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Assessment of local thin areas in a marine pipeline by using the FITNET FFS corrosion module
International Journal of Pressure Vessels and Piping 86 (2009) 329–334
Contents lists available at ScienceDirect
International Journal of Pressure Vessels and Piping journal homepage: www.elsevier.com/locate/ijpvp
Assessment of local thin areas in a marine pipeline by using the FITNET FFS corrosion module ˜ o a, b S. Cicero a, b, *, R. Lacalle a, b, R. Cicero a, b, D. Ferren a b
Dpto. Ciencia e Ingenierı´a del Terreno y de los Materiales, Universidad de Cantabria, ETS. Ingenieros de Caminos, Av. Los Castros s/n, 39005 Santander, Cantabria, Spain ´ gico de la Universidad de Cantabria, Fase A, Mod. 2003, Av. Los Castros s/n, 39005 Santander, Cantabria, Spain INESCO Ingenieros SL, Centro de Desarrollo Tecnolo
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 December 2007 Received in revised form 31 October 2008 Accepted 18 November 2008
This paper analyses the structural integrity of the marine stretch of a pipeline which is placed over a natural bay. The pipeline is part of a 30-year-old installation used for the provision of petrochemical products to a nearby chemical plant. Although there have been no relevant leaks in the past, both the visual inspections performed (revealing numerous local thin areas) and the fact that it is located in a highly sensitive place with high ecological and tourist value recommend the assessment of the pipeline in order to ensure that it is working in safe conditions and that there are no risks for the environment or the people living in the surrounding area. The assessment has been performed using the newly developed FITNET FFS procedure, whose local thin areas assessment methodology is also explained and compared to the analyses proposed by other well known procedures. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Local thin areas Structural integrity FITNET FFS
1. Introduction During the visual inspection of an approximately 1800 m long marine stretch of a pipeline placed over a natural bay, many local thin areas (LTA) were detected (Fig. 1 shows an example) in the exterior of the pipe. This pipeline, whose total length is around 5000 m, is used for the provision of petrochemical products (conversion oil) to a chemical plant and its failure would inflict substantial damage on the environment and the population living nearby. This makes it essential to assess the pipeline in order to ensure that it is working safely. The structural integrity analysis of the pipeline has been performed using the corrosion module of the FITNET FFS procedure [1– 4] and requires the following steps: structural analysis, stress analysis and assessment of the LTA (all of these are explained below). The FITNET FFS procedure has been the main result of the recently completed European Fitness for Service Network [1] and constitutes a unified European procedure for structural integrity assessment, which covers the analysis of components and structures under four main failure mechanisms: fracture-plastic collapse, fatigue, creep and corrosion.
* Corresponding author. Dpto. Ciencia e Ingenierı´a del Terreno y de los Materiales, Universidad de Cantabria, ETS. Ingenieros de Caminos, Av. Los Castros s/n, 39005 Santander, Cantabria, Spain. Tel.: þ34 942 200917; fax: þ34 20 18 18. E-mail address: [email protected] (S. Cicero). 0308-0161/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2008.11.021
Before the FITNET FFS procedure was developed there were numerous assessment procedures of a clearly national nature (British BS7910 [5] or the French RCC-MR [6]) and others whose application was restricted to a specific industry (i.e, R5 [7] and R6 [8] of British Energy and the Swedish SAQ [9] in the nuclear sector). Moreover, each of these procedures tended to focus on one specific type of failure (R5 on creep, BS7910, R6 and SAQ on fracture.), there being no procedure which allowed a component to be assessed for the various failure mechanisms. At the same time, some other industrial powers, such as the USA and Japan had their own procedures offering a simpler, more clearly defined picture. Thus, American procedures such as API 579 [10] in the petrol sector and ASME XI [11] in the nuclear industry or the Japanese JSME procedure [12] were fully defined and widely used in industry. This panorama in Europe, characterised by a lack of uniformity, an excessive variety, lack of global vision and delay with respect to other countries made it absolutely essential to develop a unified European procedure covering the main failure modes and offering European industry a tool with which to design, manufacture and manage industrial components and installations in a safer, more efficient way. Finally, the procedure is organised in three volumes (procedure [2], annex [3] and case studies and tutorials [4]) and has been submitted to the European Committee for Standardisation (CEN) for its adoption as a European standard for structural integrity.
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S. Cicero et al. / International Journal of Pressure Vessels and Piping 86 (2009) 329–334
Nomenclature Do Di l LTA M n P Q R Rb RSFa rsfECmin rsfLmin t tm
outer diameter inner diameter length or longitudinal dimension of the thin area local thin area applied bending moment parameter used for the calculation of scyl working pressure length correction factor average radius in the transversal section of the elbow (as shown in Fig. 6) average radius of the elbow (as shown in Fig. 6) allowable residual strength factor (used in API 579) minimum required strength factor for circumferential failure for the elbow system stress minimum required remaining strength factor for the nominal hoop stress pipe thickness allowable remaining wall thickness for combined internal pressure and axial loading ðmax½tmL ; tmC Þ
Concerning the assessment of LTA, the FITNET FFS procedure provides a truly comprehensive and straightforward methodology that can be applied to most common components (cylindrical bodies, elbows, spheres.), providing critical parameters (minimum allowable wall thickness, safe working pressure, safe moment estimate.) for the corresponding working conditions. When compared to the equivalent methodologies of other well known procedures some considerations can be made: – It is more general than the ASME code (ASME XI, code case N597-2), focused on nuclear components. The ASME code also analyses both circumferential and longitudinal failures but it is even simpler. Also, and considering the critical safety aspects associated to the nuclear industry, it provides more conservative results through the consideration of the yield stress as the resistant parameter (and not the flow stress as considered in FITNET). – The API 579 assessment is, at first sight, a more complete methodology, but it is also noticeably more complex in terms of presentation, document extension, and the definition and obtaining of the different inputs. Also, it is mainly focussed on pipeline systems designed following API standards. If it is applied to other systems, designed using other procedures/ codes (or whose design bases are not well known), there are certain input parameters (design criteria, geometry of the different defects, historical sequence of the measurements, defect profiles.) whose definition or availability is not always straightforward. It proposes three different levels of analysis (I, II and III), level II being the one which is comparable to the FITNET FFS methodology when external loads are acting together with the internal pressure. The higher the level is, the more accurate the results are. This accuracy can be noticeably higher than that obtained with FITNET FFS when level III is applied.
