3D Measurement and FFS Assessment for LTA in Pressure ...

1 Copyright © 2016 by ASME

Proceedings of the ASME 2016 Pressure Vessels & Piping Conference

July 17-21, 2016, Hyatt Regency Vancouver - Vancouver, BC, Canada

PVP2016-63912

3D MEASUREMENT AND FFS ASSESSMENT FOR LTA IN PRESSURE EQUIPMENT

ACCORDING TO WES2820:2015

Takayasu TAHARA Seikowave K.K. Saitama, Japan

Yoshiharu SHIMURA IHI Marine Co., Ltd.

Tokyo, Japan

Minoru NIIMURA Seikowave K.K. Tokyo, Japan

ABSTRACT The corrosion of pressure equipment such as corrosion under

insulation, CUI, is the most common problem in refinery and

petrochemical plants in recent years.

Fitness-For-Service, FFS, assessment technologies for

pressure equipment have been studied in recent 15 years, and

standardization of a FFS assessment procedure for local thin

area, LTA, has been expected by maintenance engineers of

process industries.

Based on the verification using extensive burst tests and FEM

analysis of LTA, the Japan Welding Engineering Society, JWES,

developed new FFS standard WES2820 in June 2015.[1]

This paper presents high lights of WES2820 and a FFS

assessment system consisted with 3D optical measurement

method and FFS software for LTA in pressure equipment as a

new tool for effective inspection and reliable maintenance

activities.

INTRODUCTION There are many process plants such as refineries,

petrochemical production operating more than 40 years in the

world. Pressure equipment such as pressure vessels, piping,

storage tanks have common problems of corrosion metal loss,

so called LTA and how to assess the integrity of them for

continues safe operation. In order to assess the integrity of

pressure equipment with LTA, it requires to use a proven FFS

assessment procedure and reliable flaw sizing method.

BACKGROUND From 2000, Petroleum Association of Japan (PAJ) and Japan

Petrochemical Industry Association (,JPCA) jointly established

the PAJ/JPCA FFS Assessment Committee and commenced to

propagate Fitness-For-Service assessment technology into

Japanese industries. Through recent 13 year’s activities, they

developed FFS Assessment Handbook and FFS assessment

standards for fire damage, LTA and high temperature creep

using Omega method based on API579-1/ASME FFS-1, 2007

(herein after API/ASME FFS-1) which was published on

participation with the committee members including one from

PAJ/JPCA FFS Assessment Committee [4].

From 2012, activities related to FFS assessment technology in

PAJ/JPCA FFS Committee have been transferred into two

groups;

• HPI/Task Group on Metal Loss Assessment based on

Reliability, HPI TG-MLR

• JWES Task Group on FFS Assessment Procedure for

Pressure Equipment

HPI: High Pressure Institute of Japan

JWES: Japan Welding Engineering Society

In the past 15 years, series of maintenance standards for

pressure equipment have been developed in Japan as shown in

Fig.1. And above new activities are expected to enhance future

maintenance system of pressure equipment.

HPI/TG-MLR is now developing HPI/HPIS-TR Metal loss

assessment for pressure equipment based on reliability which

was presented in PVP2015-45382, PVP2015-45658 and

PVP2015-45842.[4].[5],[6]

JWES/WES Task Group on FFS Assessment Procedure issued

the FFS assessment procedure for pressure equipment - Metal

loss assessment, WES2820, herein after the Standard, on June,

2015 after two years standardizing activities based on FFS

assessment standards of LTA developed by PAJ/JPCA FFS

Assessment Committee.

1. Concept of WES 2820:2015 [12] As of today, It is not allowed to continue operation of pressure

equipment with LTA when remaining thickness is less than the

calculated minimum required thickness according to existing

regulation such as the High Pressure Gas Safety Act due to FFS

assessment procedure for LTA have not been authorized in

Japan.

The concept of remaining strength factor, RSF which is adopted

in API/ASME FFS-1 is introduced to the Standard as simple

assessment method for metal loss thinner than minimum


required thickness. FFS assessment procedures as specified in

Part 4 (general metal loss) and Part 5 (local metal loss) in

API/ASME FFS-1 are referred as basic concepts of the

Standard with several features including improvements or

modifications as summarized below considering the specific

requirements in the regulation in Japan and user’s friendliness.

