series of parametric

residual stress distributions were developed and included in

API 579 Appendix E. However, an in-depth review of the

residual stress analyses performed thus far and a large body

of recent residual stress results from other sources over the

last few years suggest that additional work should be

performed to improve the current FFS procedures for pres-

sure vessel and piping components. An upcoming research


project will address the following issues:

² Con®rmation of some of the parametric distributions in

Appendix E.

² A clear criterion for selecting `bending' and `self-equili-

brating' types of residual stress distributions in pipe/

vessel welds.

² Development of improved residual stress distributions

for ®llet welds at corner joints, nozzle welds, and repair

welds.

² Incorporation of local post-weld heat treatment effects.

Appendix E will continually be expanded and revised as

new results become available.

8. API and ASME FFS activities

The American Society of Mechanical Engineers (ASME)

has formed a new main committee, the Post Construction

Main Committee, with a charter to develop codes and stan-

dards for in-service pressure containing equipment covering

all industries. Currently, standards development activity is

underway in the areas of Risk-Based Inspection (RBI) and

repair methods (e.g. leak sealing, boxes, patches, etc.).

In the area of FFS, API and ASME are working to create a

new standards committee that will jointly produce a single

FFS standard in the US that can be used for pressure

containing equipment. It is envisioned that once the nego-

tiations and operating procedures for the new committee

structure are complete, API 579 will form the basis of the

joint API/ASME standard that will be produced by this

committee. The initial release of the new standard will

include all topics currently contained in API 579 and will

also contain an FFS assessment procedure for the evaluation

of creep crack growth. This assessment procedure is

currently being developed jointly by the Pressure Vessel

Research Council (PVRC), Continued Operation of Equip-

ment (COE) Division and the ASME Post Construction

Committee Subgroup on Creep and Fatigue Growth, and

is being sponsored by Edison Electric Institute.

The agreement to produce a joint standard on FFS tech-

nology is a landmark decision that will focus resources in

the US to develop a single document that can be used in all

industries. This will help avoid jurisdictional con¯icts and

promote uniform acceptance of FFS technology. It also

provides an opportunity for pooling of resources of API,

ASME, PVRC, and MPC to develop new FFS technology

as required by the standards committee. Discussions are

already in progress, and suggestions have been made to

have the new standards committee meetings in conjunction

with PVRC. This would help to create a focal point for FFS

technology development in that the PVRC COE and MPC

FFS JIP have previously met at this time. In addition, the

members of the standards committee could directly

interface with members of these groups to de®ne technology

needs and help arrange for appropriate funding levels.

References

[1] API. Recommended practice for ®tness-for-service. API 579.

Washington, DC: American Petroleum Institute, 2000.

[2] API. Pressure vessel inspection code: maintenance inspection,

rerating, repair and alteration, API 510. Washington, DC: American

Petroleum Institute, 1999.

[3] API. Piping inspection code: inspection, repair, alteration, and

rerating of in-service piping systems. API 570. Washington, DC:

American Petroleum Institute, 1998.

[4] API. Tank inspection, repair, alteration, and reconstruction, API 653.

Washington, DC: American Petroleum Institute, 1998.

[5] API. Recommended practice for risk-based inspection, API 580 (in

development). Washington, DC: American Petroleum Institute.

[6] MPC. Fitness-for-service evaluation procedures for operating pres-

sure vessels, tanks, and piping in re®nery and chemical service,

FFS-26. New York, NY: The Materials Properties Council, October,

1995.

[7] British Energy. Assessment of the integrity of structures containing

defects. British Energy R-6, 1999.

[8] BSI. Guide on methods for assessing the acceptability of ¯aws in

structures, BS 7910. British Standards Institute, 1999.

[9] SAQ/FoU. A procedure for safety assessment of components with

cracks Ð Handbook. SAQ/FoU-Report 96/08, 1997.

[10] Method of assessment for ¯aws in fusion welded joints with respect to

brittle fracture and fatigue crack growth, WES 2805, 1997.

[11] Kumar V, German MD, Shih CF. An engineering approach for elas-

tic±plastic fracture analysis. EPRI Report NP-1931, Palo Alto, CA:

EPRI, 1981.

[12] Ainsworth RA, Sharples JK, Smith SD. Effects of residual stress on

fracture behavior Ð experimental results and assessment methods.

J Strain Anal 2000:53.

[13] Osage DA, Shipley KS, Wirsching PH, Mansour AE. Application of

partial safety factors for pressure containing equipment. Presented at

the 2000 ASME Pressure Vessel and Piping Conference, Seattle, July,

2000.

[14] Anderson TL, Thorwald GV, Revelle DJ. Stress intensity solutions for

surface cracks and buried cracks in cylinders, spheres, and ¯at plates.

Presented at the 2000 ASME Pressure Vessel and Piping Conference,

Seattle, July, 2000.

[15] ASTM E 1921-97 Standard test method for determination of reference

temperature, T0, for ferritic steels in the transition range. Philadelphia:

American Society for Testing and Materials, 1997.


Fad api 579.pdf

Documents

Transcript of Fad api 579.pdf