tmC tmL w
aelbow belbow l sa sA scyl sf sh ssys suts syield sz
minimum allowable remaining wall thickness to prevent circumferential failure minimum allowable remaining wall thickness to prevent longitudinal failure width or circumferential dimension of the thin area elbow parameter quantifying the elbow effect on the hoop stress elbow parameter quantifying the elbow effect on the axial stress parameter used to quantify belbow axial stress from the axial force in the bend code allowable stresses nominal failure stress of an unflawed pipe flow stress (average value between the yield stress and the ultimate tensile strength) hoop stress system stress (sa þ sz) material ultimate tensile strength material yield stress axial bending stress
analysed here. All the pipes are supported every 12 m by a solid foundation system made up of reinforced concrete piles and bracing beams. Finally, the pipes are transversally joined by structural profiles that transmit the weight of the entire system to the main pipeline halfway between every two foundations. Fig. 2 shows a scheme of one piece of the model used to make the structural analysis of the whole system, showing the geometry of the structure. It can be observed that there is an expansion loop introduced in the route of the pipes in order to avoid high stresses and/or buckling problems caused by the relatively high temperatures (þ80 C) of the chemical products and the corresponding length increments of the pipes. In total, there are eight expansion loops in the pipelines, one every 200 m, as shown in the pipeline plan view illustrated in Fig. 3. Table 1 shows the diameters and thicknesses of the different pipes and Table 2 gathers the tensile properties of the main pipeline material [13].
2. Geometry and material properties The pipeline assessed is the main one in a system composed of five secondary additional smaller pipes whose integrity is not
Fig. 1. Example of LTA located in the pipeline.
S. Cicero et al. / International Journal of Pressure Vessels and Piping 86 (2009) 329–334
331
Table 1 Diameter and thickness in the different pipes. Pipeline Pipe Pipe Pipe Pipe
Fig. 2. Scheme of a stretch of the whole structure (model used in the structural analysis).
3. Structural analysis The pipeline is subjected to the following loads: – – – –
working pressure (0.8 MPa); own weight (7800 kp/m3); weight of the conversion oil (1000 kp/m3); loads transmitted by the secondary pipes (and the fluids in their interior); – loads transmitted by a footbridge placed on top of the pipeline (90 kp/m), as shown in Fig. 1; – thermal loads caused during the provision of the conversion oil (þ60 C).
Calculations have been performed using the software Tricalc [14] and the results show, as expected, that all the straight stretches are subjected to very similar bending moments and compression loads, something that also happens between the expansion loops. This simplifies the structural analysis, with just two different bending and compression diagrams. Table 3 shows the critical combination in both the straight stretches and expansion loops (the axes are defined in Fig. 4). In the latter, the critical section corresponds to the elbow. 4. Stress analysis Considering the results shown in Table 3, the worst combination of stresses caused by the bending moments, compression forces and internal pressure is shown in Table 4 for both the straight stretches and the expansion loops. These stresses are the inputs used in the structural integrity assessment. 5. Assessment of LTA following the FITNET FFS corrosion module The owner used guided ultrasonic longitudinal waves in order to locate the main metal loss areas in the entire pipeline stretch (1800 m) and then, ultrasonic transducers were used to measure the metal loss in those specific zones. Due to the enormous number of local thin areas it was decided not to make a one by one assessment, but to establish a characteristic defect for both the straight stretches and the expansion loops. This decision, which may be considered too conservative, is quite reasonable after a visual inspection of the pipeline and the results of the thickness measurements. These revealed a minimum thickness of 4.1 mm for
Fig. 3. Pipeline system plan view (1800 m).
2400 1200 800 600 (x2)
Outer diameter (mm)
Nominal thickness (mm)
609.6 304.8 203.2 152.4
9.52 8.38 8.18 7.11
the straight stretches and 4.4 mm for the expansion loops, the corresponding values for every single straight stretch or expansion loop not being very far from these values. Moreover, the defects were located in both the intrados and the extrados of the pipe and also in the upper and the lower part. Thus, it seems reasonable to consider one characteristic defect for each of the mentioned parts of the pipeline and consider that this defect can be perfectly located in the most stressed point of the pipe. If the fitness for service of the pipeline is demonstrated with these assumptions no more calculations would be needed, given that a conservative assumption would produce a safe assessment. The last data required for the analysis were the superficial dimensions of these defects. These were not supplied by the owner of the installation, so another inspection was necessary to determine them. Since the metal loss was mainly produced in the external surface of the pipeline, a simple visual inspection was enough to estimate that the dimensions w (width, or circumferential dimension) and l (length, or longitudinal dimension) could be taken as 300 mm and 400 mm, respectively, for elbows, and 500 mm (w) and 350 mm (l) for the straight stretches. This assumption is again conservative (since these are the maximum dimensions measured) but covers both the biggest metal loss areas found in the pipeline and the possible interactions between different defects. Therefore, the problem has been reduced to the assessment of two defects: the first one is located in the most stressed point of the expansion loops (elbow) and its dimensions are 300 mm for w and 400 mm for l, the remaining thickness being 4.1 mm; the second one is located in the most stressed point of the straight stretch, w being 500 mm and l 350 mm, and the remaining thickness 4.4 mm. The assessment procedure can be applied once the geometry of the defects, the stress state and the material properties are known. In this case, the FITNET FFS corrosion module (chapter 9 in the procedure) has been used. This consists of two types of assessments: (1) stress corrosion cracking and corrosion fatigue, in chapter 9.1, and (2) assessment of local thin areas (LTA), in chapter 9.2. A comprehensive explanation of the first type of assessment can be found in Refs. [15,16], this paper being focussed on the application of the second one (which is also introduced in Refs. [15,17]). Fig. 5 [2,18] shows the flowchart for the assessment of LTA. The procedure provides the formulation, after which the analysis can be applied, for the following types of components: cylindrical body, sphere and vessel end, elbow and nozzles. It also provides guidance for interaction rules. Finally, for both a cylindrical body and an elbow, the analysis can be performed to obtain the safe working pressure, the safe working system stress or the minimum allowable remaining wall thickness. Given that for the case analysed here (and for efficiency reasons in the provision process) the working pressure is fixed at 0.8 MPa, calculations will be oriented to obtain the minimum allowable remaining wall thickness.
Table 2 Material properties (lower bound values). Material
Yield stress (syield, MPa)
Ultimate tensile strength (su, MPa)
Young’s modulus (MPa)
Steel ASTM A333
205
380
200,000
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Table 3 Main forces and moments on pipe 2400 .