• LTA assessment method using RSF concept verified in

WRC Bulletin 505 [14]and adopted in API/ASME FFS-1

will be acceptable to use FFS assessment for pressure

equipment according to Japanese construction codes

which were also standardized referring ASME B&PV

Codes

• The standard is consisted with main body with detailed

assessment flow charts, equations and figures, appendix

for calculating method of shell section for supplemental

loads, and interpretations which includes validation with

burst tests and example problem in one document. So it is

easy to understand how to assess the LTA and also study

background and/or discussion of each requirement.

• The assessment procedures in the Standard is equivalent to

Level 2 methods for general and local metal loss of

API/ASME FFS-1 which is the most convenient and

practical as FFS assessment to use both manual calculation

and computer software from thickness reading using UT

and also 3D measurement methods.

• The recommended methods of thickness measurement are

point reading, thickness profile and groove profile. It is

emphasized to include the lowest point of LTA at the first.

• Assessment of circumferential direction is based on stress

evaluation at LTA region considering overturning moment

of pressure equipment due to seismic force. In the case

when supplemental load is negligible, the Standard

provides higher accuracy of assessment calculating

circumferential stress for internal pressure than

API/ASME FFS-1 Part5.

• Remediation includes not only methods of rerating,

evaluation of FCA, repair, replacement but also detailed

assessment by FEM analysis as Level 3 assessment.

• WES7700-1~4 “Repair welding of pressure equipment” in

2012 developed by the Chemical Plant Welding Research

Committee in JWES requires that feasibility of repair

welding should be studied based on FFS assessment of

flaws in pressure equipment. An improvement of

reliability in repair welding of pressure equipment is

expected when FFS assessment procedure is clearly

specified in the Standard.

3

Chiyoda Advanced Solutions

Inspection/Testing

Repair/Replacement FFS Assessment

WES7700-1,2,3,4Repair Welding of Pressure Equipment

1. KHK/PAJ/JPCA S0851(2009) FFS Assessment Standard2. PAJ/JPCA FFS Assessment Standards for Pressure

Equipment3. HPI Z-101-1,-2 Assessment Procedure for Cracklike

Flaws in Pressure Equipment4. WES2820 FFS Assessment Procedure for

Pressure Equipment–Metal Loss Assessment

１．JPA Maintenance Standards JPI-8R-11～17２．HPI Z106,107 Risk Based Maintenance Standards

Maintenance/Safe Operation

Fig.1 Present status of maintenance standards in Japan (as of

January, 2016) [1],[4],[7],[8],]9],[10],[11]

2. Specific Requirements in WES2820 2.1 Contents

The contents of the Standard are as follows.

1. Scope

2. Applicable Codes and Standards

3. Terms and definitions

4. Symbol and meaning

5. Required information and data for LTA assessment

6. Assessment procedure

7. Applicable type of components

8. Thickness reading

9. Characterization of LTA

10. Calculation of maximum allowable pressure and RSF

11. Stress calculation of cylindrical shell with LTA subjected

supplemental loads

12. Acceptance criteria

13. Remediation

Appendix A (Mandatory) Calculation method of cylindrical

section with LTA subjected supplemental loads

Interpretation including validation with burst tests and example

problem

As of today, WES2820 written in Japanese is published.

2.2 Assessment procedure

LTA in pressure equipment is assessed according to flow as

shown in Fig.2.


Classification of component type

Characterization for general metal loss

Characterization for local metal loss

Judgment

Remediation

Judgment

Continous operation up to next inspection

LTA data

LTA data

unacceptable

acceptable

unacceptable

acceptable

General MetalLoss Assessment

Local MetalLoss AssessmentApplicable to

Type A component

Fig.2 Assessment flow of LTA

The component in which flaws is found is classified in Type A

or B (refer to Table 1) and then LTA assessment according to

the flow which shows that local metal loss assessment is

performed after general metal loss assessment. However, it is

applicable to perform local metal loss assessment only.

Table1 Classification of pressure component type

Pressure Component Type

a) Pressure vessel cylindrical and conical shell sections A

b) Spherical pressure vessels and spherical tanks A

c) Hemispherical, ellipsoidal and dished heads A

d) Straight section of piping system A

e) Pipe bends and elbow do not have structural attachments A

f) Cylindrical shell for atmospheric tank A

g) Nozzle connections B

h) Transition between conical shell and cylindrical shell B

i) Flange joint B

(Type A and B are defined as same as API579-1/ASME FFS-1)

a) Assessment of general metal loss

The LTA is characterized as uniform general metal loss and

calculated minimum required thickness tmin or maximum

allowable pressure PMAW for judgment of continuous operation

up to next inspection as shown in Fig.3..

t

きず，又は損傷

t

tam

Fig.3 Characterization of general metal loss

b) Assessment of local metal loss

The LTA is characterized as local rectangular LTA and then

judged capability of continuous operation up to next inspection

as shown in Fig.4.