Straight stretches Elbows in expansion loops
Fx (kN)
My (kN m)
Mz (kN m)
43.1 z0
50.4 107.4
75.7 55.9
Straight stretches Elbows in expansion loops
For the elbows (located in the expansion loops), the steps in the assessment (section 9.2.5.5.3 in Ref. [2]) are shown below. An explanation of the formulation can be found in Ref. [17]. For internal pressure, it assumes that the stress distribution in a pipe elbow can be approximated by the membrane stress solution to the thin shell equilibrium equations for an axisymmetric shell of revolution [19,20]: (a) Determine the allowable remaining wall thickness to prevent longitudinal failure (hoop stress): (a.1) Calculate the nominal failure stress of an unflawed pipe (scyl), following Ref. [21]:
scyl
n 1 suts ¼ 2
(1)
where suts is the ultimate tensile strength and n [22]:
n ¼
65
(2)
syield
syield is the yield stress expressed in MPa. Here, scyl is 305 MPa. (a.2) Calculate the elbow parameter aelbow using:
aelbow ¼
(3)
R 12R b
where R and Rb are defined in Fig. 6. For the case analysed, R is 299.9 mm and Rb is 1524 mm and, therefore, aelbow is 0.89. (a.3) Calculate the hoop stress, sh:
P
aelbow
saxial (MPa)
shoop (MPa)
38.5 39.4
25.2 25.2
"
syield sh; elbow sh; elbow ¼ max ; sA 0:85scyl syield
rsf ELmin
# (5)
The hoop stress should not be higher than the code allowable stress [2]. Typical code allowable stresses (sA) for pipelines are listed in Table 5. Here, the following value has been used [2]:
sA ¼ min
syield suts 1:5
;
3
(6)
sA is 126.6 MPa and rsfELmin is 0.18. (a.5) Calculate the length correction factor (Q): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi l2 Q ¼ 1 þ 0:8 Do t
(7)
where l is the length of the defect. Q is 3.66. (a.6) The minimum allowable remaining wall thickness to prevent longitudinal failure (tmL) is:
tmL ¼ t
rsf ELmin ð1Q1
ELmin 1rsf Q
(8)
giving tmL ¼ 1.39 mm.
1RRb
sh; elbow ¼
Table 4 Total stresses (maximum absolute values) due to external loads and internal pressure. The elbow values do not include the aelbow and belbow correction factors.
ðDo tÞ 2t
(4)
where P is the working pressure, Do is the outer diameter and t is the thickness. Here, sh is 28.4 MPa. (a.4) Calculate the minimum required remaining strength factor (rsfLmin) for the nominal hoop stress (sh, elbow):
(b) Determine the allowable remaining wall thickness to prevent circumferential failure (axial stress): (b.1) Calculate the axial bending stress (sz) for moment M (in this case My caused by thermal loading) of a pipe without corrosion:
sz; elbow ¼
b ¼
l2=3
Z ¼
x
(10)
0:9 tRb R2
(11)
y
(9)
where
l ¼
z
M
belbow Z
p
D4o D4i
32
Do
(12)
Di is the inner diameter. Here l is 0.161, belbow is 0.329, Z is 0.002725 m and, finally, sz, elbow is 119.8 MPa. Here it is important to note the influence of the elbow geometry in the stress state, since belbow provides stress values around three times bigger than the corresponding ones for a straight pipe. (b.2) Calculate the axial stress (sa, elbow) from the axial force in the bend:
sa; elbow ¼
F
pDt
(13)
Here, the axial stress is negligible. (b.3) Calculate the elbow system stress (ssys, elbow): Fig. 4. Axes definition in the pipeline section.
ssys; elbow ¼ sa; elbow þ sz; elbow
(14)
S. Cicero et al. / International Journal of Pressure Vessels and Piping 86 (2009) 329–334
333
Fig. 5. Flowchart LTA assessment procedure [9.1].
(b.4) Calculate the flow stress (sf):
sf ¼
tm ¼ max½tmL ; tmC
syield þ suts
(15)
2
Here, sf is 292.5 MPa. (b.5) Calculate the minimum required strength factor rsfECmin for circumferential failure for the elbow system stress, ssys, elbow:
rsf ECmin ¼
1
sf
X1
sh; elbow 2
þ X2 ssys; elbow
(16)
being
" X1 ¼ max X2 ¼
syield sh; elbow ; sA syield sf
X sSA 1s syield 2 A
sSA s2A
# (17)
Therefore, for the expansion loops and for the hypotheses considered here, the minimum thickness required to ensure their integrity is 2.30 mm. A totally analogous assessment process is provided in FITNET FFS [2–4] for the evaluation of cylindrical bodies (straight stretches) and, therefore, the calculation process is not presented here. The formulae are also analogous, but not including the geometrical factors a and b used for the consideration of elbow geometry. Also, the allowable remaining wall thickness is calculated for both the longitudinal and the circumferential failure. For the case analysed here and for the hypotheses considered, tm is 1.90 mm (tmC being the critical parameter).
! (18)
sA is the code allowable hoop stress and sSA is the code allowable axial stress, both following Eq. (6) (see Table 5). Here, rsfECmin is 0.66. (b.6) The minimum allowable remaining thickness to prevent circumferential failure (tmC) is given (as shown in Fig. 9.9 in Ref. [2]) by:
" tmC ¼ t
# rsf 1 Cmin
1 ½0:707cos1 f0:45sin Dwo 0:318Dwo 1:111
(19)
For the case analysed, tmC is 2.30 mm. (c) Finally, the minimum allowable remaining wall thickness (tm) for combined internal pressure and axial loading is given by:
(20)
Fig. 6. Elbow dimensions [1].
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S. Cicero et al. / International Journal of Pressure Vessels and Piping 86 (2009) 329–334
Table 5 Summary of typical allowable design stresses for typical pipelines and piping. Design code
Allowable circumferential stress Allowable total axial stress for non-incidental loads Allowable external stresses at full design pressure
B31.3 Chemical plant and refinery piping
B31.4 Liquid transportation systems for hydrocarbons
B31.8 Gas transmission and distribution piping systems
ISO 13623 Petroleum and natural gas industries pipelines
UTS minðSMYS 1:5 ; 3 Þ
0.72 SMYS
UTS minðSMYS 1:5 ; 3 Þ
0.54 SMYS
UTS minðSMYS 1:5 ; 3 Þ 2
0.18 SMYS
Ranging from 0.40 to 0.80 SMYS depending on location class 0.75 SMYS for onshore pipeline 0.8 SMYS for offshore riser and pipeline 0.35–0.55 SMYS depending on location class
0.45–0.83 SMYS depending on location class and product zSMYS depending on location class and product z0.58–0.77 SMYS depending on location class and product
6. Discussion Once the minimum allowable remaining wall thickness has been obtained in both the elbows and the straight stretches, the results can be compared to the measured values in order to evaluate the structural integrity of the pipeline and, finally, take the corresponding decisions (i.e, inspection periods, replacements, repairs, etc): – For elbow locations (placed in the expansion loops), the minimum allowable thickness is 2.30 mm, which is noticeably lower than the minimum measured (4.4 mm). Then, thickness has been reduced by 5.4 mm in 30 years, which represents a metal loss rate of 0.18 mm/year (considering a constant rate) and, therefore, the minimum allowable thickness would be achieved in 11.66 years. However, given that w and l also increase with time, the actual value would be lower. This fact, together with the uncertainty of the metal loss rate, makes it recommendable to perform the inspection of the elbows noticeably before the mentioned 11.66 years (i.e, 2–3 years). – For straight stretches, the minimum allowable thickness is 1.90 mm, which is still somewhat lower than the minimum measured thickness (4.1 mm). The corresponding thickness loss rate (if constant) is 0.19 mm/year and the expected time for the minimum allowable thickness to be achieved is more than 11.58 years. Again, this value is an upper bound (given that w and l also grow) and the thickness loss rate is uncertain, so it is recommended here to perform the next inspection much before 11.58 years (i.e, 2–3 years). If the LTA analysis is performed using the ASME code (case N597-2) and API 579, the results obtained are the following: – Both the ASME code and API 579 do not provide explicitly solutions for elbows when external loads are applied simultaneously with the internal pressure. Therefore, FITNET FFS provides solutions for a wider range of industrial situations. – For straight stretches, ASME provides a minimum allowable remaining thickness of 2.82 mm, and API 579 provides (considering a reasonable allowable remaining strength factor, RSFa, of 0.8) 2.01 mm. The ASME solution is more conservative, while the API 579 solution is very similar to that obtained using the FITNET FFS. The main difference between ASME and the other two procedures is the resistant parameter considered in the calculations: the former uses the yield stress and the latter uses the flow stress. 7. Conclusions Local thin areas detected in a pipeline placed in a marine environment have been analysed using the FITNET FFS corrosion module. After considering some conservative (but realistic and reasonable) hypotheses, the structural integrity of the system has been demonstrated and recommendations about inspection periods have been proposed.
As shown here, the FITNET FFS corrosion module provides a comprehensive and relatively simple methodology for assessing components (cylindrical bodies, spheres and vessel ends, elbows and nozzles) with local thin areas. This procedure constitutes an alternative to other well known procedures, providing a general methodology that covers the four major failure modes (not just the LTA assessment) and being applicable to all kinds of industries. As an example the results obtained using the FITNET FFS procedure have been compared to those obtained using the ASME code and API 579, and the differences detected have been justified. References [1] European fitness-for-service network. EU’s framework 5, proposal no. GTC12001-43049, contract no. G1RT-CT-2001-05071. [2] ISBN 978-3-940923-00-4. In: Kocak M, Webster S, Janosch JJ, Ainsworth RA, Koers R, editors. FITNET fitness-for-service (FFS) procedure, vol. 1. Geesthacht, Germany: GKSS Research Centre; 2008. [3] ISBN 978-3-940923-01-1. In: Kocak M, Hadley I, Szavai S, Tkach Y, Taylor N, editors. FITNET fitness-for-service (FFS) annex, vol. 2. Geesthacht, Germany: GKSS Research Centre; 2008. [4] Kocak M., Laukkanen A., Gutie´rrez-Solana F., Cicero S., Hadley I., editors. FITNET fitness-for-service (FFS) case studies and tutorials, vol. 3. ISBN 978-3940923-02-8, in press. [5] British Standard BS7910. Guide to methods for assessing the acceptability of flaws in metallic structures. London: BSI; 2000. [6] RCC-MR: Re`gles de Conception et de Construction des mate´riels me´caniques des ˆılots nucle´aires RNR. AFCEN; 2002. [7] R5: assessment procedure for the high temperature response of structures. Procedure R5, Issue 3. Gloucester, UK: British Energy; 2003. [8] R6: assessment of the integrity of structures containing defects. British Energy Generation. Report R/H/R6, revision 4; 2001. [9] Bergman M, Brickstad B, Dahlberg L. A procedure for safety assessment of components with cracks – handbook. SAQ/FoU report, 9/101, AB Svensk Anla¨ggningsprovning. Swedish Plant Inspection Ltd; 1991. [10] API 579. Recommended practice for fitness for service. Draft issue 4. American Petroleum Institute; 1996. [11] ASME boiler and pressure vessel code, section XI: rules for in-service inspection of nuclear power plant components. The American Society of Mechanical Engineers; 1995. [12] JSME. Codes for nuclear power generation facilities – rules on design and construction for nuclear power plants. JSME S NC1; 2001. [13] ASTM A333M-88a/ASME SA333. Seamless and welded steel pipe for low temperature service. [14] Tricalc 6.4. Ca´lculo de Estructuras Tridimensionales. Arktec; 2007. [15] Kocak M. FITNET fitness for service procedure: an overview. In: Proceedings of the international conference on fitness-for-service FITNET. Amsterdam; 2006. p. 3–14. [16] Gutie´rrez-Solana F., Cicero S. FITNET FFS procedure: a unified European procedure for structural integrity assessment. Engineering Failure Analysis. Available online, 10.1016/j.engfailanal.2008.02.007. [17] Ritchie D., Koers R. Assessment of local thinned areas. In: Proceedings of the international conference on fitness-for-service FITNET. Amsterdam; 2006. p. 281–99. [18] British Standard BS7910:2005. Guide to methods for assessing the acceptability of flaws in metallic structures; 2005. [19] Miller AG. The plastic collapse of cracked pipe bends under internal pressure or in-plane bending. Report no. TPRD/B/0806/R86. UK: Central Electricity Generating Board; 1986. [20] Bickell MB, Ruiz C. Pressure vessel design and analysis. 1st ed. Macmillan and Company Limited; 1967. [21] Stewart G., Klever F.J., Ritchie D. An analytical model to predict the burst capacity of pipelines. In: International conference on offshore mechanics and arctic engineering OMAE. Pipeline technology, vol. 5. American Society of Mechanical Engineers; 1994. p. 177–88. [22] Wallin K., Laukkanen A. Theoretical basis for correlation between irradiation induced change in yield strength and shift in fracture toughness transition temperature. VTT research report BTU072-041309; 2004.