Firstly, judgment of longitudinal section is performed to

determine PMAW from result of RSF. Then circumferential

section is evaluated based on calculated Meses’s stress

compared with allowable tensile stress σa. (Refer to equations in

2.6.2 b) .)

s

tl

c

tmm

a)longitudinal section b)circumferential section

Fig. 4 Characterization of local metal loss

2.3 Thickness reading

Three methods of thickness reading may be used upon features

of flaws and damages, type of components and assessment

procedure as shown in Table 2.

Table 2 Methods of thickness reading

Type of

flaw

Applicable assessment procedure Thickness reading

Uniform

metal loss

Assessment for general metal loss

of Type A component Point thickness

Locally

corroded

damages

or metal

loss

-Assessment for general metal loss

of Type A component

-Assessment for local metal loss

of Type A component

-Assessment for local metal loss

of Type B component

Thickness profile

Groove

like flaw

Assessment for general metal loss

of Type A component Groove profile

2.3.1 Point thickness reading

Point thickness reading is used for metal loss widely extended

in the surface. Proper numbers of point are selected to cover

extent and degree of flaw or damages.

a) In the case when COV of measured data is greater than

10%, it is judged that point reading method is not suitable

for assessment of general metal loss due to large surface

irregularity, and then thickness profile method should be

selected.

b) The distance from gross structural discontinuity to the flaw

shall be measured, and if it is less than c1.8 Dt ,assessment

for local metal is not applicable.

c) Assessment for local metal loss is not applicable when the

results of thickness profile and groove profile fall into

following cases

1) Shell parameter c of circumferential direction is larger

than 9

2) Bottom radius of groove like flaws is less than groove

Characterization flaw


depth

2.3.2 Thickness profile

The thickness profile method is used for locally corroded flaws

or damages as shown in Fig.5. The measurement grid may be

set with space ( tDtl 2,36.0min mins 　 )




C1

C2C3

C1

C2

C3

M1 M2 M3

C1

C2

C3

i) 圧力容器の円筒胴

M1 M2 M3

M1

M2

M3

ii) 圧力容器の円すい胴 iii) 球形圧力容器，球形タンク Fig. 5 Thickness profile reading

2.3.3 Critical thickness profile, CTP

C1 C2 C3

M5

M4

M3

M2

M1C4 C5 C6 C7

最小厚さ測定点の結線

軸方向CTP

射影

Fig. 6 Critical thickness profile

The CTP of longitudinal direction is developed from the profile

connected lowest points of each circumferential section of grid

as shown in Fig.6. The CTP of circumferential direction also

developed as same method as above.

2.4Characterization for general metal loss

a) Characterization of general metal loss from point reading

Mean measured thickness is calculated from thickness reading

of each point and then characterized as uniform metal loss.

b) Characterization of general metal loss from thickness

profile

Compute the length of thickness averaging, L from

longitudinal CTP and determine the average measured

thickness tam as shown in Fig.7.

cDtQL which is same as defined in API 579-1/ASME

FFS-1 Part 4

It is possible to apply simple and conservative assessment of the

flaw characterized as uniform metal loss.

t

減肉平均化長さL

3

mm

s

m2

s

m1s

am

tttt

軸方向平均測定厚さ

s

m1t s

m2t

mmt

軸方向減肉長さs

Fig.7 Characterization of general metal loss by thickness

profiling

2.4.1 Characterization of local metal loss

The longitudinal thickness and length of local metal loss are

determined in all subsections as shown in Fig.8

tc

tc

si

tli

tc

si + 1

tli + 1

の面積Ai の面積Ai+1

si si+1

si×tli=Aiとなるようにtl

iを決定

si+1×tli+1=Ai+1となるようにtl

i+1を決定

任意の2点を選択任意の2点を選択

Fig.8 Subdivision process for determining the RSF

(Effective area method)

2.5 Calculation of RSF

RSF is defined as follows

RSF= LDC/LUC

LDC : limit or plastic collapse load of the damaged

component with flaws

LUC : limit or collapse load of the undamaged component

RSF is utilized to define the acceptability of a component for

continued service and means the degree of remaining strength

of the component due to flaws in case of judgment for

assessment of local metal loss.