Contents lists available at ScienceDirect
International Journal of Pressure Vessels and Piping journal homepage: www.elsevier.com/locate/ijpvp
Assessment of local thin areas in a marine pipeline by using the FITNET FFS corrosion module ˜ o a, b S. Cicero a, b, *, R. Lacalle a, b, R. Cicero a, b, D. Ferren a b
Dpto. Ciencia e Ingenierı´a del Terreno y de los Materiales, Universidad de Cantabria, ETS. Ingenieros de Caminos, Av. Los Castros s/n, 39005 Santander, Cantabria, Spain ´ gico de la Universidad de Cantabria, Fase A, Mod. 2003, Av. Los Castros s/n, 39005 Santander, Cantabria, Spain INESCO Ingenieros SL, Centro de Desarrollo Tecnolo
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 December 2007 Received in revised form 31 October 2008 Accepted 18 November 2008
This paper analyses the structural integrity of the marine stretch of a pipeline which is placed over a natural bay. The pipeline is part of a 30-year-old installation used for the provision of petrochemical products to a nearby chemical plant. Although there have been no relevant leaks in the past, both the visual inspections performed (revealing numerous local thin areas) and the fact that it is located in a highly sensitive place with high ecological and tourist value recommend the assessment of the pipeline in order to ensure that it is working in safe conditions and that there are no risks for the environment or the people living in the surrounding area. The assessment has been performed using the newly developed FITNET FFS procedure, whose local thin areas assessment methodology is also explained and compared to the analyses proposed by other well known procedures. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Local thin areas Structural integrity FITNET FFS
1. Introduction During the visual inspection of an approximately 1800 m long marine stretch of a pipeline placed over a natural bay, many local thin areas (LTA) were detected (Fig. 1 shows an example) in the exterior of the pipe. This pipeline, whose total length is around 5000 m, is used for the provision of petrochemical products (conversion oil) to a chemical plant and its failure would inflict substantial damage on the environment and the population living nearby. This makes it essential to assess the pipeline in order to ensure that it is working safely. The structural integrity analysis of the pipeline has been performed using the corrosion module of the FITNET FFS procedure [1– 4] and requires the following steps: structural analysis, stress analysis and assessment of the LTA (all of these are explained below). The FITNET FFS procedure has been the main result of the recently completed European Fitness for Service Network [1] and constitutes a unified European procedure for structural integrity assessment, which covers the analysis of components and structures under four main failure mechanisms: fracture-plastic collapse, fatigue, creep and corrosion.
* Corresponding author. Dpto. Ciencia e Ingenierı´a del Terreno y de los Materiales, Universidad de Cantabria, ETS. Ingenieros de Caminos, Av. Los Castros s/n, 39005 Santander, Cantabria, Spain. Tel.: þ34 942 200917; fax: þ34 20 18 18. E-mail address: [email protected] (S. Cicero). 0308-0161/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2008.11.021
Before the FITNET FFS procedure was developed there were numerous assessment procedures of a clearly national nature (British BS7910 [5] or the French RCC-MR [6]) and others whose application was restricted to a specific industry (i.e, R5 [7] and R6 [8] of British Energy and the Swedish SAQ [9] in the nuclear sector). Moreover, each of these procedures tended to focus on one specific type of failure (R5 on creep, BS7910, R6 and SAQ on fracture.), there being no procedure which allowed a component to be assessed for the various failure mechanisms. At the same time, some other industrial powers, such as the USA and Japan had their own procedures offering a simpler, more clearly defined picture. Thus, American procedures such as API 579 [10] in the petrol sector and ASME XI [11] in the nuclear industry or the Japanese JSME procedure [12] were fully defined and widely used in industry. This panorama in Europe, characterised by a lack of uniformity, an excessive variety, lack of global vision and delay with respect to other countries made it absolutely essential to develop a unified European procedure covering the main failure modes and offering European industry a tool with which to design, manufacture and manage industrial components and installations in a safer, more efficient way. Finally, the procedure is organised in three volumes (procedure [2], annex [3] and case studies and tutorials [4]) and has been submitted to the European Committee for Standardisation (CEN) for its adoption as a European standard for structural integrity.
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Nomenclature Do Di l LTA M n P Q R Rb RSFa rsfECmin rsfLmin t tm
outer diameter inner diameter length or longitudinal dimension of the thin area local thin area applied bending moment parameter used for the calculation of scyl working pressure length correction factor average radius in the transversal section of the elbow (as shown in Fig. 6) average radius of the elbow (as shown in Fig. 6) allowable residual strength factor (used in API 579) minimum required strength factor for circumferential failure for the elbow system stress minimum required remaining strength factor for the nominal hoop stress pipe thickness allowable remaining wall thickness for combined internal pressure and axial loading ðmax½tmL ; tmC Þ
Concerning the assessment of LTA, the FITNET FFS procedure provides a truly comprehensive and straightforward methodology that can be applied to most common components (cylindrical bodies, elbows, spheres.), providing critical parameters (minimum allowable wall thickness, safe working pressure, safe moment estimate.) for the corresponding working conditions. When compared to the equivalent methodologies of other well known procedures some considerations can be made: – It is more general than the ASME code (ASME XI, code case N597-2), focused on nuclear components. The ASME code also analyses both circumferential and longitudinal failures but it is even simpler. Also, and considering the critical safety aspects associated to the nuclear industry, it provides more conservative results through the consideration of the yield stress as the resistant parameter (and not the flow stress as considered in FITNET). – The API 579 assessment is, at first sight, a more complete methodology, but it is also noticeably more complex in terms of presentation, document extension, and the definition and obtaining of the different inputs. Also, it is mainly focussed on pipeline systems designed following API standards. If it is applied to other systems, designed using other procedures/ codes (or whose design bases are not well known), there are certain input parameters (design criteria, geometry of the different defects, historical sequence of the measurements, defect profiles.) whose definition or availability is not always straightforward. It proposes three different levels of analysis (I, II and III), level II being the one which is comparable to the FITNET FFS methodology when external loads are acting together with the internal pressure. The higher the level is, the more accurate the results are. This accuracy can be noticeably higher than that obtained with FITNET FFS when level III is applied.