Allowable remaining strength factor, RSFa = 0.9, is provided as

same as Table 2.3 of API 579-1/ASME FFS-1 Part 4

considering basis of Japanese pressure vessel code equivalent to

ASME BPV Codes.

RSF is determined from profile of metal loss with following

formula

1)Cylindrical shell 2)Conical shell 3)Spherical shell

C1 C2 C3

M5

M4

M3

M2

M1C4 C5 C6 C7

最小厚さ測定点の結線

周方向CTP射影

flaw flaw flaw

Circum. CTP

Pass of max. metal loss

Longi. CTP

t

t

tSF

11

1 RM

RR

Longitudinal length of flaw s

Length of thickness averaging, L

Longitudinal average measured thickness

Optional 2 points Optional 2 points


RSF: remaining strength factor,RSF, of the component

Rt: remaining thickness ratio

Mt: Folias factor based on the longitudinal extent of LTA

2.6 Acceptance criteria

The acceptance criteria of component with flows for

continuous operation to next inspection are defined as follows

2.6.1 General metal loss

a) For Type A component

The smaller of Equations below

F C At : future corrosion allowance

mint : minimum required thickness

amt : average measured thickness of LTA

b)For Type B component

p : pressure for evaluation

M A Wp :maximum allowable working pressure calculated

from amt - FCAt

2.6.2 Local metal loss

a) Maximum allowable working pressure

(for longitudinal section)

b)Allowable stress (for circumferential section)

MAWp : maximum allowable working pressure calculated from

t - FCAt

a :allowable tensile stress of the material，

fH =1.0 for stress raised by supplemental load is negligible

=3.0for stress raised by supplemental load is the sum of

primary and secondary stress A

e ， B

e : Mises stress subjected supplemental loads at Pint A

and B (refer to Fig.9.)

t：measured thickness of uncorroded portion

局部減肉特性化断面

My

Mx

MTF

Fs

pMT F

Mx

My

Fs

特性化した減肉領域

My

Mx

A

(B)B

tmm-tFCA

c

θ θ

Df

D

Do

F

MT

tc

Fig.9 Circumferential section subjected

supplemental load

2.7 Remediation

When result of LTA assessment is not acceptable, continuous

use of the pressure equipment is not feasible, suitable

remediation methods must be considered as listed below.

a) Evaluation of future corrosion allowance, FCA determined

from future operation plan, timing of next inspection or

applicable corrosion protection system

b) Weld repair of LTA such as weld overlay refer to

WES7700-1~4

c) Renewal or replacement of the component

d) Rerating of operating pressure and/or temperature

e) Evaluation of structural integrity of the component by

detailed mechanical assessment such as FEM

3 3D Optical Measurement and FFS Assessment of

LTA As reported in PVP2015-45658 for round robin test of

thickness reading of LTA in the pipe by ultrasonic testing,

contact points of UT transducer may be varied and make large

scattering of measured values, especially in case of corrosion at

outer surface of pipe, variation of thickness reading may occur

due to surface irregularity in LTA/CUI (corrosion under

insulation) as shown in Fig.10.

Fig. 10 example of mean values vs. standard deviation of

thickness reading of the pipe with CUI by UT [5]

The various tools measuring corrosion have been studied and

developed longtime in US pipeline industries.

In recent years, 3D optical measurement system such as laser

light-section method, photogrammetry method and 3D

minFCAam 9.0 ttt

MAWpp

MAWMAW

SF ,9.0

min ppR

p

9.0,max a

f

B

e

A

e

H

minFCAmm 5.0 ttt

Characterized LTA


structured light method have been utilized considering

effective measurement and quick decision making based on

FFS assessment.

3D structured light method using LED is recommendable with

advantages as listed Table 3.

Table 3 Advantages of 3D structured light method

Item Advantages of 3D structured light method

Setting Very easy to set. Objects just need to be in

the range of the specified working distance.

Time to capture 0.3 sec *

Time to point cloud 3 seconds *

Resolution XY resolution with 300K

camera : 0.2mm~0.4mm.

Unable object to

scan

Objects with no specula reflection.

Errors Multiple of resolution.

Best fitting objects Objects in short range.

Training Half day training required for better scanning

*: in the case of Seikowave method,3D TOOLBOXTM [12]

3D optical measurement system can measure surface exposed

profile of LTA but cannot directly measure remaining

thicknesses. In order to determine remaining thickness at LTA,

it is necessary to measure thickness at the portion other than

LTA by other method such as UT and deduct depth of LTA

measured by 3D structured light method. In case of

measurement of internal corrosion, inside diameter of the pipe

needs to be large enough to enter the system

(approx.300mm),otherwise the pipe cut in half to expose the

corroded surface if it is allowed.