tmC tmL w
aelbow belbow l sa sA scyl sf sh ssys suts syield sz
minimum allowable remaining wall thickness to prevent circumferential failure minimum allowable remaining wall thickness to prevent longitudinal failure width or circumferential dimension of the thin area elbow parameter quantifying the elbow effect on the hoop stress elbow parameter quantifying the elbow effect on the axial stress parameter used to quantify belbow axial stress from the axial force in the bend code allowable stresses nominal failure stress of an unflawed pipe flow stress (average value between the yield stress and the ultimate tensile strength) hoop stress system stress (sa þ sz) material ultimate tensile strength material yield stress axial bending stress
analysed here. All the pipes are supported every 12 m by a solid foundation system made up of reinforced concrete piles and bracing beams. Finally, the pipes are transversally joined by structural profiles that transmit the weight of the entire system to the main pipeline halfway between every two foundations. Fig. 2 shows a scheme of one piece of the model used to make the structural analysis of the whole system, showing the geometry of the structure. It can be observed that there is an expansion loop introduced in the route of the pipes in order to avoid high stresses and/or buckling problems caused by the relatively high temperatures (þ80 C) of the chemical products and the corresponding length increments of the pipes. In total, there are eight expansion loops in the pipelines, one every 200 m, as shown in the pipeline plan view illustrated in Fig. 3. Table 1 shows the diameters and thicknesses of the different pipes and Table 2 gathers the tensile properties of the main pipeline material [13].
2. Geometry and material properties The pipeline assessed is the main one in a system composed of five secondary additional smaller pipes whose integrity is not
Fig. 1. Example of LTA located in the pipeline.
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Table 1 Diameter and thickness in the different pipes. Pipeline Pipe Pipe Pipe Pipe
Fig. 2. Scheme of a stretch of the whole structure (model used in the structural analysis).
3. Structural analysis The pipeline is subjected to the following loads: – – – –
working pressure (0.8 MPa); own weight (7800 kp/m3); weight of the conversion oil (1000 kp/m3); loads transmitted by the secondary pipes (and the fluids in their interior); – loads transmitted by a footbridge placed on top of the pipeline (90 kp/m), as shown in Fig. 1; – thermal loads caused during the provision of the conversion oil (þ60 C).
Calculations have been performed using the software Tricalc [14] and the results show, as expected, that all the straight stretches are subjected to very similar bending moments and compression loads, something that also happens between the expansion loops. This simplifies the structural analysis, with just two different bending and compression diagrams. Table 3 shows the critical combination in both the straight stretches and expansion loops (the axes are defined in Fig. 4). In the latter, the critical section corresponds to the elbow. 4. Stress analysis Considering the results shown in Table 3, the worst combination of stresses caused by the bending moments, compression forces and internal pressure is shown in Table 4 for both the straight stretches and the expansion loops. These stresses are the inputs used in the structural integrity assessment. 5. Assessment of LTA following the FITNET FFS corrosion module The owner used guided ultrasonic longitudinal waves in order to locate the main metal loss areas in the entire pipeline stretch (1800 m) and then, ultrasonic transducers were used to measure the metal loss in those specific zones. Due to the enormous number of local thin areas it was decided not to make a one by one assessment, but to establish a characteristic defect for both the straight stretches and the expansion loops. This decision, which may be considered too conservative, is quite reasonable after a visual inspection of the pipeline and the results of the thickness measurements. These revealed a minimum thickness of 4.1 mm for
Fig. 3. Pipeline system plan view (1800 m).
2400 1200 800 600 (x2)
Outer diameter (mm)
Nominal thickness (mm)
609.6 304.8 203.2 152.4
9.52 8.38 8.18 7.11
the straight stretches and 4.4 mm for the expansion loops, the corresponding values for every single straight stretch or expansion loop not being very far from these values. Moreover, the defects were located in both the intrados and the extrados of the pipe and also in the upper and the lower part. Thus, it seems reasonable to consider one characteristic defect for each of the mentioned parts of the pipeline and consider that this defect can be perfectly located in the most stressed point of the pipe. If the fitness for service of the pipeline is demonstrated with these assumptions no more calculations would be needed, given that a conservative assumption would produce a safe assessment. The last data required for the analysis were the superficial dimensions of these defects. These were not supplied by the owner of the installation, so another inspection was necessary to determine them. Since the metal loss was mainly produced in the external surface of the pipeline, a simple visual inspection was enough to estimate that the dimensions w (width, or circumferential dimension) and l (length, or longitudinal dimension) could be taken as 300 mm and 400 mm, respectively, for elbows, and 500 mm (w) and 350 mm (l) for the straight stretches. This assumption is again conservative (since these are the maximum dimensions measured) but covers both the biggest metal loss areas found in the pipeline and the possible interactions between different defects. Therefore, the problem has been reduced to the assessment of two defects: the first one is located in the most stressed point of the expansion loops (elbow) and its dimensions are 300 mm for w and 400 mm for l, the remaining thickness being 4.1 mm; the second one is located in the most stressed point of the straight stretch, w being 500 mm and l 350 mm, and the remaining thickness 4.4 mm. The assessment procedure can be applied once the geometry of the defects, the stress state and the material properties are known. In this case, the FITNET FFS corrosion module (chapter 9 in the procedure) has been used. This consists of two types of assessments: (1) stress corrosion cracking and corrosion fatigue, in chapter 9.1, and (2) assessment of local thin areas (LTA), in chapter 9.2. A comprehensive explanation of the first type of assessment can be found in Refs. [15,16], this paper being focussed on the application of the second one (which is also introduced in Refs. [15,17]). Fig. 5 [2,18] shows the flowchart for the assessment of LTA. The procedure provides the formulation, after which the analysis can be applied, for the following types of components: cylindrical body, sphere and vessel end, elbow and nozzles. It also provides guidance for interaction rules. Finally, for both a cylindrical body and an elbow, the analysis can be performed to obtain the safe working pressure, the safe working system stress or the minimum allowable remaining wall thickness. Given that for the case analysed here (and for efficiency reasons in the provision process) the working pressure is fixed at 0.8 MPa, calculations will be oriented to obtain the minimum allowable remaining wall thickness.
Table 2 Material properties (lower bound values). Material
Yield stress (syield, MPa)
Ultimate tensile strength (su, MPa)
Young’s modulus (MPa)
Steel ASTM A333
205
380
200,000
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Table 3 Main forces and moments on pipe 2400 .
Straight stretches Elbows in expansion loops
Fx (kN)
My (kN m)
Mz (kN m)
43.1 z0
50.4 107.4
75.7 55.9
Straight stretches Elbows in expansion loops
For the elbows (located in the expansion loops), the steps in the assessment (section 9.2.5.5.3 in Ref. [2]) are shown below. An explanation of the formulation can be found in Ref. [17]. For internal pressure, it assumes that the stress distribution in a pipe elbow can be approximated by the membrane stress solution to the thin shell equilibrium equations for an axisymmetric shell of revolution [19,20]: (a) Determine the allowable remaining wall thickness to prevent longitudinal failure (hoop stress): (a.1) Calculate the nominal failure stress of an unflawed pipe (scyl), following Ref. [21]:
scyl
n 1 suts ¼ 2
(1)
where suts is the ultimate tensile strength and n [22]:
n ¼
65
(2)
syield
syield is the yield stress expressed in MPa. Here, scyl is 305 MPa. (a.2) Calculate the elbow parameter aelbow using:
aelbow ¼
(3)
R 12R b
where R and Rb are defined in Fig. 6. For the case analysed, R is 299.9 mm and Rb is 1524 mm and, therefore, aelbow is 0.89. (a.3) Calculate the hoop stress, sh:
P
aelbow
saxial (MPa)
shoop (MPa)
38.5 39.4
25.2 25.2
"
syield sh; elbow sh; elbow ¼ max ; sA 0:85scyl syield
rsf ELmin
# (5)
The hoop stress should not be higher than the code allowable stress [2]. Typical code allowable stresses (sA) for pipelines are listed in Table 5. Here, the following value has been used [2]:
sA ¼ min
syield suts 1:5
;
3
(6)
sA is 126.6 MPa and rsfELmin is 0.18. (a.5) Calculate the length correction factor (Q): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi l2 Q ¼ 1 þ 0:8 Do t
(7)
where l is the length of the defect. Q is 3.66. (a.6) The minimum allowable remaining wall thickness to prevent longitudinal failure (tmL) is:
tmL ¼ t
rsf ELmin ð1Q1
ELmin 1rsf Q
(8)
giving tmL ¼ 1.39 mm.
1RRb
sh; elbow ¼
Table 4 Total stresses (maximum absolute values) due to external loads and internal pressure. The elbow values do not include the aelbow and belbow correction factors.
ðDo tÞ 2t
(4)
where P is the working pressure, Do is the outer diameter and t is the thickness. Here, sh is 28.4 MPa. (a.4) Calculate the minimum required remaining strength factor (rsfLmin) for the nominal hoop stress (sh, elbow):
(b) Determine the allowable remaining wall thickness to prevent circumferential failure (axial stress): (b.1) Calculate the axial bending stress (sz) for moment M (in this case My caused by thermal loading) of a pipe without corrosion:
sz; elbow ¼
b ¼
l2=3
Z ¼
x
(10)
0:9 tRb R2
(11)
y
(9)
where
l ¼
z
M
belbow Z
p
D4o D4i
32
Do
(12)
Di is the inner diameter. Here l is 0.161, belbow is 0.329, Z is 0.002725 m and, finally, sz, elbow is 119.8 MPa. Here it is important to note the influence of the elbow geometry in the stress state, since belbow provides stress values around three times bigger than the corresponding ones for a straight pipe. (b.2) Calculate the axial stress (sa, elbow) from the axial force in the bend:
sa; elbow ¼
F
pDt
(13)
Here, the axial stress is negligible. (b.3) Calculate the elbow system stress (ssys, elbow): Fig. 4. Axes definition in the pipeline section.
ssys; elbow ¼ sa; elbow þ sz; elbow
(14)
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Fig. 5. Flowchart LTA assessment procedure [9.1].
(b.4) Calculate the flow stress (sf):
sf ¼
tm ¼ max½tmL ; tmC
syield þ suts
(15)
2
Here, sf is 292.5 MPa. (b.5) Calculate the minimum required strength factor rsfECmin for circumferential failure for the elbow system stress, ssys, elbow:
rsf ECmin ¼
1
sf
X1
sh; elbow 2
þ X2 ssys; elbow
(16)
being
" X1 ¼ max X2 ¼
syield sh; elbow ; sA syield sf
X sSA 1s syield 2 A
sSA s2A
# (17)
Therefore, for the expansion loops and for the hypotheses considered here, the minimum thickness required to ensure their integrity is 2.30 mm. A totally analogous assessment process is provided in FITNET FFS [2–4] for the evaluation of cylindrical bodies (straight stretches) and, therefore, the calculation process is not presented here. The formulae are also analogous, but not including the geometrical factors a and b used for the consideration of elbow geometry. Also, the allowable remaining wall thickness is calculated for both the longitudinal and the circumferential failure. For the case analysed here and for the hypotheses considered, tm is 1.90 mm (tmC being the critical parameter).
! (18)
sA is the code allowable hoop stress and sSA is the code allowable axial stress, both following Eq. (6) (see Table 5). Here, rsfECmin is 0.66. (b.6) The minimum allowable remaining thickness to prevent circumferential failure (tmC) is given (as shown in Fig. 9.9 in Ref. [2]) by:
" tmC ¼ t
# rsf 1 Cmin
1 ½0:707cos1 f0:45sin Dwo 0:318Dwo 1:111
(19)
For the case analysed, tmC is 2.30 mm. (c) Finally, the minimum allowable remaining wall thickness (tm) for combined internal pressure and axial loading is given by:
(20)
Fig. 6. Elbow dimensions [1].