Using same pipe with LTA for round robin test as shown in

Fig.10 with specification listed in Table 4, LTA in the pipe

was measured by 3D structured light method

Fig.11 shows measurement of LTA in pipe using

3D structured light method. Phase-shifted pattern is projected to

the target surface.

Fig.12 shows 3D data with no texture of outer surface of pipe.

It is easy to develop colored map of LTA depth in short time as

shown in Fig.13.

Table 4 Specification of pipe with LTA

Pipe

with

LTA

O.D.

(mm)

Thk

(mm)

Service

(Year)

Design

Temp.

(Deg-C)

Design

Pressure

(MPa)

Allowable

stress

(N/mm2)

165 7.2 37 85 4.2 92

Fig.11 Measurement of LTA in pipe using 3D structured light

method

Fig.12 3D data (no texture)


Fig.13 Colored map of LTA depth

Fig. 14 shows critical thickness profile, CTP, of the LTA in

pipe for FFS assessment of LTA.

Fig.15 shows LTA map by CSV (Excel)

(CSV: Comma Separated Value)

Fig. 14 CTP of LTA in Pipe.

Fig.15 LTA map by CSV (Excel)

It takes almost 0.3 second for point captured and 30 second

for point cloud.

CSV data is directly transferred to FFS assessment software.

Fig.16 shows data of grid points transferred from CSV data.

Based on input data as shown in Fig.17 and measured

remaining thickness data from CSV , the judgment of FFS

assessment of the pipe based on WES2820-2015 is easily

obtained using the computer software, uni-FitnessTMTM as

shown in Fig.18. .

In this case, general metal loss assessment shows acceptable.

Fig.19 shows FFS assessment after future 3 years operation.

When FCA for future 3 years operation is set 0.30 mm, general

metal loss assessment shows not acceptable, but local metal loss

assessment shows acceptable in both MAWP and allowable

stress.

CONCLUDING REMARKS 1) WES2820-2015 was developed for FFS assessment method

of pressure equipment with metal loss in Japan.

2) In order to perform FFS assessment of LTA in pressure

equipment in short time with the highest reliability, the

assessment system combined with LTA measurement using

3D structured light method and FFS assessment software

according to WES2820 was also developed.

3) LTA measurement using 3D LED system is very effective

in case of detailed measurement and continuous FFS

assessment in short time.

References [1] WES2820-2015, Fitness-For-Service assessment procedure

for pressure equipment – Metal loss assessment

[2] API579-1/ASME FFS-1, Fitness-For-Service, 2007

[3] FFS Assessment Standards for Pressure Equipment,

PAJ/JPCA FFS-S, 2011

[4] T.Kaida et al., Development of Fitness-For-Service standard

for pressure equipment with metal loss based on reliability,

PVP2015-45832

[5] T.Tahara et al., Study on LTA measurement for FFS

assessment, PVP2015-45658

[6] M.Ozaki et al., Buckling strength of towers having partial

metal loss on shell under overturning moment, PVP2015-

45842

[7] JPI Standards JPI-8R-11~17, Japan Petroleum Institute

[8] Risk Based Maintenance, HPIS Z106, Z107, High Pressure

Institute of Japan

[9] Repair welding of pressure equipment, Part 1~4, WES7700

-1~-4, The Japan Welding Engineering Society

[10] Standards to determine next inspection interval based on

Fitness-For-Service assessment for high pressure gas

equipment, KHK/PAJ/JPCA S 0851(2009)

[11] Assessment Procedure for Crack-Like Flaws in Pressure

Equipment-Level1 and Level2, HPIS Z 101-1 and -2, 2011

[12] US Patent, US8, 976, 362B2, Seikowave Inc., USA

[13] IHI Marine Co., Ltd., Japan

[14] J.L.Janelle et al., An overview and validation of the

Fitness-For-Service assessment procedures for local thin

areas, WRC Bulletin 505

Top views

Longitudinal CTP


Fig. 16 Input data of the pipe

Fig.17 Data of grid points transferred from CSV data

Fig. 18 Judgment of FFS assessment of the pipe

Fig.19 Assessment after future 3 years operation

3D Measurement and FFS Assessment for LTA in Pressure ...

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