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Table 5 Summary of typical allowable design stresses for typical pipelines and piping. Design code
Allowable circumferential stress Allowable total axial stress for non-incidental loads Allowable external stresses at full design pressure
B31.3 Chemical plant and refinery piping
B31.4 Liquid transportation systems for hydrocarbons
B31.8 Gas transmission and distribution piping systems
ISO 13623 Petroleum and natural gas industries pipelines
UTS minðSMYS 1:5 ; 3 Þ
0.72 SMYS
UTS minðSMYS 1:5 ; 3 Þ
0.54 SMYS
UTS minðSMYS 1:5 ; 3 Þ 2
0.18 SMYS
Ranging from 0.40 to 0.80 SMYS depending on location class 0.75 SMYS for onshore pipeline 0.8 SMYS for offshore riser and pipeline 0.35–0.55 SMYS depending on location class
0.45–0.83 SMYS depending on location class and product zSMYS depending on location class and product z0.58–0.77 SMYS depending on location class and product
6. Discussion Once the minimum allowable remaining wall thickness has been obtained in both the elbows and the straight stretches, the results can be compared to the measured values in order to evaluate the structural integrity of the pipeline and, finally, take the corresponding decisions (i.e, inspection periods, replacements, repairs, etc): – For elbow locations (placed in the expansion loops), the minimum allowable thickness is 2.30 mm, which is noticeably lower than the minimum measured (4.4 mm). Then, thickness has been reduced by 5.4 mm in 30 years, which represents a metal loss rate of 0.18 mm/year (considering a constant rate) and, therefore, the minimum allowable thickness would be achieved in 11.66 years. However, given that w and l also increase with time, the actual value would be lower. This fact, together with the uncertainty of the metal loss rate, makes it recommendable to perform the inspection of the elbows noticeably before the mentioned 11.66 years (i.e, 2–3 years). – For straight stretches, the minimum allowable thickness is 1.90 mm, which is still somewhat lower than the minimum measured thickness (4.1 mm). The corresponding thickness loss rate (if constant) is 0.19 mm/year and the expected time for the minimum allowable thickness to be achieved is more than 11.58 years. Again, this value is an upper bound (given that w and l also grow) and the thickness loss rate is uncertain, so it is recommended here to perform the next inspection much before 11.58 years (i.e, 2–3 years). If the LTA analysis is performed using the ASME code (case N597-2) and API 579, the results obtained are the following: – Both the ASME code and API 579 do not provide explicitly solutions for elbows when external loads are applied simultaneously with the internal pressure. Therefore, FITNET FFS provides solutions for a wider range of industrial situations. – For straight stretches, ASME provides a minimum allowable remaining thickness of 2.82 mm, and API 579 provides (considering a reasonable allowable remaining strength factor, RSFa, of 0.8) 2.01 mm. The ASME solution is more conservative, while the API 579 solution is very similar to that obtained using the FITNET FFS. The main difference between ASME and the other two procedures is the resistant parameter considered in the calculations: the former uses the yield stress and the latter uses the flow stress. 7. Conclusions Local thin areas detected in a pipeline placed in a marine environment have been analysed using the FITNET FFS corrosion module. After considering some conservative (but realistic and reasonable) hypotheses, the structural integrity of the system has been demonstrated and recommendations about inspection periods have been proposed.
As shown here, the FITNET FFS corrosion module provides a comprehensive and relatively simple methodology for assessing components (cylindrical bodies, spheres and vessel ends, elbows and nozzles) with local thin areas. This procedure constitutes an alternative to other well known procedures, providing a general methodology that covers the four major failure modes (not just the LTA assessment) and being applicable to all kinds of industries. As an example the results obtained using the FITNET FFS procedure have been compared to those obtained using the ASME code and API 579, and the differences detected have been justified. References [1] European fitness-for-service network. EU’s framework 5, proposal no. GTC12001-43049, contract no. G1RT-CT-2001-05071. [2] ISBN 978-3-940923-00-4. In: Kocak M, Webster S, Janosch JJ, Ainsworth RA, Koers R, editors. FITNET fitness-for-service (FFS) procedure, vol. 1. Geesthacht, Germany: GKSS Research Centre; 2008. [3] ISBN 978-3-940923-01-1. In: Kocak M, Hadley I, Szavai S, Tkach Y, Taylor N, editors. FITNET fitness-for-service (FFS) annex, vol. 2. Geesthacht, Germany: GKSS Research Centre; 2008. [4] Kocak M., Laukkanen A., Gutie´rrez-Solana F., Cicero S., Hadley I., editors. FITNET fitness-for-service (FFS) case studies and tutorials, vol. 3. ISBN 978-3940923-02-8, in press. [5] British Standard BS7910. Guide to methods for assessing the acceptability of flaws in metallic structures. London: BSI; 2000. [6] RCC-MR: Re`gles de Conception et de Construction des mate´riels me´caniques des ˆılots nucle´aires RNR. AFCEN; 2002. [7] R5: assessment procedure for the high temperature response of structures. Procedure R5, Issue 3. Gloucester, UK: British Energy; 2003. [8] R6: assessment of the integrity of structures containing defects. British Energy Generation. Report R/H/R6, revision 4; 2001. [9] Bergman M, Brickstad B, Dahlberg L. A procedure for safety assessment of components with cracks – handbook. SAQ/FoU report, 9/101, AB Svensk Anla¨ggningsprovning. Swedish Plant Inspection Ltd; 1991. [10] API 579. Recommended practice for fitness for service. Draft issue 4. American Petroleum Institute; 1996. [11] ASME boiler and pressure vessel code, section XI: rules for in-service inspection of nuclear power plant components. The American Society of Mechanical Engineers; 1995. [12] JSME. Codes for nuclear power generation facilities – rules on design and construction for nuclear power plants. JSME S NC1; 2001. [13] ASTM A333M-88a/ASME SA333. Seamless and welded steel pipe for low temperature service. [14] Tricalc 6.4. Ca´lculo de Estructuras Tridimensionales. Arktec; 2007. [15] Kocak M. FITNET fitness for service procedure: an overview. In: Proceedings of the international conference on fitness-for-service FITNET. Amsterdam; 2006. p. 3–14. [16] Gutie´rrez-Solana F., Cicero S. FITNET FFS procedure: a unified European procedure for structural integrity assessment. Engineering Failure Analysis. Available online, 10.1016/j.engfailanal.2008.02.007. [17] Ritchie D., Koers R. Assessment of local thinned areas. In: Proceedings of the international conference on fitness-for-service FITNET. Amsterdam; 2006. p. 281–99. [18] British Standard BS7910:2005. Guide to methods for assessing the acceptability of flaws in metallic structures; 2005. [19] Miller AG. The plastic collapse of cracked pipe bends under internal pressure or in-plane bending. Report no. TPRD/B/0806/R86. UK: Central Electricity Generating Board; 1986. [20] Bickell MB, Ruiz C. Pressure vessel design and analysis. 1st ed. Macmillan and Company Limited; 1967. [21] Stewart G., Klever F.J., Ritchie D. An analytical model to predict the burst capacity of pipelines. In: International conference on offshore mechanics and arctic engineering OMAE. Pipeline technology, vol. 5. American Society of Mechanical Engineers; 1994. p. 177–88. [22] Wallin K., Laukkanen A. Theoretical basis for correlation between irradiation induced change in yield strength and shift in fracture toughness transition temperature. VTT research report BTU072-041309; 2004.